turbo and carb question

[TimZ]

no, creating a restriction is how they increase pressure. The basically pinch the line to bring the pressure up. This reduces the flow volume, but increases the fuel pressure. Think of putting your thumb over the end of a hose to make it shoot out further. That's due to the pressure increase. How you would reduce pressure with a device like that without a bleed off escapes me.

 

Niether type of regulator can increase the pressure above whatever pressure the deadheaded pump can produce. However, this "base" pressure is generally much higher than what you are probably used to thinking about for EFI systems. This could range from ~45-50psi for a stock n/a pump, to well over 100psi for some aftermarket pumps.

 

In general, the more the pump is flowing, the lower it's output pressure gets. The bypass regulator regulates the pump pressure by bleeding the majority of the pump output back to the tank in order to acheive the desired pressure. This type of regulator can change pressures pretty quickly, since there is always fuel flowing through it, and it just needs to change the amount of fuel getting bypassed to adjust the pressure. The main limitation here is the size of the return line - if it is too small, the regulator will have problems reducing the pressure beyond some point.

 

A non-bypass regulator is kind of the opposite of the bypass regulator. It achieves its pressure reduction by pinching off the fuel supply to the carbs. This will raise the pressure upstream of the regulator, but the pressure downstream will start to drop was the carbs fill with fuel. The problem here is that the only bleed mechanism is the carb itself. This works okay with the normal constant pressure needed for an n/a carb, but when you start trying to follow the manifold pressure (or carb inlet pressure in this case), there will be problems when the fuel pressure needs to drop quickly. For instance, when you quickly let off the throttle after a full boost run, the carbs won't be flowing much, so it will take a while for the fuel pressure to fall. In the meantime, you'll be flooding the bowls with 14-20psi of inlet fuel pressure - not good.

 

BTW, if you use a bypass regulator to get to 4psi base pressure, a stock n/a EFI pump will probably be all you would ever need, since it will flow a crapload of fuel at 10-15psi. This is a much lower pressure than it normally has to supply, so that's about the best condition that you could ask for for that pump.

[Tony D]

My suggestion would be to NOT reference the plenum pressure, but to reference manifold pressure.

 

Why?

 

When you drop-throttle, even with a blowoff valve, there is somewhat of a spike in plenum pressure, and this can sink the floats momentarily, causing a slightly elevated float bowl level, and a drop-throttle enrichment stumble.

 

If you reference the manifold pressure, as soon as you drop throttle, or lift throttle, your fuel pressure drops precipitously. This allows for a sealing of the needle valve quickly, and a bit better fuel level control.

 

now, if you want to really get into it, putting modulator rings in the plenum above your throttle plates will allow for a on-boost enrichment without using humongo jets giving a better idle and off-boost economy and drivability.

 

For the effort of converting the DGV's to Turbo usage, buy a Megasquirt, and a stock turbo setup and go EFI.

 

I ran a blow-through carb setups from 85-on in the Z, and as soon as Standalone EFI bacame economical, I converted gladly and NEVER looked back! I've run blowthroughs on VW's since the late 70's.

 

EFI is better.

[veritech-z]

posted by TimZ: This will raise the pressure upstream of the regulator, but the pressure downstream will start to drop was the carbs fill with fuel

 

How does this explain the old "put a socket over the vacuum line on your efi pressure regulator, and tap it with a hammer until the pressure goes up" method of increasing fuel pressure? Big_Phil said he got his up to 50psi or so, and I've heard other stories like that, but isn't base fuel pressure something like 45psi with a factory pump? I know mine is, since that's what it did with a pressure regulator screwed into my fuel rail and nothing running but the fuel pump...If I read your explanation correctly, I shouldn't be able to increase fuel pressure at the fuel rail higher than the stock pump will provide? Oh, wait a sec, the fuel rail comes before the regulator on the efi setup...so that would still work with the way you explained it...disregard question, just thinking out loud.

[TimZ]

My suggestion would be to NOT reference the plenum pressure, but to reference manifold pressure.

