Keep in mind that the instantaneous airflow will be greater as each cylinder fills. Most manifolds that fit under a stock hood wont have much plenum volume to buffer this out. Basically itll negate the vortecs as there will be vacuum present further up the line at the TBI when the cylinder is filling, causing a loss in power. You need the larger TBI. Itll help out across the board at WOT. Those injectors are ok, but the stock pump wont sustain anywhere near 18 psi under load. It just doesnt have the capacity to sustain this. It probably couldnt get to 18 PSI even if you dead-headed it.

Without these, you might as well have an LG4.

 

The pump needs to be able to pressurize to (say) 18 psi AND it needs to be large enough to deliver the volume (a.k.a. mass flow rate) that the engine needs.

Most people here will tell you that you should replace the stock pump with (at the very least) an L98 GM pump, or an GM/ACDelco LT1 pump.... or the Walbro equivalent.

Quote:

If I kept everything
----->65lb/hr injectors, stock TB, stock fuel pump
Bumped fuel pressure to 18psi (if it is capable) couldn't I get the max HP up to the 5,500rpm mark without going static.

It depends on how much air is moving into the engine at that rpm, and THAT means it depends on the load presentged to the engine. In neutral you can rev to 5500 rpm... but you only need (guess) 20 hp to overcome the internal friction of the engine at that speed.... so you need very little fuel and therefore you need a very small throttle opening to get to that speed. Not much air, not much fuel, no external load and only internal friction gets you to 5500 rpm.

I'm using that impractical example to show that it's not enough to say you want to rev to 5500 rpm and ask if 18 psi is enough.

Quote:

I understand the concept of the larger throttle body & higher pressure fuel pump to make more HP, but truth is if it isn't going to help unless I rev to 6,000rpm than (to me) it isn't worth the time and money right now.

Fast355 showed in his 355 build that he made 279 rwhp... which is approx 330-340 fwhp, and that was done with the sbc TBI. Yes the pressure drop across the TB was too high at the rpm for peak power so it means he could have used a larger TB --- but it still got the job done. So it can for you too. So airflow won't be the problem, but fuel will. The vFPR is a good bandaid to give you increase FP when you need it (at WOT) so IMO you ought to listen to Fast's advice. There is nothing about his builds that's costly, so you don't need to spend excessively but IMO you do need to learn DIY EPROM skills.... because you will need to more-or-less duplicate what he did.

 

Yeah, ignore me, listen to Fast and kdrolt. I'm not the one with that motor in an f-body . Oh, and JFYI, I've tried a VAFPR and it only worked well with a near stock chip. But hey, again, listen to Fast on this one because kdrolt says you should . (yes some hostility can be read into this reply but it's also a joke)

 

Last time I heard most people werent running 500 CFM 2 barrel carbs on their 350's unless some sort of racing regulation mandated it. Id personally rather have a 700 CFM TBI on my HO then a 500 CFM TBI. Its just doesnt make sense. Its definatly the ultimate in "penny wise, pound foolish" to spend 2k on a motor, save a few by using the stock TBI, and only get a portion if its rated power output. The ecm wont care either way and will work equally well with each.

Get the 2" TBI and dont look back. With my 350 with the old smog heads it pulled at least 2 inHg and upwards of 3 in Hg if I revved it. Way too much pressure drop. Keep in mind that the measurements I took where the average readings, and not the instantanious readings, which where likely to be much higher.

Also, the VAFPR might be a bad idea on a computer that has no compensation for it. It could potentially really throw things off. Id leave it untill the proper code could be implemented on the ECM to allow for compensation.

 

For someone that has done a lot of DIY ECM work, how much sense did it make to use the vFPR with a stock chip?

And, did you spend any time trying to rework the chip while still using the vFPR? Fast355 did, and he isn't having a fuel problem (at least that we know about --- I don't know if he used the WB O2 when he was testing on the dyno). The FP he is using and the injector size seems consistent (by math, good to first order) with what his engine needs.

I used the abbreviation IMO, that he should listen to Fast355 for the reasons stated --- air flow is a problem for the sbc TBI but it's a minor one. Fuel delievry to that engine is a big problem though, a comment you agree on.

