Does size really make a difference??

[85diplomat]

Is there a substantial difference between the mercruiser 4" and 3" risers?
I have a 85 Diplomat( see Doug's 85 mate pics) and need to replace one of the risers. I want to keep the through prop exhaust but want the best flow for the money. Would it make a difference to switch to a 4" "Y" as well. I'm probably gonna pull the motor again this year to fix a small oil leak and replace the lower shift cable, so this winter would be a great time. I have the specs of engine and such on the same posting.


Doug

17' checkmate "Cheap Thrills"

[85diplomat]

Is there a substantial difference between the mercruiser 4" and 3" risers?
I have a 85 Diplomat( see Doug's 85 mate pics) and need to replace one of the risers. I want to keep the through prop exhaust but want the best flow for the money. Would it make a difference to switch to a 4" "Y" as well. I'm probably gonna pull the motor again this year to fix a small oil leak and replace the lower shift cable, so this winter would be a great time. I have the specs of engine and such on the same posting.


Doug

17' checkmate "Cheap Thrills"

[vinny p.]

While upgrading to a 4" exhaust will certainly not hurt anything, I think that you have to ask yourself if your motor requires the extra breathing. Is the motor stock? What power do you have? Are you planning an engine upgrade? Basically, I would not run a 3" exhaust behind a Merc 525 EFI, but if I had a 3.0 liter 135 horsepower motor, I would feel comfortable with a 3" exhaust.

[85diplomat]

no the motor is not stock, if the calculations are correct, it around 300 hp. It's bored .30 over Flat top Hypertectic pistons 9.5/1,Comp Cam extreme marine cam, Air gap intake, 600 cfm elelbrock carb. Stock 68cc heads, 194/170 valves.It runs strong but gets a decent amount of blow by build up in the lower end..Installed a breather and a PCV valve and it no longer pushes the oil out the dipstick. I'm thinking the exhaust is backing up in the system, and hoping the better flow will release some more top end.

17' checkmate "Cheap Thrills"

[vinny p.]

Your motor sounds similar to a motor I built for a friends boat, with the exception of the cylinder heads. We used a set of 292 casting # 2.02 turbo heads. The motor made 360 h.p. on a dyno.

Anyway, in your case, the exhaust will definetly help. But, I can't see it helping your blow-bye problem. A 3" exhaust is just not that restrictive. If you suspect the motor has blow-bye, I would suggest getting definitive #'s by performing a leak down test. That will certainly tell you the condition of your internal engine parts.

[Eriktheviking]

I think you have some worn internal parts in your motor.Here is a list of possibilities.Worn rings,valve guides or possibly pitted cylinder walls(If it wasn't winterized properly,the walls can develope surface rust.)Hypereutectic pistons are cast alloy.Cast pistons expand and contract a lot so rings have to be gapped properly to compensate for it.Forged pistons are tougher and don't expand as much.So if you have to rebuild it you might want to consider them.Vinny talked about 2.02 heads,they would really make a difference if you could get some.You could go a litte bigger on the carburetor too. Make sure its a marine version. And yes there is a big difference between 3" and 4" risers.For the cost of new risers you might be able to get a decent set of used aluminum high flow exhaust off ebay.With the cam,intake & carb changes you really need to upgrade the exhaust.If you saw how small the area is where your exhaust flows thru your alphas lower unit you wouldn't believe how restrictive it is.Keep us updated on how you make out.

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87 ENFORCER CUSTOM EDITION,454 MAGNUM SS ALPHA W/CUPPED 22 PROTESTOR PROP

[85diplomat]

I will check it out this winter but it did it since it was freshly rebuilt ( year ago). It has solid compression, I noticed on a lot of high compression motors they use evacuators to remove pressure in the lower end. Thought maybe it had something to do with the higher compression. Are the 292 casting heads vortec? I would have to replace the new intake if that is the case. The carborator is rated for much more cfms than I thought the motor would require. It's bored out to a 314. Do you think I will have to go to through hull to realize this motors potential?

Thanks for the feedback!

17' checkmate "Cheap Thrills"

[175checkmate]

I don't know if it will make a differance, but twin stainless steel dumps out the back and the sound to go with it. Also better take off as there will be no cavitation from the exhaust at the prop.

http://www.checkmate-boats.com/images/avatars/GusAvatar2.jpg


1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop

[85diplomat]

Love the sound but the "police" on the lake I like to go to doesn't!

17' checkmate "Cheap Thrills"

[vinny p.]

The 292 casting # heads are not Vortec heads. They are old style, 4 bolts around the outside of the valve covers type, and angle plug.

[Eriktheviking]

Gil makes a 4" silencer with a swirl thru design that really quiets it down but gives it a good tone and decent performance.Another good ebay item.I picked up a set for $50.00 at a swap meet.

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87 ENFORCER CUSTOM EDITION,454 MAGNUM SS ALPHA W/CUPPED 22 PROTESTOR PROP

[85diplomat]

Thanks for the info!!! a lot of my upgrades to the boat are from e-bay.

17' checkmate "Cheap Thrills"