GT-40 compression |
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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Posted: June-08-2018 at 4:27am |
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I bought my 99 air nautique last year and forgot to get a compression test just for
Baseline numbers. When I got it dewinterize this week the dealship did a compression test and was surprised by the numbers for a boat this old. 156 was the lowest and 165 the highest. I asked what was normal and they said 125. Is that right?I know the PO said they had done an engine rebuild but had no paperwork so I didn’t know if one had been done or not but the dealer said it’s definately not a standard GT-40 head and it looked to them from the numbers they were getting that it’s definatly modified and might even be a 302 head giving a smaller space thus a higher compression ratio? My concern is should I only put premium gas to avoid predetenation? They also said by how fast it revs it seems like I might have a modified cam as well. I bought new AIV to replace the old one as the engine sometimes gets into a loping idle that ends in either the engine dying or I give it enough throttle to get the engine above idle and then it smooths out and runs great. The dealship seems to think the ecm just can’t keep up with the modifications to the engines at idle. But I think if the ecm can make the engine run great at every speed but idle I’m hoping the AIV can fix that. I’ll let you know after I replace it and test it out on the lake again. |
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i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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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.
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W
i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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tryathlete
Platinum Member Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797 |
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Perhaps to verify the possible combustion chamber mod you could take a few comparative photos of your engine and a known standard GT40 using a Boroscope into the spark plug openings.
Short of pulling a cylinder head and cc-Ing the chamber and piston top, this may be an easy way to determine if you’re just looking at “high” compression readings or in fact you’re dealing with a modified engine. What leads you to believe the engine is modified? My stock GT40 revs to slightly above 5300 rpms with a repaired 644 and will GPS at 46-47 if I let it go that fast (which I won’t do for more than a few seconds). I run Premium ethanol free gas even though I don’t believe it’s necessary. |
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halfnelly
Senior Member Joined: January-14-2013 Location: Maitland, FL Status: Offline Points: 253 |
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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):
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Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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I'm a little surprised at your numbers I would think they would be better. My SS runs 46.2 at 4700 with its Acme 380. Do check the heads for 3 or 4 bars.
Pete where are you getting ethanol free gas,Wisconson ? |
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DVskier
Senior Member Joined: September-04-2014 Location: Seneca SC Status: Offline Points: 449 |
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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.
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shierh
Gold Member Joined: April-29-2015 Location: Florida Status: Offline Points: 605 |
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Build up on piston top and combustion chamber can raise your numbers.
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Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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I have- you cannot buy ethanol free gas in northern Illinois anywhere. No problem though all my boats are over 21.
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81nautique
Grand Poobah Joined: September-03-2005 Location: Big Rock, Il Status: Offline Points: 5771 |
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Agreed, Only time my boats get ethanol free is if we are traveling and fill up at a marina that carries it. Ethanol blend pump gas is all you get here in Illinois for at least 15 years. I honestly have never had a fuel issue using it. |
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8122pbrainard
Grand Poobah Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41040 |
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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. |
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8122pbrainard
Grand Poobah Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41040 |
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I've never had a problem ether. I feel there are times that people use it as an excuse when they can't figure out what's wrong with their engine. |
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tryathlete
Platinum Member Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797 |
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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. |
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Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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Iron Pete. I had that problem with the Florida ethanol eating my tank liner,but have never had any trouble here other than a Stihl weed whip. 7 months the nautique sat with it in,it started right up ,nock on wood.
Alan - Casey's in LeRoy has non ethanol. |
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8122pbrainard
Grand Poobah Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41040 |
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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. |
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tryathlete
Platinum Member Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797 |
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Old ethanol mixed fuel isn’t the stuff you want in your tank. It’s junk after many months of water absorption and breakdown.
