NA 3.1L=>head & camshaft questions. No shortcuts, max

[zredbaron]

bradyzq --

Thanks for the graph analysis! It's interesting that the curve shifted to the right. There are a lot of changes between these motors: outer-springs only, freshness of fuel, Stahl headers, no airbox, lightweight rocker arms and retainers, twin idler gear, and an adjustable cam pulley (which looks to be around 1 degree in my pictures, I think it was at 0 degrees in 2008?). Power doesn't really fade much at the 7500 RPM redline, either, which I didn't expect. Stahl headers, I suspect, allowing the head and cam to perform as they would.

Yes, I'm running Electromotive's XDI ignition. I will be upgrading to their TEC-3R or whatever they'll call their current version when I get to ITBs. Currently my high RPM A/F ratio goes way rich because the 40 DCOEs with 36mm venturis can't supply the engine's air requirements above 4-5000 RPM. Oxygenated fuel helped!

 

 

Interesting... I was starting to suspect "reversion" and came across another hybridz post (sometime during the hours and hours of searching/reading) where another member addressed their "bog" issues completely by adjusting the cam timing.  My engine builder spec'd out the cam timing with three different settings on my adjustable cam sprocket.  One of my recent tuning changes was to retard the cam a touch to see if it improved the wide open throttle (WOT) below 4k situation.  Retarding the cam changes intake valve opening and closing times relative to piston position, so it has an impact of the amount of reversion.

 

Ah... so when you pulled off the rocker cover, the number one valve (exhaust) was as pictured: valve keepers missing and valve bent (it looks bent to me...).

 

You mention a rock or a nail... Were you running without an air filter?  How about a pic of the number one spark plug?

 

No air filter, no hood, just air horns. Exactly as shown in the video. At the time of the failure, I was driving behind traffic at about 10-15 mph @ 3k RPM, an easier speed for bouncing pebbles to have a fighting chance, perhaps even thrown up from the old slicks a couple feet away. Let's not forget I was in AR's Ozark Mountains, where more mud and rocks are flung around than just about anywhere else. Whoops?

EDIT -- I'll have to get back with pics of the spark plug when able. I think the exhaust valve looks bent too, but I'm not experienced enough to pronounce it so.

Yes, I suspect reversion between the intake manifold and the exit of the carburetor is the bulk of what we're experiencing under 4k, and I'm pretty sure that's what Dan Baldwin was describing as well. So did retarding the cam help driveability? I understand in theory it might, but how did it drive? That's why I have the adjustment available... After all, I'm not seeking a WOT high hp motor but a wide graph with power under the curve for autocross.

ryant67 --

You're absolutely right about those airbox rivets! They don't last... the vibrations ream out the fiberglass and the rivets fall out. I drilled the problem-rivets out and replaced them with nuts and allen head screws in 2008.

EDIT -- The airbox wasn't on the car when the incident occurred because it conflicts with my shock tower modifications for strut braces. The box is old and now it won't ever fit anyway, so I'm trying my hands at a fabrication experiment with it. It'll be a while before I return to it, but this is how much I ended up cutting out in order to use the box with a Canon manifold and my strut tower modifications. I cut rectangular shapes because I'm going to use sheet rubber so the box can "be flexible" when removing or installing, but pop back out into full shape once the maneuvering is complete, returning the air box to nearly the same performance characteristics. Leaning towards clenching the sheet rubber with fiberglass patches on both sides, but I'm not sure how that will hold up and will need to experiment first.

Edited May 9, 2015 by zredbaron

[bradyzq]

Yes, I'm running Electromotive's XDI ignition. I will be upgrading to their TEC-3R or whatever they'll call their current version when I get to ITBs. Currently my high RPM A/F ratio goes way rich because the 40 DCOEs with 36mm venturis can't supply the engine's air requirements above 4-5000 RPM. Oxygenated fuel helped!

 

We the Hybridz collective have talked about this (airflow limits of sidedrafts vs ITB EFI) often, but when doing the math, your 235whp at 6500RPM requires 51ish cfm per cylinder, which, in a 36mm unimpeded veturi, would travel in the neighbourhood of 53mph. Not very high or demanding numbers, are they? At 4500RPM, the numbers are less than 75% of that. I wonder if there is something else at play here. Maybe an overly strong aux venturi? We don't hear them mentioned often. They must have an effect on fueling, though, or they wouldn't be there.

[NewZed]

 Currently my high RPM A/F ratio goes way rich because the 40 DCOEs with 36mm venturis can't supply the engine's air requirements above 4-5000 RPM. 

