Replacement Pistons for an HP500 EFI
 

Question for all you guys who know a lot more than I, I am having my HP500 EFI rebuilt and part of that included getting it bored 30 over.

My question concerns the replacement pistons, these are flat top as opposed to the domed that were stock. When I questioned about the change I was told that if I had stayed with a domed piston, the compression ratio would have gone up due to being bored 30 over, going to a flat top 30 over piston keeps the compression ratio nearly the same as stock.

Is this true?

Re: Replacement Pistons for an HP500 EFI
 

I could be wrong, but I don't think the C/R would increase enough to cause any harm. I personally would stick with the same style (domed) as original pistons.

Re: Replacement Pistons for an HP500 EFI
 

It appears that there is approximately .2 difference (increase) in compression ratio when boring to 4.500 from 4.470, if you were to use the original dome configuration. I would have gone with the domed pistons and lived with the slightly higher c/r. Just my opinion.
I'd also say that your engine builder knows what he's doing. Again, just my opinion.

Re: Replacement Pistons for an HP500 EFI
 

Well if he knows it would cause a change ask him exactly what the ratio changed to. A tad more might not be a bad thing.

Re: Replacement Pistons for an HP500 EFI
 

Yup. He should know without getting out the catalog, if only because he's already looked in the catalog.

Re: Replacement Pistons for an HP500 EFI
 

I was told that the "experts" at JE/SRP pistons told them that when he ordered the 30 over pistons to replace the orginals. I have been told in this forum that JE is the OEM for the pistons to Merc HP. I am just checking that the JE/SRP guy is thinking right.

In any case who knows after a couple of years of trouble free boating I may add a blower and having the compression near stock and not slightly over would be better, right?

Re: Replacement Pistons for an HP500 EFI
 

Keep in mind that your builder is probably decking the heads also which will raise the compression ratio more.

Re: Replacement Pistons for an HP500 EFI
 

Is your builder decking the blocks? I have seen blocks where the pistons were .020 down in the bores. 0 decking the block offers a noticable increase in performance as well as helping to control detonation.

Re: Replacement Pistons for an HP500 EFI
 

If you plan on adding a blower, then go flats.

Re: Replacement Pistons for an HP500 EFI
 

15 decked off the block.

The reason for all of this is that the motor over heated in 1999 and although worked on four times since then it has never been fixed right or completely. It is a long story that has been told here on OSO several times, but the motor over heated the first year do to clogged low water pickups, compounded by a faulty temp gage (would not read above 190-200) and a non functional alarm buzzer.

This time I went to a good and trusted mechanic, he is rebuilding the motor as if it were his. I am just trying to keep it stock as I just want to get a couple of years running without major motor troubles (Motor has broke every summer since 2001).

By the way I have learned my very expensive lesson, I have added a water pressure gage as well as an oil temp gage, any one or two of the monitoring systems fail and I will still know what is going on and when to shut down and accept the humiliation of being towed.