I went to the track on Fri and ran my fastest time ever. Yeah that's good, but it was 75 degrees outside. As the night went on and the temps dropped the car ran slower and slower. I lost two tenths by the end of the night (with a temp of 63). In the top of second gear around 4500-5000 the car would stumble and my rpms would slow for about a second. What could be causing this?

The only thing I could come up with is maybe after my LT1 cam install the car is running lean up top (running 12.5-13 psi). It seems to pull fine up to 5500 in 1st but second dies badly when it gets colder. The colder air would require more fuel...but why do I only feel it in the top of second?

 

9 to 13 psi is the correct range from the factory for the TBI engines. So you are running almost stock for a static fuel pressure.

IMO you identified your problem. You have an LT1 cam, you have intake mods, and you have exhaust mods. If you have a 305 (you didn't say) then your engine wants to be putting out 220 to 230 hp and if you have a 350 then it would be more like 240+.... but you are not providing the fuel needed to reach these levels, so the air:fuel ratio is too lean.

I assume that you are still using the stock injectors, so that means you need to raise the FP. That should help but it won't help you in a load-dependent sense. In other words during normal driving, you would want a fuel pressure close to what you have right now. But at the track, or during strong acceleration, you'd want an extra 2-3 psi higher than what you have now. The vFPR can provide that, because it is a load-dependent adjustable FPR.

You may also need to use the injectors from the Caprice LO5 9C1 copcar because they pump 65 #/hr, and you probably have LO3 injectors rated for around 50.

Lastly, even with the above changes, you won't get optimum performance because you won't be running an ECM that's tuned for your mods. So you can get most of the performance that your mods can attain, but you won't necessarily have it run in an optimally smooth manner. THAT is the reason why a few people on this forum keep hammering on the notion that a custom chip is needed once you get past the most simple mods.

Read some of the old posts on this forum. They'll cover all of this in more detail, and it will save everyone else from having to answer the same old questions. Oh and report back when you try the track again with more fuel pressure.

AND keep in mind that the stock fuel pump may be old/tired, and when the FP gets increased to around 15 psi, that the pump may not delivery fuel very well because the stock pump wasn't designed to pump into a 15 psi load. You might then need to use a TPI pump that is designed for higher pressure and higher fuel delivery (like the L98 one).

 

Boy that sure sounds like my problem. What were your MPH on the runs that it stumbled at the top of 2nd?
I have the STOCK cam and heads and have this problem...I can't even imagine what it's like with an LT1 cam!

Ken, the L03 injectors are 40 lb/hr, so he does really need at least 55s from the truck L05.

My305, install a fuel pressure gauge, set it up to run a couple drag races and go to the track. let me know what your fuel pressure does when the car bogs at the top of 2nd. Mine doesn't do much, maybe ~1psi change. But I still take issue with my stock fuel pump since I know I am at the threshold for what it can deliver (~15psi, opened exhaust/intake, stock L05 otherwise). Not that I expect more from a 152k fuel pump.

 

Thanks for the help!

I do have a 305 L03...sorry I left that out...I'll have to change my sig

I ordered Auto Meters fuel/air ratio gauge to mount inside so I can watch my mixture during runs. If I'm going lean I'll at least know fuel starvation is my problem and then be able to tune to correct it.

I have already been thinking about doing a few runs with a fuel pressure gauge to see if I have pressure drop at WOT. I've modified my stock regulator so I can go upwards of 13 psi but I was unsure if the stock pump would keep up. I have heard about the vFPR but I have not seen a website/part number for it so if anyone could help me out with that....

I have also read alot of posts about custom chip burning and I am planning that to be among my next purchases because my goal it to hit solid 13's with no juice.

 

My mph drops to ~91 instead of almost 93. And I seem to lose about 2-3 tenths depending on how cold it is. I ran about a month ago on a freek cold day (43 deg) and averaged around a 15.0. Then ran a 14.7 in 75 deg weather last Fri with no other changes.