This logic seems backward or warped.  Seems like you've decided that choked off air supply is the problem and are trying to force observations to fit that model, with the "reversion" (whatever that really means, especially at high air flow) and the above.  Sometimes it's hard to do but you might try letting go of that idea and see if something else makes sense.

[zredbaron]

I think we're talking about two different modes of operation. I have two separate RPM ranges with two separate breathing challenges, and they can't quite be interchanged.

 

Below 4k RPM, as per Dan Baldwin's comments and several of our dyno plots, with big cam overlap the engine simply doesn't breathe well. Reversion is perhaps the issue at this in-between mode of operation where the intake system is very unhappy and VE is very low. This is a mainly a function of the camshaft duration and is worsened by running triple carburetors as opposed to EFI. I imagine the aux venturi of the DCOE exacerbates this issue relative to ITBs, since they have more of a straight-through design.

Above 5k RPM (or so), for my engine specifically, I'm undercarbureted. The main venturis can't supply an adequate volume of air, and this is observed on my A/F graph as it tapers more rich as RPM increases. Said another way, since I'm running 36mm venturis, as RPM continues to increase past ~5000 rpm, my VE decreases. To the best of my knowledge, this will still occur with other venturi sizes and even ITBs, the graph shape will simply taper differently. Perhaps with ITBs this can be almost negligible if the correct sizes are chosen for a given application.

But hey, this is a layman's understanding, and this layman doesn't have a very good track record at the moment...

[steve260z]

Well, if the carbs are too small for the motor wouldn't you just see less than max power and possibly a "choking off" of the motor.? Not wanting to rev past a certain level? What you are saying regarding the AFR ratio assumes once the carbs are at the limit regarding air flow they somehow continue to provide more fuel as the air volume remains the same. How is that possible?

[zredbaron]

One or more of us are barking up the wrong tree, no doubt! Haha. Good stuff, learning is underway!

[significantly edited for readability, this grew rather long! I made no assumptions about technical backgrounds.]

 

No, I'm not saying the carburetors provide more fuel as the demand for air volume remains the same --  I'm saying the carburetors provide a constant amount of fuel, but when severely restricted, the *mass* flow rate of oxygen doesn't keep up with increasing demands for volumetric flow rate of air. I'm saying the more an intake system is restricted, the less laminar the airflow through a wider operating range. Less laminar fluid flow is less dense, which means less O2 is available for combustion. Same volume of "air" carrying less O2 mixed with the same amount of fuel, and the engine goes measurably rich.

 

Let's take a step back. The carburetors come in different housing sizes (40, 45, 50) and offer different venturis. The manufacturing and performance industries both recognize that the airflow requirements to realize 150hp through 2.4L are different than realizing 300hp through 3.1L, both in terms of peak power and drivability. The airflow adjustments are with our intake shape and size, the venturis in the case of sidedraft carburetors, do we agree? None of us are talking about having to upgrade our fuel pumps or fuel lines, we only make fuel adjustments with jets. (The air corrector jet is not a "corrector" but the finest last-stage adjustment; it cannot provide adequate adjustment range if an improper venturi or main jet is selected.)
 

Practically, if we had a fully-tuned race engine running 50 DCOEs and we were to then place 40 DCOEs on this engine and severely restrict it, I'm here to argue that there is no collection of jets that allow this 40 DCOE engine to have a remotely flat or stoichiometric A/F line ~5000 RPM and above, because the carburetor body does not allow proper venturi selection. It tapers way rich no matter what, making jet selection simply a matter of how early or late (RPM) you want to cross the stoichiometric line (main jet) and at what angle (air correctors). I choose to enter the region lean, and split the difference, knowing it will be going way rich unavoidably.

Why this limit exists is another discussion, and here's my take. The short version: the air is "stalling" on the way into the engine.

 

The detailed version: Fluid is of course any composition of gas and liquid, a good example being "air." Mixed composition fluid flow through an open system is very, very complex. It isn't simply water through a hose (same in, same out) and it isn't a wing moving through the fluid, or air. It's both and neither. In addition to its static variance in composition, pressure and temperature, fluid flow does all sorts of hard-to-control things: it can compress and expand, it can speed up and slow down, it can be laminar and dense or turbulent and unpredictable. In subsonic fluid flow, the faster a fluid travels, the more head loss or parasitic loss is encountered (drag or friction, essentially). As velocity and its associated losses continue to increase, the laminar layers of fluid flow are further separated from one another and the fluid flow becomes more and more turbulent and less dense. This means that above and below the one and only one ideal speed (the one speed with the fewest losses for a given set of conditions), as speed diverges further from the sweet spot, *mass* flow rate continues to decrease. For example, the reversion range might be for speeds too far below ideal, and the stalling range would of course be for speeds too far above ideal.

