|What compression should my P55M have?
Post Number: 41
|Posted on Tuesday, February 26, 2008 - 10:07 am: ||
quick question and a little update...
I have searched and read for hours through this board and can't seem to find a direct reference to it (found it for P66 but not sure if this is the same).
I checked the compression today and with both sparks off, using the dynastart, both my cylinders were coming up at 45 psi (or on this old dial LBS./(square symbol)" - if that means the same thing)
Compared to what I have read here that P66 should have this seems quite low?
aside from this though, with a drain of remaining old dregs in the tank and a fresh fuel change (to high octane)-
a VERY IMPORTANT tightening up of the bolts fixing the carby to the block (they were a little loose thus with a little carby wobble letting in air- stupid me not tightening it properly when I put it back on)
the engine started and ran and restarted beautifully (without airostart as I had resorted to recently).
The old compression tester (frayed and worn rubber) was very hard to get a good seal going, so I am hoping that the compression is actually a little higher than recorded...
However, when in gear it still doesn't quite feel like it getting full power. Next step down the line after some more much needed (and delayed because of engine trouble) work on the boat itself I might try to remove the muffler (sort of doable) and see how gunked up it is.
First off though, while the engine is actually going, is to take Altair out for a decent run before I start pulling things off the engine again.
After all, I think that was what I got the boat for... wasn't it?
Post Number: 343
|Posted on Tuesday, February 26, 2008 - 10:15 pm: ||
I don't know of a specific compression range for these old engines, most that we deal with are hand cranked. When hand cranking we look for resistance and "bounce" When pulled against compression it should bounce back like a spring. If it bleeds down and pulls right through compression you have a problem.
Post Number: 48
|Posted on Tuesday, February 26, 2008 - 10:33 pm: ||
45 PSIG would mean that the compression ratio is about 3 to one, which to me seems low. ( 45 / 14.7 )
I would have expected more like 4 or 5 to one.
Post Number: 42
|Posted on Wednesday, February 27, 2008 - 05:27 am: ||
Thanks JB and John,
I will try the resistance and bounce thing next I am out there. I do know there is a lot of resistance and bounce- I do not know if it will bleed and pull through though- so I will check.
John, I couldn't be sure if the old rubber was sealing properly- and I suspect it wasn't. First time attempts I was getting 35-ish psi, then with two hands and full weight it went up to 45. If I can get my hands on a newer tester I will try it again.
as a side note, I am about to search through my boxes of paperwork to find the old receipt for the last engine rebuild that happened maybe five years ago (got it from the last owner) to see what was done.
I seriously doubt the engine has done much work since then (it used to be a racing sailing boat- and I certainly haven't given it much work), and would find it hard to believe that they would not have addressed anything such as problematic low compression when they had it out. The bill for this rebuild if I recall correctly was something above $2000, so I assume they did a fair bit.
On the other hand, whoever did the work on it quite obviously missed the hole in the carby float (with 1/4 full of petrol) that had caused the carby to flood and the previous owner to permanently place a ice cream bucket under the carby to collect all the leaking fuel- so anything might have happened or not happened.
Anyway, whatever the case it is running now, and starting easily, so that has taken a load off my mind.
Thanks for the help guys.
Post Number: 248
|Posted on Wednesday, February 27, 2008 - 06:09 am: ||
Hans............I do not have any information for compression readings on the P55 but the following is verbatim from a P66 info sheet so you could certainly use it as a guide .
''The compression gauge reading when the engine is cranked by hand at 180 rpm should be approximately 85 p.s.i. '' ,end of quote!
From the sound of it your engine is in need of new rings.
Post Number: 11
|Posted on Wednesday, February 27, 2008 - 09:50 am: ||
I have checked the P5/P55 Manuals, they do not mention a specific compression reading. However, the manual reads, " Be careful that each ring goes back in its own groove. Before re-fitting, place the ring in the cylinder (1/2-in. down the barrel); if the gap exceeds 1/32-in. a new ring should be fitted." Also, "Put the piston upside down in the cylinder, the skirt flush with the top. With a feeler gauge measure the clearance between piston and cylinder bore. Lower the piston 1/4-in. and measure again. If the difference exceeds .003-in. a regrind is necessary." Piston rings go in +.010, +020, +030-in. These will help ascertain the need for more work. There you are, from the horses mouth as it were. Mark