Would any of you rather go with the slp 1 3/4 on a stock head,cam,intake L98 with a 100 shot or the 1 5/8?The car allready has a high flow cat and flowmaster catback.Also i was thinking of putting a cutout on the car before i do the headers but where would i put it at before the cat or after?

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New owner of a 88 gta with 72k miles just started to modify it.

 

A 100hp shot will add 150 lb ft of torque at 3500rpms. This is also the approximate RPM where the engine is at its highest volumetric efficiency and making maximum torque. This is also the approximate shift recovery RPM for a 700R-4 equipped car.
The 150 lb ft of torque represents approx a 42% increase in power at 3500 RPMs and makes the original engine act like a physically larger engine.

I say go with the 1.75" headers.

ODB

 

Traditionally a cutout goes in front of the cat, but for closer to stock applications, you might want to put it after the cat to retain some backpresure and to keep from buring up your valves. Id also agree with ODB in that if your gona run 100 shot or bigger you would want to run the 1 3/4 headers. 1 5/8 are good for stock or cars that are gona stay close to stock and that care more about torque then HP.