 

What I don't know, is how far into the turbulent regions of fluid flow we are going. Reversion is a specific form of turbulence, and so is stalling. This discussion is much of why I would like to dyno the 40 DCOEs "fully tuned" and then 45/50 DCOEs fully tuned. ("Fully tuned" meaning the best I can do with the limitations of my jets, venturis, emulsion tubes, timing, etc.) The dyno plots would offer some of the data this discussion currently lacks.

Similar fluid limitations are also present in propellers & pumps and wings & turbines. Cavitation occurs with propellers and pumps when a liquid can't move as fast as the conditions are asking it to (pressure difference is too great). Stalling occurs with aircraft wings and a turbines (an air pump comprised of thousands of tiny wings) when the pressure difference is too great, and the airflow goes from laminar to turbulent. I find it helpful to make the correlation that stalling is to gases what cavitation is to liquids; liquids and gases are both fluids, and both fluids respond unfavorably to excessive pressure differentials.

The crankshaft and pistons are an air pump. Not quite a water pump, not quite a turbine. A piston-driven [compressible] air pump, like for inflating tires, is very crude in terms of fluid flow performance, or VE. The pulsing of fluid flow screws everything up bigtime, which is what centrifugal pumps and turbines were designed to overcome (but the rotary engine does not). Valvetrain and check-valves do not dampen pulsing, they only truncate or rectify it.

 

I submit that with small enough straws and big enough slugs at a high enough RPMs, the airflow stalls (the pump cavitates if you like) as the flow goes from laminar to turbulent. This stall would be occurring I suppose in a toroid shape across the plane of the venturis or air horns or both and the stall would probably extend for several centimeters downstream before recovering to more laminar flow, but that's just theory of course. The molecules simply can't get into the hole fast enough, and since a gaseous mixture is compressible they end up fighting each other on the way in. This allows fewer total molecules through the door, just like a panicked room full of people relative to an orderly group. No really, it's for your health, people: try and stay calm.

 

It's like the opposite end of peeling out, but through your air intake system (the first molecules from world to car) rather than mechanical power output (the first molecules from car to world). You tried to access/wield more force than physics would allow. "You suck too fast, you drive slow now."

 

When fluid flow can't keep up, all bets are off. In supersonic fluid flow this manifests by all relationships becoming inverted (venturi principle is inverted). I'm not suggesting supersonic velocities are reached in our intake systems, I'm simply highlighting that you can't put fluid into a single conceptual box, or it will find a way to spring a leak and remind us that we're still figuring fluid out more and more every year. That's why fluid is also a verb; nothing else behaves like it. The moment we make a rule for it, we need to provide rules for its exceptions, which we normally haven't discovered yet.

Sounds a lot like the laws of man, but let's leave the politics out of it! Haha!

Cheers, fellas. However this fog lifts, let's clear the air!

Edited May 13, 2015 by zredbaron

[bradyzq]

One or more of us are barking up the wrong tree, no doubt! Haha. Good stuff, learning is underway!

 

No, I'm not saying the carburetors provide more fuel as the demand for air volume remains the same --  I'm saying the carburetors provide a constant amount of fuel, but when severely restricted, the *mass* flow rate of oxygen doesn't keep up with increasing demands for volumetric flow rate of air. I'm saying the more an intake system is restricted, the less laminar the airflow through a wider operating range. Less laminar fluid flow is less dense, which means less O2 is available for combustion. Same volume of "air" carrying less O2 mixed with the same amount of fuel, and the engine goes measurably rich.

 

Let's take a step back. The carburetors come in different housing sizes (40, 45, 50) and offer different venturis. The manufacturing and performance industries both recognize that the airflow requirements to realize 150hp through 2.4L are different than realizing 300hp through 3.1L, both in terms of peak power and drivability. The airflow adjustments are with our intake shape and size, the venturis in the case of sidedraft carburetors, do we agree? None of us are talking about having to upgrade our fuel pumps or fuel lines, we only make fuel adjustments with jets. (The air corrector jet is not a "corrector" but a fine, last-stage adjustment; it cannot provide adequate adjustment range if the wrong venturi is selected.)

