Thanks!!

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

It depends on a lot of things. Engine design, gearing, tire size, etc. A typical street car will benefit best with 1 1/2" headers. Mild performance can use 1 5/8" and a race vehicle can use 1 3/4" headers and up.

Headers are like carbs. Bigger isn't always better. Smaller tubes help low end torque and a street engine needs that the most. Tube size is an extension of the exhaust port. Small exhaust valves and unported heads don't need 1 3/4" tubes. Exhaust velocity will decrease and performance will drop. Large tube headers are for high rpm horsepower. Now we're talking over 5000 rpm. Below that and the large tube headers will actually hurt performance.

I'm not trying to hurt SLP or Hooker with their 1 3/4" headers that everyone seems to like but if most of the cars that use them switched to 1 5/8" headers, they would probably gain more torque and really notice it on the street.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
LS6 Big Block buildup now in progress

Best results before the 383 blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

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

 

I totally agree....I have read more than one dyno header shoot-out that backed this up.

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

 

Thanks 87 IROC, good information!!

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

I just saw two dyno sheets from a 383. When changing from 1 7/8" to 1 3/4", the engine picked up 80 HP and 95 # T. You wouldn't think that 1/8" difference in tube size would effect performance to this degree, but it will.

For a 350, the 1 5/8" is generally more than enough to support 500 hp.

 

So every1 recommends the 1 5/8 to the 1 3/4 for 350 TPI??

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

[This message has been edited by IROCZTWENTYGR8 (edited October 29, 2000).]

 

Go with the 1 5/8

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89 FORMULA 350 Edelbrock Cat-Back ,March Pulleys,Super Coil SLP Airfoil,Taylor 8mm Spiral Pro,MSD 6AL ,Edelbrock TES's ,Hi Flo Cat ,Crane AFPR ,Shiftkit w/Hurst lockup

 

hooker 1 3/4 headers and Borla XR-1 mufflers netted 7 hp increase over the 1 5/8 headers from 402hp to 409hp.

info from CHP magazine, GM Goodwrench 350 article.

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Rick
90 Camaro RS 305 TBI
350 on the way!!

 

What rpm was that increase achieved at? Anything above 5000 rpm would never be seen on the street and 400+ hp isn't really a street engine. A typical street V8 engine is around 160-300 hp and that's where 1 1/2 and 1 5/8" headers are designed to be used. Hedman also makes 2" small block headers but I doubt they would work well on anything less than 600hp.

Since he wants headers on a TPI engine I have to assume this isn't a race engine and TPI dies off above 5500 rpm so 1 3/4" headers are a waste of money for that engine.

My 383 had 1 5/8" headers and ran fine to 6500 rpm.

[This message has been edited by Stephen 87 IROC (edited October 29, 2000).]

 

87 IROC, when I get this car, I want to mod just about everything with performance stuff, but keep it looking stock. I want to be around 11's-12's. I'm going to do all the engine stuff, cam, heads, all that. Thanks!!

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

Almost forgot that 1 5/8 Hookers can be had between $100 to $200, where as the Hooker SuperComp 1 3/4 cost $370 bucks !!! and over $500 bucks w/coating.

Listen to Stephen, how long do you spend at >5000 RPM's, maybe a few seconds ?

Good Luck,

Ron C. Terry




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'82 Trans Am
'81 Camaro Z-28
'94 Vette LT1 Coupe

 

About the only reason to get 1 3/4" headers on a street engine/mildly modded is because you PLAN to build it bigger later. This will save you buying headers twice. But, as many have said before, until you start getting over 400 HP AND do a lot of racing over 5,000 rpm, you won't notice the difference and may actually decrease bottom power.

This is extremely important to remember with the TPI system, which is designed around "small volumes, high velocities". I believe that 1 3/4" headers should be also spoken in the same breathe as "Superram or MiniRam" induction system.

 

I remember reading in one of the 4 million magazine articles that I have that on the TPI 350s, they always suggest using the 1 3/4" headers. They have found that the 1 3/4" always make more power at ALL RPM on the TPI cars than the 1 5/8. They also said that they don't know why, because 1 3/4 tend to suck on carbed 350s unless they are designed for very high rpm usage. For some reason, the TPI likes bigger tubes. I read that article right after I bought my 1 5/8 headers

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The IROC Homepage
<A HREF="http://www.rit.edu/~jli4307/camaro" TARGET=_blank>
View the restoration of an 85 IROC</A>
"I didn't know a bored out Ford could go so slow" -Shenandoah

 

We need to keep in mind whether you're using full length headers where you can choose your length and collectors for your application, or whether you're using shorty headers (which are the only street legal ones I know of).

Instead of reposting what I've written a few times, give SLP a call, talk with "Joey". You'll find that even for a stock 350, they'll recommend their 1-3/4's, over their 1-5/8's. Price is the same....I can't see tooling costs for the headers to be an issue.

 

So is it 1 3/4 or 1 5/8 for a really modded 350 TPI??

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Looking For:

87 IROC-Z 350 TPI
84 TRANS AM 305 H.O.

 

SLP 1 3/4"!

 

if you really plan on getting into the 11's the you will probably benifit from the 1 3/4 headers. and even though they are coated, if you have the bucks, don't be hesitant to coat them either. it will lower your engine bay temps and add a couple hp. YES ADD A COUPLE HP by coating them. proven on the dyno on the race motor, which at the time was probably a similiar cam profile setup to what you might have.

Anyways, for under 400 hp daily driven cars, they will be much happier with 1 5/8 headers. i plan on having about 400 hp, in a daily driven car, and i will be using 1 5/8 headers to maitain my lower rpm power.

i have just seen too many motors loose a helluva lot bottom end for some top end.

Steve