 

That's about the most damning evidence I've ever seen. Aside from launch issues on a cold track with cold rubber, one would think that cold weather would be a tremendous help in both ET and trap speeds. Your results showed the opposite.

If you can get to the track again, it would be useful to raise the static FP with the quarter mod BEFORE you run, and then return it to stock and run again, to see what effect it has. I think we know what will happen, but nothing like trying it for real. And you don't have to wait for the AFR gauge to come arrive/install.

Of course if you run 14.5 @ 94 mph (or better) with the quarter mod, maybe you shouldn't bother putting it back to stock. Just keep in mind that the quarter mod may still be a band aid solution, and that a load-sensitive FPR (like the vFPR) would be better, and the vFPR plus a custom ECM would be the best.

The part number for, and discussion on, the vFPR is on a LENGTHY thread. Search for it using the keywords vFPR or vaFPR.

And remember that the fuel pump for your car was installed to support 170 fwhp, so it may not be up to the task and it's probably old too.

 

Thanks for help...I was hoping for the 14.5-14.6 area after my last couple mods but was very disapointed to run in the high 14's (or 15's) on my first couple times to the track. I knew that cold weather = denser charge = faster times...so it comfirmed for me something was wrong. I'll be sure to do a search on vfpr...

Thanks again...

 

what are you running for timing?
you might want to try testing more timing and dont just blindly add fuel.. change one or the other and make a pass and see if your mph goes up or down. Dont pay attention to your ET.

 

"Blindly" adding fuel is not a bad idea. If you add pressure (hence more flow), and the car responds with higher MPH, you know you're going in the right direction. If you keep raising the static FP and it keeps running better and better at WOT (MPH), then you probably need a vAFPR to provide a pressure range for you.

Now, blindly adding timing is NOT a good idea. That seems to be EVERYone's favorite idea on this board, and I don't think it's such a cool idea. Sure, he has an LT1 cam, and sure, he DOES need more timing than stock, but what does that mean? Adjusting the base timing changes the timing everywhere. He may need 7 more degrees of advance during cruise but only 3 (for example) during WOT. And if you go too far, we all know what happens.

Too much fuel pressure and about the worst thing that can happen is fouled plugs.

Anyway, I guess do advance the timing 2 degrees at a time, BUT ONLY IF you have access to a scanner, like the XP240, to monitor the knock sensor. If you cannot monitor the knock sensor, then you are blindly adding timing (a no-no).

 

I am running 7 degrees advanced running 93 octane and octane boost. I also constantly run my fan between runs to keep the engine as cool as possible. I don't hear/feel any pings but I know a guy that has a scanner that I can use to check my current knock count.

I got mt Autometer fuel/air mixture gauge today and just came back from a late night test drive. It was really something to see all the tuning the computer does and I really think it will help in my tuning. I didn't get it up to WOT for very long but it did shed some light. I seem to be running a little rich at idle (.650V), running on the lean side while cruising (.200-.350V), and hit almost rich at WOT (.700-.800V). I did notice that as the RPM's went up my mixture started to ease out of the rich and into more of the stoich area (.700-.650).

This has lead me to a comple conclusions. One, I could save some gas by running a little leaner at idle. Two, I seem to need more fuel up top because even in my short runs up to 50mph I could see the mixture was no longer in the "rich" area.

I know the gauge will be really helpful at the track while I'm doing my runs so as I'm having the problem I can watch my mixture. I went ahead and ordered a vafpr from GMdirect and that should help me solve the two problems I have identified. The chip burning will have to come after I iron out the current problems.

Now that I have these rough numbers...(the suspense builds)...WHAT DOES IT ALL MEAN???

 

btw, I arrived at the timing after LT1 cam install from what other thirdgeners where running...which I'm sure it could always need some tuning. It ends up being ~38 degrees total advance if I remember right.