 

Practically, if we had a fully-tuned race engine running 50 DCOEs and we were to then place 40 DCOEs on this engine and severely restrict it, I'm here to argue that there is no collection of jets that allow this 40 DCOE engine to have a remotely flat or stoichiometric A/F line ~5000 RPM and above, because the carburetor body does not allow proper venturi selection. It tapers way rich no matter what, making jet selection simply a matter of how early or late (RPM) you want to cross the stoichiometric line (main jet) and at what angle (air correctors). I choose to enter the region lean, and split the difference, knowing it will be going way rich unavoidably.

 

Why this limit exists is another discussion, and here's my take. Fluid is of course any composition of gas and liquid, a good example being "air." Mixed composition fluid flow through an open system is very, very complex. It isn't simply water through a hose (same in, same out) and it isn't a wing moving through the fluid, or air. It's both and neither. In addition to its static variance in composition, pressure and temperature, fluid does all sorts of hard-to-control things: it can compress and expand, it can speed up and slow down, it can be laminar and dense or turbulent and unpredictable. In subsonic fluid flow, the faster a fluid travels, the more head loss or parasitic loss is encountered (drag or friction, essentially). As velocity and its associated losses continue to increase, the laminar layers of fluid flow are further separated from one another and the fluid flow becomes more and more turbulent and less dense. This means that above and below the one and only one ideal speed (the one speed with the fewest losses for a given set of conditions), as speed diverges further from the sweet spot, *mass* flow rate continues to decrease.

 

What I don't know, is how far into the turbulent regions of fluid flow we are going. Reversion is a specific form of turbulence. This discussion is much of why I would like to dyno the 40 DCOEs "fully tuned" and then 45/50 DCOEs fully tuned. ("Fully tuned" meaning the best I can do with the limitations of my jets, venturis, emulsion tubes, timing, etc.) The dyno plots would offer some of the data this discussion currently lacks.

 

Though not quite comparable, similar fluid limitations are also present in pumps, propellers and wings. Cavitation occurs when a liquid can't move as fast as the conditions are asking it to (pressure difference is too great). An aircraft wing also "stalls" when the pressure difference is too great, and the airflow goes from laminar to turbulent. You could think of stalling is to gases what cavitation is to liquids if it's helpful, I certainly do. Liquids and gases are both fluids, and both fluids respond unfavorably to excessive pressure differentials.

 

I submit that with small enough straws and big enough slugs at a high enough RPMs, the airflow stalls / cavitates as the flow goes from laminar to turbulent. This stall would be occurring I suppose in a toroid shape across the plane of the venturis and the stall would probably extend for several centimeters downstream before recovering to more laminar flow, but that's just theory of course. The molecules simply can't get into the hole fast enough, and since a gaseous mixture is compressible they end up fighting each other on the way in. This allows fewer total molecules through the door, just like a panicked room full of people relative to an orderly group. No really, it's for your health, people: try and stay calm.

 

It's like the opposite end of peeling out, but through your air intake system (the first molecules from world to car) rather than mechanical power output (the first molecules from car to world). You tried to access/wield more force than physics would accommodate. "You suck too fast, you drive slow now."

 

When fluid flow can't keep up, all bets are off. In supersonic fluid flow this manifests by all relationships becoming inverted (venturi principle is inverted). I'm not suggesting supersonic velocities are reached in our intake systems, I'm simply highlighting that you can't put fluid into a single conceptual box, or it will find a way to spring a leak and remind us that we're still figuring fluid out more and more every year. That's why fluid is also a verb; nothing else behaves like it. The moment we make a rule for it, we need to provide rules for its exceptions, which we normally haven't discovered yet.

 

Sounds a lot like the laws of man, but let's leave the politics out of it! Haha!

 

Cheers, fellas. However this fog lifts, let's clear the air!

 

Wow, had to read that one a few times! 

 

We will learn whatever is hidden up in those trees.

 

I suggest that ALL the principles behind a working carb are volume, not mass, based, at all times. So, density changes are not directly tunable without changing parts (jets, etc.). I also suggest that you will not be able to strip the oxygen out of the air under any conditions encountered in a carb.

 

Air volume flow will remain proportional to mass air flow for any given set of atmospheric conditions at the carb inlet.

And mass air flow should be proportional to power.

And since the venturi is not changing size, power is proportional to air speed though the venturi.

And air speed will be proportional to pressure drop through the venturi.

Pressure drop at the venturi should be proportional to fuel flow if there was only one jet and nozzle.

