Most Hp from stock exhaust.(350 Mag)

[joseph m. hahnl]

What do you think is the max HP achievable , with a completely stock center riser mercruiser exhaust manifold? (350 mag)

[biggiefl]

Just like with cars...no exhaust is going to add more than 10% and that is REALLY pushing it. I don't believe the claims about exhaust HP. Many here report no gains by adding a $2500 system. I don't notice much of a loss running with mine closed vs open. At 50mph my speedo does not move if I open the exhaust and that is pretty restrictive. The best exhaust vs Stock even on the sickest SBC would be a couple mph.

[maddad]

Joseph, I'm NOT an expert, but once when staying at a Holiday in, I read that the stock exhaust will start to have issues once you get to the duration needed to make 1-1.1 hp/ci in a 9.5/1 350ci sbc. That would be about 380 hp. Then reversion at idle and heat at WOT start killing other parts.

[MOP]

Most of what you are asking depends on the cams overlap, IMO you could with some thought get 300+ with no sweat. Then when your budget allows add after market exhaust for a "little" added boost. My Tempest 300 had stock exhaust, I added Gil headers and saw little to no improvement, I have since upgraded to a mild bullet proof 383. The stock 300 did 61.3 the 383 does 65.5, I do not care much about speed. I wanted a good cruise and reasonable economy which I got!

[younger]

The exhaust plays a major roll in performance. In the sbc more than 230 degrees duration at 0.050 lift starts to create revertion. That is when the performance exhaust is required so the cooling water enters the exhaust sream further down the line. This is the major reason. As far as flow the 4" merc riser has been on every motor from 4.3 to the 502. I have run the standard merc and glm manifolds with 4" risers with no problems to about 375-400 hp. Just not to much cam or the motors will start gettin water in the cylinders! Remember it is the hole package that makes the motor produce power. There is a difference between a high performance marine engine and a mildly warmed over stocker.

[mrfixxall]

comp cams xm270 cam,a set of vortech 64 cc cylinder heads, a eldelbrock rpm air gap,holley 750, and a set of emi thunders with the cast alunimum risers will give you 5-7 mph..

[biggiefl]

"comp cams xm270 cam,a set of vortech 64 cc cylinder heads, a eldelbrock rpm air gap,holley 750, and a set of emi thunders with the cast alunimum risers will give you 5-7 mph.. "

Over what engine and how much $$$? Also what about in salt? No way can you run the aluminum stuff you lilly pad killers run.

This on a stock 260 SBC would make about 40-50hp...I doubt that would make 7mph difference.

[BUIZILLA]

I think it will make a 5-7 difference... no reason why not

[HallJ]

Just for reference,

I built a 9.5:1 350 with Vortec heads, 2.02 int. 1.6 exh. valves, mild port work, 320 EFI intake, 220/220 dur .448/.448 lift on a 112deg lobe sep, cam.

On the dyno with 1 3/4 long tube headers the max power was 395 hp. with the stock '88 350 mag exhaust it made 364 hp.

Damn near 100 hp more than the 270hp 350 mag I started with.

Jeff

[biggiefl]

Yes but you built it. I am thinking a stock 260hp with a semi mild cam like a 270, intake and vortech heads is maybe a 300-320hp max. I don't think 40+hp will add 7mph. Ok it might on a really light hull.

[VetteLT193]

[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

If you are asking how much power you can make with the stock exhaust in place... a lot. Depends on a few factors. N/A Merc sells a 383 scorpion with stock exhaust @ 350HP. If you are talking turbo / supercharged it will be much higher than that.

[mrfixxall]

Registered User

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Join DateAug 2011Posts244

Yes but you built it. I am thinking a stock 260hp with a semi mild cam like a 270, intake and vortech heads is maybe a 300-320hp max. I don't think 40+hp will add 7mph. Ok it might on a really light hull.


Vortec heads are good for 40-50 hp + depending whats done to them, the chosen cam is good for 25-35 hp depending where its degreed at. air gap intake is good for 15 hp the exhaust with the aluminum risers are good for 10 hp and - 100 ;bs in the azz of the boat and the Holley is good for 10+ hp over the oe carb.

A member had the same combo and his 18 went 70 ish..

[joseph m. hahnl]

So, 400 hp would be achievable with the stock exhaust. Fix are you saying just the riser or the entire exhaust manifold. I'm Looking more to change heads and intake to Vortec . Not Sure if I should go with cast iron GM performance or go with aluminum Eddlebrock.I prefer to leave the stock roller cam in it. I don't want to pull the motor out. just looking to get a little more torque to turn a 4" bigger screw. I want a little better cruise rpm. Not looking to compete with MR.X or anything like that.

[biggiefl]

I would like to see that setup pull 360-370hp over a stock 260..bottom end might not be too happy if it actually happens. I will hae to look into my new Jegs catalog...I am still thinking 300-320 with steel Vortec heads, 270 cam and some headers and an intake/carb. I am way more rdical than that on my Vette and she is almost 400hp...crank.

[BUIZILLA]

Jeg's for boat parts?