hey I just went out and made some test runs with my firebird and it seems to be falling out of the powerband above 5000 area, i never really made good solid pulls with it before, but i dont think that i should be falling out of the band. it trapped at 94 mph and 15.5 s E.T. on my Mr. dyno (yea not anywhere near a timeslip, but it consequtively is about .75 second slow, and the more info the better)

it's got a stock stall because the car wouldnt move with the high performance stall in it, and the rear gears are 3.23 however soon I'll put in the 4.10s sitting in my kitchen

-motor is a 71 350, 9.4-1 compression
-Lunati BM-II cam 230*/230* @ .050 .480"/.480" lift
-487X corvette heads
-weiand xcelerator (high rise aluminum) intake
-holley 750 VS carb (lightest secondary spring) 70 primaries, stock metering block secondaries
-roller rocker arms, polylocks
-MSD super conductor wires
-Mallory distributor

I was thinking of a few things,
too lean of main jets?
timing over or under advancing?
head flow not high enough? but they're corvette heads so I'd think they'd be high enough
overcarbureted?

any thoughts are appreciated

 

Check the timing particularly the advance curve and check the carb. tuning.
May not be getting enough gas.

 

In addition to fuel/timing, look at the springs. What kind of springs on the heads? If you are still running stock type, the are probably floating > 5,000 rpm, killing high rpm power.

No factory installed sbc head flows really well, Corvette or not. But they won't cause a sudden drop in high rpm power like floating valves will. But you won't get the full potential of a 230* cam unless they have been well ported.

You are giving up low rpm torque with that cam, especially with relatively low compression ratio, and the heads are killing high rpm power. A 218 cam would be a better match for your engine as is, more fun to drive and a quicker ET. But with 4.11s coming, sounds like you are building for high rpm power. You might want to consider better flowing aftermarket heads or Vortecs. With 230* cam, consider 72cc or even 64-68cc chamber heads to kick up compression ratio over the 75cc 487x heads.

 

Thanks man, I bumped up the jets and that definitely helped, now at 75 mains.

update: the 75s and the timing curve both helped out immensely, however, would it be of any interest to get a "high flow" power valve for the carb? and about how high should i go on the jets? I'm not really one for carb tuning, i can just make do, so details, and idiot proof instructions are awesome.

 

and also thanks to angel71rs, but i have some further q's too

I replaced the springs with something like 105 lb closed springs er something like that i cant remember exactly, but i got them semi-performance and brand new when i built the motor, so i dont think it's the springs. however it's my first motor, so i may be way off, correct me if i'm wrong

what level of porting would be necessary? I know i've seen some articles on simple vortec head porting and such, but for say, what could be gained with simply opening up the runners and polishing it all out? I've got the tools for porting and polishing just I've been afraid to do it...

or would i be way better off with finding some cheap performance heads? I say cheap because I'm now in college and just beginning to finish up the car, and i dont really want to spend too much more on it

 

I would think the carb is the biggest thing holding it back, not getting enough fuel would definately cause a problem-75 still seems pretty lean IMO, try going upto 78 and see what happens. I want to say with that combo I ran similiar to yours I was in the high 70/low 80 range for jetting.

Gears are a must as well as a bigger stall that flashes in the 32-3500 range. If you plan to run the 4.11's definately get some 27-28" tall tires out back.

total timing should be all in by 3000rpm and you'll have to track test to find the sweet spot for your combination.

I ran high 12's with almost your same set-up, worse heads (305 heads with bigger vlaves and no port work) but I also had an 830DP on top with 3600 stall and 3.73 gears with 27" tires out back. Ran a performer rpm intake and it had a noticable difference pulling up past 6000 when I switched to the open plenum vic jr, before it nosed over at 5500 every pass.

 

I would try and get the most out of what I have before changing anything. Keep working on the carb. Lots of books out there on tuning Holley's, would be a good learning experience.

 

ok i'm at 78's with the jets, and i changed to the purple secondary spring, cuz when i went over the 75s i picked up a stammer above 2000, like a flat spot and then a stumble every 700 rpm er so, very annoying, that was with the lightest secondary spring opening at 1670 er somethin and completely open at 5750. Now i changed to the one just lighter than stock starting at like 1700 and finishing at 6950 rpms, and then 78 main jets, no more stammer, gonna do a pull tonight hopefully, but it seems to have significantly more power, thanks IHI and Z28SORR, and I may need more help, but I'm just going to keep at it, thankfully it seems to be the carb and nothing major

and a sidenote, you reminded me of this IHI, I was installing the stall but....I have a 2700 stall for a t350 i got on ebay, the issue is i have a t350c (82-86 t350 with a lockup converter), and when i installed the high stall, she wouldnt go into gear, put the original back in and it works fine.... now i talked to a tranny guy in town here and he had never heard of rebuilding a t350c for performance, and apparently there is a diference between lockup and non lockup converters besides the converter clutch... should i just look at finding a non lockup t350 in the junkyard?

 

I would go to a non lockup tranny
im running a 327 with a th350 non lockup
3500 stall and stage 2 shift kit rear gears are
342 and a set of 7 year old 295 50 15 gt qualifier
tires and im running hi 12s

 

my engine setup is
327 large jurnal
.240 dome top pistons
76cc smoger heads with mild port and polish
cc ex284h @50 240 246 dur .507 .510 lift
edelbrock eps dual plain intake and 600cfm carb
heddman 17/8 heddars