 

So, based on that, kinda sorta, if the engine keeps one getting richer and richer, it's because more and more fuel is getting drawn into the carb by higher and higher airflow. Have you measured vacuum in the intake manifold at WOT at higher engine speeds? That would help tell you if the carbs were really a "severe restriction."

 

EDIT: Basically, this is stating what steve260z said a couple of posts up, but in a much less efficient way....

EDIT#2: Errr, it's my description that's the less efficient of the 2.

Edited May 13, 2015 by bradyzq

[zredbaron]

We're definitely honing in the discussion, at the very least! I thought your descriptions were just fine, Brady. You and Steve are much better at being concise than I!

 

I suggest that ALL the principles behind a working carb are volume, not mass, based, at all times. So, density changes are not directly tunable without changing parts (jets, etc.). I also suggest that you will not be able to strip the oxygen out of the air under any conditions encountered in a carb.

100% agree! 

 

 

Air volume flow will remain proportional to mass air flow for any given set of atmospheric conditions at the carb inlet.

And mass air flow should be proportional to power.

And since the venturi is not changing size, power is proportional to air speed though the venturi.

And air speed will be proportional to pressure drop through the venturi.

Pressure drop at the venturi should be proportional to fuel flow if there was only one jet and nozzle.

This is where we diverge. Volumetric flow rate will be proportional to mass flow rate, yes, but I'm saying they will diverge farther and farther from one another with higher demand/restriction ratios (velocity continues to increase). You seem to indicate they are a given proportional relationship for given atmospheric conditions.

From what I can tell, I have different conclusions because you and Steve are fixated on the tools we have and how to use them, and I'm fixated on the details of our physical limitations as a basis for engine design. (I just edited my last post and worded the phenomenon as more of "stall," if that changes anything.)

I haven't measured vacuum, but I haven't observed anything that looks like measured vacuum would be a helpful direction, either. I have a vacuum canister to help with braking (it does nothing). I also have a MAP (manifold absolute pressure / vacuum) sensor hooked up to my ignition, so that it can vary advance with sensed load. I've dynoed it with and without the MAP sensor hooked, up, and it doesn't really affect the engine performance. Again, "good vacuum" can only be observed with proper carburetor and venturi selection, and not all camshafts are equal. If you're suggesting an experiment could be set up to observe where the "stall" occurs, I agree! I don't have any vacuum data for that, no.

 

 

"Rich" is a condition involving combustion chemistry and nothing else. If the engine is rich, there is more fuel than there is available oxygen, period. There are no other variables such as volumetric efficiency, conditions, etc. -- those determine how we get there. The bottom line in combustion is chemistry, so only the balanced stoichiometric chemical equation matters. The carburetor parts and/or EFI are how we benefit from obeying the stoichiometric equation; the parts are not how we bend the laws of physics to our will.

 

A carburetor is a "dumb" device that simply responds to pressure differentials, we agree. It has no idea what the composition of the "air" is, therefore it's up to the human to mechanically choose the appropriate jets, venturi, etc. Its purpose is to deliver a metered amount of fuel based on pressure difference. This pressure difference is a very crude approximation of engine demand, and it knows nothing about the air density. This approach is over 100 years old, let's not forget! Engine demand is responded to with a metered amount of fuel, and delivery is unverified. Too little or too much, Goldilocks, you get what you get.

 

Enter EFI, and this entire conversation is now unobservable. We are still subject to the laws of physics, but now the symptoms are nearly invisible. An O2 sensor immediately senses that there is a shortage of O2 (or abundance) and adjusts how long the injector stays open on the next engine cycle, balancing the stoichiometric equation as per the computer's programming. When the car used to fall flat on it's face and say "choose a different venturi, I need more air, stoopid" now the computer smooths it out and I don't have to feather the throttle. I'm simply "not in my power band yet" because my cam is so aggressive. Wrong, but partially correct. The power band is still restricted, and the computer feathered the fuel delivery instead of my foot feathering the throttle plate.

 

Neither the computer nor the jets addressed the physical limitation that the venturi is the wrong size or that VE was inadequate for engine demand! From what I can tell, you and Steve are talking about fluid flow like it's a "given day" type of static variable. Yes, that is a huge part of the foundation, but the science that's dedicated to it studies dynamics, not statics. I keep hearing "tune the carburetor, they're very tunable." I agree, but I don't hear anyone acknowledging that if we put a single SU carburetor on a big block V8, that carburetor wouldn't be tunable. They're only tunable on the right applications!