 

Why?

 

When you drop-throttle, even with a blowoff valve, there is somewhat of a spike in plenum pressure, and this can sink the floats momentarily, causing a slightly elevated float bowl level, and a drop-throttle enrichment stumble.

 

If you reference the manifold pressure, as soon as you drop throttle, or lift throttle, your fuel pressure drops precipitously. This allows for a sealing of the needle valve quickly, and a bit better fuel level control.

 

now, if you want to really get into it, putting modulator rings in the plenum above your throttle plates will allow for a on-boost enrichment without using humongo jets giving a better idle and off-boost economy and drivability.

 

For the effort of converting the DGV's to Turbo usage, buy a Megasquirt, and a stock turbo setup and go EFI.

 

I ran a blow-through carb setups from 85-on in the Z, and as soon as Standalone EFI bacame economical, I converted gladly and NEVER looked back! I've run blowthroughs on VW's since the late 70's.

 

EFI is better.

 

I agree on the plenum pressure spikes causing problems.

 

My main reasoning for suggesting using plenum pressure was due to the low base fuel pressures involved - basically, any manifold vacuum higher than about 8inHg (-4psi) would result in zero fuel pressure to the carbs. If you use plenum pressure, at least when the engine is pulling vacuum, the plenum pressure is approximately 1 atm (zero vacuum), and the carbs will get the proper ~4psi. Am I missing something here?

 

The bottom line here is pressurized carbs are just plain problematic, and any fix you try to make will have pitfalls.

 

I ran a pressurized carb setup back in the late 80's too, and like Tony said, EFI is better. WAY better.

[TimZ]

How does this explain the old "put a socket over the vacuum line on your efi pressure regulator, and tap it with a hammer until the pressure goes up" method of increasing fuel pressure? Big_Phil said he got his up to 50psi or so, and I've heard other stories like that, but isn't base fuel pressure something like 45psi with a factory pump? I know mine is, since that's what it did with a pressure regulator screwed into my fuel rail and nothing running but the fuel pump...If I read your explanation correctly, I shouldn't be able to increase fuel pressure at the fuel rail higher than the stock pump will provide? Oh, wait a sec, the fuel rail comes before the regulator on the efi setup...so that would still work with the way you explained it...disregard question, just thinking out loud.

 

 

Also, remember that the stock regulator is a bypass regulator - the quote you showed for me was referring to the non-bypass style.

 

And, no, you should never be able to make more pressure than the pump can provide when deadheaded (i.e., zero flow). The stock n/a pump is internally bypassed at something like 45 or 50psi. The stock turbo pump is somewhat higher, like 55 or 60psi.

[gjc5500]

i have considered efi. too much. i dont like to use used fuel system parts. so id be looking at new intake, reg, pump, injectors, MS, fuel rail, ect. it would add up really fast. and ontop of that, efi turbo setups r common. i want my car to stand out. this is the same reasoning behind me staying with the l6 instead of swapping a v8 in there. i no its probelmatic, but IMO it will be worth it.

[veritech-z]

posted by Timz: Also, remember that the stock regulator is a bypass regulator - the quote you showed for me was referring to the non-bypass style.

I knew you were talking about non-bypass at that point, but the principals of fluid pressure in a tube increasing by putting in a restriction should be the same for either type...denting the top of the regulator must not be causing a restriction then, but just changing the pre-load on how much fuel is able to bypass the rail? Sorry I kind of turned this into an EFI discussion, I thought I had it figured out, but now I see that I wasn't as clear as I thought...I had been under the impression that the pressure regulator was able to INCREASE fuel pressure above the base pressure by restricting it, and in the case of a bypass type regulator, it could either restrict it to increase pressure (according to how I was viewing it) or allow the fuel to bypass to reduce pressure. Part of what was causing this is the fact that I have seen few pumps advertised as having much higher pressure than 45psi. For instance, the aeromotive A1000 is a common EFI upgrade pump, and it is advertised as flowing 600lbs per hour at 45psi, and the A1000 bypass regulator is advertised as being able to be adjusted from a base pressure of 30psi to 70psi, and then still allowing boost to increase pressure at a 1:1 ratio, and it says it was "designed specifically for use with our a1000 fuel pump" which should only allow you to have a maximum system pressure of 45psi...

