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Custom Cylinder head building.. What's Involved?


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I know they are in balance at idle - i've verified it several times with the syncrometer.

 

When I had my SUs I used the fast idle screw to balance them at part throttle and there was a slight difference from idle. Tonight I'll rig up a stick on the throttle and test it to see. Thanks for the suggestion Drax240Z.

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I just checked the carb balance. The front was off about 1/3 of a unit on the synchrometer at about 2500 rpm so I adjusted that. On the run afterwards I noticed that the sound was louder. Hmm, louder with the air cleaners off, that doesn't sound like detonation. So I tried going up a hill in 3rd or 4th at around 1k rpm. The sound is still there, and completely regular, like what a cracked exhaust manifold would sound like. There is none of the randomness that I have heard in the past when my old engine would sometimes ping.

 

Based on this I'm pretty sure it is NOT detonation. That would explain why retarding the timing to zero (from about 13 where it is now) didn't make the slightest bit of difference. I'm not sure why it sounded louder last weekend when it was so hot. Perhaps it was just me expecting it to be worse due to the heat, or perhaps whatever is making the sound in one of the carbs just does it more when it's really hot.

 

I'm thinking I'll swap the bigger cam in and while I have the carbs off take a close look for anything strange. While I have the head off, are there any tell-tale signs of pinging I would see if it really was detonation?

 

Dan, Jon, Drax240Z, does this make sense to you?

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You've pretty much got me stumped. I don't know that the filter would or wouldn't change the volume of pinging, never tried it out. I think it's hard to mistake an exhaust leak for pinging IME. I know Webers pop out of the carbs when they're cold (at least downdraft Porsche Webers do), but that doesn't seem to be your problem. I don't think you'll find any evidence of detonation unless there has actually been some damage, like a piston that looks like its pitted or melting or something. Assuming no obvious damage, it will be interesting to see what happens with the bigger cam.

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It's not detonation from what you describe.

 

See if you can measure with a dial indicator on the furthest throttle lever away from where your throttle shaft is actuated, the amount of deflection in the system as the throttle is applied. If you are using heavy return springs or a small diameter throttle shaft you will be surprised at how much change there can be there.

 

Is your set of carbs a matched trio, or are they pieced together from 2 or more sets?

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See if you can measure with a dial indicator on the furthest throttle lever away from where your throttle shaft is actuated, the amount of deflection in the system as the throttle is applied. If you are using heavy return springs or a small diameter throttle shaft you will be surprised at how much change there can be there.
I can see flex in the shaft and some small movement between the shaft and the bushings. However, I took that into account when I set the linkage by taking up the "slack" before adjusting. I probably should replace the throttle shaft and bushings, it just hasn't made it to the top of my budget yet :) Do you think that would be the source of this noise?

 

Is your set of carbs a matched trio, or are they pieced together from 2 or more sets?
All 3 covers list "TIPO: DCOE2". Below that is "No. 4F" on the front carb, and "No. 4K" on the 2 back ones. What is the significance of the differnt letter (F vs. K)? Perhaps the jetting as shipped from the factory?
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Any chance it could be a loud rocker? I've got a clicking noise coming from my valvetrain that's pretty annoying, even though I adjusted all my valves when I changed the headgasket...
No, don't think so. The sound is completely dependant on the throttle position. It's only present from around 20% to 50% if I had to guess. Foot off the gas, or at WOT the sound disappears.
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  • 2 weeks later...
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how in the hell do you bend the laws and 110% ve?

 

Silent,

“CLICK!!!” It just now clicked what you meant by this question…

As usual, another Paul (BRAAP) Ruschman dissertation…

 

 

By taking advantage of the Helmholtz cycle, engine designers have been able to achieve greater than 100% VE at the extreme of engine development, i.e. “natural” supercharging. (I can’t remember if it is spelled Helmholtz or Helmholz. I do apologize for not knowing, as I should.)

