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Riley
05-25-2004, 04:58 PM
I've followed a lot of conversations on these board in regards to marine exhaust manifolds. I've also read Dennis Moore's book, and I understand that it is desirable to isolate the individual exhaust pulses to improve flow. I guess I'm still having a hard time understanding how a stock Mercruiser manifold with a 3" tall riser and stock elbow, dumping into a 4" diameter, 2' long thru-hull exhaust, can be very restrictive. I would think the volume of the stock system would go a long way towards reducing any flow restrictions.

Does anybody have any dyno proven figures for updating to an aftermarket exhaust system, specifically on the sbc? I know Jamie Root was planning on doing some testing this summer, but I thought maybe somebody else here on the boards would have, or know where to locate, some solid data. I'd even like to hear any "real world" stories you all might have, as far as how much performance you gained by going to an aftermarket exhaust manifold.

Dr. Dan
05-25-2004, 05:13 PM
:bonk: Hey Chris...Glad to see you're alive and kickin....I talked to Bud about 3 weeks ago...BTW.

I am installing some CMI's in our 22, I am anticipating a mild increase in performance...something in the neighborhood of 140+Horsepower Gain....I'll let ya know in a few weeks when it get wet for the first time. :rlol:

:D Doc :smash:

Rootsy
05-25-2004, 05:51 PM
well chris,

i have asked this question a lot with few results.. as i see it.. they use this system on the SCORPION 377's which make around 380 crank hp without a very stout valvetrain...

so as far as restrictive on a sbc... probably not... short run, big tubes not much back pressure...

now with seperating pulses what you are attempting to do is SCAVENGE.. or get one pulse to pull the exhaust gasses from an adjacent cylinder, etc. the problem is.. with most systems you need a LONG runner to do this effectively.. something in the 30 inch +/- neighborhood...

so the aftermarket stuff, short of a CMI or Lightining or Stelling, etc will not truly due this... but the individualized runners help channel the gasses out... basically taking a big open chamber which lends to very turbulant flow and making it more laminar in nature... kinda like taking a mob of folks on the street and putting them in nice straight lines..

i do feel that i've picked up at least a slight bit with the GLM's under my imco risers... i never truly opened my 16 up by myself in cumberland but i did manage 74.7 @ 5000 rpm when running next to the turbo guys... the boat is capable of 5500 with just me and half a tank of gas... proven this spring before cumberland...

now whether this is from just HP gain or weight reduction is hard to tell.. i mean i dumped like 65 lbs off the back.. and i did a NEW carb last fall and further jetted it leaner this last week... i am also running a different prop now... so this is not truly an apples to apples comparison... but DAMN does she sound good :D

what i do know is that she was running extremely strong at cumberland.. WITH PASSENGERS... better than ever before.. especially on saturday... and it was REALLY hot.. so go figure...

i truly want to do some benchmarking on the dyno...

JR

Forrest
05-26-2004, 08:04 AM
When you buy an aftermarket exhaust manifold system for a small-block anything, especially in a small boat like a 16 or 18 Donzi, buy it primarily for weight reduction. The loss of hundred or more pounds off of the stern will help your performance much more that any exhaust efficiency gains that you may or may not get from the system. If you do get a three to five HP increase from the aftermarket manifold system, just view it as an added bonus. As Root said above, you will need long tubes for real HP gains.

txtaz
05-26-2004, 09:22 AM
Doc, You're scaring me. 140+ HP gain is mild??? :bonk:
Wes

MOP
05-26-2004, 10:02 AM
Most of Dans HP gains is what is between those headers, stage #I will be a sweet machine. Stage II could be scary! :checkered

Riley
05-26-2004, 05:30 PM
Thanks for the info, guys. As always, the Donzi boards come thru with some useful information!

One other quick question: Does the water pouring out of the exhaust have any type of scavenging affect, by helping pull the exhaust out of the manifolds? Or does the water actually hurt exhaust flow out of the manifolds?

Rootsy
05-26-2004, 07:02 PM
you are pushing the slow moving water with the high velocity exhaust pulses... therefore creating back pressure.. the water acts as a muffler though... fairly negligable in any losses or gains in a true thru-transom system...

BigGrizzly
05-26-2004, 08:01 PM
Whell I have seendynoed several sets of different hedders. there are three typs of hedders. the tube type like CMI stellings and lighting also Harding makes a good set. of these CMI makes the most by 15 hp extra on a 625 hp motor. Then there are the generic imco EMIi Stainless Marine Eddie marine type and the CMI Elbow top of these what is surprising that the Stainless and CMI Elbow top arw within 10 hp of each other. The thied type are the stock replacement type merc and other these are heavy and restrictive in the risers and ttend to overheat the motor. My boats have the tube type because of the engine configuration, cams etc that I use. I would use Stainles Marine if it weren't for that and the fact my mom bought the CMI's for me. The best part about Stainles MArine is they stand behind their product. When you buy Stainles Matine products you know they are a good product

ALLAN BROWN
05-27-2004, 07:40 AM
I get a lot of reports from users (both drugs and exhaust systems) and the consensus is: The Mercruiser riser for the big block is the same for the small block. It is so restrictive on the big block that it prevents reversion in many cases. It is not restrictive as much on the small block with near-stock engines. It is restrictive on big HP small blocks and small block lookalikes. We promise zero improvement on the 310-330 hp big block, and the 255-260 hp small block. Big gains on 365hp Magnum Big block and over 370 hp small block. You guys are correct about the weight saving. I personally think that the dumbest thing I have EVER seen in my 53+ years in the business is the wet manifold/riser connection. Guaranteed to eat your motor. Why would anybody make it like that? Maybe to guarantee replacement sales? Hmmm.....