http://www.aeromotiveinc.com/pdetail.php?prod=3

http://www.aeromotiveinc.com/pdetail.php?prod=11

[Tony D]

My main reasoning for suggesting using plenum pressure was due to the low base fuel pressures involved - basically, any manifold vacuum higher than about 8inHg (-4psi) would result in zero fuel pressure to the carbs. ... Am I missing something here?

 

The bottom line here is pressurized carbs are just plain problematic, and any fix you try to make will have pitfalls.

 

I ran a pressurized carb setup back in the late 80's too, and like Tony said, EFI is better. WAY better.

 

There at one time was a device called the "Cagle" FPR, also Cartech sold the Italian Regulator used on the Maserati Bi-Turbo, and they both used spring referenced pressure to maintain fuel pressure differential over boost of 3-7psi.

 

Key was you set the pressure at idle so that was your referenced starting point. 3-7psi at idle. This is how you keep pressure at idle vacuum. It also realizes that under drop-throttle spikes of 20+" Hg, you WILL effectively drop fuel pressure to ZERO, letting those floats seal positively against their seats and preventing that fuel bowl level spike.

 

it's an art screwing with the carbs for boost. Anybody who says "carbs are easy" hasn't tried tuning them to the extent necessary for optimal preformance!

 

Anyway, that is what I think you were missing, TZ. Many times with EFI, you will set fuel pressure to a known pressure at "0" manifold pressure, and the spring will compensate for the differential at all other pressures. (source of all noobs wonderingwhy they have 40psi static fuel pressure when the book says 36psi.... They are missing the differentiation between static fuel pressure and idling fuel pressure, even on EFI!) In a carburetted application, you have to have the fuel pressure set with the egnine running at idle to give proper fuel pressure at idle. You could, I suppose, fudge and set something like 7psi static, but the directions for the regulator said to set it at idle. Always follow the directions!

 

 

In the "old days" there were all sorts of caveats you had to obey when checking fuel pressures (at idle, at 2000rpm...) and stuff like that (Timing at idle with vac advance removed and EGR port plugged....)

 

BAH! EFI is better....

[Tony D]

As for elevated pressures, mos JDM conversion of carbs used stock turbo EFI pumps. This goes to veritechs Q on the Aeromotive pumps...

 

The FLOW the stock pump is capable of producing goes UP as pressure decreases. So a pump that can only accomodate 250HP at an EFI pressure of 55psi, may indeed be able to comfortably support 350-375hp at a carburetted pressure of 23-27psi fuel pressure (22psi+3psi for the carbs differential). I know this firsthand.

 

Now the Aeromotive advertizing maybe misleading, it may not. While the pump gives a RATED FLOW of X GPH or X PPH in their advertizing, that is only ONE POINT on their flow curve. At 70psi, it may well produce enough flow to support an engine. Chances are VERY good though, that the flow rating is decreased considerably at 70 psi when compared to it's delivery at 45psi.

 

The stock 280ZX Turbo EFI pump WILL produce 60psi. But PRESSUREdoes you absolutely NO GOOD whatsoever if no flow accompanies it---and that is about where the stock 280ZXT pump "deadheads" or in otherwords does not flow any more. Pressure versus Flow are inverse quantities, and as flow increases, pressure decreases. Also vice-versa. So just because they give you a rating of something at 45psi it doesn't mean it won't make 70psi, with flow. You need ot get the flow curve from the manufacturer. Some pumps don't have internal reliefs like the ZXT pump does (which is why it stops at 60psi and internally recirculates), so they can still flow, but they draw a buttload of amps. nothing is free. It all takes horsepower to flow a fluid through a tube. Hydraulics is hydraulics. Nothing changes the laws of physics.