What is this Helmholtz cycle you ask? In lamen, the Helmholtz cycle is basically taking advantage of the fact that air has weight, (the mass of the air), and when that air gets set into motion, (flowing through a port), it now has energy. To fully understand and appreciate this, try this experiment. Take a long piece of PVC tubing, at least 5 foot long 10 foot is even better, or any tubing/pipe for that matter, say between 1/2 to 1” in diameter. Now blow through it and very quickly either put your tongue in the pipe or close your lips to stop the air flow. Notice how it tried to suck your tongue/lips though the pipe and you could feel a slight reverberation as well? That is the Helmholtz cycle you just felt. Those vibrations are pressure waves going back and forth inside of the pipe. The longer the pipe, the more effect it has. You also would’ve noticed that the strongest pulses were the first ones and they diminished over time, (rather rapidly). Try sucking through it and stopping the flow quickly as well. This time, the first thing you noticed this time is that the pressure wave tried to push your tongue/lips back into your mouth. Now lets apply to this to internal combustion engines.

 

 

In the intake port of our engines, when the intake valve opens, the air gets moving due to the fact that the piston is moving down the bore, drawing the air into the cylinder, and the air flow moves quite fast as the port is small in comparison to the cylinder being filled, (just like when you sucked into that pipe). When the valve shuts, (you used your lips or tongue to stop the flow of air very quickly, but in the engine, this is exaggerated MUCH more as the pressure differentials are much greater, therefore the air flow is much greater than with your lungs on long piece of pipe), this column of air is still moving and now builds up pressure behind that valve, this pressure wave then surges back up the intake tract, then comes back, this would be the second wave, then it does this again, but with less energy, this would be the third wave, and the cycle repeats itself again and again. If we were tuning our intake tract for the second, third, fourth or even the fifth wave, then it would be at the point where either of these waves is just approaching peak pressure that we would want the intake valve to open, therefore allowing this column of air and its kinetic energy to “shove” even more into the cylinder than the piston could “suck” in on its own, ie. Greater then 100% Volumetric Efficiency, or VE for short

 

 

Now remember, Torque output of any N/A engine is directly related to displacement AND VE. Have you ever noticed the Torque figures quoted for NASCAR engines, (I’m sure they are under stated as well)? For a given displacement, NASCAR is achieving some insane VE numbers from the lowly 2 valve push rod engine. There are other top echelons of Motorsports utilizing multi valve technology that are getting even more natural supercharging affect than NASCAR on N/A applications. Now keep in mind, engines that are able to produce this kind of power are VERY VERY narrow in the band they run efficiently. Run these engine below their optimum RPM range, and a nicely built/tuned hot street engine could kill them, say 2500-5000 RPM.

 

I’m sure many of you are familiar with the L-98 TPI engines GM produced in the mid ‘80’s for the Camaro’s and Corvettes. In a good state of tune with a nicely matched cam, those engines pulled like freight trains! The power band was peaky, peaking at around 3000-3500 RPM, and then the torque curve would take a steep dive after that, running out of breathe by 5000 RPM. They could make as much torque as a carbureted big block that was 50-75 cubes larger. That was due to the LONG intake tract. The compromise was that they could not breathe above 5000 RPM no matter what you did. Some people have been able to get them to breathe at 5500 or so, but that is it, (yes they will all free rev to past 6000 but the power curve is DEAD up there, even with WAY radical cams, exhaust, big tube intakes, etc.) At the other end of the spectrum would be the LT 1 that replaced the GM L-98 TPI engines in the early ‘90’s. The LT 1 used extremely short runners, and as such, their peak torque output was much less, though the power band was much flatter across its operating range and because of the short runners, it could rev higher therefore allowing it to make more HP even though it couldn’t match the torque output of the TPI engines.

(Torque is measured, HP is calculated. HP is calculated as Torque over time. The more torque you make at a higher RPM, the more HP you generate. Or if you can’t make as much torque as the next guys, at lest be able to make some torque at a much higher RPM and you will have a higher HP. It is the Power to weight ratio that wins the race, Torque is what you feel in the seat of your pants.)

 

At any rate, this should kind of illustrate the idea behind intake design and tuning as it utilizes the Helmholtz cycle. Exhaust tuning is no different and all of this applies there as well, long tube vs short tube headers, etc…

 

Hope this helps answers that question.

