A couple weeks ago i swapped to L98 manifolds and dual cats from the tiny TBI manifolds and single 2.25" T and cat. I kept the same dynomax 2.5" cat back on it. previous---new
60- 2.157 - 2.233
330 6.282 - 6.297
1/8 9.655 - 9.624
mph 72.94 - 74.46
1/4 15.05 - 14.89
mph 90.58 - 93.18

I'd say conditions were pretty equal, but overall i'd guess the new slips conditions were a little worse, and that's why it struggled more out of the hole. Still a tenth and a half and 2.5 mph isn't too bad for still using stock pieces. I just put a fromaster cat back on it w/ the improved Y design and honestly i feel more SOTP from that than from the N10. I'm gears, converter and headers away from 13s w/ a stock LB9. All i have so far is a ported plenum and a little bit of playing with the chip.
And for anyone who wants a better exhaust but doens't want to deal with headers this shows that even bigger manifolds and the improved Y design of the dual cats can be worth a lot over the stock T junction set up.
...ed

ps, in case anyone is confused why i had to swap exhaust manifolds. In the 90-92 non G92 305 TPI cars GM used the small TBI style manifolds and single 2 1/4" cat instead of the peanut cam to derate the power. Thats why i'm right into a good HP range just by swapping to the good factory exhaust.

 

Before someone says, "You lost torque because you reduced backpressure...":

That is a significant difference in the 60 ft, were you having traction problems? You obviously made it all back and more by 330.

My bud here keeps telling me dual cats are better than a single 3". You seem to be validating his claim.

Good work! Gotta love the "budget" racer approach.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. 2-1/2" cat-back, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker 2055 headers, 3" Catco cat & 3" cat-back, Spohn SFCs).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

No it's not a lost torque issue, the car feels stronger than ever all over the board. It's a matter of it being a heavy convertible with 2.73 gears, so the slightest difference in air density makes a big change in how it gets off the line. The same thing happened a few weeks ago on the old set up where in hot/humid air i couldn't get better than a mid 2.2 60' (and i was running 15.3s that night so theoreticaly it might be closer to a half second gain)

 

Those times are with a 305, automatic, ragtop? Wow, that's impressive. You even had a poor launch. What else have you done?

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92Z28
89GTA

 

The only 'mods' are a ported plenum, a little bit of playing with the PROM and a 2.5" dynomax cat back (at the time of the runs, now a fromaster.)