S-24

I`m replacing 260/350 engine with GM350/330hp crate engine. Iīve got some really valuable information allready from few OSO members. But I still have few
questions regarding exhaust-system. The engine is
not going in as itīs: cam is changed to crane`s 109821
roller, 2.02" & 1.60"stainless valves. Intake is Air-Gap W/600cfm Edelbrock. So question is; if I use GLM cast
manifolds w/stock 4"risers and thru-prop exhaust, how
much power I`m throwing away, if any? Someone must
have made similar setup, and changed exhaust later?
What were the gains, mph or rpm? There seems to be
two schools for this subject; no need for thru-hull for
mild sb engine, and yes it will need it? So if someone
has put better manifolds, but first ran thru-prop and
then thru-hull, what did you gain? My boat weights
apprx.4500lbs and is 24ft. If you have made some
test`s, let the real world results flow

S-24

Liberator21

The magazine Family and Performance Boating did a test on the GLM manifolds. They took a 21' sport boat with a stock 350, and bolted a set on. Before and after tests were done all in the same day (Changed manifolds at the ramp), and they saw a 2 -3 mph increase in speed. That was going from stock center rise with through hull exhaust to the GLM manifolds. I still have the article.

Paul

cuda

I'm happy to hear that, I bought GLM's for a small block I'm building.

S-24

Thank you for your input guys, I made the manifold purchase based on that article. But I still like to hear if
anyone has just changed stock manifolds to GLM, and is still using thru-prop, any gains by that way? We have strict noise laws here in the other side of the world.
And if not necessary, I wouldn`t like to put extra $$,
to switchable system. By the way, has Bravo bigger
thru-prop exhaust outlet than Alpha?

Thanks

S-24

Sonic30ss

GLM manifolds, where are they available????

S-24

As for what I know, www.mooreperformance.org

cuda

I bought mine through mooreperformance also.

Steve H

I just ordered two sets for BBC. I will let everyone know how they work

Steve H

220BR

For what's it worth, when I put my 383 in my SeaRay with Alpha, I plumbed a fitting at the base of the Y-pipe so I could actually measure back pressure in the exhaust system. The bottom line is that the copper tube that is connected to the fitting will spit water and exhaust (so I know its not blocked) but not even 1psi of back pressure will register (idle to WOT). I've yet to get a more sensitive gauge to really check it (now, 0-15psi fuel gage). I'm running the merc exhaust snout that opens on trim.

My preliminary conclusion is that better manis and thru hulls only help with exhaust wave tuning and not with back pressure for SBC

jaroot

i've always pondered the benefits of aftermarket exhaust for the sbc. I am not willing to fork over 1500+ bucks for only a 100 lbs of weight savings. I can find very little info on real tests and benchmarking. None of the manufacturers publish any data for the sbc but info for the BBC runs rampant.

I run stock merc center riser manifolds with IMCO M-72 pipes... i am sure with a true tubular header i could see significant gains but vs say SM or imco or emi or gil manifolds.. just how much hp is there to be realized?

Merc uses the stock manifolds on the scorpion and manages to develop 350 pshp... equal to roughly 375 - 380 crankshaft hp...

Now i fully understand that very little scavenging is going on in the stock exhaust but as far as back pressure is concerned with the sbc i don't think there truly is enough to tax a 4 inch thru-hull setup... i just don't believe the exhaust gas volume is there.. unlike a BBC... or big inch SBC...

if someone can feed me some REAL data or actual comparisons, say from a dyno... i'd be EXTREMELY interested. If in fact there are measurable gains to be achieved, other than weight savings, i might write the check then...