I would like about 300 hp, would 1 1/2" headers support this ? Thanks

 

Yeah they will. You might see gain from going to a 1 5/8"..but at the 300hp level it won't be a hell of alot.

As a very general rule for matching header primariy size to a motor:

300hp and under use 1 1/2"
300-400hp use 1 5/8"
over 400hp use 1 3/4"



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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
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ICON Motorsports

 

If you can you should go for 1 3/4" headers as that way your set for more house already.

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1989 Trans Am GTA, 5.7L TPI, TH700-R4, T-Tops, dual cats, 9-bolt rear, 3.27, Dynomax muffler, K&N
1992 Camaro RS, 5.0L TBI, T-5, T-Tops, 3.08, L69 Dual Snorkel, K&N - Sold

Theme song for my GTA Cake - The Distance

 

i thought L98's were supposed to use 1 3/4 anyways.

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88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT

 

I had 1 1/2" shorties on my 383 but had poor heads on it. When I put some better heads on the engine and went to 1 5/8" full lengths, I was running 11.8's at 117 mph. The 1 3/4" would have been nice but the extra cost didn't justify the larger size. Unless it's a full race engine, 1 5/8" should do you fine.

Most people who install 1 3/4" headers on their cars would probably make more HP and torque if they went to 1 5/8" since driving on the street doesn't put you in the RPM range to get the full use out of the larger headers.

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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

At 300 hp, 1 1/2 will be just fine. 1 5/8 will see very little gain on the top end and probably lose a little on the bottom.

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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, currant red metallic paint. Lots of other stuff...
82camaro

 

I want to build up an L98 to run 11's-12's and still be good on the street, and I think I'll be putting SLP 1 3/4's and 3" exhaust on it. They should be able to get more higher RPM HP from the TPI while a good TQ converter and a cam that will work good for both, makes use of that lower RPM TQ that the TPI makes also.

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89 RS

STILL Looking For:
An 87 IROC-Z28 350 TPI


[This message has been edited by IROCZTWENTYGR8 (edited August 27, 2001).]

 

TPI dies out after 5500 rpm and that's where 1 3/4" headers will start to show an improvement. Unless you're constantly spinning the engine to 6000 rpm or have some sort of supercharger installed then just go with 1 5/8". A large tube header will kill the bottom end torque that a TPI produces.

Bigger isn't always better. Headers are an extension of the exhaust ports in the head. Small unported heads won't benifit from large tube headers and will actually create a drop in low end torque. That low end torque is what you use driving from stoplight to stoplight.

I wouldn't even consider 1 3/4" headers for a SBC unless you have a minimium of a 406 with aftermarket heads.

 

I agree with you Steven....There was a 406 buildup last year in one of the mags. It had Iron eagle heads putting out 425-450 horsepower. They tried swapping a set of 1 5/8 for the 1 3/4 headers.

They gained some topend but the over all numbers dropped. So they used the 1 5/8s it was going to be a street motor and they were going for torque. It still made almost 450 horsepower with the 1 5/8s

Lingenfelter uses the stock corvette 1.5 factory headers on his vettes because they cost little in the way of power. His words not mine.....

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Black 90 IROC, L98, A4, 323 gear. SuperRamed 406 in the works!