How much ET am I losing with 2.73s?

2

22lbChevelle

Member
Joined
Jan 14, 2005
Wondering how worthwhile a change to 3.42's or 3.73's would be considering my combo in my signature? Powerlogger files show that I'm not getting full boost (24psi) until 2 seconds into the run, and I'm launching with about 3psi. Shifts are at 4100.

Ive got stainless tube headers I think are ATR, a conversion to 4 bolt turbo, external wastegate that had about a 5 psi spring, and some Air Research t4 turbo I still haven't identified. Is this exhaust setup along with 2.73s killing my launches?

60ft - 2.029
330 - 5.362
1/8 - 8.160
MPH - 87.32
1/4 - 12.682
MPH - 107.75

PS: Tires are 275/60R15 MT ET streets with no slip, converter unlocked, temps 115
 
With a 2.029 60ft you definitely have some room to improve. I had 308 in my 68 bird with same size tire you have and that thing had a terrible 60ft I later changed to 3:45 and I shaved a good. .3 off the 60 ft time. If I remember the chevelle has a suspension similar to the turbo Buick Have you upgraded the control arms in the rear and are you running air bags to help tune the suspension in? Why are you only launching at 3psi? If you’re not able to hold the car with more then 3psi you may want to look in to a line lock or something so you can get more boost for the launch.
 
I doubt the gears are what's causing the slow boost rise....Actually, the engine loading will be higher w/ those gears, resulting in an improvement in egt. [Egt being a component in determining turbo spool performance.]
 
i would say yes a switch to a 3.23-3.42 would help out considerably. So would shifting higher than 4100 rpm, even a stock setup likes 4900-5000 or so.
 
Im only launching with 3psi because it felt like I was about to push through the lights if I tried to build anymore boost. I used to be able to hold 8psi but Ive redone the brakes all new and now I cant for some reason. I think maybe I have a small exhaust leak though too. How bad can a faint exhaust leak hurt spool up?

I have since switched to a much stiffer spring in the wastegate, my minimum boost is now like 19psi. I figured this might help spool up, what do you think?

I gotta figure out how to adjust the tranny to shift higher, I imagine only shifting at 4100 isnt the best.
 
Having an exhaust leak will make a big difference in spool up. I made a rubber cap with air fitting to go over the end of the downpipe and I hook up my air compressor to the fitting and put 20 psi into the headers to check for leaks. I like doing this way because I can check for leaks without the car running and when its cool. When I made this set up and tried it I found a couple of exhaust leaks I never knew I had and had never heard. Are you running any sort of MBC? If not a MBC with a spring and ball set up like the one RJC sells will help with spool up also.
 
Im running an external wastegate with a stiff spring that gives about 19psi alone, then I have a pressure regulator that i can turn by hand inside the cabin that takes boost pressure from a vacuum port and applies it to the bottom of the wastegate effectively increasing its spring pressure and allowing more boost.
 
What elevation is the track that you are running at? If the air is thin, you won't make as much horsepower.
 
yes swap them out to 3.42s My 68 400 firebird had 2.56s in it, a swap to 3.23s REALLY woke the car up !
 
If you are running a stock converter with those gears, it is a wonder you are even getting the time shown. :confused:
 
Listen to Nick, the stock conveter with a 4 bolt turbo is likely the problem, look on the side of the turbine housing and see what the A/R is, post up the number
Mike
 
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