You had an experience with the vFPR but it seems you didn't go very far with it. Chris apparently did. So is your solution any better?

You advocate the larger TBI (fine) and raised FP. What FP should he aim for, you didn't say what number?

Will the idle/cruise manners be adequate --- can the injectors flow a small enough mass of fuel for idle/cruise AND satisfy WOT for all rpms?

A learned NH neighbor who has dealt with this problem before says no -- that it's too hard to achieve both (idle/cruise vs WOT) with only 2 injectors when you are shooting for 350+ fwhp, because if you tune for one extreme (WOT acceleration) then you give up having good idle/cruise behavior because the injectors flow too much fuel. Any mods to the 350 HO engine is going to push it past 350 fwhp, so the situation only gets worse to solve with TBI.

Conversely if you tune for idle/cruise, you lose out on the top end and go lean. I accept his comments as facts, so the only way (IMO) to solve the problem is to either use more injectors (dual TBI) or to vary the FP with load. Dual TBI seems hardest to do --- the vFPR does not seem nearly as hard to do. And Chris (Fast355) semed to get the vFPR working fine together with the ECM work. And so did GM (that's where the vFPR came from, from use on the bbc TBI engine).

That's the basis for my suggestion, to listen to Fast355. Now lets hear yours JP.

What FP are you using, how large are the injectors, how well did it run this summer, and how did it do on the last emissions test? IOW does it provide the fuel under all operating circumstances (idle, cruise, WOT, emissions)?

swerve-driver seemed to get good results with a fixed FP and ran 13.8. OTOH a 350 hp v8 in a 3rdgen ought to run near 13.0 (LT1 4thgen Fcars did it with 285 fwhp), so me thinks there is still a lack of fuel delivery.

Fuel delivery at all rpms and loads is the big problem for high-hp TBI engines with only 2 injectors, and that's the core issue. At some point, a fixed FP won't do the job under all circumstances. It's debatable if you, or SD, have reached that point already.

 

No offense taken. Just giving another way to do it, as well as reinforce that we both say he needs more pressure. Regardless he needs the bigger injectors at more pressure. Just two different ways to get the pressure.

BTW, my 305 was pulling down to 93 KPA at higher RPMs with a 454 TBI unit. A CFM-Tech spacer, Hypertech Powercharger and a 3x14 open element took care of this. 98 KPA @ WOT now. Noticeable improvement on High End HP.

JP-I know what you mean on the VAFPR. I still like to use them due to the emissions standards down here. I have always had trouble with the emissions being TOO high on HCs with the injectors at the fixed higher pressure.
You also mentioned that you would keep the stock TBI if you weren't racing to 350 HP. That is sound advice, IMHO. No argument there.
You also mention the F-body as some signifigance. A TBI motor has pretty much the same requirements no matter what it is in, within reason anyway. Be it a F-body, a B-body, a G-body, a truck, or even a G-Van.

 

When I get back from the track I'm going to reply to this. kdrolt, you need to stop making assumptions through implication. It makes you look stupid and I know you aren't. Stubborn like me but stupid you are not.
Be back in a while, gotta go track tune an Lt4 hot cammed 350 vortec TPI in a friends 88 IROC and a bolt-on 96 ImpSS.

 

Theres so much affected by the fuel pressure its not even funny. The PID gains, AE, steady state fueling, etc. are all effected by the injector flowrate. Without compensation for the BPW it could get interesting, depending on what the dynamic range of the regulator is. Id say its better to leave it off untill the UTBI code comes out. The best solution may be to balance the idle and WOT fueling. If JP got his combo to work well, and I got my engine to run well with the same TBI and injectors, I cant see it being a problem for the poster, whos running the same engine as JP.

 

I am one person that is one of the thought that there really aren't too many idle problems with big injectors. Obviuosly I'm not near the power levels of a 350 HO, but I have a decent little 305 with TPI heads and a ZZ4 cam. Now, I'm running BBC injectors (80# or whatever they may be) and I'm running fixed fuel pressure at about stock pressure, based off the position of my FPR. Now, most would say this 305 with ZZ4 cam wouldn't need much fuel at all at idle and that BBC injectors would either drown it or be erratic with too low of pulsewidths. That is not the case though. I have lowered BPC in the PROM to match the injectors and just by flattening out the spark curve on idle and working over the VE curve, I've achieved a rock steady idle.