 

Hope that makes it clearer to someone...

[Tony D]

OH WOW! I just clicked on the Aeromotive links and saw their advertizing is dead on in line with what I was trying to explain. See how they are giving N/A and Forced Induction HP ratings that are different for the SAME pump? Similarly they give different ratings for EFI versus Carburetted engine applications, and they give the flow chart for the pump to correlate flowrate against pressure...

 

Now you know why!

 

Just look at the flow curve from them and deterimne if it will meet your needs. That is a big pump.

Funny thing is they recomend the 16302 "fuel pump controller" for long trips to keep fuel cooler. It's a fuel pump modulator that cuts voltage below a predetermined RPM.

 

I guess those 81-83 Datsun Engineers were on to something back then by using the same strategy to "keep noise down at idle" in the GL models of the ZXT!

[TimZ]

Key was you set the pressure at idle so that was your referenced starting point. 3-7psi at idle. This is how you keep pressure at idle vacuum. It also realizes that under drop-throttle spikes of 20+" Hg, you WILL effectively drop fuel pressure to ZERO, letting those floats seal positively against their seats and preventing that fuel bowl level spike.

 

Anyway, that is what I think you were missing, TZ. Many times with EFI, you will set fuel pressure to a known pressure at "0" manifold pressure, and the spring will compensate for the differential at all other pressures. (source of all noobs wonderingwhy they have 40psi static fuel pressure when the book says 36psi.... They are missing the differentiation between static fuel pressure and idling fuel pressure, even on EFI!) In a carburetted application, you have to have the fuel pressure set with the egnine running at idle to give proper fuel pressure at idle. You could, I suppose, fudge and set something like 7psi static, but the directions for the regulator said to set it at idle. Always follow the directions!

 

Yep - that's what I was missing - I tend to think of base pressures as the zero vacuum settings. It's been a long time since I messed with carbs.

 

Now I'm gonna ask another dumb question ...

 

If you set the regulator to give ~4psi at idle, and your idle vacuum was, say, 16 in Hg, wouldn't result in ~12psi of fuel pressure at 0 vacuum (thus, 12psi plus boost)?

 

Wouldn't that be close to enough pressure to blow the needles off their seats and flood the float bowl? Or did the Cagle regulators hold a static pressure until the manifold pressure exceeded atmospheric?

[veritech-z]

I was looking at that Aeromotive pressure map, and I think I see what might have been misleading to me. Evidently the A1000 pump is capable of 70psi, but the flow reduces to 400gph. I had assumed they were advertising the maximum pressure, when in fact they were advertising the flow rate at a typical fuel injection pressure. I'm still a bit confused about why pressure does not increase with restriction, though. I thought I remembered from physics that if you reduce area available to flow, velocity and pressure should increase, but flow volume should decrease...Is it due to something going on with the pump itself? Some type of bypass valving or something? I know you need a certain volume of fuel to make a given amount of horsepower, but you also need a certain amount of pressure to overcome the boost (is that still just base system pressure + boost pressure, or does adding fuel injection change that?), which is why I was hung up on pressure capability. I appreciate you guys taking the time to explain this, by the way, this has been really helpful so far (to me, at least).

[TimZ]

I was looking at that Aeromotive pressure map, and I think I see what might have been misleading to me. Evidently the A1000 pump is capable of 70psi, but the flow reduces to 400gph. I had assumed they were advertising the maximum pressure, when in fact they were advertising the flow rate at a typical fuel injection pressure. I'm still a bit confused about why pressure does not increase with restriction, though. I thought I remembered from physics that if you reduce area available to flow, velocity and pressure should increase, but flow volume should decrease...Is it due to something going on with the pump itself? Some type of bypass valving or something? I know you need a certain volume of fuel to make a given amount of horsepower, but you also need a certain amount of pressure to overcome the boost (is that still just base system pressure + boost pressure, or does adding fuel injection change that?), which is why I was hung up on pressure capability. I appreciate you guys taking the time to explain this, by the way, this has been really helpful so far (to me, at least).

