4inch exhaust - 5.7L GM

helmling · Post by **helmling** » Sat Mar 17, 2012 10:58 pm

Any advantage to using all 4inch exhaust hose from the riser ells to the wye then back to the muffler? Currently have 4x3 reducers at the ells then into 3x4x3 wyes then 4inch hose to mufflers
5.7L mercruiser twins in F30 Flybridge Express being repowered

captainmaniac · Post by **captainmaniac** » Sat Mar 17, 2012 11:06 pm

Every time you hit an expander or reducer, you increase back pressure and turbulence. Keeping it at 4" all the way through is best, if you can do that.

Big D · Post by **Big D** » Sat Mar 17, 2012 11:14 pm

Welcome aboard Helmling. I'm with CaptainM; go with 4" all the way if you can.

What's in her now? Are you repowering with Merc 5.7L or pulling those out for something else? If you're changing it up, there are a few things that need to be considered.

helmling · Post by **helmling** » Sat Mar 17, 2012 11:28 pm

She's a 1977 I've owned about twenty years. Original MIE 233s were replaced with used 1989 GM mercruisers about nine years ago. Considering new longblocks, non vortec, so all my accessories will bolt up, most of which are in good condition.
And thanks guys for your prompt replies.

k9th · Post by **k9th** » Sat Mar 17, 2012 11:30 pm

Welcome aboard.

While captainmaniac is not totally accurate, I agree with his advice, stay with 4 all the way. If you go from 4 to 3 you will increase back pressure since you are restricting flow, and restriction to flow is what builds pressure. When you go from 3 back to 4 you will reduce pressure because you are alleviating the restriction which will lower the pressure.

Staying with 4 all the way will also minimize turbulence in the flow which will maximize efficiency.

prowlersfish · Post by **prowlersfish** » Sat Mar 17, 2012 11:38 pm

That was the common set , then as the HP went up they went to 4" right out of the riser , If you running 300 hp or above I would do it . if not I would let it be , unless you need to replace it anyway . I don't think you will notice any difference.

Big D · Post by **Big D** » Sun Mar 18, 2012 12:05 am

Does anybody else have the setup he's moving to? I'm wondering if his current wheels will affect recommended WOT rpm for the 5.7Ls, and will he have to switch them up. Can't recall the specs on both those engines.

prowlersfish · Post by **prowlersfish** » Sun Mar 18, 2012 12:29 am

He already has 5.7 that he is replacing with the same , He has 3" eshust coming out each riser into a y with a 4" outlet . And he is looking at changing it to 4" all the way.

helmling · Post by **helmling** » Sun Mar 18, 2012 12:40 am

Had the original props 14x11 Supercup 976 refurbished when the GMs were installed - WOT reached what was expected.
Considering new GM longblocks (non vortec) and running the Q-jets after rebuild, also new electronic distributors.
What advantage would Vortec 5.7s provide?
Have not ruled out fuel injected(?) Existing wiring concerns!!

alexander38 · Post by **alexander38** » Sun Mar 18, 2012 7:34 am

the biggest thing injected engines have over Carbed ones is gas savings they seem to get better burn for the buck. But to get that you get the heart burn of a ECM that runs your engine. And they're still working the bugs out of the things. I've seen a couple boats turn in to dock queens after a re-power.

and in keeping with your tread I'd stay with 3 to 4. Go with this thought they did it that for a reason, it cost them more to do it that way when the boat was made. So the engineers did it for a reason..

helmling · Post by **helmling** » Sun Mar 18, 2012 9:59 am

Good point alexander38 - one consideration not mentioned is the MIEs were raw water cooled and my used replacements are fresh water. Never had any overheating problems with the 3 into 4. Wyes need replacing and I will add 45deg ells at the inboard side of both motors to clean things up. My 3inch hose is in good shape. Are fiberglass fittings the best way to go?

alexander38 · Post by **alexander38** » Sun Mar 18, 2012 7:56 pm

the 3 to 4 glass Tee's on my boat are fine after all these yrs. Check yours at re-power if they seem brittle I'd replace them. In the big picture they don't cost much for what they do.

rooferdave · Post by **rooferdave** » Wed Mar 28, 2012 7:46 pm

this is what I did to my 5.7s they had the 4" to 3" and back to 4" so this is what I built

rooferdave · Post by **rooferdave** » Wed Mar 28, 2012 7:48 pm

here is another view (before I added and double clamped everything)

alexander38 · Post by **alexander38** » Wed Mar 28, 2012 8:48 pm

looks nice Dave, but you really should double up the hose clamps. I like these.

http://www.teleflexmarine.com/products/ ... series720/