First start up after the cam swap it sounded like a muscle car with a big lopey cam in it. After tuning for the injectors, the idle is almost no different than when it had a stock cam. It is super smooth at about 700RPM, pulls decent vacuum, and 128 BLM. It doesn't smell rich at all either. Now I suppose that without a WB O2 it could still be rich, but this cam doesn't have much overlap, so I believe the BLM readings. I'm not positive what my BPWC was but I'm thinking its around 80 or so (car is at home along with all the bins). If I can get this motor to idle pretty easily and solid like a stock car with BBC injectors, I say that the low pulsewidth problem is not a real one. Maybe I'll be proved wrong. But honestly, all I ever read about on here was that it would be a problem but never saw anyone experience said problem. So until it starts running erratic at idle in my car, I see it as a disproved theory in my case at least. Just my $.02

 

I guess I agree with all of you in a certain way...

I have witnessed the VAFRP work great with a stock chip & cam.
For me, it wouldn't be worth the trouble to try and tune around the FP. The thing I don't understand is why kdrolt, you mentioned that in Fast355's 355ci build up the stock TB was capable of producing the hp numbers, but 355 also says "listen to us" you need more fuel pressure and a new pump. If the motor will rev to 5000rpm with the stock pump and TB without going lean and you don't advocate a larger TB what advantage is higher FP going to give. I'd have to use smaller injectors with too high of FP in order to not drown the stock TB.

It really isn't a matter of taking sides at all, there are too many variables that allow different things to work for different people. I, personally, am trying to discover which path leads to the least headaches. But I really appreciate everyone jumping in.

 

I have to agree with you on that. I have a feeling my engine will run even better with the compensation taken into account in the code. Really though I had no other choice on the matter since my engine makes upwards of 370 FWHP at 6,000+ RPM yet still needs to idle, get decent mileage, and pass emissions. Although the basic changes were made in the vacuum vs. air/fuel ratio table in Open Loop. Nearest I can tell through trial and error, that also effects the calculated A/F mixture in closed loop. That table was setup in relation to calculated pulsewidth. After changing from a well tuned table at a steady fuel pressure, calculating the BPW from various vacuums, then adjusting it based off % change, I was able to make the A/F mixture nearly dead on. I made some final tweaks to the VE table using the BLM correction method, verifying the A/F mixture with the Wideband. I did the same with AE, although it was much trickier to get tweaked. It is still far from perfect but it is close enough that it still runs great SOTP. No hesitation, delay, stumble, etc.

 

There are differences in FUEL and AIR on a TBI. It is not like a carb. Air Flow is the easy part. Fuel is tough. Your engine will pull air, no problem. Only thing is, it needs more fuel to go along with this air. My analogy goes back to Nascar. They can still make huge HP with a restrictor plate and a 390 CFM holley. They just have alot of vacuum in there intake at HIGH RPMs. They would make more power with a 1000 CFM carb. My old engine made good power using the stock sized TBI. I had much larger injectors in it though. 454 Injectors at 15-17 PSI. The stock for the engine 68 lb/hr injectors just would not cut it. Even at 18 PSI. Now I have the same 68# injectors running at 32 PSI @ WOT, which gives the fuel I need. Throttle body size has nothing to do with "Drowning the TBI". The actual fuel flow @ WOT changes very little when you go to the bigger injector. The difference is when you slightly crack the throttle, the necessary TPS enrichement changes greatly.

Take a look at these two graphs from Airdeano on Fullsizechevy.com. They are scaled in TPS voltage Vs Airflow. These are measured at 28 in/h20.

Small Block TBI.



Big Block TBI

 

OOPS!

 

That is what I am saying, I NEVER said that throttle body size has anyhting at all to do with drowning the TBI.

What I was trying to get at is that if the engine will rev to redline without going lean at stock FP what would be the advantages of using higher FP?

 

Thats just it, there is no possible way that the a HO 350 will go all the way to redline with stock 350 injectors and pressure and not go lean. It just won't happen. Get a WB or even look at your NB readings, they will show lean if your running stock injectors and pressure no matter how much you tune your chip you can only hold the injector open for so long. Keep in mind if your going lean enough to get your NB sensor to switch back across your .45 volt center line your leaner than 14.7 to 1 and this will burn pistons very quick.

