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1989 Mercruiser 7.4 rebuild suggestions

kjnstl

New member
I have pulled a pair of Mercruiser 7.4s out of my 1989 32' Wellcraft St. Tropez and would appreciate some suggestions on refresh plans...

I would like to increase low rpm torque to get on plane quicker and be able to maintain plane without having to keep the 4 barrels open. Increased fuel economy would be great.

Can I achieve this with Cam/piston combo?
Carburetor modifications, rebuild or swap? Note: no room for any high rise manifold.
Should I consider different heads or have them reworked?
Should I go to the expense of making it a stroker?

Any suggestions with proven results would be greatly appreciated.

KJ
 
I have pulled a pair of Mercruiser 7.4s out of my 1989 32' Wellcraft St. Tropez and would appreciate some suggestions on refresh plans...

I would like to increase low rpm torque to get on plane quicker and be able to maintain plane without having to keep the 4 barrels open. Increased fuel economy would be great.

Can I achieve this with Cam/piston combo?
Carburetor modifications, rebuild or swap? Note: no room for any high rise manifold.
Should I consider different heads or have them reworked?
Should I go to the expense of making it a stroker?

Any suggestions with proven results would be greatly appreciated.

KJ

Ayuh,... Merc built Alota 7.4ls in '89,... What, Exactly are ya startin' with,..?? the ole 330hp,..??

What carbs ya got now,..?? Quadra-jets,..??

For what yer wantin' to do, you Don't want a hi-rise intake, 'n ya probably wanta use the peanut port heads...
Donno if there's any Vortec type castings available for a BBC or not....
If so, they'd be a Great choice, especially with matchin' pistons to achieve 'bout 9/ 9.5:1 or so compression ratios..
 
They are 340HP, Quadra-jets and peanut heads. I have seen on engine sites where they are building 415HP engines with low end torque. I have been told that this is accomplished with cam and piston combination. Anyone have experience building and engine like this? or am I better off just rebuilding to original specs?
 
I assume that you have V Drives and the boat is stern heavy. A 32 ft Wellcraft St Tropez should have no trouble getting on and staying on plane without opening the 4 barrels. Your boat is like a lot of stern heavy boats in that they are built with trim tabs that are much to small. Your Wellcraft could probably use 36" X 12" Bennett trim planes with 2" turned down edges and be able to stay on plane @ 2800-3000 rpm with no problem. My 39 ft Sea Ray Express with 454 Crusaders was like your boat until I changed the 28" X 12" trim planes out for 30" X 15" with turned down edges. I also added 15" X 12" fixed trim planes in the center of the hull below the prop pockets. Results are the boat will run up on plane @3000 rpm 4 barrels closed @ 19 mph burning 27 gph (on flow meters). If you mototrs were running ok and you are going to all this work and expense you may be spending tons of $ for little gain. Also, if you have the prop tip clearance you will get better performance with props that are 1-2" larger in diameter with a little less pitch. Good luck.
 
I have torn one engine down and there is surface rust in several of the intake and exhaust ports. I presume that the exhaust manifolds had not been replaced and are faulty. Question that I want to pose is... since these are 24 year old motors which were salt water cooled am I wasting my time and money rebuilding them?
 
If the blocks are 24 years old and have run in salt for that long they are way overdue for replacement. Still many rebuilders out there with reman blocks such as Jasper. I would consider converting to FWC, the engines can then run 160* thermosats and will be more efficient. You will increase resale also.

Still think a 32 ft St Tropez has plenty of power with 340 big blocks if propped and tabbed correctly.
 
I went with reman long blocks. I replaced carbs (rebuild Quads), exhaust, starters, fuel pumps, water circulation, oil and transmission coolers and rebuilt the raw pumps. I am having an issue at about 3000 RPMs on both engines. They both start surge until I advance to open the secondaries. So I need to open secondaries sooner as in adjust seconary pull off (?), different rods and jets, timing, or ???
 
If the blocks are 24 years old and have run in salt for that long they are way overdue for replacement. Still many rebuilders out there with reman blocks such as Jasper. I would consider converting to FWC, the engines can then run 160* thermosats and will be more efficient. You will increase resale also.
I agree, you'll not want to over-haul any engine that has been salt water cooled..... especially after 24 years of it.


I went with reman long blocks. I replaced carbs (rebuild Quads), exhaust, starters, fuel pumps, water circulation, oil and transmission coolers and rebuilt the raw pumps. I am having an issue at about 3000 RPMs on both engines. They both start surge until I advance to open the secondaries. So I need to open secondaries sooner as in adjust seconary pull off (?), different rods and jets, timing, or ???
The Q-jet secondary throttle plates open progressively with the primary throttle plates.
The air door above the secondary throttle plates is what's probably goofing you up.... perhaps that and/or the metering rods.

Also, no mention of having replaced your ignition systems.
If you've not check your progressive ignition advance, this would be wise to do.
Your progressive advance must coordinate with the RPM, or you won't pull any power from these.

Look up your OEM ignition advance curve. Not just BASE advance, but the progressive and total advance.
Strobe your timing marks progressively up to approx 3,200 RPM, and see what the ignition spark lead is.


Your OEM specs will look similar to this curve.
These curve graphs are typically minus base advacne in the vertical scale, so when doing the math, the BASE advance must be added to the curve values.
When strobing your timing marks dynamically, BASE will automatically be included.




Edit... I see that you have another thread going.

In this thread you mention; "new ignition components on old distributor bases original ingnition modules".
What brand and type are these, and are they using mechanical advance?
If these are mechanical advancing types, then perhaps your 24 year old flyweight systems need some attention.

.
 
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