I have the GM vac FPR that has been modified to make it adjustable.

The regulator works as it should when I don't go over 26 psi. 20 psi at idle, 26 at WOT.

If I adjust the pressure higher than 26 the following happens: The pressure drops when I rev the engine, vs going up as it is supposed to.

Is this caused by the fact that vac can't overcome that high of a spring pressure?

Or do I need to install a stiffer spring to increase the pressure before I begin increasing it?

Or, is the answer to install an aftermarked FPR and eliminate the GM unit?

If I install an aftermarket unit, what fittings do I need to adapt the regulator to the GM fittings?

Also, I'm having trouble calculating what my fuel pressure should be with the 75 lb GM injectors (454). I already have the TB modified with the 2.2 inch bores, as seen on the 454SS website.

Also, would switching to a single plane manifold help alleviate high RPM leanout? Walt Sherwin (I'm sure you have heard of him!) recommended I use a single plane with the TBI; matter of fact, I bought a manifold (Holley Street Dominator) from him that he was going to use in a blown TBI setup, but moved on to other things.

A friend did my engine combo on desktop dyno, and I can produce a theoretical 500 HP at 5000 rpm.

I know the 94 and 95 454 trucks used a different fuel pressure reg that produced a pressure of 26-32 psi, but I was unable to find this info as the DIY EFI site no longer has this info. Could I just use the spring from this regulator in my current vac ref unit for increased pressure and adjustability? Does anyone have the P/n for this reg handy?

Thanks

Bill

 

i used the vafpr for a while earlier this year. what is the aplication/ engine specs? i am aware of three springs available. the one that came with vafpr was way too strong for a 350 engine. I think the vafpr is used on a marine engine 502 cid? by base idle pressure was set at 11 lbs.

 

If you are using the stock TBI pump that is your problem. That stock pump cannot support that amount of PSI at WOT. Change to a high volume unit and I bet you get the pressure you need. 26psi is an afull lot for wot. Most modded TBI combo's need 17 at most. It may be slightly different with those injectors.

 

is it possible the fuel pump can't keep up at that pressure?

and yes, IMHO a single plane is the way to go wtih TBI.

(wieand 7525 over here )

 

good point. i upgraded pump on mine. no issue with FP per dyno run. was running at 12/1 at 6000 rpms...

 

What did you upgrade it with?

 

you need this
http://www.top-downsolutions.com/pro...products_id=39

and this

http://www.top-downsolutions.com/pro...products_id=97

 

Fuel pump from a 1985 chev vette. i think puts out 22 lbs to TBI regulator. prior pump was a 82-84 chev vette. common upgrade. or the LT1 pump works too. higher pressure yet..

 

I'm using the Holley 12-903 pump. I thought that the pump was the problem also, but it proved not to be.

 

what is your source of vacuum?

 

The port on the front of the TB that was originally used for the PCV valve. This port pulled the most vac at idle, hence I used this one

 

Sorry guys!

I guess I wasn't clear as what my app was.

Its a 90 454SS pickup.

Stock shortblock with 50K miles. Compression is 8.0

CC 264 HR cam.

781 iron heads with 2.19 and 1.88 valves

Edlel shorty heads with true 3 inch duals.

RPM Air Gap manifold with Holley TBI adapter, 1 inch phenolic spacer, modifed TB with 2.2 inch bores.

I know this isn't a 3rd gen Camaro, but there is a wealth of TBI experience here .

I know this combo has potential as the truck has been a 13.42 at 99.9. This was with a single exhaust, Edel TBI intake, stock converter, and 3.73's. Fuel pressure at that time was 16 at WOT.

I know with the intake change, addition of true duals, and cold air intake, there is much more air moving thru the engine. Before, the engine would run out of breath at 4800, now it pulls strong to the 5200 rating of the cam..

 

So, can anyone help calculate what fuel pressure I should need to feed 500 HP 454 CI at 5000 RPM?

 

At what RPM and only 2 injectors? If so, be ready for some serious idle issues, 4 barrel tbi is for anything over 400hp.
Check to make sure you aren't bottoming out the spring.