 

 

The pressure does increase due to the restriction - you're just confusing where the pressure increase happens. The pressure between the restriction and the pump does indeed rise, and the flow decreases accordingly. However, on the other side of the restriction you have less flow in the same size tube, so the pressure there is decreased.

 

It works the same in the "thumb on the end of the hose" example. The pressure in the hose is increased, the flow is decreased, and the velocity of the water is greatly increased at the restriction, due to trying to flow the same amount of water through a much smaller orifice. As soon as the water leaves the orifice it's pressure becomes the same as the atmosphere around it.

[veritech-z]

I got ya now. I was thinking that it increased the pressure across the system, not just up to the point of restriction. For the purposes of carbs, that isn't really useful, but for a fuel injection system where the regulator comes after the rail (like in the hammering on top of the regulator example) that would work to increase pressure. If the real goal is fuel flow, what is the point of raising pressure (aside from compensating for boost)? Can you compensate for a smaller flowing injector by bumping the pressure up, or is it really just a pointless modification?

[TimZ]

I got ya now. I was thinking that it increased the pressure across the system, not just up to the point of restriction. For the purposes of carbs, that isn't really useful, but for a fuel injection system where the regulator comes after the rail (like in the hammering on top of the regulator example) that would work to increase pressure. If the real goal is fuel flow, what is the point of raising pressure (aside from compensating for boost)? Can you compensate for a smaller flowing injector by bumping the pressure up, or is it really just a pointless modification?

 

A bypass regulator never adds restriction - it simply modulates how much flow gets bled off. You can look at this as forcing the pump to flow more and thus go to the lower pressure point on the graph. As such, the bypass regulator does effect the pressure across the entire system, both before and after it. This is a fundamental difference in the way it regulates pressure as opposed to the inline style.

 

When you hammer on the top of a bypass regulator (or crush it or whatever), all you are doing is changing the preload on the internal spring, and thus changing it's operating point. This doesn't add any restriction to the system (unless you really hammer the crap out of it ).

 

The flow through the injector increases or decreases proportionally to the square root of the pressure increase or decrease across it. So, if you double the pressure across the injector, you increase it's flow by a factor of 1.414. The reason for modulating the fuel pressure with manifold pressure is simply to keep a constant pressure across the injector. This way it will act predictably, regardless of the manifold pressure. If you didn't do this, the injector would flow more than its rating at idle, and less than its rating at full boost, which is pretty much the opposite of what you want. (You often see newbs pulling the reference off the FPR, thinking it's richening the mixture, not realizing that it has the opposite effect on boost)

 

Oh - and just as an FYI, injectors are pretty much always rated for flow with a 3bar (43.5psi) pressure differential across them.

[Tony D]

Yes Tim, the fuel pressure at "0" manifold vacuum would indeed be a pressure-referenced 12psi in the inlet fuel line.

 

BUT...

 

When you go WOT, the fuel level in the float bowls will DROP precipitously anyway, admitting fuel at an alarming rate. This is why the manifold referenced Cagle gave more power than other regulation methods. The normal fuel system would simply maintain 3-4psi at WOT, and fill the carbs as usual. A Cagle-Equipped vehicle would act like an early 240Z, as you hit WOT, the fuel pressure raised, pumping in fuel MUCH faster than it normally would, keeping the float bowl level more consistent, if not a bit higher than normal. It was not uncommon in a Cagle-Equipped vehicle to drop one or two jet sizes because the float bowl level did NOT drop under WOT, and this led to better off-WOT fuel economy as well. As SOON as you lifted the throttle in the least bit, that fuel pressure is CUT drastically, and the float bowl needles seat positively.

 

The early 240Z worked similarly with a fixed orifice (very misunderstood!) tHE FLOW of the fuel pump went up with engine speed. At idle the thing would run 3psi. At 6500rpm, the restriction in the return line would make that extra volume from the stock mechanical pump turn to pressure (7+psi at that speed), and the pressure differential across the needle and seats in the SU's would increase. Basically you dumped twice as much fuel into the bowls as you could at idle, and this is how they got away with those small SU float bowls.... But this digresses...

