Sbc performance exhaust

[kzcollectr]

I'm at the point now where i'm getting ready to buy the exhaust for my Formula 272 project. I have been looking at IMCO , stainless marine, Harden marine,etc
I'm hoping to get some input from people who have had sucess with their systems. or possibly have a system for sale. My engines are 350hp smallblock vortecs thru transom exhaust. I hear the high performance exhaust gains are minimal with these engines any input from the experienced would be welcomed here as well. i dont mind spending the money for a good exhaust system but if the gains are minimal would i be better off with a GLM stock replacement. thanks guys.

 

[Chris]

You'll see modest gains with just about any exhaust system if that's all you're changing.

20 to 30 horsepower is going to be tough to measure, let alone noticeably feel- and enjoy. You may get a very minor fuel usage decrease, but that will be traded by your desire to use the new power.

What an exhaust will do for you is allow you to put more cam in your engine. Stock exhaust mixes water into the exhaust stream very close to the engine. With a stock, low-performance engine this isn't a problem. But as you move to a more aggressive cam a phenomenon known as reversion occurs. With any cam, there's a point where the intake and exhaust valves are open at the same time. This is called overlap. The hotter the cam, the bigger the overlap. With moderate overlap and at idle, the engine will draw some exhaust back into the cylinder before the valve closes. With more overlap, the more exhaust gets drawn back. If that exhaust is full of cooling water and you've just come off plane from a hard run and you're idling, that water is getting splashed back on a very hot exhaust valve. And that's bad.

To solve this, the typical aftermarket exhaust mixes the water further out. Or in the case of max performance, you have dry exhaust- fully water jacketed and the water dumps externally.

An exhaust is a good first step. It allows you to do all the other performance upgrades you might want to do. But before doing it, sit down and figure out where you want to go. If you drop 5 grand on exhaust (assuming twins), then you want more, that's cams and a new induction system- probably heads. Next year it's big-inch shortblocks since the bottoms you have now are RPM and HP limited. Now you're looking at drive upgrades. Decide now if you want to put 20 or 30K into the engine compartment, or if you want to just enjoy the boat for what it is and put that $$ towards something that's alrerad what you want it to be.

 

[kzcollectr]

Its going to stay an alpha drive boat so that is going to be my limiting factor the blocks are going together as we speak. 4 bolt main vortec roller blocks nodular cranks new powdered metal rods w/arp bolts speed pro flat top hyper pistons w/trflon coated skirts fresh 040 bore block decks set to 0 with felpro .042 marine headgasket new vortec 906 heads 64 cc chambers 1.94 intakes and 1.5 exhaust I hear the intake runners are 165ccs intake manifold is an aluminum highrise from pleasurecraft marine dual plane with bronze water passages the carbs are New Holley 650 spreadbores #80552 Ignition is mercruiser thunderbolt IV I have not chosen the cams yet but I'm partial to comp cams and looking at the extreme marine line. I want the biggest possible up to the point of reversion. So exhaust and cams need to be purchased so they work together.
The boat is 6000 lbs alpha 1.5s with mirage plus 21s the boat has k planes also. The 350 hp is just a honest guestamate on my end.

 

[Anchored]

Just for a benchmark, I built an engine similar to yours for my Donzi.

With the Mercury exhaust manifolds, it made 364 hp. With 1 3/4" long tube headers it made, 395 hp.

Jeff

 

[Chris]

It sounds like an aftermarket exhaust is going to be a necessity. Otherwise you'll be leaving some significant power on the table. The Alpha isn't going to love that horsepower, but if you don't holeshot it you should be OK. I'd rig some drive coolers on it and definitely do very regular oil changes.

I would strongly suggest contacting Bob Madara at Marine Kinetics. He can grind you a cam to your exact setup. You won't pay anymore than you would for an off-the-shelf cam. I know lots of people that have used Bob, including myself. Not one of them ever disappointed.

marinekinetics(at)rochester.rr.com

 

[sledge]

What year 272? do you need the 3" spacers? Fresh or salt water?

Fresh water go with Lightning headers for a good value. If you need the 3" taller it shouldn't be a big deal. And you'll be set for a big cam if you want.

Salt water, consider either forgoing a big enough cam to cause reversion, or get Stainless Marine. SM can accomodate the taller riser requirement and modify the tails for reversion. Or go with a non-reversion cam and the cheaper exhaust like GLM; don't forget the spacer if you need it.