 

Hmm, lets say the 75#/hr is at the 9-13 psi range, we'll use 11 psi. For 500HP at .5 BSFC (safe value) and 85% DC the injectors need to flow:

500 HP / 2 injectors * .5 BSFC / .85 duty cycle = 147 #/hr per injector.

Thats a lot of flow.

Lets try 30 psi:

75 #hr * sqrt(30 / 11) = 124 #/hr

Not quite enough. Try 40 psi:

75 #hr * sqrt(40 / 11) = 143 #/hr

Close, so it looks like 42-45 psi would do the trick. . . If you want to run it that high. May also want to get a 4bbl or bigger injectors.

If the true BSFC is lower (a possibility) then less flow is required. Lets see the flow for a BSFC of .45:

500 HP / 2 injectors * .45 BSFC / .85 duty cycle = 132 #/hr per injector.

RBob.

edit: (heh, Jon's posting while I was still typing. Like Jon mentioned a 4bbl route would probably be better. I don't know if I would put 40 psi into a GM TBI unit.)

 

GM injectors are rated at 12 lbs. the largest i know of are 90 lbs. when i upped FP in error i blew diaphragm gasket twice with VAFPR. holley rates theirs at higher FP. do they offer a larger unit? my single plane manifold had a dedicated spot to run vacuum. i did not run off TB. maybe it makes no difference.

 

So, what are these 90 lb injectors? Anyone have a p/n?

I know it has been mentioned before, but Walt Sherwin advises that the injectors can be run up to 70 lbs without problem.

What problems occur at idle with 40 psi pressure? I was playing around with the fuel pressure and had it up to 45 at idle, and didn't notice any problems with the injector spray patterns.

Also, the fact that everyone says there are problems with high pressure at idle is the reason I wanted to use a vac referenced fpr.

I really don't think airflow is going to be a problem with the single plane and the 2V TB bored out to 2.2 inches.

Walt advised me that there were higher flow injectors produced for the foreign market, but they were no longer available. Anyone have any knowledge of this?

I know 4 bbl TB or carb would be the easy way out, but I'm trying to push this TBI setup to the limit.

Thanks guys

Bill

 

an interesting note to this:

i e-mailed holley just this week for some clarification on flow ratings for the TBI's, and other information along those lines. although they didn't answer my question they did give me the following response.


Fuel Injection systems are not as critical on the air flow as the
Carburetor. Carburetors require draw from the engine to pull fuel through
the Carb. Also Carbs are flowed with fuel at Holley. The fuel will change
the air flow. The TBI systems are based more on the Injector capacity to
flow fuel.
670 CFM TBI will feed up to 275 HP. Due to the capacity of the Injectors
to flow fuel. They can flow 85PPH with 21 PSI pressure.
The 650 CFM four barrel will feed up to 150-325 HP.
700 CFM will feed 250-440 HP
900 CFM small Injector will feed 375-525 HP
900 CFM large Injector will feed 450-600 HP.
When the air flow demands past the 700 CFM TBI we go to the 900 CFM at
that point.
You will need to pick the TBI kits by the HP of the engine.

 

 

no part # on hand but that are used in the later model SS454 try gmpartsdirect.com for a decent price.

 

It is my understanding that these are basically the same injectors I have, just set up to run at a higher pressure.

The stock ones for the 90 are rated at 75 lbs hr at 12 psi, and the ones for the 94 and 95 are rated for 75 lbs at 26-32 psi.

I just looked at a rebuild kit for the 94-95 and it uses the same gasket as the 90-93, so I don't think elevated pressures are a problem as far as the fuel channel.

Again, the info on the injectors was provided by Walt Sherwin.

Yes, Walt did mention that the higher flow injectors were for the Brazilian market. Anyone have an idea where I could get those?

 

i thought i read that GM rates all injectors (tbi) at 12 lbs FP. Hoilley rates at higher FP. there are two injectors used in 454 SS one 90 lbs (late model)and the other either 80 or 85 lbs(early model). i suspect you are aware of this. not sure what is in marine engines but would like to know...