How much speed gain, anybody got a guess?
 

Against all odds, despite overwhelming forces of evil, I'm actually getting to start the exhaust swap out on my 311. Here's what I'm doing.

Boat: 89 311 SR-1 with 454 Mags running Hydromotive Quad IV 24" props

Losing the silent thunder swim platform, and installing new 6 inch thru hulls.
weight saved probably 175/200 lbs. I would guess. I would think that the engine would breath a little better with the box gone.

Going with EMI Thunder exhaust system, aluminum risers.
Weight saved 65 lbs. per engine/total 130 lbs. saved there
This system has to be more efficient than the stock Merc. exhaust.


Total weight lost: 300 plus pounds
Should gain some rpms with the more efficient exhaust and no box on the back.

I've tossed it around with some friends down here and concensus of opinion appears to be 3 maybe 4 mph.

What do you guys think? Any guesses?

Re: How much speed gain, anybody got a guess?
 

250-300 rpm and 2mph

Re: How much speed gain, anybody got a guess?
 

Russ,

With the stock 454 Magnums, I think 3-4 mph is a bit optimistic. Probably 2-3 mph at best. If the engines were modified (intake, cam, heads) you would see more of an increase after opening up the exhaust. Would still be delighted to see you squeeze more out of it, though! Go ahead ... prove me wrong. :D

Re: How much speed gain, anybody got a guess?
 

Scott, as a rule I see around 69/70 (GPS) at just about 4900 rpms. When real light, in 1 ft chop, a couple of times I've seen 70/71, but that's only a couple of times. I always go out with a full tank of fuel.

Remember, I also run in salt water, which I think is a little "faster" than fresh.

When I bought the boat it ran 63/65, but the prop switch from Mirage 23" to the Quad IV's and the silent thunder mods I did gave me a total of about 5/6 mph gain. Primarily it was the props, it got the ass end higher out of the water so much less drag.

I'd be happy with three mph. I'll be estatic to lose those damned rectangular exhaust tips. I HATE those, always have.

Next year, new motors. Kidney stone ordeal killed that idea for this year. Out of work for eight weeks did that.

Re: How much speed gain, anybody got a guess?
 

jsut make sure you check jetting with the new exhaust. don't want to run lean and melt things down, or the new engines may come sooner than you think.

speed follows a squared law ( technically cubed, but the third order coef is small) anyway.
to gain 3 mph based on your current power and speed you need
sqrt(70^2/73^2*365) ....ching...ping...bang..petooie: = 397 hp per engine required.....

for 5 mph gain..... 419 hp

Re: How much speed gain, anybody got a guess?
 

I'll take a stab at it, since I have done the same exact exhaust conversion that you are doing on a previous boat, though it was a single. My guess would be that you probably would see around 2 a mph gain on the top end. However hole shot and possibly midrange cruising gains would probably be the most noticeable.

Another mph gain (and fuel savings) would be if you didn't carry that much fuel. Do you use that much each time you go out? If not run around 1/2 tank and save about 500 lbs that way (160 gal tank - 80 gal * ~6lbs gal = 480 lbs) . With your other changes that could drop your weight close to 800 lbs :eek: ! That's alot!

Re: How much speed gain, anybody got a guess?
 

Jay, your right. I always heard it was better to carry as much fuel in the tank at all times to minimize condensation. Is this a myth? If so, I can easily cut it down. I usually burn about a half a tank every time I go out.

Re: How much speed gain, anybody got a guess?
 

I haven't heard of the condensation issue, though I guess it's possible. That's why you have a water separating fuel filter anyways. I only top off the tank if going out for an extended run, and haven't run into issues to date. Most of my boating buddies do not fill their tanks and haven't had problems. Think of it this way too- actually you're burning extra gas with a full tank because you are lugging 500 extra pounds, therefore degrading performance and increasing fuel consumption. My 311 "sits" in the surf with a full tank as opposed to running over the tops of waves.

Re: How much speed gain, anybody got a guess?
 

I've never filled my tank up, because I'd have to get a home equity loan to do it!

Re: How much speed gain, anybody got a guess?
 

Isn't THAT the truth....

I saw you had posted and I went "uh oh, here it comes.....you took it easy on me. What's the matter, don't feel well? :D