 

VE isn't some subtle, in-the-weeds technotalk that can be left for the engineers. It's huge. Our Datsuns' OHC intake system is from the dark ages, and so are carburetors. We can't limit our thinking to the words printed in Weber books decades ago, technology moves way too fast these days for that.

 

Let's not forget that Koenigsegg and Ferrari are all about intake VE via variable length manifolds:

http://en.wikipedia.org/wiki/Variable-length_intake_manifold
 

VE changes so much at different RPMs and different pedal positions, that intake manifolds themselves have been discovered to be one of the most significant areas for improvement in VE, which gives us both power and fuel economy. VTEC is another way that intake VE limitations are addressed. Multiple valves are another. ITBs vs. EFI is another. We mask our VE limitations with a vacuum advance distributor or vacuum sensor + pedal position for EFI, but both are simply patches to our gaps in fluid flow performance. Variable intake manifolds and VTEC address the limitations. Talking about tuning the carburetor or the computer is a patch; talking about carburetor selection or the intake system's design addresses the limitation.

 

Do the old carburetor books talk about how important intake VE is? In the language of 1985's understanding of 1965's technology, they mention it, you bet. In the language of what Formula 1 and EcoBoost are doing in 2015, those books don't have any idea what we're talking about these days, and they'll guide us back to yesteryear if we let them.

Wow, Weber DCOEs are over 50 years old! That's pretty crazy.

Edited May 13, 2015 by zredbaron

[bradyzq]

I understand all the improvements in engine breathing are breathtaking. Pun intended.

 

But what Steve and I are trying to say is that you're running too rich because of too much fuel, not due to a lack of oxygen (air).

 

For example, in a modern, EFI'd spark ignition gasoline piston engine, if you're cruising along the highway at an AFR of 10:1, would you say that the engine needs more oxygen and you must therefore turbo/blower/nitrous it? Unlikely. You would say that it needs less fuel and remove some in the fuel table.

 

I was suggesting measuring MAP at WOT to see if pressure starts dropping where you think the choke/stall point may be.

[zredbaron]

Hah! Round and round we go!

 

But what Steve and I are trying to say is that you're running too rich because of too much fuel, not due to a lack of oxygen (air).

 

Ok, let's try the simple route.

Where is the excess fuel coming from then, if there isn't a lack of oxygen (air)? And if that were the only variable at play, then why can't I correct the condition with smaller jets?

Are you proposing that 40 DCOEs are adequate for all applications?

[steve260z]

Have you addressed the fuel level? At high rpm the signal is at the highest and could be pulling additional fuel. I read back a few pages. Do you have an O2 sensor or getting the AFRs just off the dyno? Just wondering if you can check the AFR easily...Another thought is ignition consistency at high rpms. (A lot of thoughts round here)

Edited May 14, 2015 by steve260z

[madkaw]

I'd say those carbs are too small--- period. Put some bigger ones on with the same jets and let the numbers do the talking  

[bradyzq]

I wish 40's were adequate for all applications!

 

Assuming the carbs are jetted, adjusted, and operating correctly, the extra fuel should be drawn in by pressure delta due to extra airflow. Are you suggesting that the air speed in the center of the venturi, where the fuel is drawn in, keeps increasing, but the overall air volume decreases due to non-laminar flow towards the outside?

[zredbaron]

Steve -

Fuel level? Do you mean the carburetor floats? Yes, precisely measured as per Weber guidance on plastic floats. I agree a bad float level can ruin an A/F graph, but I feel strongly this is not the issue. Again, the A/F graph responds to my jets very predictably, same as in 2008, just not enough since the venturis and are wrong and mains do what they can with what they've got. If it were floats, the A/F ratio would fall apart and my jets wouldn't affect it nearly as much. In my experience, the main jets raise/lower the whole A/F graph, but don't really change its shape much. Venturis, emulsion tubes, float levels and air correctors adjust shape. Anyone experience these differently? (In all objectivity, I really only have experience tuning "an undercarbureted stroker," so perhaps my experience is skewed?)

 

No 02 sensor, the exhaust was never completed. The only data I have for either RIP motors is the 2015 dyno A/F graph. I agree with your point about high RPM ignition, but I really don't think that's a variable with Electromotive ignition. Very consistent, precise, powerful and long spark... so personally I for road tuning I turn exclusively to the carburetors once the timing is dialed in for a given build.

 

madkaw -

Thank you! Agreed. Too small, let the numbers do the talking and compare the tales of the tape.
EDIT -- I do have a MAP sensor installed, after all. If a dyno could receive the signal from my sensor, we might actually get some data?