Gasoline Expiration - Ethanol Blend Fuels Have a Short Shelf Life. An Alcohol Fuel Test Kit will give you the data you need to keep engines safe with E10 gasoline. E10 Gas has a shelf life of only 3 months. Ethanol alcohol fuel blends have a shelf life of only 90-100 days, under ideal environmental conditions. |
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Blamey
Gold Member Joined: August-18-2015 Location: White Plains,NY Status: Offline Points: 631 |
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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. |
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96 Super Sport
Previously: 95 Sport Nautique, 1980 Ski Supreme |
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81nautique
Grand Poobah Joined: September-03-2005 Location: Big Rock, Il Status: Offline Points: 5771 |
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good to know Gary, that would work when we go down to Clinton a few times a year. Thanks |
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Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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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. |
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DVskier
Senior Member Joined: September-04-2014 Location: Seneca SC Status: Offline Points: 449 |
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I guess y'all will have to deal with it (ethanol fuel) then. Perhaps a political influence conspiracy exists in IL because of the corn farmers. I use Stihl chainsaws, weed eaters, leaf blowers and in the owners manuals they recommend ethanol free fuel only. Also PCM recommends ethanol free fuel for all PCM engines.
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8122pbrainard
Grand Poobah Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41040 |
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Gary, They grow weird corn in Florida! |
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tryathlete
Platinum Member Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797 |
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My 30 year old McCulloch chainsaw starts by the third or fourth pull on ethanol free gas after a year of sitting. Try that with e-10 and see where it gets you. There’s a good reason to keep crappy ethanol that absorbs 6000-7000 ppm of water that disturbs many additives in blended fuel AND lowers octane. Just because you can’t hear detonation doesn’t mean it’s not happening. |
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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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.
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W
i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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Dealership told me the top end is not standard GT-40 head. They don’t know what it is but they do know what is not. They also do a lot of older air nautiques and said mine just runs better than most. They would say it excellent if it wasn’t for the occasional loping loop the engine gets into when idling which doesn’t happen every time; and anytime it does happen to where it kills the motor on restart it always idles smoothly. It’s like the recovery from a load that was put on or released from the engine like into or out of drive or reverse, sometimes not always, gets the engine to start hunting and instead of the ecm being able to dampen that hunting or loping out, it gets progressively worst until it kills the engine.
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W
i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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Hagan
Newbie Joined: August-13-2017 Location: Albany, OR Status: Offline Points: 17 |
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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. |
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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Well I put on the acme 422 prop today and took the boat out for a spin top speed at wot was 41.1 or 1 mph faster than the previous prop. Rpm max 4700. So either I have a smaller combustion area which would explain the higher compression numbers or I have more weight in the boat than I realize like maybe wet foam? I do have a fresh air exhaust put on it so that takes off a couple of miles per hour but I wouldn’t think 5 mph. I guess a quick easy way is to go weigh the boat on the trailer at a weigh station and see how heavy she is. What is the weight of an air nautique 99 suppose to be? 2700lbs? As far as the trailer I could after the weigh drop off the boat in the lake and then go weigh just the trailer and I’d know the boat weight. One thing I did notice is when I chop the throttle now the boat stops much quicker.
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W
i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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MrMcD
Grand Poobah Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3600 |
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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. |
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Gary S
Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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Keep in mind not every boat is going to weigh the same,the ones before or after yours on the line could be totally different as to how many layers of glass and how wet or dry the layup was. Then add in being built in the old factory meaning older production methods being different than the more controlled methods possible today. Anything listed in a brochure is most likely an average. There is hope though,you could write the factory and see if they will look up your boat. There are detailed records of your build,I was told mine are 21 pages long, included is the original weight. You ever going to post some pictures of the end of your heads so we can see what they are? |
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throttle out
Senior Member Joined: May-11-2015 Location: Chelsea, MI Status: Offline Points: 153 |
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my 89 with the base 351 245 hp motor and a 422 runs 47 mph gps confirmed @ 4800 rpm
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1989 Ski Nautique 2001 351 PCM w/ 1-1.23
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curiouslibra
Newbie Joined: January-10-2018 Location: Dallas Texas Status: Offline Points: 26 |
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How do you post pics from an iPhone? Option to paste from my photo library doesn’t come up.
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i l l "Don't argue with idiots, onlookers will not be able to tell the difference between the two of you, as the idiot drags you down to their level and beats you with experience." |
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KENO
Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 10703 |
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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? |
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