2.7 Exhaust Crossover Requirement

[andycook]

I am still working on plans for my exhaust on my 74 2.7 in my SC. I have seen posts that speak of the need for a "crossover" pipe between the exhaust banks. Is this true? What purpose does it serve?
My thoughts are individual exhaust systems for each side. Any thoughts would be appreciated.

[project935]

Shouldn't be needed - pre-cat 2.2s-2.7s didn't have any crossovers other than the muffler. Not sure if it merged or was separate in the muffler.

Depends on what exhaust headers your using. I don't think std 2.2-2.7s had any crossovers thru the thermal reactor years. 3.6L 964s have a exhaust crossover. 3.6L 993s only have a heat crossover.

[Bill Verburg]

The 74 2.7 used 2 separate banks of headers connected only at the muffler
like this


The corregated ss pipe in the pic is a mod needed for heat when using a 964 engine w/ the early 2.7 or SSI exhaust in an early chassis, it is also called a heat crossover. It would not be used on an SC or early 2.7.

These are SSI which are very similar in appearance to the '74 2.7 exhaust


The Carrera and 964 used an exhaust w/ both of the previosly mentioned Xovers as pictured here


the SC oem used an exhaust Xover(#11) but not a heat Xover


There is also another kind of xover as pictured here on race exhausts

[andycook]

WOW BILL!!!

Either you know your ***** or you slept at a Holiday Inn express last night! Thanks for all the pics and info. Now I have even MORE ideas to work with, That is awesome. Much appreciated.

I don't suppose that damaged race exhaust is for sale is it?

Awesome info, Thanks again. And also, alas, another East Coast Pelicanite!!

[unclebilly]

A cross over pipe gives more bottom end torque on a dual exhaust system on a V-6 or V-8...

[jluetjen]

Unclebilly;
I'd be careful with such a broad generalization since there a different kinds of V6's (odd-fire or even-fire) and different kinds of V8's (90 degree crank and 180 degree crank). Because of the different firing patters of each, I would think that the exhaust requirements or affects onf each would be somewhat unique.

BTW, our "Boxer" 911 motors would be considered to have an even fireing pattern.

[Bill Verburg]

90&deg V8s benefit from an exhaust Xover because they are not even firing, they always have 2 cylinders on the same bank firing sequentially

This is the firing sequence for a small block chevy, note the 2 -6 , 3 - 1 firing sequence.


Some Xovers , for example vertical twin m-c engiines are used for noise abatement purposes.

The race exhausts I shoved aren't really Xovers they are joined pipes strategicaly placed to augment the flow through them by combining the exhaust pulses of 1 cylinder w/ the pulses from another. Sort of like tuning a symphony.

[randywebb]

"WOW BILL!!!"

He has the best database of pics ever seen.... not to mention the brake and wheel research he's done. There is an accolade thread from a few months ago.

For more info on engine layouts, the books by Phil Smith (?) and Colin Campbell are good recreational reading and don't have the math in newer books.