MILD THUNDER

We get really excited about head flow, porting mods, and all that stuff. And spend alot of money on it too! For some reason though, exhaust just seems to be an afterthought.

Maybe, its because, most guys build a combo, bring it to their local dyno guy, he dyno's it with race car headers, and it makes the number. But most dont' dyno with their actual exhaust they are running in the boat, so they don't see the losses on paper.

Tim's build is fairly close to BCK's. Esp when BCK had the victor jr 4150 carb combo. Cams close, compression close, bck had the 315 afrs, tim had the 325 afrs. Almost a 50hp difference. I'd be willing to bet, if tim had dyno'd with some 2.25" open race headers with a decent collector, those numbers would have been alot closer to each other.

I think its all a package. And it should be a package, of how it goes into the boat. I would like to think, if Tim was to upgrade exhaust, intakes, carbs, and maybe a cam change, he'd have a noticeably better performing boat. I don't think changing just ONE of those things, is going to do much for him. What I'm saying, is if he had a 700HP engine, dynoed with accessories, marine exhaust, flame arrestor on, etc.

Come to think of it, thats how I dynoed my engines this time around. At 6000rpm with old combo, dyno sheet showed 798HP. That was with 2.25 race car headers, no accessories. This current combo, with accessories, my exhaust system (1 7/8 stellings with tails that had baffles in them), my flame arrestor, at 6000RPM, the dyno sheet showed 782hp. At first I thought, well that sucks. I lost 16hp with the new combo. But then I ran the boat, with the same props, and realized , it didnt suck at all! I gained a solid 4-5mph. As we know, to gain 4-5mph in a straight bottom 38ft boat, with no other mods besides engine, takes a decent amount of power.

Baja Rooster

I gained 5mph in my little ski boat just from rejetting the brand new QF carb. Once the hardware is set in place then it's a matter of finding where I'm losing performance, and from what I can tell, that's a bagillion places on a boat and the most tenacious will have the best results. On motorcycles the most gains are had on the exhaust side of things. My last racebike I went from 165hp to over 185hp with little more than timing the cams but mostly blending in the exhaust ports. It was pretty significant.

Full Force

Ok, so as far as headers go, I will see what I find, but what is the brand to have? Stellings? CMI?

MILD THUNDER

Going from a full length header to a shorty style header, cost almost 50hp on this 600ish hp 427 ci. If that were the case on say, a healthy 540ci build, its almost safe to say, the money spent on fancy cylinder heads, may have been better spent on a good exhaust system?

https://www.youtube.com/watch?v=O1bXPNy1Yn4

Full Force

Sell me yours lol

bck

M/T - We did put the flame arrestor on mine to see if it made any difference- there wasn't any. I'm running CMI big tubes which might even be bigger than the dyno headers dumping into 5 in. tails dry to the very tip, so I figure there shouldn't be any loss there. I have the 305 heads. I was planning on running all accessories and marine exhaust for the final dyno but lost interest and just wanted it finished.

SB

Same place smashed a header's primary tubes near closed all over the place and reported near 0 hp loss.

Just as a reminder on some of these tests...lol

Full Force

I will say that when looking at boats everyone running 700 hp ish or more have headers unless boosted...

SB

Me Being a broken record, NA the more overlap you have, the more exhaust id's and lengths are critical.

That said, we need more tests with SM's exhaust to put a better finger on the issue/ no issue. They seem to be fine on approx 600hp 502's. I don't know of data past that.

F-2 Speedy

I dyno'd mine in full marine dress, CMI sweepers and tails that were going in the boat, why do it any other way ? because several, maybe most, of the shops are not set up to run wet exhaust, its really that simple.