Fuel management

[tbtapper]

I've been racing a GenII for about a year now and have finally gotten "reasonably" competitive with the handling of the car. Few fellow competitors will dust me in a corner (if I have, can afford, new tires).
What happens is the fast guys (Stubbs, BMW, Williams, 240Z, other Speedsource7's) will pull 5 to many lengths on the straights at RA, CMP or Roebling.

I do not believe I have an internal "problem" with seals etc, no smoke, hard starting or oil usage. Clearly, the issues are exhaust and fuel/timing management.

Since we're unable to piggy back computer controls I'm interested in what others have done to control fuel and ignition.

I realize I'm asking for what many may consider "secrets". Then, don't respond. This is a hobby for me not a business so I look at it as such.

I have the attitude that I would help anyone who asked me for any of the little knowledge I possess in regard to my hobby.

With that said anyone willing to offer ideas or suggestions will be greatly appreciated.

Remember, if we don't stick together pretty soon ITS will be all BMW's and Honda's winning and we'll never get a tire certificate!

Travers
ITS 19

[paulydee]

Well it is not as simple as fuel and ignition tuning. That is a large part of it. You have to start from a strong foundation. The way the engine is buit dictates how much HP potential is availible. The seal clearances are what make for a strong motor. Even though an engine may not smoke or hard starting issues, you may not be making the most possibel HP.

After the engine the exhaust setup is somewhat critical. There have been many discussions on the subject, but just having any header/presilencer/catback setup will not always get you the most HP potential.

After that then the fuel and timing come into play. An adjustable fuel pressure regulator is vital in this regard, as well as a dyno with a wide band O2 sensor. Then you can tune the fuel and ignition to maximize HP potential.

I am kind of at the last stage of this process myself.

[bldn10]

Travers, since you are being pulled on the straights, are you sure you have the optional 5th gear? Can we also assume it's a pro-built (or equivalent) S5 motor, etc.? How old is the rebuild? I am the 3rd owner of an early SpeedSource car and it still does not have any after-market A/F or other tuning components. Indeed, I have not touched the timing in the season and a half I have had the car! I did change out my fuel pump this year (w/ another OEM one) and it made a difference - have you checked your fuel pressure? I don't recall being passed by any ITS car on any straight since. Of course, over there you racing against some of the best ITS cars and drivers in the country so don't feel too bad.

[C. Ludwig]

First thing to do is a compression check on the engine. Don't beat a dead horse. The very next thing to do assuming the short block is reasonably solid is hit the dyno to find your baseline. Hard to know where you need to go without knowing where you're at. A little searching of the net or various books on the subject will lead you to ignition timing in the 20-25* total advance range and A/F ratios in the 12.5-14:1 range for best power. The proper way to find out what works is at the dyno. Make a few runs to establisha baseline and then slowly tweak one thing at a time. Start with A/F and slowly lean it out until the power falls off. In my experience there's enough of a safety margin with an IT motor that you can go lean enough to make the power fall off before you'll hurt the engine. Find the ratio that gives best power. Next move to the ignition timing and slowly advance it a couple degrees per pull until the power doesn't advance any more. Lock it down. Go back to the fuel and play with it a bit to see if the engine wants a bit more or less fuel with the new timing. In the end don't get hung up on the actual number you put down. Dynos vary from one to the next even within like makes of dyno. Use it for a tuning tool. Lap times will be the ultimate judge.

It's the total package though as Paul and Bill said. Have all that but don't have the 5.12 gear and the proper OD gear in the trans? You won't be as quick. New tires will get you more corner exit speed and thus more speed down a long straight.

[Andy Bettencourt]

Understand also that the RX-7 isn't the fastest on the straights. Our team car, has been the fastest ITS RX-7 at the ARRC for the past 2 years. Williams 240Z pulls it by a car lenght on the back side and the BMW's obliterate it by 5 lengths. Understand this was pre-restrictor plate E36's but word has it they aren't making much difference. Rebello power is a sweet thing. So is Speedsource and Bimmerwold.

Having said that, we took another team car that holds the track record up here at NHIS, and with some dyno time, found 4whp with just fuel pressure adjustments. We think the Racing beat racing header is the best but the overall best package we have tested is the ISC 13B set-up.

Do you have any dyno numbers you care to share? 180whp is where you need to be if you are going to even dream of hanging with some of teh cars you have listed. The best way to get to that level is a Speedsource motor, the best exhuast on the market and some fuel pressure tuning. DynoJet numbers + or - a couple.

As usual, Bill is correct - 5.12 and the optional 5th gear is critcal.

AB

[tbtapper]

Thanks for the insightful responses. All good ideas and things to do over the winter.

I assume you use an adjustable fuel pressure regulator to modify the fuel ratio?

I don't have an adjustable pressure regulator nor do I have any idea what my fuel pressure is so there is an "opportunity" for improvement.

Over the winter I plan on adding a FP gage as well as a realtime AFR meter so I can observe what's going on in the real world.

Also, I don't have a 5.12 (4.88 Torsen). My 5th gear ratio is weird in that it only makes about a 200 rpm difference compared to 4th. I would run out of revs all over the place if I was using a 5.12 I am rebuilding a "regular" tranny with a typical OD 5th for backup and use at places like RA.

I will schedule a dyno session as soon as some of my race pals are heading over to set a base line.

Thanks

Travers

[C. Ludwig]

Originally posted by tbtapper@Aug 17 2005, 04:28 PM
My 5th gear ratio is weird in that it only makes about a 200 rpm difference compared to 4th.
58567


Might want to keep that to yourself. That can't be a legal ratio.