351 Compression
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=1678
Printed Date: May-18-2024 at 10:03am
Topic: 351 Compression
Posted By: skimartin
Subject: 351 Compression
Date Posted: May-17-2005 at 7:09pm
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I recently bought a 1987 Ski Nautique 2001 with Ford 351 engine. Compression in all cylinders is 120. Is this considered acceptable? ------------- Ben 
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Replies:
Posted By: Bradley950
Date Posted: May-17-2005 at 7:32pm
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The important thing is not what the value is, but that all cylinders are close to each other with no more than a 10% variance between any two cylinders. I cannot remember what a good value is, but my engine ranged from 125 to 135psi. That was with only 200hrs, so you should be pretty good.
-Brad- ------------- Brad Miller bradley950@yahoo.com |
Posted By: Guests
Date Posted: May-17-2005 at 9:06pm
| I just did mine. You can see what I got in my thread "83 2001 problems" I had one cylinder that was around 135, and the others were around 150. Like brad said, it is more the deviance from the other cylinders that makes the most differences. BTW, my boat supposedly has 835hrs. I say that b/c I haven't seen my hour meter move yet, just make noise. |
Posted By: skimartin
Date Posted: May-19-2005 at 1:53pm
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Thanks for the input - My engine has close to 1000 hrs on it and we started having problems with acceleration. 1) We rebuilt the carb - No help. 2) We did plugs, wires, points, etc - No help. 3) We tested with an alternate fuel source - No help. 4) We noticed some of the new plugs were fouling and checked the compression to see if rings were bad (120 in all cylinders). Now, we're thinking its leaky valve seals causing the plugs to foul. Anyone have comments or suggestions? ------------- Ben
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Posted By: jameski
Date Posted: May-19-2005 at 5:44pm
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Maybe your timing chain / gears have worn to the point that you need to time your engine by ear and not by timing light. The factory setting may not apply anymore and you may actually be way under-advanced. ------------- current boat http://www.correctcraftfan.com/diaries/details.asp?ID=1977 - 94 Sport Nautique previous boat http://www.correctcraftfan.com/diaries/details.asp?ID=601 - 78 Martinique |
Posted By: Bradley950
Date Posted: May-19-2005 at 10:11pm
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Does your engine smoke when you first start it up. Valve seals will leak when you shut down and the next time you start the cold engine it will smoke for a minute or two. If thats not the case then you my be running to rich. Maybe your electric choke is malfunctioning. What about your coil is it really old? It may not be as hot as it once was. Have you cheaked for vacuum leaks? Like Jameski said it could be as simple as your timing being off. Let us know!
-Brad- ------------- Brad Miller bradley950@yahoo.com |
Posted By: 64 Skier
Date Posted: May-20-2005 at 9:51am
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Probably worked this a lot harder than necessary, but when I rebuilt and bored my 289 we measured 165 psi. Ford racing thought this was max for a pump gas engine. They also said 120 was very common and I've seen 90 run very strong.
Probably need a little head work on your engine if your fouling plugs. Friend pulled an engine in a 70's Mustang...like yours...good compression...he only had the heads re-done and his boat will outrun mine. ------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: skimartin
Date Posted: May-20-2005 at 1:45pm
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The poor acceleration does not occur until after a few minutes of test starts/stops (after the plugs foul). If it were timing, wouldn't the poor acceleration occur regardless? There is some smoke after starting and the coil has not be replaced to my knowledge. ------------- Ben
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Posted By: jameski
Date Posted: May-20-2005 at 1:53pm
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What color smoke? Black is too much fuel. Blue is oil. ------------- current boat http://www.correctcraftfan.com/diaries/details.asp?ID=1977 - 94 Sport Nautique previous boat http://www.correctcraftfan.com/diaries/details.asp?ID=601 - 78 Martinique |
Posted By: Guests
Date Posted: May-20-2005 at 2:38pm
| get a chevy |
Posted By: jimbo
Date Posted: May-21-2005 at 1:50am
| I'd be happy with 120 psi. Mine is running in the 90s. The plugs looked ok and it has lots of power and it doesn't smoke. I was thinking about doing the rings this winter but unless I see some problems, I'll probablly wait. Speaking of power, I don't have to give it much thotle to get a skier out of the water. I'm amazed at how much power these boats have. |
Posted By: Jim_In_Houston
Date Posted: May-21-2005 at 3:09am
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64 Skier,
How did you get your compression up so high? What heads are you running? I just replaced the heads on my 289, the bottem end is stock. My compression is in the 90s. The stock pistons are dished. ------------- Happy owner of a '66 and a '68 Mustang |
Posted By: 64 Skier
Date Posted: May-22-2005 at 1:31pm
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Jim, we used flat top pistons. Original heads, but with larger valves. I like the old head with big valves for the low end torque and swirl/fuel mixing that you get with the old design. Gonna play with new heads and exhaust this summer after I remove the foam/water form the flooring.
Did you bore your engine and end up with 90# or is it the original equipment? ------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: Jim_In_Houston
Date Posted: May-23-2005 at 11:23am
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64 Skier, I did nothing to the bottom end of the engine. The bore is original as from the factory. I was totally shocked when I pulled the heads and found almost no ridge ring. The boat consumes no measureable amount of oil while running. The previous owner obvoiusly treated the boat very well. This winter I may rebuild the lower end just to improve my warm fuzzy feeling about things and I may want to change pistons. I have no idea what pistons to install. I just would like a little higher compression. 90LBS just dosen't sound right to me.