 

I have NJ emissions and I passed the HC with 25% to give on my engine with the 65# @ 28psi. The pulse width wasn't that low! I took out some timing and made sure my cat was nice and hot. Then there was the fact that I had my PID routines adjusted to run RICH. My o2 signal is below and it passed the HC test. This is vortec heads doing what they do best (fast burn).


Did you read the previous posts in this thread?
I said "I tried a VAFPR and it only worked well with a near stock chip." There are no implications in that sentence that suggest I didn't spend any time with it. I spent months trying to get it to work the same as higher fuel pressure because I 'thought' that because some person a LONG time ago suggest a problem at idle with his TBI tune might be because of his fuel pressure being high. It wasn't the case. As for it making sense to start with a stock GM bin file... well you obviously don't do much tuning because that's the best place to start. I made over 40 calibrations trying to get the VAFPR to run right and it just so happened to run best with a "NEAR STOCK CHIP."
You say Fast isn't having a fuel problem but earlier in the thread;
Lying to the ECM and a super lumpy VE table isn't how a novice should start tuning his engine. In fact lying to the ecm should be a last resort, not the first.

Yes, I think my solution is better because I've done the testing with this motor in an f-body and found the fixed FP to be much better. It's not only easier to tune but it removes the complexity of having ALL of your transients skewed by a variable inj flow rate that can't be well compensated for with the stock code.
Give me a break. I don't need to echo what has already been said a hundred times by me and others. I run 28psi and if you read this thread THAT is implied!
Well your learned NH neighbor is wrong. He must not be very good at tuning because I'm sitting here now with an almost identical vehicle as Mav and I've passed emissions testing in NJ with 28psi of fuel pressure at IDLE/CRUISE/WOT. My BLM's are all within a 5 spread (125-130) during the drive to the inspection station, during inspection, and return home.
Your doing that whole "numbers game" thing again. The 350ho isn't going to make push it past 350fwhp with any mods. Heck, I can think of a few that would bring the HP DOWN .
Why are you accepting his comments as facts and not listening to ACTUAL facts? Don't you trust that what I'm telling you is true? I don't have any reason to lie on this subject. You use the word, "seem" a lot. It "seems" as if your neighbor needs a quick schooling in the TBI voodoo .
GM didn't put the VAFPR on all of their bbc's. Have you looked at the bin's of the engines with the VAFPR? I have, but hey, listen to Fast355 and your "learned NH neighbor" because I must not be somebody worth listening too

I think you need to re-read this whole thread and every other post I've made on the subject of tuning TBI.
BTW, I don't think I recall EVER seeing where Mav needed to pass emissions testing. If he does, he hasn't said it so why would the VAFPR be better than fixed FP? Even then, I got my fixed high FP to pass and I've said it before in at least 3 threads on this subject of TBI fuel limited. Do a search for all of my suggestions.
Comparing LT1 4thgen's to a 3rd gen makes a bad comparison. There are numerous reasons why a 4th gen is faster than a 3rd gen with the same power. That's a whole nother thread in and of itself but let's just agree to disagree.
How is it debatable if I've reached that point already? You must not be thinking to compare ET's in 2 different vehicles with so many differences. How about looking at the trap speeds to compare horsepower, ET has too many variables where as trap speed is basically HP and vehicle weight. I don't consider Mav's engine to be debatable considering I've run the same engine with fixed high FP and not had any emissions (as if that were a consideration) or drivability issues!

edit: JFYI and everybody elses; I've tuned 2 other TBI vehicles with VAFPR's and ALL of them have removed them and gone to fixed high FP. ALL of them are passing NJ emissions testing and one of them is a 454SS with heads/cam/intake/headers/exhaust. His truck has run mid 13's with GM heads and is running out of fuel up top. 90# injectors at 20psi. We're going to 30psi to see if we can't get into the 12's with an all GM long block. But hey, again, don't listen to me, I don't know what I'm talking about when it comes to tuning TBI. [/bragging]

 

It's hard to argue with that...

Thanks for all the help though, EVERYBODY that has been posting, even through the arguments there is a lot of good info.

