GT-40 compression

Also forgot to mention I got a acme 422 prop that I’ll put on and hopefully get a higher rpm and speed than what I’m getting now on an OJ which I think might be original which is 40.1 mph (gps) at 4900 rpm. Hoping for 42 at 4950 or 5000.

Standard GT40 heads will have 3 bars cast into the front, GT40P heads (from a V8 Explorer) will have 4 bars. The regular GT40 heads will have around a 64cc chamber, while the GT40P heads have a smaller 60cc chamber. Swapping the 3 bar GT40s for GT40P heads is a popular upgrade on boats, as the straight spark plug design of the GT40P head doesn't interfere with marine manifolds like it does in an automotive application, and the smaller chambers help bump the compression a bit. Here's a set of standard GT40s (which would've been stock on your motor):

Hey Gary S try pure-gas.org to find ethanol free gasoline. I have found the site to be accurate for locating the only acceptable fuel for my boats.

I have- you cannot buy ethanol free gas in northern Illinois anywhere. No problem though all my boats are over 21.

Gary,
All the stations premium up here is ethanol free. Even the big ones like Shell and BP. Then there are a few that all their grades are ethanol free.

With the length of time my boat sits and the time it takes me to consume the last of the gas I put in my boat in October, I’m not willing to deal with ethanol in my gasoline. I can get ethanol free gas in Twin Lakes, WI at the Mobil on Route 12 just before it turns into the autobahn-like divided highway heading towards Lake Geneva.

Those GT40P heads: are they iron or aluminum? I thought somehow they were aluminum. Edit: I missed the pic above: I see they’re iron.

Gary S wrote: Iron Pete. I had that problem with the Florida ethanol eating my tank liner,.

I do see that they now have tank liners for ethanol. I looked into it when I was considering lining the tank for the launch.

I had compression around 125 on the GT40 on my 96 Super Sport.

Engine ran strong. I would hit around 45MPH on my 422 before I would hit the rev limiter (5500 I think).

With the stock OJ prop I ws able to hit 47MPH.


I was told that compression can vary depending on the tool and method used and to not pay much attention to the numbers.

8122pbrainard wrote: I do see that they now have tank liners for ethanol. I looked into it when I was considering lining the tank for the launch.

Mine was a liner for ethanol Pete. What ever the blend is for Orlando area did not like it. I have had it for years in two tractors,a motorcycle and a car up here with zero trouble.

Gary S wrote: 8122pbrainard wrote: I do see that they now have tank liners for ethanol. I looked into it when I was considering lining the tank for the launch. Mine was a liner for ethanol Pete. What ever the blend is for Orlando area did not like it. I have had it for years in two tractors,a motorcycle and a car up here with zero trouble.

Gary,
They grow weird corn in Florida!

I have to go on a trip but when I get back I will check the prop that’s on the boat now just to see what pitch in diameter it is I do know it’s OJ but I haven’t got the numbers around it I’m really hoping to see what the ACME 422 prop does as far as bringing up my speed and rpm’s. The boat came with two propellers in the one that was on it originally on the boat was very low pitch I think they were doing wakeboard riding with it when I took that prop off and put on the other prop that came with the boat it increased my speed and lowered my rpm’s for every speed that I was crusing at. I wonder with my air nautique if I have the slower hull design than the tstc2 hull? I do know the bow seems to me to ride very low. I’ll try to take pictures next time I’m out. With no one in it and full tank of gas it sits what appears to me as normal. But when less than a 1/3 of gas then it sits with the aft floating higher up than the bow. I will usually fill the fat sacks with some water to help weigh down the backend which raises the bow somewhat. I know people have talked about soaked foam but I thought it was more in the back than front and would think the back where all the water runs to would be the first to go? I’m disappointed as well that the boat only does 40mph at wot and at a loss as to why so low of numbers but gps don’t lie.

halfnelly wrote: Standard GT40 heads will have 3 bars cast into the front, GT40P heads (from a V8 Explorer) will have 4 bars. The regular GT40 heads will have around a 64cc chamber, while the GT40P heads have a smaller 60cc chamber. Swapping the 3 bar GT40s for GT40P heads is a popular upgrade on boats, as the straight spark plug design of the GT40P head doesn't interfere with marine manifolds like it does in an automotive application, and the smaller chambers help bump the compression a bit. Here's a set of standard GT40s (which would've been stock on your motor):

Explorer heads are not a direct swap. 302's use 7/16" head bolts and 351w use 1/2". The heads will have to be drilled out to accommodate the larger bolts.

My GT40 in a 95 Ski Nautique Hull will push 5,200 RPM and sometimes 5,300 RPM with a 224 prop which is very similar to the 422. I just bought a new 422 so I will try that soon but I think you have an issue with your engine. Should be able to weigh your boat and figure out if it is heavy.   I don't know if the newer hulls had an issue with wet foam.
I thought that was mostly in the older boats with wood stringers.   Pre 1993?
On an older 1978 351W I did install a set of ported and polished 302 HI PO heads that had a lot of work done to improve flow.
They woke up my 351W big time. I drilled out the head bolt holes in my garage and they never had an issue in the many years I used them.
That engine did demand Premium fuel though, it pinged pretty good if I tried to use Regular fuel.
When I sold it the new owner had it checked out at the local Marine dealer.   They hooked up a water hose, jumped in and fired it up.   The Dealer started cracking the throttle to see how it sounded and he looked at me and said, " holy crap what have you done to this engine".   Except for the heads it was bone stock but like you said, it revved really quick and sounded much stronger even at 1,500 hours.
If I were you I would run premium and smile. The extra snap in the engine is worth it.

my 89 with the base 351 245 hp motor and a 422 runs 47 mph gps confirmed @ 4800 rpm

It sounds to me like you think you have a higher performing engine than stock and a boat that doesn't go as fast as you think it should. Bit of a disconnect there

And you also have a loping idle. Some people might say that's a standard feature of a GT 40 sometime in it's life.

There are plenty of threads here on CCF about GT40's with a loping idle.

Why don't you take the Fresh Air Exhaust off at least temporarily and run it so you'll know just what difference it does or doesn't make.instead of making a guess or assumption like you're doing now?