I agree with you about the old heads. Nothing wrong with them at all. I am just hung up the way Ford uses pressed in rocker studs on the old heads. That drives me nuts. I had to get away from them. Are the flat top pistons Ford stock? Anyone know what pistons should be used with GT40P heads to safely get around 120 Lbs compression? ------------- Happy owner of a '66 and a '68 Mustang |
Posted By: 64 Skier
Date Posted: May-23-2005 at 12:42pm
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Jim,
If you changed heads to GT40's and the volume increased then your compression was reduced by that and not engine wear. Even the gasket type can reduce the compression since it obviously affects volume. I needed some smart guy's to help out so I got a good Speed Shop to help me out. I also called Ford Racing and Holman & Moody (Jimmy Tucker) since I had the old solid lifter cam which can be tricky with a new piston. I had to make sure I bought the right piston to use with the old heads/cam to get 140-165 PSI. My engine had more of a mountain than a ridge so the boring/pistons were necessary. Between a reputable speed shop and Ford Racing or H&M you'll get the right combination for your engine. ------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: David F
Date Posted: May-23-2005 at 5:51pm
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Jim:
FWIW: back in '89 when I rebuilt the 351W in my '77 Martinique, I was most interested in torque. So I bored the engine 30 over and installed flat top pistons. I then consulted with my machine shop (the Owner mostly built up racing engines and had the machining service to help ends meet) who advised that I go with a RV camshaft to help low end torque. He gave me the part number and I purchased it. Incidently, I have never checked the compression on the engine, but let me tell you that engine pulls! With the boat just on plane, if I stab the throttle, the torque on the prop causes the stern to slide sideways as it hooks up...simply awesome. The heads are bone stock with no porting or polishing at all. |
Posted By: Jim_In_Houston
Date Posted: May-23-2005 at 6:30pm
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Yeh, I kinda knew going in the the heads where going to knock down my compression. My real reason for going to the heads was to get a good set of well built heads. I like the rocker assemblies. I also kinda knew that when I do rebuild the lower end I will have an opportunity to change pistons. I just don't want to spend time putting clay on the piston tops. I think the problem is not too many folks put GT40Ps on an old 289 so info is limited - I could be wrong. 302s are fairly popular and maybe they have the same rods and block deck height as a 289 - I just dunno.... Interesting though, huh? ------------- Happy owner of a '66 and a '68 Mustang |
Posted By: 64 Skier
Date Posted: May-24-2005 at 4:16am
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David...we did the same thing...lotsa torque.
Jim, the 260, 289 and 302 are pretty much the same except for the crank and with my solid cam at "0" lash the playdough still had room so we bolted her up and then ran 020 lash. Like you....I don't like the original heads and will try to work in a set of GT40's or some other premium to keep the compression but give me some more top end breathing ability. Right now the bottom end is very impressive and if the new heads compromise my pulling torque, I guess I'll put them on a Bronco. ------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: Tim D
Date Posted: May-24-2005 at 11:44am
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The 260,289,and 302 are very similar. Same bolt pattern for intake and the heads which are interchangeable. The bore and stroke is different but the 260 has a different bell housing pattern, the bolts are not as close. My Mustang has 160 PSI, Keith Black pistons. ------------- Tim D |
Posted By: Jim_In_Houston
Date Posted: May-24-2005 at 12:37pm
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Tim, what RPMs do you top out at? Obviously you run 93 - 95 octane fuel. ------------- Happy owner of a '66 and a '68 Mustang |
Posted By: Tim D
Date Posted: May-24-2005 at 6:38pm
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I don't know, really. I very seldom run above 3500 rpms. My cousin built the motor for me, he said I could hold it a 6000 and not hurt it. I know it wont tach that high in gear. It also has roller rocker arms and a melling oil pump, which tachs out the oil pressure past 80 psi at idle. If I had a 4 barrel carb and some of those high rise exhaust manifolds, I would see a tremedous difference, maybe it would porpoise then and scare me. ------------- Tim D |
Posted By: 64 Skier
Date Posted: May-26-2005 at 11:34am
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Jim in Houston...article on heads...the GT40's have 13 more cc's, sorry but the picture did not paste.
Although the slightly larger GT-40 valves are hard to detect in this comparison (GT-40 on right, 289 Hi-Po on left), the difference in combustion chamber size and shape is hard to miss. The larger 65.5cc combustion chambers in the GT-40 head, compared to 52-55cc for vintage 289s, are what causes a slight compression loss when the new heads are used on a vintage short-block. ------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: jimbo
Date Posted: May-26-2005 at 5:21pm
| Can you tell if you have GT-40 heads with out pulling them off the engine? |
Posted By: 64 Skier
Date Posted: May-27-2005 at 3:23pm
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Yes, the OEM's have 3 parallel marks on the end as I recall...not sure about the aftermarkets. This data/fact was on another post. Good Luck, sorry I can't help you more.
------------- 64 Skier 66" HO VTX and 67" HO Triumph http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC |
Posted By: Guests
Date Posted: June-06-2005 at 12:17pm
| So where did the skimartin idiot go?? What fixed his problem. I say just rebuild the whole thing instead of piece by piece, or hire someone who knows what is going on??? hahaha |
Posted By: Jim_In_Houston
Date Posted: June-08-2005 at 3:17pm
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About marks on a GT-40P head: I think the GT-40s have 3 bars cast into the end of the head and the GT-40Ps have 4 bars - I have to go look to make sure. The GT-40Ps are different heads than the GT-40s. ------------- Happy owner of a '66 and a '68 Mustang |
Posted By: captan1
Date Posted: June-09-2005 at 2:50am
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Thats right, GT40 have 3 bars, GT40P have 4 bars, I have the P's on my boat, still run middle grade with no problems. I'm not sure what the compression is now, up some from the D80E heads I took off.
It runs good, I think my timing stops at around 32 deg. I just ordered a ACME prop for my 78 Ski Nautique to replace the 13X14. ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=452&sort=&pagenum=1&yrstart=1978&yrend=1978 - 1978 Ski Nautique |