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GT40 Low Compression

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DHMcFadin View Drop Down
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    Posted: August-16-2019 at 9:16pm
Well I was hoping to get this engine through two more seasons but looks like I have a problem. I’ve been chasing a lack of power issue. I’m not burning oil and I’m not getting water in the cylinders. No missing either. I developed a small exhaust leak and prior to tear down, I decided to do a compression test now I’m at a loss and I’m not sure what route to take. Could I possibly have bad head gaskets that aren’t bad enough to cause water to leak into the cylinders but bad enough to cause compression loss? The plugs that came out of the cylinders with the lowest compression aren’t black either. I’ve had the new plugs in for about 20 hours.

1. 140
2. 125
3. 120
4. 155
5. 120
6. 130
7. 145
8. 165
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Gary S View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 10:31pm
Got or have access to an air compressor? You could do a leak down test on it that would tell you exactly where the compression is going. They are roughly around 40 bucks at Harbor Freight. Just possible that it only needs a valve job.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DHMcFadin Quote  Post ReplyReply Direct Link To This Post Posted: August-16-2019 at 10:57pm
Originally posted by Gary S Gary S wrote:

Got or have access to an air compressor? You could do a leak down test on it that would tell you exactly where the compression is going. They are roughly around 40 bucks at Harbor Freight. Just possible that it only needs a valve job.


This engine has over 3000 hours. I think it’s last overhaul was at 2000. Hopefully all it needs is a valve job. Good idea. Thanks!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: August-17-2019 at 12:06am
Wow that is up there it's done good!. What you might want to do is just check around for a set already done while you have some time. I had a set of regular heads done many years ago by a local shop,it ended up quite expensive. I bought a set of GT40P's about 8 years ago with new castings, larger valves ready to install for the same price. It's been a while since I've seen anyone here buy heads but in the past some have used Clearwater Cylinder Heads and Tri State Cylinder heads. Certainly would not hurt to check prices.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-18-2019 at 8:09pm
With your engine and the variance you have I think I would opt for a leak down test first.
I usually don't advise this but it would be good to know where the leak is before any work is started. Since you had an overhaul done it is possible the cylinder bores are not all good and maybe the rings are the source of your leak.
The leakdown will help show if it is rings or valves leaking.
Hope this helps.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-18-2019 at 10:22pm
How much lack of power are you talking about?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DHMcFadin Quote  Post ReplyReply Direct Link To This Post Posted: August-18-2019 at 10:40pm
Originally posted by KENO KENO wrote:

How much lack of power are you talking about?


It was pretty bad, max 3600 rpm, full throttle would bog down to a crawl on a lock to lock turn. I installed the new Ford performance injectors but hadn’t driven the boat prior to making this post. This weekend I had a mechanic friend look over the engine who also helped me re-align the prop shaft. He looked at the compression numbers and said everything was actually in line. Took the boat out on Saturday afternoon and Sunday morning. Holy carp this gt40 has balls now. Turned 4800 rpm with an Acme 654 at 45 mph. Replacing the original injectors with injectors that actually work was my issue all along. The idle also smoothed our completely. I think my amazon compression gauge is not quite accurate either. That’s what I get for buying cheap.

Boat runs like a bat outta hell. This will get me two more seasons. In my meantime, I’ve decided on the replacement engine for this boat. I’ll give you a hint- Ford 427 small block, multi port fuel injection, 530 hp, 525 ft lbs torque. Built by Prestige. Cheaper than dropping in a ZR409 and LOTS more fun! 80 series trans will handle it no problem. Very excited about the future of this boat! I think I’m gonna keep the gt40 engine cover and have it repainted to say “GT427” lol!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-18-2019 at 11:05pm
Glad it's running good cause those compression numbers didn't sound so bad at all

Not textbook perfect, but nothing that would turn it into a dog
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2019 at 5:24am
If he had bad injectors the lousy compression readings might have been under leaking injectors that thinned out the oil on those cylinders. Rings need good oil to seal a cylinder.
Glad it is running strong again.
How many hours are on this engine?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2019 at 6:56pm
Originally posted by DHMcFadin DHMcFadin wrote:

[
In my meantime, I’ve decided on the replacement engine for this boat. I’ll give you a hint- Ford 427 small block, multi port fuel injection, 530 hp, 525 ft lbs torque. Built by Prestige. Cheaper than dropping in a ZR409 and LOTS more fun! 80 series trans will handle it no problem. Very excited about the future of this boat! I think I’m gonna keep the gt40 engine cover and have it repainted to say “GT427” lol!