 

The stock 350 TBI engine ran with 9-13 psi from the FPR, so using 11 psi as a typical value, and you using 28 psi means that you are dumping approx 59% more fuel in under all circumstances (incl at idle). This means you have the potential to make approx 320 fwhp (59% above what a 9C1 LO5 made from the factory with 11 psi FP and 65 #/hr injectors), so that fits in nicely with your engine combination.

If you had a stock cam (which you don't) with that fuel pressure, then you would need to reduce the PW by the inverse of 1.59 to keep the mass flow rate of fuel the same as stock, which means a 37% reduction. I don't know if the injectors can tolerate that much of a reduction in PW at idle/cruise to maintain a smooth idle. But with the warmer cam you are using, you have more overlap so less air/fuel gets into the cylinders at idle, so you must be able to tolerate it at the expense of burning fuel in the cats.

That means your cats are making up for what you cannot tune around at idle, and perhaps at cruise. If that's the case, it may work fine (for you) but I don't like it. It leads to premature failure of the cat because the cat was designed to burn fuel not burned in combustion, and not designed to make up for the situation described above.



Less ignition advance means more burning in the exhaust (cats) so you succeded in passing emissions but you did it in a way that might be acceptable for you, but might not be accepatcle for all here.



If you are plotting raw voltage then the DC equivalent stands near 500 mV, which looks a bit rich and therefore consistent with the above discussion. Since I'm seeing crosscounts then it looks like this is a NB trace.



Sorry for my confusion. I know the stock chip is the place to start (but thanks for reminding me); I didn't know, or had read and then forgotten, that you had spent considerable time trying to get it to work.



How do you know his VE is super lumpy? I didn't see that in the thread above. Did Chris send you the file?



It looks like you do have a tune that works, but you are living with the rich mixture at (probably) all conditions of idle/cruise and letting the timing (reduced) and cats clean up after you. So your method also has it's flaws, and therefore it isn't obvious to me if you have a solution any better than what Chris seems to have sucessfully done with the vFPR.



Thanks for clarifying.



He might be. OTOH he's was doing DIY EFI when you were still in high school, and maybe earlier than that. He has a lot more experience than you, so I'm inclined to listen to what he says. I'm even more inclined to believe it if I can reconcile his findings with what my engineering judgement (with numbers!) tells me should happen.



I'm not so sure about that, but we can agree to disagree on this point.



I knew very little about TBI or TPI engines 3 years ago, other than what I already knew about ICE (internal combustion engines). I did a lot of reading before I started posting. I accepted a lot of posts as "truth" when I got here... but then I couldn't find any evidence to back up the claims. Swirl ports heads = junk (a favorite of yours) was one of them, on the heels that TPI (iron) heads are good. Stamped steel rockers = junk was another. Then there was fixed driveline loss (yours). I didn't just jump in and disagree with you (or anyone else for that matter); I waited until I had found enough contrary evidence, hopefully here on TGO, before I disagreed. Then I posted.



I know that. But here, as well as elsewhere in other threads, you've been given contrary examples and you ignore them. I'm listening to you here, I know you have something that works (it's obviously tuned to make the numbers of the 350HO) but it does so with compromises at idle/cruise and uses timing and cats to make up for it. I never saw you mention that little detail, but I did. So it might not be the best way to go. I can see that (both sides), I accept it, but it doesn't mean that I have to think your way is better. Most cars driven on the street spend most of their time at idle/cruise, not WOT. So I don't like the idea of tuning for WOT at the expense of longevity (in the exhaust system). IMO.



I never said they did. The vFPR came from bbc marine application.



Maybe.



I always assume that they do, because after all, DIY EFI efforts technically violate the federal laws of emissions (Clean Air Act), so it helps to be responsible and therefore in keeping with the notion of clean air.



Hummm. Same power, but the 4th gen is faster. Hummm. 3rdgen has a lower Cd, and same power (after mods) but is slower. Sorry but that doesn't match the physics.



I couldn't agree with you more.

MOD EDIT: Discussing the topic and attacking an argument is fine, bringing other posters in and commenting on them is not.

 

I think the origional posters questions have been answered. Now its just decending into another arguement. You knew it was coming...