Header size

[zook]

Hey:

My stock early style heat exchangers have completely disintegrated. A good friend of mine gave me a gas heater, so i thought i would install some headers. I've seen recommendations of 1 5/8 primaries for a 2.7 instead of 1.5s. Any opinions of this as well as brands? My motor is the base non emissions 74 911 but with electronic fuel injection.

Thanks.

Pat

[tonythetarga]

I'm not an engineer, but I would say that 1 3/4 is too big for any performance gain. I believe that the 1 1/2 would give you some improvement if that is what you're looking for.

[zook]

The stock heat exchangers are 1 1/2

[Steve@Rennsport]

Stick with 1.5" headers. Anything else kills the low-end and mid-range power without any gains on top.

[zook]

Steve:

Thank you for your reply. Is there any advantages to an aftermarket header instead of the stock one besides weight? What brand? I guess i could just cut the exchangers off of the stock headers right?

Pat

[RWebb]

do you need heat?

[zook]

No, i don't need heat. I have a gas heater.

So, if i stick with the 1.5" size, is there any advantages with an aftermarket header?

pat

[tonythetarga]

Originally Posted by zook

No, i don't need heat. I have a gas heater.

So, if i stick with the 1.5" size, is there any advantages with an aftermarket header?

pat

Headers are usually equal length tubes designed to accelerate the exhaust gases. The stock heat exchanges, whether you strip off the heat exchanger part and just leave the pipes, are unequal length and not designed to act as headers. So, the basic answer is that if you want to increase some hp, then headers will offer you some. How much depends on what else, if anything you are doing. I might suggest to dyno the car before and after and look at what you have achieved.

[Bill Verburg]

Just to be clear, stock 2.7 header/heat boxes were 1.5" OD, 1 3/8" ID

I would use SSI which are sized as stock and made of a very durable alloy, most aftermarket headers are made of mild steel which doesn't last long.

Here is a pic of SSI w/ part of the heater box cut away.

[RWebb]

since you don't need heat, you can save weight by using tube headers only

Steve Weiner has some that are high enuf quality to satisfy him - they would have to be shipped acrors the US tho. I dunno of any manf. on the East Coast for these.

otherwise, SSIs.

[Jack Olsen]

Originally Posted by tonythetarga

Headers are usually equal length tubes designed to accelerate the exhaust gases. The stock heat exchanges, whether you strip off the heat exchanger part and just leave the pipes, are unequal length and not designed to act as headers. So, the basic answer is that if you want to increase some hp, then headers will offer you some. How much depends on what else, if anything you are doing. I might suggest to dyno the car before and after and look at what you have achieved.

That's true for your Carrera, but not the early (pre-1975) exhaust. They're equal-length headers -- the SSI design is essentially a knock-off of the early exhaust. That there are heat exchangers welded around them doesn't really impact their function.

Emissions laws killed the good exhausts. The 911SC, Carrera and 964 exhausts were compromises made to fit with catalytic converters and other junk. The problem got fixed, somewhat, with the 993 short-tube headers.

[tonythetarga]

Thanks Jack,
Good to know. I wondered that after seeing the cut-away Bill posted of the SSI's.

[boyt911sc]

Originally Posted by Steve@Rennsport

Stick with 1.5" headers. Anything else kills the low-end and mid-range power without any gains on top.

Pat,

Pm me if you want to try a 1.5" headers (equal length) in your car. I could bring it to the barn after my vacation here in Phoenix, AZ. Will be back home next week.

Tony