Is 7: 1 compression too low for a procharger in a boat? I need a new crank rods and pistons for my boat and am considering a 572 drag boat 7 to 1 short block vs rebuilding mine. The procharger is an M-4 unit with the 504 intercooler. THe 502 was a 9 to 1 motor and came out of the hole good but I was concerned about the low compression of the 572. What do you guys think?

The 454cid Merc 525SC has a whopping 7.5:1 compression.

What combustion chamber size is that 7:1 rated at ? Some of these big knarly blower engines don't use standard 118-119cc combustion chambers, so you need to do some calc's with the heads you are going to use.

Main thing will be camshaft timing and effects at idle/very low rpm especially if running wet exhaust.

I built my 540 procharged motor at 8-1,it left approx 8% less bottom end power on the table compared to 9-1 BUT I run almost 11 psi of boost on pump gas and never had any detonation problems and because I'm turning a smaller pulley than most the boost comes up earlier with STRONG midrange tq. 7-1 would definately be less than ideal,biggest thing is idle qualityand low range but even if you give up some bottom end tq it is still a 572 so it shouldn't be no pig either. Run a few degrees more timing and some extra boost,should still be STRONG on the top end,Smitty

I am running a 572, 8:1 with a M-4 running 12psi boost she idles fine. Sometimes I have a half hour of channel idle.

Randy

Thaks for the replies. Arctic I do not know what CC the heads.the motor is together and they dissassemble for me when I buy. Dont you have a 572? What is your compression? I dont know if I will go that direction as of yet, I have a good 502 block thats needs the crank, rods, and new 030 pistons. I found a deal on this motor for $4000, shortblock only. Other wise I need to buy the parts for my block and then 8-8.5 : 1 is my goal. What about cranks and rod selection. I was thinking Callies dragonslayer/Lunati pro mod rods/JE. Ever heard of Diamond pistons?
Randy, youir combo sounds like a success. Do you run pump gas? What kind of heads cc and so on? Iron heads or alum? I have some ported Brodix 315cc's I would like to use.
I am logged in at a different computer Hence the Eyeburn. Hpjunkie

Find out what head CC's your Brodix's are and then find out what piston is in this shortblock. Do the math to find what your compression ratio would be.

You don't want to find out the 572 was built to have 7:1 compression with 100cc comb chambers and then you put 118cc chambers on, which would drop compression by a bunch.

That's all.

BTW: 7.5:1 is really as low as most would go on a SC build up. 7.0:1 should be 100% considered the ABSOLUTE lowest. These are Roots type compression ratios, but can be made to work with centrifugals. Just spin the SC faster and the engine will come alive quicker.

7.5 is what Merc use to build there 600SC's to. I upped it to 8.1 when I rebuilt them and never had problems.

GlH ... did you notice a difference? If so what else did you change?

SB good point.

Your responses have kinda led me back to my 502/540 shortblock rebuild thing. The heads are better suited pushrods are correct, and Ill know what ive got. I neednew roller lifters? I think. I have cranes with about 2-250 hours on them on a .632 lift. cam . Are the morels that much better? where do you get them? Greg

RmBuilder is the Morel guy. Good news is there is even better Morels made this year. Bad news is, this time of year may be super hard to get them quickly. Give RM a shout.

If you need new reciprocating assembly, then going 540cid will be no more $ than staying at 502.

More dollars than 502 why? If I am not using stock crank and rods I think its about the same cost. I have gotten quotes on the stock crank and 7/16 rods for $350. Then I need to rework them and buy bolts. The cranks need to be cut and polished. All of this is going to be about another 3-400 clams. So thats my thought on the upgrade on the cubes. Greg