You're gonna need bigger props too...or see if your 21s can be bent to 23s.

 

[kzcollectr]

I have heard of Bob Madera never had any dealings with him I guess now is the time to introduce myself. The boat is an 84. As far as the drives go I was planning on showers and I do frequent fluid changes. I cant say the boat wont be run hard but it definately wont be abused. I will try to get a hold of bob asap. The boat will have limited salt water use. I have no idea about spacers yet the previous setup was thru prop exhaust.

 

[kzcollectr]

Hey Chris thanks for the link to Bob Madera he will definately be doing the cams. He was very happy that we have not picked an exhaust yet. His recomendation was the stainless marine manifolds with the individual inner exhaust pipes that go all the way to the riser. I have checked all manufacturers that i know of and stainless appears to be the only one. Does anyone know of any other companys that offer that type of manifold. All seem to have individual fingers that go up around 3". Bob says that set up does funky things to exhaust flow. thanks Paul.

 

[Chris]

Tony put them on his Cig 28. Stainless made him a set of tails that go back, down and dogleg out through the transom, giving him max protection against reversion and they exit at the waterline for quiet idling. I don't remember how much they were, but they were a fraction of what we guessed they'd cost.

 

[kzcollectr]

If I go with that set up he can grind a cam that will come in around 3200 and provide max torque around 5400 and put out 400hp.

 

[Dangerous Dave]

I put Stainless Marine manifolds and risers/tailpipes on my 28 ciggie and I am very happy with them. They are dry all the way to the tip and they are 4". I had fully dry Gil's before but the manifolds failed due to corrosion. I have had the SM's for about 3 years, I use them exclusively in salt, I flush religously, and they still look brand new. I would highly recommend them. They shipped me the manifolds first, I installed them and measured for the tailpipes, then they shipped the tailpipes and they fit perfectly.

 

[Tony]

The full length tails were only a few hundred bucks more than the standard HP risers,

 

[pullmytrigger]

I got the Revolution Marines and Ive been very happy with them...good value......right out of the hole they def pulled better than stock Merc when I bolted them on to the stock engines......picked up a bit up top too.....maybe 200rpm....pics are on the new mills

 

[jrz]

Great performance and price.

 

[Teague Custom Marine]

CMI SMALL BLOCK! Best bang for the buck! We have them on super blow out rite now, that is while supplies last.

Satin Kit - $2199.97pr +tails
Polished Kit - $2499.97pr + tails

TEGAUE CUSTOM MARINE
661.295.7000
www.teaguecustommarine.com
​

 

[Expensive Date]

CMI SMALL BLOCK! Best bang for the buck! We have them on super blow out rite now, that is while supplies last.

Satin Kit - $2199.97pr +tails
Polished Kit - $2499.97pr + tails

TEGAUE CUSTOM MARINE
661.295.7000
www.teaguecustommarine.com
​

Bob saw you also have the IMCO thumper on sale for the small blocks.06
6.2s with the new style dry riser gasket.It looks like these will bolt up with minimal changes am I correct?And is there a way to eliminate the thru prop and just go thru the existing thru hull without pulling the motors can I block off the tubes that run to the props somehow?

 

[4mulafastech]

Kind of on the same subject... What are the pros/cons of running the exhaust thru the transom only Vs thru both the transom and the outdrive?

My 292 has the Corsa Quick-&-Quite Y-pipes (non-switchable) that flows both thru the transom and outdrive. Is this quieter than running thru transom only? Isn't it slightly less restrictive allowing the exhaust to flow out of both places? Does having some of the exhaust flow thru the prop help the boat plane out better due to the exhaust 'bubbles' helping the prop spin up a bit in rpm? In other words would the engine have more of a tendency to bog down with no exhaust through the prop?

Sorry for all the questions, but I have not been able to find a clear explanation on this subject. Thanks.

 

[Madpoodle]

Bang for the buck, Stainless marine. If ya want bling, they aint it. If you want reasonably priced proven performance exhaust, they are the answer..

 

[Expensive Date]

Bang for the buck, Stainless marine. If ya want bling, they aint it. If you want reasonably priced proven performance exhaust, they are the answer..

They won't work on the newer Mercury cooling systems with out major re plumbing.Other than that I agree.

 

[BajaIceBreaker]

My motors are 525 HP and I replaced my stock cast iron's for Revolution Marine aluminum. They are super light, and much easier to put on/take off. That's as far as the incrase in performance gets me. But I would buy them again.