I figure a link to this marine engine would be some good reading
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-21-2019 at 7:46pm
Something to keep in mind when buying a marine engine. If the cam has too much overlap it will start sucking water from the exhaust into the chamber and cause engine failure.
Make sure your engine is designed to run with Marine wet exhaust.
This type failure has happened to many people, it is not a new discovery.
I do not know at what point in cam overlap it happens. I think they call it inversion.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DHMcFadin Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2019 at 12:32am
Their marine version has a 112 LSA cam so reversion isn't an issue. The point I drove across to Prestige was I wanted something extremely reliable and smooth while having enough power to jump out of water. Below are the actual specs of the engine I think I will purchase. They are going to mate the trans and exhaust manifolds as well so this thing is ready to drop in.





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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2019 at 1:23am
A few things for you to think about

If you want it to be reliable and smooth, I'm not sure I see a new flywheel in that list but you'll be needing one or some balancing work on the old one, but they know that.

And I'd hang on to that heavy old timing cover, you might need it if you're gonna retain your original cooling system

Make sure you know the oil pan depth so you can figure out much clearance you'll have to the hull

Tall valve covers and PCM exhaust manifolds may have some fit issues.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DHMcFadin Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2019 at 1:38am
Thanks Keno.

There will be a new flywheel installed and the oil pan is the aftermarket 351 oil pan sold by Discount Inboard Marine. It’s 1/2 inch deeper than the oem pan on my gt40. I have 6” of room under my current oil pan. The cooling system, pulleys, and belts will also be installed by Prestige. I will be sending them a new raw water pump, HO alternator, and pulleys. They are going to be figuring out the rest. I really was impressed with the conversation I had. The guy I spoke to knew a ton of about these marine motors and really provided a lot of insight in why each component was selected. He’s also the one that asked me to send him the trans, manifolds, water pumps, alt, and pulleys as the build cost covers setting all that up. All of the other engine builders I talked to say they would build the engine and then it was up to me to figure out the rest. Prestige said they would build a true plug and play power plant. I was impressed.

But it isn’t paid for yet and I may not go this route. I won’t be ordering anything until I get the rest of the restoration sorted out. Lots of planning to do but I am really excited for the future of this boat. I really appreciate all of the excellent advise I have been given on this forum. Awesome community!

Originally posted by KENO KENO wrote:

A few things for you to think about

If you want it to be reliable and smooth, I'm not sure I see a new flywheel in that list but you'll be needing one or some balancing work on the old one, but they know that.

And I'd hang on to that heavy old timing cover, you might need it if you're gonna retain your original cooling system

Make sure you know the oil pan depth so you can figure out much clearance you'll have to the hull

Tall valve covers and PCM exhaust manifolds may have some fit issues.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MrMcD Quote  Post ReplyReply Direct Link To This Post Posted: August-22-2019 at 4:43am
Lots of very nice parts inside that engine. I like the Cleveland Mains on a HP 351W based engine. The large Windsor Mains work great in truck type applications but cause oil overheat issues if run at high RPM for any length of time. Large mains work for hard working truck engines, and Ski Nautiques very well but you don't see them drag or circle track raced unless they convert to Cleveland Mains. Ford guys have done that trick for at least 30 years now. AFR heads are top notch and offer a nice power increase while being dependable. The cam listed only shows it is a Roller, does not indicate overlap. You will have to know if the cam choice works with wet manifolds at some point before purchase.
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