 

The second thing to consider in the blowthrough fue lregulation on a carburetted car (and it was touched on earlier in this post) is the differential between plenum pressure and float bowl pressure. Hopefully you see how the fuel pressure will be 12psi, and how that helps with keeping jet sizes smaller. Now, if you restrict the flow of boost to the throat of the carburettor versus what goes into the float bowl (we are talking fractions of an inch of water) by using a device called a "modulator ring" you end up stepping down a few more jet sizes because this differential RAISES the float bolw fuel level by keeping plenum pressure on the petrol in the float bowl, while the throat is running through the modulator ring at a slightly less pressure. This raised level allows the on-boost characteristic become slightly richer WITHOUT using a larger jet in the main well. Dellorto had Turbo Emulsion Tubes that also helped with this (OEM Maserati Bi-Turbo Carb). By using the modulator rings, you raise the fuel level in the main jet well, making it easier for the fuel to get into the throat. in extreme cases you can actually pump fuel from the float bowl up high enough to spray it through the orifices straight into the carb throat. Usually modulator rings are sized very closely to the size of the main venturis in the carb throats. This means that under N/A operation, the rings had no effect on fuel level, but when under boost the whole throat bacame slightly depressed pressure-wise and the fuel level in the jetwells raised.

hks took a different approach in their surge tank, by using a calibrated orifice in the upper plenum. Boost first entered an upper plenum that was vented to the float bowls, and then went through three orifices cast into the box to the lower plenum that was where the main throats were. This allowed yo uto immediately enrichen the fuel mix because the float bowl was pressurized slightly before the main carb throat, and then the differential between upper and lower plenum was maintained throughout the on-boost operation.

As well as Corky Bell's Cartech stuff was, this was not in their plenum, and is why transitional periods on the old Cartech Plenums always had a slight lean pop at low loads (if they were jetted for any sort of economy.) I found that I could run main jets in some cases FIVE STEPS smaller when using an HKS Plenum (or SK Plenum as well, similar design but different engineering) than when running the "simple box plenum" like Cartech cast, or that most people make out of 2X4 Aluminum Extrusion.

 

Outside of the VW Scene, I find almost nobody knows of Modulator Rings. They all look at me like I'm crazy. The difference in driving an HKS Surge-Tanked Blowthrough and a Cartech Blowthrough was like night and day on part-throttle cruise!

 

But to repeat: BUT EFI IS EVEN BETTER!!!

[TimZ]

Yes Tim, the fuel pressure at "0" manifold vacuum would indeed be a pressure-referenced 12psi in the inlet fuel line.

 

BUT...

 

When you go WOT, the fuel level in the float bowls will DROP precipitously anyway, admitting fuel at an alarming rate. This is why the manifold referenced Cagle gave more power than other regulation methods. The normal fuel system would simply maintain 3-4psi at WOT, and fill the carbs as usual. A Cagle-Equipped vehicle would act like an early 240Z, as you hit WOT, the fuel pressure raised, pumping in fuel MUCH faster than it normally would, keeping the float bowl level more consistent, if not a bit higher than normal. It was not uncommon in a Cagle-Equipped vehicle to drop one or two jet sizes because the float bowl level did NOT drop under WOT, and this led to better off-WOT fuel economy as well. As SOON as you lifted the throttle in the least bit, that fuel pressure is CUT drastically, and the float bowl needles seat positively.

 

Okay - that makes sense. Thanks, Tony.

 

I guess where I was headed with the question was to figure out whether the pressure-referenced regulator that we were talking about earlier (Mallory #4309) would work for the original poster, provided that he referenced it to manifold pressure and set it up to give 4psi at idle. From this last post, it sounds like it would.

 

And yes, EFI is still better.

[gjc5500]

prolly gonan go efi. but i hit another speed bump. my brake booster decided to die today on my way home from skool. so therte another 100 bucks that i need to get