FPR vacuume line with supercharger

[redrocco]

I have a 93 SC400 engine and a custom supercharger setup installed in my 86.5 supra. Charger Is a a whipple 1600ax and I am down to the last few things before it goes out to the dyno. I have a megasquirt 2 ECU fro engine manigment and retained the stock ecu for the non mangement things. Upgraded to 440cc 7M-GTE injectors but kept the stock FPR. I know I need to connect the MAP sensor post charger and the break booster pre charger but what about the FPR? Ime guessing pre charger so it never sees boost and works like stock but I would like to be shure. Any help would be appreacited, If possable could someone post up there supercharged 1uz vacuume diagram so i can compare to what I have done.

Thanks,

Brett

 

[Zuffen]

FPR attaches after the charger.

This is so the fuel pressure retains the difference between manifold pressure and fuel pressure.

Say you ran up to 20psi of boost and you didn't pick up boost pressure for the FPR the effective fuel pressure would drop from 43psi to only 23psi when you're at max boost. This would give poor atomisation of the fuel and you'd run lean.

 

[cribbj]

MAP and FPR need to sense manifold pressure, after your supercharger.

The brake booster needs a vacuum source.

 

[BigKriss]

Brett, A one way check valve of substance-something similar to what Alfa Romeo use-will solve the vacuum problem for the brake booster. Under boost there will be no effect on the booster as the check valve will handle the positive pressure.Once the throttle snaps shut, all the vacuum you would require. The std. fpr would suffice IF you have the capacity in your injectors. As Rod indicates, there will be slight fall off as boost acts on the injector but this really is minimal. A 23 pd differential across the injector head is not going to relate to a real world differential of 23 pd in fuel pressure drop. The ideal would be a rising rate fpr. This again does not lift fuel pressure pound for pound much above 8 or 10 psi, depending on the fpr. I use the Italian Malpassi setups here. I seldom use a rising rate fpr on my turbo setups as I usually go for a larger injector and the programmable ecu takes care of so called diminished fuel supply due to boost.This all determined at the time of programming and tuning. When injectors are relatively small, ie: within 130% of std. size, I would use a rising rate fpr. The above is from real world experience. Hope it helps a little.

 

[JustenGT8]

That's some pretty crap advice there BigKriss......why does any FPR have a vac line? Because even in n/a the pressure differential across the injector is important. If you ran enough boost and did not use vac signal to increase fuel pressure it is theoretically possible to stop fuel completely. You MUST reference the reg to manifold pressure (unless it's an in tank non return system).

 

[BigKriss]

"why does any FPR have a vac line?"- The vac line on a std production engine is there to reduce fuel pressure under vacuum conditions such as deceleration and idling. This has one function and one only, to reduce fuel consumption and emissions. No other reason. As soon as vacuum is removed from the fpr the spring exerts its pressure on the diaphragm fully. This causes fuel pressure to rise to the designed running pressure. "If you ran enough boost and did not use vac signal to increase fuel pressure it is theoretically possible to stop fuel completely. "- Bud, you got the cat by the balls here. You dont use vacuum on a fpr to INCREASE fuel pressure-you use a positive pressure from the boost referenced intake manifold under pressure to increase fuel pressure. This increased pressure does not always work on a std. fpr as it is not designed to work like that. Its designed to reduce fuel pressure by means of a vac. signal under certain circumstances only. Now a rising rate fpr is designed to to work with pressure to increase fuel pressure linearly with boost pressure. Unfortunately in the real world it does not rise linearly with boost pressure but does drop of as boost rises. Anyway, I'm not here top swing dicks with you guys, simply adding my little bit of experience from building a few turbo engines in my life. Of course nothing as great as your racer-I am in awe of it-just street driven cars. My humble turbos can be seen on Youtube-search Xenithonman. BK.

 

[cribbj]

I think we're all saying about the same thing here, but we're throwing around some commonly used terms, a little out of context.

Justen used the term "vac line", but I think we all knew he meant a reference line from the manifold. He didn't literally mean a "vacuum" source to the FPR for a boosted car.

BK is advocating a RR FPR, however I think he's a little offbase with the statement that a RR "increases fuel pressure linearly with boost". By definition a RR regulator does not increase fuel pressure 1:1 according to manifold pressure. It does it more by a multiplicative factor, ie 1.25:1 or similar. This is the whole reason why you use them with original injectors in low boost situations, so you don't have to change the injectors to larger ones. The RR reg gets the injectors to perform like bigger shooters.

Here's a graph that shows the differences between a regular 1:1 reg and a 1.25:1 rising rate reg. They both produce the same fuel pressure from vacuum to atmospheric, and once the motor goes on boost, the normal reg will produce a 1 bar increase in fuel pressure for a 1 bar increase in manifold pressure.

However, the RR reg pressure doesn't do a 1 bar increase in fuel for 1 bar boost in manifold pressure. It does a 1.25 bar increase in fuel pressure, for a 1.0 bar increase in manifold pressure. That's why it's called a rising rate reg. Some of these things, like (I think) the Cartech FMU can even do a near exponential rising rate.

Now y'all feel free to take pot shots at what I just said!

 

[redrocco]

"As soon as vacuum is removed from the fpr the spring exerts its pressure on the diaphragm fully. This causes fuel pressure to rise to the designed running pressure."


Thats the main thing I needed to know. I was unsure if the stock FPR would be ok under mid to high boost. I have replaced the 250cc stock injectors with some 440cc injectors, and have a stand alone EMS with dual 4 bar map sensors. (it uses one for manifold pressure and the other for atmosphere for barometric correction). So I don't think I need a rising rate regulator. I do have a EIP tuning FMU unit kicking around just in case.

 

[JustenGT8]

yup all clear there John

....maybe i read too much (or too little) into BK's post? but reads to me as if "the pressure differential across the injector" isn't that important, which is obviously is. Anyways seems we are on the same page and just for reference BK, the stock UZ FPR will handle at least 12psi of boost just fine.

 

[XR8tt]

The main part with fuel pressure is its constant so it can be tuned..
Using the right or big enough injectors with stand alone ECU is best...

I generally don't like Malpassi as the bloody things NEVER return back to the same fuel pressure at idle.. BUT they do keep an engine a live if fuel is an issue.. It's old tech and more trusty on superchargers where power / boost is more lineal..
Where a turbo can build boost at lower rpm under load..
The percentage of rising rate must be considered ...
If it's 12 to 1 ?? Your fuel pump is going to have a VERY hard time supplying fuel at say ?? 15Lb boost...
40Lb line pressue + 15Lb boost X's 12 = 220 Lb..

This type of fuel supply is best with low boost levels..

I thought he was talking about brake booster when it came to vacuum / boost??