FAST Mid/LOW RPM Miss

D

dhegge

New Member
Joined
Sep 6, 2001
Guys,

I have posted several times before I have a twin turbo (PT52) SBC that makes over 1000HP at WOT. It uses the FAST Sequential Fuel w/Individual Cylinder Control. We are trying to tune the car for driveability. The Manual T56 transmission seems to make the car surge/miss at mid RPM more so that the automatic used too. As you might expect the car has huge 84LB injectors in it to satisfy the WOT HP. We have tried several things to tune the miss out of the car. Nothing seems to help with the fuel injection.

The manifold is a converted Vic JR EFI setup (Short Runners) so on a whim we unhooked the fuel injectors and put a 850 carburator on the car. The miss at MID rpm went away along with the richness at idle. The car will fog the shop in nothing flat if we are not careful.

So my question is am I going to have to give up some HP and go with smaller injectors to get this more streetable/driveable or can I tune this out of the car.

With indivdual cylinder control it seems to help to take some fuel out of #3 and #5.

Any thoughts!
Doug
 
Now let me preface all of what I am about to say with this warning. I dont know very much at all about FAST systems, the learning curve starts in about a week.

That said, if a carburetor solves the miss, it sounds like you are just a touch too lean at a given cell in the map. If you have big jumps in the VE table( or big jumps in the fuel map on a DFI) the ECM sometimes will have a problem finding the "happy" medium between the cells. For instance, if you have say a value of 35 in one cell and it goes to 50 in the next, the difference is 15. The drivers may not be able to make that transition.Try to isolate the cell and then make sure the cells around it are not so big of a jump.

I know a VE table or fuel map is a 3 dimensional graph if you were to plot it but, I always looked at it as a linear graph and the peak of that line was where max torque occurred in the RPM range.

With ICC you may be taking the wrong approach, look at it from the perspective of what the motor needs not just the individual cylinder.

I may be wrong. Hope it helps.
 
Reggie's idea of trying to minimize the difference in values in adjacent cells is right. The values used in this example (35 vs. 50) aren't yet to the point where I would suspect them to cause tuning trouble but the idea is there. The problem lies in the fact that as the ECU transitions from a "small" cell to a "big" cell there can be very abrupt, large fuel changes for very small engine RPM or manifold pressure changes. The same concept applies to the timing and target a/f tables as well.

I don't think you are at the point where it isn't going to be a driveable car. I've seen lots of cars similar to the combo you described that Grandma could drive. :D I will admit though that tuning a stick car is more challenging for me than an automatic so I know where you are coming from.
 
Yes I learned last year the importance of a smooth transition in the VE table. The values only differ by 1 or 2 between adjacent cells. The miss only occurs at light load about 45KPA at 2800 RPM. In that area I have tried to be especially careful with VE transitions. and still we have a miss.

I have been told by a couple people that my Manifold (Vic JR) could be the problem. I guess there are essentially two kinds of manifold design those for a wet system (Carb) and those for dry (EFI). From what I understand that dry/EFI systems use a much longer Intake Runner. The Wet Vic JR. has extremely short runners, I have a high lift cam and very open exhast as in 1 7/8" 321 stainless headers/collectors/turbos with no mufflers.

I will keep trying to tune the car. The miss is driveable it is just annoying :)

Doug
 
Miss Resolved... set IDLE TPS Value to be one greater than TPS at idle and boost referenced the AFPR to the manifold... and wolla the miss went away! The car runs and sounds great.

Thanks,
Doug
 
You didn't have the pressure regulator hooked to manifold vacuum? That'll do it every time. ;)

Your whole tuneup will likely change now. 45 psi of injector with, say, 25 psi of boost = 25 psi fuel pressure. Referencing the regulator to the manifold will keep the fuel pressure pretty much equal under all vacuum and boost levels.

Glad it's working - keep us posted.
 
Yeah,

As you said we had to retune the whole car, but tuning for WOT is so much easier than tuning for driveability thanks to the WB02 and closed loop/correction. The car is now totally driveable and runs great. With the current street tune it makes 675HP at the rear wheels and it is so smooth. As a matter of fact the engine/trans/gears is so smooth you can easily start off in 3rd gear! It pulls out of the whole in 3rd about like the old motor did in first :)

Now that we have it running correctly of course it is time to change it again. I need to replace the Vic JR manifold with a TPIS MINIRAM for hood clearance. So that will require another retune although I don't think it will be hard. Also we are finally hooking up the RacePAK computer w/the 8 individual EGT's once thats done we will be using the ICC to dial in each cylinder. I have been told that the MINIRAM manifold tends to run lean front to back I hope to use the ICC to tune that out of the manifold.

