sutphen 30

for everyone,the 23.6 is a little misleading,,you need to subtract out the Atm pressure of 14.5inHg,then the reading of vacuum we all see on the gauge of the engine is ~9.1" of vacuum.

Bonzen

Trash will have to attach the KO/EO and KO/EO files, as I don't have permissions yet to attach files haha.

Pliant

Your injector size with numbers. Your pushing big power at high psi for tbi

http://www.thirdgen.org/forums/tbi/3...i-buildup.html

C.)Proper fueling

Fuel Injector requirement- To mathematically calculate the injector size you will need to have to make your HP is pretty simple. Lets say our engine is capable of 340 FWHP. How much fuel will it need? Simple. Take 340 HP divide it by .85(Max Safe Injector Duty Cycle for TBI), then divide by 4 (easier than saying number of injectors and BSFC). On a TBI engine with two injectors it will always be 4 (.50 lb/hr/HP and 2 injectors) That gives you 100 lbs/hr per injector. Oops, GM only ever made a 90 lb/hr injector in the US, and I have 2 68# injectors, what do we do. Well we take the stock GM fuel pressure of 12 PSI and raise it. How much though. Well this simple equation will help a lot. New flow rate = Old flow rate x SQRT (New Pressure/Old pressure) Lets switch this equation around. We know the old flow rate and the old pressure and the necessary flow rate. Just solve for the new pressure.

100= (Injector size(68)) x SQRT (X/12)
100=SQRT(X/12)x 68
100/68=SQRT(X/12)
1.471=SQRT(X/12)
1.471 squared=(X/12)
2.163=(X/12)
2.163 x 12 = X
X=25.956 PSI

The closest production regulator to this is the GM 30 PSI unit used on the later 454 trucks/vans/suburbans. It is GM P# 17113079, $64.74 gmpartsdirect.com. Using the 30 psi regulator will give you 108 lbs/hr using 68 lb/hr injectors.

Click the image to open in full size.

You might wonder why you need more than 100 lbs/hr. It has to do with a little routine called Acceleration Enrichment (AE). Lets say you are accelerating at 3,500 and 1/2 throttle. Suddenly a Honda comes along begging to be destroyed. You mash the gas, the manifold goes dry because you just sucked the fuel out of it. The engine bogs and backfires. One kill for the Honda. Lets just say not having enough fuel to cover AE is not fun.

This is a chart of approximate flow and what 30 PSI will do to it.

Engine---12 PSI----30 PSI
2.8/3.1-----33---------52
4.3---------45---------71
5.0---------55---------87
5.7---------61---------96
5.7HF------65---------103
5.7HF------68---------108
5.7 96 Van 72--------114

Just to give you an idea 10 lbs/hr is roughly 20 HP per each injector. For each 10 lb/hr for the pair is 40 HP more potential.

Bonzen

I have done about 1000 hours of research on injector size, & TBI mods. Talked to all the back east TBI experts on message boards and on the phone. Everything you are saying makes sense and is true, but I really think I'm inline with what this motor requires. Bottom line is the old motor ran perfect with the TBI setup and big injectors (approx. 80-90pph) with zero hesitation at all. The old motor ran on 87octane 100% of the time. The new motor gets 91octane 100% of the time. The new motor is only over-bored .030, with a little more compression and a slightly hotter cam. The TBI on the new motor is identical in everyway except it is slightly over-bored for more CFM and the fuel pressure regulator is user adjustable. Right now I have the FPR set for 28psi which should be plenty. The current injectors are 80-90pph(at 13psi). I have swapped over the old injectors just for the hell of it and the problem is the same. I have also swapped over the old FPR spring and no change. I have played with fuel pressure up and down the scale with no positive change. Even bumped it up to about 34lbs and lowered it down to about 22lbs. All visible on my gauge and it holds steady. I'm estimating the motor has about 375hp which the TBI should be able to keep up with fine by my calculations. If the plugs are any clue(always black and wet looking), then it is running on the rich side. The old motor actually ran muct leaner, according to the plugs always being a light tan color.

94-95 7.4L TBI units had injectors which are flowed at 30psi. When those injectors are tested at 13psi (actually 1bar which is 14.7psi) then they are rated at ~45lb/hr). Of course if they are flowed at 30psi then they will be closer to 90lb/hr which makes sense because it needs to support the 7.4L engine! It's a direct cross reference to a Quicksilver part number. All marine units regardless of V6 or bigger ran at 29-32psi of pressure.

Here is the throttle body I'm running:

http://www.ebay.com/itm/54mm-PORTED-...3D231047944257

And lets not forget, at WOT it runs over 4500rpm and high 50's with zero stumble or backfire. so it's getting enough fuel there somehow.

Either way the wideband AF sensor is coming within a few weeks so we will know for sure(or rule fuel out) soon. I'm still not convinced its some dumb ignition issue that I just cant track down...

Thank you for the info though, I do appreciate yours and everyone's time to help me hypothesize on this issue!

Pliant

Good luck sounds fun....here's a nice tool if you follow that link closely I believe your injectors are going static. Have ever wondered why high ho uses 4 injectors.

https://www.rbracing- rsr.com/calcinjpulse.html

Bonzen

Link was dead...can you check it and post again? Id be interested in reading it.

Pliant

https://www.rbracing-rsr.com/calcinjpulse.html

Trash

First off I have a clerical error I must address in my first response and was unable to edit. The tach reading for WOT was 4000 rpm (vice 3400 as previously stated) while the computer said 4800ish.

Regarding KOEO MAP I believe it was around 98 kPa. WOT kPa did show signs of "hey I need a little more air" as it topped at around 95 kPa.

At this time I'm 80-90% confident there is nothing drastic in the tune causing this issue, but until wide band comes around this is only a guessing game.

It is entirely possible the injectors might be going static on acceleration, but we need wide band to see what the motors doing (is there an echo in here?).
The only counter to that line of thought is I've had him approach the problem zone very slowly and hold TPS steady so I don't believe we are even tripping the AE algorithm into action.

SB and others, thanks for your thoughts and chiming in on this issue. Ultimately we just want to get a fellow boater fully up and running.

Bonzen

I filled out the calculator at https://www.rbracing-rsr.com/calcinjpulse.html and it says I have enough injector at 85% duty cycle to support almost 400hp. The injectors are rated at 13psi, so at my adjusted 28psi which they currently run, they are equivalent to a 117lb injector.

Cant attach the screenshots of the spreadsheets...Can I get a moderator to speed up the process? lol

Trash says that a wideband will not only fix the problem, but will leave all the ladies in awe because we all know the ladies love expensive looking brightly lit gauges on the dash...

SB

iTrader: (1)

Don't tell them about the TBI, cause that is the unsexiest thing ever strapped to an intake manifold.

Funny and serious. LOL.