I just installed a 2 1/2 inch cutout in my I pipe before the Flowmaster.

It made a .2 and 2 mph difference in the 1/4 at the track this past weekend. See this.....

http://web.camaross.com/bb/Forum12/HTML/011137.html

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Mike L.


1987 IROC Z 5.7 TPI A4 3.27 Borg-Warner
four wheel disc

Mods:[i] 2300-2500 Stall Converter, Shift Kit(GM parts), TPI Specialties Stage 3 PROM, Modified Airbox w/ K&N's, homemade cold air, Relocated MAT sensor, Gutted MAF, 160* thermostat, MSD 6A, Accel 8mm Wires, bypassed TB coolant, Flowmaster 3 chamber single 3" in/out muffler,cutout. 3" MAC mandrel intermediate, custom dual !cat Y-pipe. airfoil, ported plenum. !smog, few others I forgot...???

**BOYCOTT LAPEER DRAGWAY**

 

You had to apples & orange it to get those numbers.

Compare your 2nd and 4th runs, where the only difference you report was the cutout. Then, you gained .04 seconds and lost .57 mph.

You probably would have seen that kind of scatter just from run-to-run variation. In other words, the cutout made a difference in noise, but most likely made absolutely no difference in performance (especially considering where you put it and the fact that it's smaller than your pipe). Shifting technique (you were lifting to get the 2-3 shift) and throttle position (you probably didn't get the accelerator to the same 3/4 spot in 3rd between runs) probably played a much bigger part. If you had barametric pressure, temperature and humidity for each of those runs, you'll probably see a correlation there as well.

Sorry if that bursts your bubble, but I've spent a little time at this.

BTW, RT makes no difference in times unless you stage differently.

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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).

 

A 3" would have been better.

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89 RS

STILL Looking For:
An 87 IROC-Z28 350 TPI

 

I doubt the difference between a 2 1/2" cutout and a 3" cutout would make much of a difference in this case. Though, a 3" makes a whole lot more sense if you're installing it on a 3" intermediate pipe...

 

It would.

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89 RS

STILL Looking For:
An 87 IROC-Z28 350 TPI