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At any rate' date=' this should kind of illustrate the idea behind intake design and tuning as it utilizes the Helmholtz cycle. Exhaust tuning is no different and all of this applies there as well, long tube vs short tube headers, etc…

 

Hope this helps answers that question.[/quote']

 

good write up!

 

im trying to understand this, and its making more sense every time i read it. any books you know of that describe this kind of science for efficient engine tuning using air and exhaust pressures and momentum?

 

also, im thinking about the exhaust tuning, and what i can visualize is the whole "backpressure" idea that a smaller diameter exhaust creates backpressure, thus creating better torque in a N/A setup.

 

but i've been one to challenge that fact, and turn it to myth because i believe that in order to tune exhaust properly, it has to be at a diameter at which that resonance you speak of (with the blowing of air) is actually sucking the air out efficiently due to its mass that it carries with it, and not pushing air back into the combustion chamber to create more torque like the say, or even pushing air into a big gaping hole of perhaps... an overly large exhaust/header/collector setup? it doesnt make sense to me that way... and this helmholtz idear is the technical term ive been trying to find info about for a long time to prove my understanding of energy/mass/velocity

 

is this somewhat correct?

 

the whole vaccuum/pressure velocity measuring thing starts to make more sense now that i think of tuning the exhaust properly.

 

i've been on this for years, telling people its NOT backpressure, and im starting to feel a bit more reassured now.

 

 

 

oh and another thing... is the added efficiency of putting air back into the intake runners in order to create more momentum the reason why the Miller Cycle, or 5 stroke, engine is as efficient as it is? its 5th stroke is made to push remaining air back into the intake tract so that it can create that same velocity so it can suck more air into the next compression cycle using the superchargers ability to force that wandering air back in at a higher pressure rate then it was carrying itself.

 

is this correct? forgive me if i dont make sense. 5 in the morning =)

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By taking advantage of the Helmholtz cycle' date=' engine designers have been able to achieve greater than 100% VE at the extreme of engine development, i.e. “natural†supercharging. (I can’t remember if it is spelled Helmholtz or Helmholz. I do apologize for not knowing, as I should.)

What is this Helmholtz cycle you ask? In lamen, the Helmholtz cycle is basically taking advantage of the fact that air has weight, (the mass of the air), and when that air gets set into motion, (flowing through a port), it now has energy. To fully understand and appreciate this, try this experiment. Take a long piece of PVC tubing, at least 5 foot long 10 foot is even better, or any tubing/pipe for that matter, say between 1/2 to 1†in diameter. Now blow through it and very quickly either put your tongue in the pipe or close your lips to stop the air flow.[/quote']

 

Hi Paul, I'm afraid I'm going to have to disagree with this. There isn't a Helmholtz cycle. What you are referring to is a Helmholtz resonator, which is an enclosed volume of air with a single openning. The classic example of how Helmholtz resonance works is blowing across a soda bottle and hearing the sound it makes. The volume of the bottle and size of the openning control the frequency of sound.

 

If you look up water hammer you will get a much more likely scenario for what goes on in an intake manifold and what is responsible for what are often called ram tuning effects.

 

Cary

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I have been playing around with the Helmholtz equation for some time now (FSAE) and havent gotten good results with it...espically because there are so many different variations of it around...i've seen examples of its useage with old mopar enignes wit "ram intakes" and through FSAE...but i dont know of anyone that has gotten a good grip on this idea besides the basics of how it works...has anyone used this idea on a NA setup?

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  • 1 year later...

If we were tuning our intake tract for the second, third, fourth or even the fifth wave, then it would be at the point where either of these waves is just approaching peak pressure that we would want the intake valve to open, therefore allowing this column of air and its kinetic energy to “shove†even more into the cylinder than the piston could “suck†in on its own, ie. Greater then 100% Volumetric Efficiency, or VE for short

------------------------------------

 

Hence why cylinder head design, intake design/tuning and camshaft profile are important.....