 

Assuming the carbs are jetted, adjusted, and operating correctly, the extra fuel should be drawn in by pressure delta due to extra airflow. Are you suggesting that the air speed in the center of the venturi, where the fuel is drawn in, keeps increasing, but the overall air volume decreases due to non-laminar flow towards the outside?

I'm not following you. I've been talking about a lack of airflow, you previously presented that I wasn't rich from a lack of air but excess fuel... and when asked about the source of this excess fuel the answer is excess airflow? How did we get here? Where is this excess airflow coming from? The carburetors that are undersized?

No, my quick summary is that fuel delivery rises consistently as a mechanical response to rising pressure differences, which is a crude approximation of pedal / demand. At some speed, air delivery isn't able to keep up with the rising demand, so increasingly inadequate air volume is paired with adequate fuel as RPM increases past the speed in question. The concept I point to, for the time being, is that the laminar airflow "stalls" on the way in and becomes turbulent, which drastically decreases air density and therefore total O2 available for combustion per unit volume.

 

To your point, yes, I am suggesting if we had a MAP graph across the RPM range, instead of seeing a consistent shape as RPM rises, we would see a dip in MAP from the moment the carbs became restricted. This dip in pressure would send less fuel, yes, but would contain *significantly* less air density than that same pressure signal normally contains (when the intake isn't "stalled"). Remember, we can produce 1 atm of pressure with temperature, density, speed, and shape. The metered delivery of fuel in response to the pressure is only appropriate for one and only one air density. Combusting "less fuel" with "way less air" results in a rich condition, and the farther these two grow apart, the more rich we go.

If we still see it differently, may I ask you to compare an ideal carb to a restricted carb in your mind? Perhaps our fog is here? Haha.

• If the 40 DCOEs are inadequate, what is the variable that is inadequate?
• What happens to the A/F ratio when this variable is inadequate and why?
• Would one carb would have limited adjustability with jets and why?

Edited May 14, 2015 by zredbaron

[zredbaron]

By the way, when a turbine is stalled (again, a turbine is a glorified air pump), the solution is to slow it down until the stall clears (laminar flow restored), and then spool it back up. It will never, ever suck in laminar flow ever again until you slow it down below the speed in question.

 

This is no different than power sliding; we must remove power until we regain performance, or traction. Adding or maintaining power will continue the lack of traction indefinitely.

 

This is peeling out through our intake. I am submitting that If a turbine can be stalled, a carburetor can be stalled. All pumps are subject to cavitation, and a 4-stroke engine is a pump.

Edited May 14, 2015 by zredbaron

[zredbaron]

Ok, time for the Weber charts! Clearly not much of a social life at the moment.

 

We don't know where the dotted lines are between successful and failing modes of operation, and the books can't talk about what's going on when I'm still running the same carbs from the stock 2.4L I had 17 years ago. They can't talk about it because it's literally off their "here's what is possible" chart, which is not something to brag about, but rather affirms that larger carburetors are needed.

 

Since I'm off the flow chart, the high cfm demand isn't possible, so it fails to occur. This is BAD! [it "stalls?"] This is one of the worst things to happen to a carburetor. Not tunable. Jets and floats won't help you, because the pistons are too big and too fast. Wrong application. Fail.

Pointing at venturis is not a mental attachment, it's an interpretation of the Weber charts. The attached chart only depicts the upper capability of flowrate (WOT), it does not tell us what happens when demand from a larger engine exceeds it. Note that chokes in a 40 DCOE perform wildly different than the same size chokes in a 45 DCOE body! (The exit of the body is a different diameter, which is clearly very influential.)

If I were keeping the 40 DCOEs, I'd consider trying the 34mms chokes as per the aux venturi note in the book. I still might. It's my understanding the aux venturi helps more with mid-throttle positions at lower RPMs than WOT in the power band, but I'm not certain. You're right, we don't talk about it much! Based on the book, I'm guessing it would help my pre-4k RPM stagger and minimally cap peak hp even further.

 

This chart shows us that a 45 DCOE might even be too small for me, but I'd like to see the 48 DCOE and 50 DCOE graphs for comparison. (I'd prefer to not be at the plateau range of operation.) The benchflow performance of my head was 205 cfm / 235 cfm, depending on some detail of their test that I don't recall. Not sure which number to go with, but both exceed the 40 DCOE's capabilities by a longshot.

Edited May 14, 2015 by zredbaron

[zredbaron]

Way past time for an update.

