351 Compression

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-

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?

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-

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.

get a chevy

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.

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?

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.

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.

Tim, what RPMs do you top out at? Obviously you run 93 - 95 octane fuel.

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.

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.

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.