 

More modern engines have variable valve timing as well as variable intake tuning...to help widen the Band of peak VE%. Good post Paul.

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  • 4 months later...
Guest agentc73

This is why equal length Intake runners make more power. If one cyl is making more power than another(or vice versa), the whole engine overall torque output goes down.

And to re-iterate, the same applies to the exhaust pulses.

However, carb intakes with a plenum work better than out of the box if they are "tuned" to apply this principal, i.e, plenum spacers, ect to the engine's goals. This effect in a plenum is the TOTAL length of the runner, from the valve seat to 1/2 inch past the runner inlet at the plenum. If an intake valve is drawing charge, and the wave is at the carb, VE goes down.

Vacuum can effect the air and or fuel charge as well. The fuel charge can "reciprocate" between the cylinders in the midst of Intake valve openings, if not in the plenum's volume/runner length's optimum rpm range. That is, in carb applications. For FI, it's just air being stolen.The individual cylinders "steal" fuel charge from each other, pulling other's cylinder's charge BACKWARDS through the runners. Sorry boys. That includes the SU's.

 

This effect in the side drafts is the TOTAL length of the runner, from the valve seat to the very tip of the "Velocity stack"(and, actually, @1/2 inch PAST the inlet!!). The TOTAL length and DIAMETER is VERY IMPORTANT!! LONGER=LOWER RPM, SHORTER= HIGHER RPM. A huge cam and short intake, plus stubby stacks=no low RPM torque! Side drafts (WHEN TUNED PROPERLY, ECT) will make more power because the cylinders are equally charged, and the intake events don't effect each other at all. IF PROPERLY TUNED AND MATCHED to the cam, heads, comp ratio, exhaust, TOTAL LENGTH of the runner, side drafts or ITB will make MORE power than any other N/A setup, at ANY RPM range.

The 110 VE short runner Intakes are often bouncing the Charge into the cyl @ 4th bounce from the last time the valve opened. At a given RPM...

Remember, the waves are moving @ the speed of sound.

Oh. Engines with cylinders grouped in 3's can use this effect best. Esp. if the plenums are grouped in 3's, equally spaced in the firing order.

How to make Horsepower Vol II, by David Vizard. Excellent read.

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  • 5 years later...

The round exhaust ports, (with the liners still intact), are excellent for mild race and hot street engines. Also of note, All the round port heads, (N-47, MN-47, and P-79), have a smaller intake port. At approx 1/2 “ into the port, the port takes on a “D” shape, this helps bias the port centerline which in turn helps to aim the incoming air flow toward the center of the cylinder. This is great, but for maximum effort high RPM breathing, that smaller port is a hindrance to overall flow. These “D” shaped intake ports compliment the round exhaust ports nicely for mild to hot street, and mild race applications. Especially for a street engine that is asked to have some civility at part throttle conditions like having to drive through town back and forth to work and in stop and go traffic on your way to your favorite bonsai back road. These "D" shaped intake ports and round exhaust ports found on the N-47, P-79, and MN-47, in my opinion, offers the best performance with the least compromise in around town drivability and low end and mid range torque.

The square exhaust ports of the N-42, E-88, and P-90, allow for considerable exhaust port alteration allowing the engine to breathe freer at the higher RPMS, though low rpm performance will suffer as a result. The Intake ports of these same heads are also larger than the intake ports in the N-47, MN-47 and P-79, so less carving.

 

In short, the N-42 offers more “all out” potential, but at the cost of low rpm and part throttle drivability. This same radical head on a street engine that is to be driven around town would be a poor choice. This N-42 works best at WOT and above 4000 RPM, where as the round port heads do their best, (depending on how they are set up and built), from as low as 1500 RPM up to 6500 and will have decent manners around town.

 

Hi,

 

I have been doing research about how to build the right engine for me and came upon this post.

 

If I read it correctly, for someone like me who will be driving around town, and driving across the country, no racing, and really wants decent performance in the 1500 - 6000 rpm range I would be better off with a N47 based engine?

 

Thanks (I appreciate knowledge from experts),

 

Rick

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