PDA

View Full Version : Most Hp from stock exhaust.(350 Mag)



joseph m. hahnl
09-25-2011, 10:28 AM
:boggled: What do you think is the max HP achievable , with a completely stock center riser mercruiser exhaust manifold? (350 mag)

biggiefl
09-25-2011, 11:01 AM
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
09-25-2011, 12:33 PM
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
09-25-2011, 01:08 PM
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
09-25-2011, 03:46 PM
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
09-25-2011, 04:51 PM
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
09-25-2011, 08:13 PM
"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
09-25-2011, 08:46 PM
I think it will make a 5-7 difference... no reason why not

HallJ
09-25-2011, 11:22 PM
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
09-26-2011, 10:06 AM
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
09-26-2011, 03:34 PM
[ 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
09-26-2011, 06:22 PM
Registered User http://www.donzi.net/forums/image.php?u=12162&dateline=1313690863 (http://www.donzi.net/forums/member.php?12162-biggiefl)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
09-27-2011, 06:13 PM
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:yes:. Not looking to compete with MR.X or anything like that.

biggiefl
09-27-2011, 08:21 PM
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
09-28-2011, 08:03 AM
Jeg's for boat parts?

handfulz28
09-28-2011, 09:34 AM
I am still thinking 300-320 with steel Vortec heads, 270 cam and some headers and an intake/carb.

You do realize that Merc/Volvo advertise 300-320hp for their 350ci offerings these days? They're Vortec motors, but less than ideal cams, stock exhaust and probably an EFI tune that leaves some room for improvement.

A hotter cam, maybe a tad more compression, optimize the fuel/spark and another 50hp doesn't seem far fetched.

BUIZILLA
09-28-2011, 09:47 AM
if the stock exhaust was so bad, why did Mother Merc use it on the bigger 6.2 350-400hp Scorpion line?

HallJ
09-28-2011, 01:06 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Does your Minx have the 270 hp motor in it?
If so, the bowtie intake on it is probably the best your going to find in a dual plane.
There's an aluminum version if you want to get rid of weight.

Jeff

joseph m. hahnl
09-28-2011, 05:05 PM
Does your Minx have the 270 hp motor in it?
If so, the bowtie intake on it is probably the best your going to find in a dual plane.
There's an aluminum version if you want to get rid of weight.




Yes allegedly it is a 270 HP 350 Mag with the Bow tie heads and intake. I believe the Webber "Eddy" is a 650 4 Barrel. The intake manifold bores are slotted so instead of four bores, there are 2 slots on each side.

I think there is room for improvement on the intake side. And Like Buiz said they run stock exhaust on the Scorpions "like these". These claim to be dynoed at 650 each

http://www.donzi.net/forums/attachment.php?attachmentid=64948&d=1304016969 (http://www.donzi.net/forums/attachment.php?attachmentid=64948&d=1304016969)

mrfixxall
09-28-2011, 05:32 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]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:yes:. Not looking to compete with MR.X or anything like that

you wont be near the 400 mark on just the head and intake and carb swap..loosing the factory exhaust will lighten up the back of the boat 100 ish pounds,the emi's are loud!! and yes i was talking about replacing the manifolds and risers..if you don't run salt water their is no reason not to run a aluminum intake also and if you do choose to go this route make sure its a air gap intake weather its a edelbrock or a professional products cross wind vortec intake,the PP intake is cheaper then the edelbrock.the intake is also good for a 30 lb loss in weight as well...remember when replacing with the vortec heads you will need to get self aligning rocker arms.

if it were me i would be pulling the engine out,it is way easier to do the work on a engine stand then working over the side of the boat..if i really work at it i can have the motor out in about a hour..back in in about 3-4 hours..you may want to set aside some money to upgrade the engine cooler to a bigger one or to even add one,,some of the small blocks did not have oil coolers..Also if you dont want to change the cam you could always change the rocker arm ratio's,this will give you about .030 more lift going from a 1.5 to a 1.6 rocker arm.

HallJ
09-29-2011, 01:14 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Yeah your right, it has a Rochester pattern on it. Go for an Aluminum one.

Jeff