Gannz

My stock small block exhaust has a 3" spacer between the riser and manifold. What is the purpose of the spacer? Is it only to line up to the Y based on how high/low the engine is mounted?

If I change to an aluminum manifold with a standard height stainless riser can I just eliminate the two stock rubber couplers and stock elbow and use a rubber exhaust elbow to connect the riser directly to the Y?

Reason I'm asking is because I can't justify the price of a new set and it's almost impossible to find a used set with a 3" taller riser.

dereknkathy

i have a set of aluminum GLM exhausts. good shape. 1100 new. worth 500 used. I will take 300.with 3 inch riser spacers. they are there more to get the exhaust a certain height above the waterline. rubber elbows also sound like home depot items. they don't work. neither do PVC. don't ask how I know. there is a 5th amendment for that...

Gannz

Waterline. Doh! That makes sense.

PM sent on the GLM's. Not sure if I really need to change/upgrade the exhaust or not but can't go wrong losing 100 lbs. off the stern.

I'll be starting my 383 build soon. I think I have a good grasp on the actual engine but I'm still unsure of what I really need for the exhaust. Goal is around 400 hp and 450 lb/ft. I'll be on an Alpha or I'd probably shoot for more. My LT4 hot cam leaves a lot on the table but with all the other components I think I'll still hit the goal. This is almost identical to my build except I have a Holley 750 DP and no headers...

http://www.superchevy.com/how-to/148...-engine-build/

What's confusing about the exhaust is whether to go thru the prop, thru the hull, or use a diverter. Some people say going through the prop with a stock or aftermarket exhaust on a built 350-383 will choke it off. Others say they've changed from stock, thru prop to aftermarket aluminum/stainless and/or thru hull and the only thing that changed was the noise level. And these are all in the 400 hp and 5k rpm range. If running 5-600 hp and 6000-6500 rpm's I could understand needing a high performance exhaust and going thru the hull.

I do have an exhaust tube instead of the bellows which I've read helps a little because you're not trying to force it all through the prop.

Here are some of the comments I've read on the subject and these are all specific to built up small blocks in the 400 Hp range. I have a bad habit/disorder of reading too much and over thinking things. LOL...

"Your limiting factor is not the fact that it is a stock exhaust manifold, or worrying about reversion, but rather trying to exhaust out the outdrive. That is your choke point."

"I just finished a 383 build and I am very familiar with this whole process. I shared all my info every step of the way and had some very good results. The idea of going to 383 for a performance gain but doing it through the prop seems beyond silly when I know the expense involved.

If you want performance from a 383...you can't go through the prop."

"Think of it this way, you can put anything on those exhaust ports and it won't matter because you are choking it with the through prop exhaust. When I switched to my 383s, I got rid of my through prop exhaust because it restricted my power, I also got rid of my captain's call. I had to do both of those things to make the power I wanted. I'm so glad I did! I'm very pleased. "

"I ran my first 383 with trick flow twisted wedge G1, 10:1 comp, CS XE264HR-10 212/218 hyd roller and the Kodiaks through the drive and when I finally put through hull on it, you know how much I gained????? Nothing but better sound. REalisitcally that was probably 330-350hp (corrected) to the prop (around 410 at the motor) and it didn't make a difference. I don't know how much more restrictive the Volvo transom is, but with a cam comparable to that I would just run it through the prop."

"That cam will like to live around 5200-5300 RPM. I see you're reasoning for staying away from the Alum manifolds now. All I can tell you is that I don't think the stock manifolds will flow as you want them to. It's going to be the barrier to a lot more HP. In fact, I don't think that motor will see 400 hp with the stock manifolds. Just my opinion. I keep getting my eyes opened up to how important it is to have good exhaust on the boat and when I expect my jetting problem will go away with the addition of the thru hull exhaust I'm preparing to put on, along with some more speed With the Dart heads I wouldn't be surprised if you pick of 2 or 3 MPH. If you don't pick up top speed I'd say you really need to get some better exhaust. So I'm still sticking with the better exhaust."

"the boat went 56mph at about 4600 RPM. The Edelbrock performer intake didn't bump up the top end but really aided in hole shot. Then the Comp cams 260H cam and roller tip rockers came. 62 MPH. Next a set of Kodiak Exhaust manifolds with stainless risers. 65 MPH. After that the stock center-bolt heads were worked over with some port work on the intake and exhaust along with port matching. 67-68 MPH. After that the 383 came with unported trickflow twisted wedge heads and a Comp Cams XR264HR roller cam with all of the other running gear. 73MPH (the builder said the motor was in the 410-420HP range. So what I figured along the way was 10hp = 1mph. Seemed pretty consistent along the whole mod process. I say you are going to need to put good exhaust on it despite you unwillingness to spend the money. By the way these figures were for a boat that was 2600lbs dry stock (less with all the alum parts now) and 3/4 take of gas and 2 people. At least you have thru hull exhaust, I finding out now you just can't put more than 400 hp thru the prop."

"I upgraded that package with Vortec heads 2.02/1.60 valves, shaved to 60cc combustion chambers, thinner head gasket(.028 compressed) yielding 10.0:1, alot of time porting the exhaust ports, Edelbrock RPM air-gap, Holley 750 vac secondaries, Comp x-marine cam 224/[email protected], 1.6 roller rockers and Stainless marine exhaust. This motor will turn a 23Powertech to 5600rpm @ 67GPS. I actually installed the Stainless marine exhaust at the end of last season on the first motor and realistically didn't see any improvement, maybe 50rpm's or so. I'm sure they make a bigger difference with the better breathing motor I have now.
So short story long, invest in a good set of heads and worry about exhaust later."

"Anything in the 170-195 cc would work well with the correct cam profile. For a small block I wouldn't worry too much about what exhaust you run unless you were routinely above 6000 rpm."

BUP

iTrader: (9)

height needed to prevent waterline from reverting back into the cylinders

Crude Intentions

iTrader: (1)

Lt4 hotcam is on a 112 Lsa. You will need aftermarket exhaust for that. Stock exhaust will likely revert.

SB

iTrader: (1)

If a a 2" rubber PVC cap can close off the exhaust outlet on the Alpha transom and the same on the inlet to the Alpha drive, is that big enough for your exhaust to flow ?

I know this ,because I just did this to remove air going thru the drive to stop prop slip during planing with a prop with the bellmouth removed. Boat has thru transom exhaust of course. None thru the drive.

Gannz

I've heard that before but lots of guys run em and I've never read of any actual cases of reversion with that cam. I've been running it in a built 355 for 2 seasons with no issues. But thanks for looking out!

Gannz

Don't know but I'm guessing that's a rhetorical question so, um, no? LOL

I just upgraded my exhaust on an F150 so I know that 2.5" is better than 3" for low end torque with a Y and single out or dual out setup. And that has restrictive cats that we don't have to deal with. On true duals, 2.25" is better for low end. You're using that cap as a plug? Probably closer to 2.25" OD?

SB

iTrader: (1)

Nope, the cap as a cap. But has to stretch a little to get on. The aluminum lips on the drive are thick...so probably 2" ID.

Gannz

Ok. I do have the exhaust tube instead of the bellows. I understand that helps flow a little better because it's not trying to push it all thru the prop? Not good enough for 400 hp?