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View Full Version : Does size really matter?


Richard Cranium
05-29-2003, 08:30 PM
The size of your exhaust that is.
I am in the process of (more or less) of converting my Vindicator to thru hull exhaust and while I don't really expect any performance gains from this. I don't want to lose performace either. My question is...is 4" exhaust ok for a small block (5.7) or should I stick with the 3"?

Most of the tips I find used for sale are 4" and the ones I really want are side exit and they only come in 4". I eventually want to go with a big block but that isnt in the cards this year at least.

laster
05-29-2003, 09:21 PM
I just don't see a down side to going with the 4". It's not like the exhaust is "tuned". :cool:

Speedwake
05-29-2003, 09:43 PM
A swift flow of exhaust gases through the pipes does help prevent reversion and I'd hate to think that increasing diameter over what is recommended would lead to reversion problems.

I would consult with Mercury (assuming they built the engine)

Richard Cranium
05-29-2003, 10:04 PM
I'm probably thinking about auto exhaust and it may not be the same in boat land...but it seems like having too much flow (ie not enough back pressure) causes a loss of torque.

Not a Merc...its an Old Mother Clunker (OMC).

laster
05-29-2003, 10:35 PM
Jeff, I would think that the reversion situation would depend more on the cam and maybe the manifolds than on the size of the tips. Richard, I don't think that back pressure is an issue on most marine engines. The chief benefit of aftermarket exhaust manifolds (on a stock motor) is that they flow better - less back pressure. It (decreased backpressure) MAY affect the amount of torque at lower RPMs but should result in a net increase in HP at higher RPMs.

crmax
05-29-2003, 10:39 PM
you do tune your engine by the size and length of the exhaust, but in your case I don't think it would matter to much. and yes you will loose some torque as your power band changes thats the reason they use 3" for small blocks. you can always drill it larger later. again these are small changes that you may not see any changes from

birdog
05-30-2003, 07:25 AM
Merc used to use 4" for 350 Mags...3" for 260's...I don't
know if they still do. If you are going to Big block later
you will need 4" anyway

Iggy
05-30-2003, 09:08 AM
My 350 MAG does have 4" pipes.
If it's too big a diameter I would think that the exhaust gasses woud not have enough velocity to prevent reversion.

RCHEVELLE71
05-30-2003, 09:17 AM
Good question,

Do you really need two 4" pipes for a 350 turning approx. 5000rpm???

In Car land we would run 3" duals, and call it done, now thats for a small block capable of turning up to 6000+rpm, plus long lengths of pipe, and mufflers.

I am planning on doing this soon, and had the same question.

If he were planning on going Big Block soon, and can deal with the loss of low end torque til then, I say go with the 4", but I plan on staying small block, would 3" be the best for me?????

Tejas Raz
06-24-2003, 08:21 PM
Running 4" pipes on my 350 Magnum Checkmate. Go big man.