Thanks,
Doug
 
I think you are taking a step backwards performance-wise from the Victor Jr. to the TPI intake, but let's hope for the best. There's no way the current setup will fit under the hood you want?
 
Yeah It is about 2.5" to tall to fit comfortably under the stock hood (9" from the top of the manifold to the top of the 100 Degree elbow). I really would like to keep the stock hood on the Vette. A cowl induction/high rise hood is a pain to look over especially when you already sit so low. If I have to I can go that route Toledo Pro and others make high rise hoods that will accomodate the additional height.

Here is my thinking/reasoning which may be invalid it often is :) First off the MINIRAM is a short runner (HIGH RPM) manifold more like the Vic JR than a conventional TPI setup. The runners are approx 4" long. I know people who are making huge power using a stock LT1 manifold. Which GM modeled after the MINIRAM I believe I heard somewhere. I have heard people complain about the loss in low in torque that the MINIRAM exhibits compared to a TPI manifold... expected. I suspect that the w/the Vic JR I am giving up some low end torque as well. But both the MINIRAM and Vic JR are designed to run heard and breath hard with RPM.

The MINIRAM is actually the new improved MINIRAM-II w/58mm Billet TPIS Throttle Body. We have made our own fuel rails for the MINIRAM-II to support the additional fuel for the HP.

Anyway with the Vic JR EFI conversion setup that I have know the motor makes 1000HP@6000RPM easy I am sure we could get 1150+ out of it if we pushed it a little more R's and a few more pounds of Boost. Especially since we made the 1000HP with low fuel pressure due to the AFPR problem!

I can't use 1000HP on the street or track. No way to hook it up with the road race setup that is on the car. So I am willing to drop down to 800 or so at the crank which will give us a nice drivable 700+ RWHP. Another problem is the huge torque the motor makes (890) I don't want to be replacing transmissions everyday so I am trying to get the torque down to around 500RWTQ. for every day driving. This is easily done by dropping the boost from 20PSI to about 10 PSI. Which of course is desireable for pump gas.

So the plan is to have two FAST programs one Drag Race and the Other street. With the change of a program/tires and resetting the EBC from 7PSI to upto 20PSI we should still be able to make the big power and use it at dragstrip.

I will know more about the MINIRAM by the end of the week. If that does not work than the next step will probably be a sheet metal type manifold. Probably a Holden, but maybe a Wilson.

If we still can't get what we are looking for it will be time for the new hood and to go back to what we know works. Vic JR EFI!

Any thoughts, comments or concerns welcome as always.

Thanks,
Doug
 
i chopped .850 off the bottom of the elbow and i dont remember how much i had milled down from the intake plenum opening of the victor but it was a lot LOL

i did it to fit under my stock camaro hood which gives me approximately 9.75" total to work with

i'd link you to pics but my DSL provider went out of business and am waiting for my new provider to be up, then you can see the pics or i could email you a select few so you could get the idea
 
I would love to see pics... please Email a few! doug@jdhcon.com is my Email address. I thought about chopping up the elbow and intake, but I need like 2.5" so I would have to chop a lot. Might even have to take some out of the center of the Elbow. I already had the MINIRAM so I thought I would give it a chance. If it does not work... it will not be a problem to put the Vic JR back on the car.

My experince has been that when the Boost is there just about any Manifold will make power. It is the low end torque on the bottom end before boost that I am trying to preserve. I don't find the Vic JR to be a low end torque manifold anyway.

Thanks for the Pic Offer,
Doug
 
Okay I see how you got .850 off I don't know how I would be able to get a full 2" off... also I noticed that you must be very tight between the Ford style TB/IAC and the Water Inlet. Do you have any pics of the manifold on the motor yet?

thanks,
Doug
 
i dont plan on running an IAC for one and i mocked up a water neck and i have one that will fit

i believe my machinist took off around .300 off the manifold

these aren't recent pics....i have sinced torqued down the heads and the intake/elbow are at the porter for some work

http://home.earthlink.net/~tt3rdgen/motor/motor_small.jpg
http://home.earthlink.net/~tt3rdgen/motor/motor2_small.jpg
http://home.earthlink.net/~tt3rdgen/motor/motor3_small.jpg
http://home.earthlink.net/~tt3rdgen/motor/motor4_small.jpg
http://home.earthlink.net/~tt3rdgen/motor/motor5_small.jpg
 
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