 

In April I pulled my engine, determined to have it open within a couple of weeks. After several phone calls and inquiries for an L6 engine builder in the pacific northwest, I chose to go with Joe Harlan of Top Tech Motorsports.

 

Joe was very clear that I would wait for a while. Story of the industry, it was longer than we thought. My engine sat on the floor of my storage unit for about four months until he was able to receive it and open it up.

 

When we spoke about what might have happened to the engine, Joe repeated the word "sad" more frequently than anything else. His conclusion is that the valve lash was set wrong across the head, every exhaust valve was coming into solid contact with the piston and to make matters worse, the twin idler pulley was evidently installed absurdly tight. He felt strongly that the build suffered from a lack of attention to detail throughout, and pointed to the valve reliefs being reversed (in/ex) for each cylinder, which is part of the problem lacking clearance (I don't see this reversal reflected in the pics). Lash was wrong, regardless. "Sad."

 

Clearly only a matter of time before something gave. I still recall vividly how concerned I was when I was breaking it in--the engine sounded labored and unhappy from the beginning. Guess I was right. Wish I was wrong!

 

The suspected sequence is rocker arm failure at the point of its largest weight relief (sparing camshaft damage?), valve collision into spark plug and compressed sideways between the piston and the combustion chamber of the head, piston and head both lose to the valve, throwing bits of metal into the intake plenum and into the next two cylinders, hence the oil going down those exhaust runners as well.

Here are the pictures of the pistons:

Edited May 9, 2016 by zredbaron

[zredbaron]

So where are we now?

 

Joe uses a shop to repair catastrophic damage like this. Didn't get any pictures of the head damage, unfortunately. Evidently this shop uses a laser technology to map out another combustion chamber and duplicate it on the damaged cylinder. Neat!

 

The head came back last week:

Parts beyond repair:

• Valves, guides, spring retainers, block, bearings, pistons, 3 rocker arms

Survived for another day:

• Camshaft, crank, rods,

On order:

• New JE pistons via Rebello. As per Joe: .210" exh pocket depth, .190" intake pocket depth. Arrives January.
• 3 rocker arms and 12 titanium valve spring retainers via ZCCJDM. Arrive when they arrive.
• Competition oil pan via DP Racing. Arrives January. Or when it arrives.

Planned but still pending:

• Send head, pistons and valves to SwainTech for thermal and friction coatings
  • As per Swain: greatly increases engine longevity plus a guaranteed 2-5% hp gain for an NA engine if you do all coatings. Can thermal coat intake manifold, too. Not now.
  • CC, exhaust port, valve face and radius side, and piston top all get a thermal barrier coating
  • sides of the piston get a low-friction coating
  • The technologies: http://swaintech.com/race-coatings/race-coating-descriptions/
  • The options: http://swaintech.com/race-coatings/automotive-coatings/automotive-coatings-price-sheet/

 

 

(Can you tell I want this engine to last?) Hah! 

[ryant67]

Glad to see an update, and here is hoping that third time's the charm!  

Seems like a solid plan, with a few incremental improvements again over your previous build.  Great to see some pics of your E31 head, and I bet you are super glad that your head and camshaft survived, as they would be very very difficult to replace at this point. 

Curious that there was so much contact damage on your old pistons, surely that would have made a very obvious noise indicating that something was terribly wrong?  But watching the video you posted, it isn't obvious...  very harsh result.   

[zredbaron]

Thanks Ryan. Infinitely grateful the head and cam survived!

 

 Curious that there was so much contact damage on your old pistons, surely that would have made a very obvious noise indicating that something was terribly wrong?  But watching the video you posted, it isn't obvious...  very harsh result.   

 

I feel the same way! When I first started the engine and tinkered to synchronize the carbs (linkage), I was so intimidated by its sound that I cut the engine off several times, generally worried that it was about to damage itself. I was even so stressed that I asked my father and uncle to leave -- they were shouting their opinions over the engine, which didn't help.

 

I brought that concern to the chassis dyno, where I hoped to confirm something one way or another. Men far more experienced than I smiled at the sound and said everything sounded just fine -- awesome, in fact. "I ain't ne'er heard a six-shooter sound like that, boy!" (--probably not the most technical reply available, but fully Arkansas! Haha.)

It's really hard, and costly, having a strong instinct of the truth but lacking enough grasp to articulate it in a useful way -- even harder to tell if you're the only one that's right or if you're the only one being arrogant. Similar trend with my carb theories!

Edited December 19, 2015 by zredbaron