421 SBC - something holding it back at the track !

[SBC-68-FIREBIRD]

Thanks Boys, these are the sorts of replies I am after, little things I can try using what I have to pick up a bit here & there.
I am well aware heads & cam are small but I also said I wanted something very similar to my old engine that will last for years with nothing required other than oil changes. I dont have the time or the $ to be changing engine components.

The pump fuel we use here in Australia is 98 octane but I think its rated a little different to yours in the US.

The carby is a 950QFT. It was put on straight out of the box & the jets have not been changed as everything was fine when on the dyno. Will have to have a look at the paperwork to get the jet sizes, cant remember.

The X pipe is 2 x 3" mandrel bends with with a slice taken off both & welded together.

[gruntguru]

No question your power at the strip is less than the 580 you had on the dyno. What has changed?
- 1 3/4" press bends flow a lot less than 1 7/8" mandrel bends.
- Mufflers?
- Fuel supply? Does it feel weaker towards the second half of the track? A simple flow test through the needles/seats - bowls off the carb - hang them over a bucket - run the pump.
- Air flow over the air cleaner at high road speed?

[steve cowan]

Very 1st thing id be doing is a run/s with the exhaust unhooked. Unless your 3" x pipe is 'very' good it's csa and shaping may be at its limit for your hp. A little more cam, more rocker ratio bigger heads , the same lsa has effectively given you more overlap area which does not like exhaust restriction. Going to bigger primaries(less velocity) with the same restriction down wind will not help.
If you want to keep/ish it quiet for the street and keep the same type of setup just step it up to a 3.5 x pipe and continue at 3.5 back to the mufflers then step it back down just before the existing mufflers.

Craig,
very good suggestion with dropping the pipes at the track,
i dont run x-pipe on my car and i have been told by plenty of people it will make more power with the x-pipe and run faster,i run the same as Scott as in straight 3'' to aerochamber mufflers dumped at the diff,my theory is the less exhaust bends the better and even with a 3'' short exhaust system the expansion-pressure waves are GONE and its not like dropping the exhaust OFF.i might be out to lunch on my theory and should research the subject a bit more,any thoughts on my theory ??
there might also be a MPH in tyre pressures IE- higher tyre pressure with small amount of wheel spin off the start may increase mph at the stripe by having the engine rpm higher sooner,
the main problem will be that our street meet and test and tune tracks here in Brisbane and Sydney are unprepped and so inconsistant its almost a joke but thats the way it is

[F-BIRD'88]

Your dyno tested engine horsepower is "corrected" power. Look on your dyno sheets for the correction factor or the weather station readings of that test.
This is the real raw power actually made that day.
(It will be less than corrected power)
The power actually made at the track is also uncorrected real power of that day.
A high correction factor will make the dyno test look good and thus the track perf seem soft.

What was the dyno correction factor?

You'd want to "correct" the track ET- and mph based on the weather data for that day to see how fast it should run under SAE standard corrected air specs.
60f deg dry air 29.92 in hg

Eg if it WaS 85f deg 75% humidity and 28.0" pressure(station pressure) it is going to run slower than if in corrected standard air.

[F-BIRD'88]

You already have a hole in the he hood . Why not try a areo carb scoop on the carb? Ya it will need jetting fine tuning at the track.. If the track is a non preped track get some real deal bias ply type wrinkle wall drag slicks. Like M/T ET streets or Hoosier Quick time Pro or M/T drag slicks.
A lot easier to hook than radials.
Leaf spring cars like a taller wrinkle wall slick eg 9x28 or 9x 29 and stiff gears as opposed to short wall radials.

Car setup and track prep is critical with radials for launch traction. Slicks are much more forgiving and consistent.

[F-BIRD'88]

Your "98 octane" pump gas is about equal to our 93 octane r+m /2 rated pump gas.. Jazz it up with a good gulp of racing gas blended in for 11.7:1 CR.

[F-BIRD'88]

A mechanical Street roller in the 254-265 range (.050") on 106 LSA will put some fire under your ass.

Eg" custom grind comp extreme energy cam.

[SBC-68-FIREBIRD]

My mistake.
The carb is a QFT850.
Primary jets 76
Secondary jets 84.

I very rarely run at street meets because the track is normally pretty crappy but the last one was the day after a test n tune & was pretty good. 99% of the time I will run at events where the track is prepped up real nice.

[SBC-68-FIREBIRD]

Put some fire under your ass

As good as it would be to have the extra HP from a solid roller I am not pulling what I have apart to spend probably $1500 or more to gain a few tenths. I want to just work with & get the best out of what I have.

I tried to get a pic of the X pipe tonight but car is low so cant get under there tonight.

[steve316]

Your carb is part of the problem; I know you don't want to spend extra money for no gain. Borrow a good 1050 dominator and try it out. I use a 9375; it doesn't make as much hp on dyno, but takes the car down the strip quicker. the new 3ard gen hp makes more tq & hp but doesn't move the car as fast. I am not saying this your only bottle neck; try it and you may find part of what your car wants.

[BrazilianZ28Camaro]

Try the cheap things first!

Remove the exaust and install 18" extensions to build that low end torque to launch well, specially with 3.73 gears and 3600 lbs.

I've seen cars gain 40 RWHP just removing the exaust with crappy mufflers!!

Have you tried run w/o the air filter? Can you fab a foward facing scoop and just duct tape it to the hood to test? A mounth of 14"x4" would be adequate.

[SBC-68-FIREBIRD]

My next run back to the track I had intentions of dropping the pipes to what difference that makes.
Then will play with jetting.

I have not tried it without the air cleaner. I am sure they dont let ya run without a filter here in Sydney & due to hole in the hood I cant really hide it.
I had contemplated making up some forward facing scoop just for the track but have not got to it yet.

As I have said in previous posts, I wont be pulling the engine apart, I am only going to mess around with the bolt on little things, just trying to get the most out of what I have because for whst it is, it is a great street car, I drive it everywhere & it does everything real nice.

Thank you all for the replies.

[geraldtson]

Man what you need is MORE GEAR!! 4.88 minimum.

[SBC-68-FIREBIRD]

4.88 gears & out of rpm at 3/4 track & reving at 4000 plus rpm while cruising around of a weekend. Its a weekend cruiser so I dont think 4.88's are ideal.

[67RS502]

you've got a variety of responses.
your mph is showing loss of power, even with different gears and stalls it should mph better.
Since my 420 ( and my car) is similar to your set up, heres the issues I see with your setup:
1. Heads... those heads are just too small, pinch is around 2.1", you need around 2.4-2.5" csa, have them ported! this will be worth a ton of power.
2. Cam is on the small side, which will limit power and rpm, my HR cam is a few degrees bigger, but don't believe all the nonsense of HR cams no power no rpm bs, I shift at 7200-7300, with matching valvetrain hr cams will make good power.
3. its going to want 1 7/8" headers, but 1 3/4" isn't hurting it that bad, about 20hp.
4. make sure your fuel and exhaust system is up to the job.