Header size question

[88 Club Sport]

I need some advice on the correct size of headers for a stock 3.2. From what I have read, 1 3/4 are best....would most agree?

If that is the case, are we talking ID or OD. Is it safe to say the if they are 1 3/4 OD, that's pretty close to 1 5/8 ID ?

Thanks.

[gestalt1]

if it is a stock or close to stock 3.2, 1 1/2" - 1 5/8" is best. you do not want to go to big, the low - mid power will suffer. the measurements are o.d. dimensions. my 3.2 short stroke with hot cams, individual throttle body injection ets will have 1 5/8" headers. smaller tubes keep the velocity of the exhaust gasses up for proper scavanging. the exhaust of one cylinder basically helps pull the next firing cylinder's exhaust out of the combustion chamber. this scavanging is lost somwhat with large diameter and/or un-equal length header tubes. the larger diameter pipes are for high rpms on larger engines: 3.6-3.8l. it is better to be on the smaller side than the bigger side.

[Bill Verburg]

I agree w/ everything gestalt1 said. And will add that w/o cams that have overlap, equal length headers don't have much to work w/. In that case you are going for flow volume rather than flow volume and accoustic tuning. As a result you get almost the same bang for a lot less money on stock 3.2s w/ stock headers, euro premuffler and a free flow muffler.

Most header manufacturers describe their products in terms of OD SSI are 1.5" OD(38mm) and 1 3/8"ID(35mm) w/ 2"(51mm) OD collector 1 7'8"(42mm) ID this is good for a street 2.4 -3.0
As the design rev range or displacement increases the pipe size must also increase.

By the time you get to 3.6 B&B 1 3/4"OD/ 1 5'8" ID are more appropriate.

A 993 Cup header is 42mm ID. This is a good bench mark because the factory actually made a true equal length header for their streetable 3.8 track car.

[88 Club Sport]

Thank you both for your info.

I already have a eruo pre-muff and a sport muff and it sounds as if Bill is indicating that I may be gettign close to header results as it is.

My 78 has a '85 3.2 and a '86 915. I am putting it togehter to Club Race as a '84-'86 US 3.2 wich it essentially is from a spec stand point The local club racers have indicated that I am leaving 20 hp on the table by not ditching the heat and going w/a header system.... they all use headers.

Guess I'll have to think well about this before I spend the money....it may be better spent else where.

[Bill Verburg]

Shaving the next few 10ths is always an expensive proposition, whether it's exhaust or tires or supension.

Like the wise man said " Speed costs money, how fast do you want to go?"

[RaceProEngineer]

Craig,

One of the most overlooked, underestimated aspects of "opening up the exhaust" is throttle response! On the street, it is a non-issue; but on the track, it can mean the difference between driving fast and winning - especially in a super-competitive class like F-Stock.

Bill is absolutely correct (as usual) in that the HP and torque can be similar between a well-thought-out exhaust system and an pair of expensive race headers. But that is where the similarity ends. Before ignoring your racing friends' advice, here are a few other factors to consider:

*Throttle uptake (speed)
*Throttle decay (rate)
*Additional weight of full exhaust system
*Location of that additional weight (as if the 911 wasn't tail-heavy enough!)
*Heat retention near the motor

Ed LoPresti