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graham
19-07-01, 09:06 PM
To all,

Should a Chevy manifold pipe be just big enough to clear the exhaust port size or must they be a certain size, what is ideal??

Graham

RigMan
20-07-01, 07:29 AM
Exhaust manifold size should be big enough so an to not restrict flow from the combustion chamber, but not too big such that exhaust gas velocity is too low thus reducing scavinging effects. Things that will determine manifold size are cylinder displacement, max revs, exhaust design and length. Typical size of Chevy manifolds are 1 5/8" and 1 3/4". Ideally these manifolds should be of equal length in order to 'ballance' the cylinder bank. Just as with the intake side of the engine, think of the engine as a 'wind instrument', changing the runner lengths will change the pressures of the 'wind instrument' thus changing the sound and performance!


RigMan:P

imported_admin
20-07-01, 12:28 PM
Having learnt the hard way about headers on my Chevy, I can tell you that Rigman is spot on.
You should run 1 3/4" primary pipes of exactly the same length which should be no shorter than 28" & no longer than 30". I have some very nice, ceramically coated 1 3/4" headers, but which are all too long and unequal length. On the dyno recently at CES in Buckingham, the difference between my headers and their's (which were 1 5/8" & 28" in length) was that my engine lost 46 bhp & 49 lbft !!!
Thus, I will be getting some new headers made very soon .....

graham
20-07-01, 10:04 PM
Cheers, but sorry do you mean each pipe has to be 282 long, if so blimey it'll be half way across the other side of the road, the distance between my furthest port and the side pipe join is only about 12" mmmmm.... confused now.

graham

imported_admin
21-07-01, 10:00 AM
>Cheers, but sorry do you mean each pipe has to be 282 long, if so
>blimey it'll be half way across the other side of the road, the
>distance between my furthest port and the side pipe join is only about
>12" mmmmm.... confused now.
>
>graham


Each primary pipe should be 28 to 30 inches in length (i.e. from cylinder head to exhaust collector). This usually means that the two middle pipes will have a series of bends in to allow for the more direct route they take compared to the outer pipes. Hope this clarify's things slightly?
Simon

graham
22-07-01, 08:24 AM
Cheers now I understand, always a learning curve for me. I guess if I cannot get them perfect it'll run but perhaps with not full power potential, I mean I am not racing the thing but it still needs to sound and idle ok. I will see how I get on, oh by the way you said simon thatyou may have some made, let me know who by if poss.

Thanks again Simon and Rigman. I am over to the garage now!

Graham

ps well is there anything better to do on a Sunday morning??

imported_admin
24-07-01, 11:54 AM
The two companies that you should speak to are :
1. "Exhausts by Design" - Graham Frankland, 01908 507117. They are in Nash, near Milton Keynes.
2. 'GDS Exhausts' in Brackley.

Talk to them both and see what you think.
Hope this helps, let me know.
Simon