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Dan Dillmann
05-07-2001, 11:46 AM
I just put a set of high compression heads on my '87 Merc 220 EFI. Cold compression was 125-135 psi with the stock heads and 150-160 psi with the new heads. Is this compression too high to run on 93 octane? Will this lean out the engine? I ran the boat for about 15 minutes and did a plug reading. I put new plugs in and when I checked them after about 20 seconds of WOT they did not have any color, they looked just like they did when I installed them. Do I need to run the boat longer before checking the plugs? How long do I need to run at WOT before checking the plugs? What should and shouldn't the plugs look like? Can I do anything to my processor to make it run richer? Thanks a lot for your help, I'm a little nervous about running the boat until I'm sure it's not going to blow up!

'87 Vegas XT/'87 Merc 220 EFI

Markus
05-07-2001, 01:29 PM
I put a link to some pictures of spark plugs on my links page + some info on two stroke jetting. (see below) Maybe that will be of some help.

Jay Smith
05-07-2001, 08:08 PM
160 is too much compression for pump gas, are you using thick head gaskets? If not you could use the fishing boat gaskets .075 vs Hi Per thin about .045 and will cut some of the compression. I don't go any more than 145 on pump gas at high RPM around 34.5 CC's is safe on pump gas even if some yo yo at the station put 87 in the 92 tanks !

Jay @ JSRE

Dan Dillmann
05-09-2001, 11:29 AM
Will the high compression heads lean out my engine? Can you hear detonation in an outboard? I am using the thick head gaskets already. Can I get a even thicker gasket? Thanks for the help.

Jay Smith
05-09-2001, 04:28 PM
Dan, An outboard is NOT like a 4 cycle car motor. When you hear it it's smoked! If your running the thick gaskets already you can have the pockets deepened to creat volumn which will lower compression but that is sometimes pricey, I would just look for a used set of heads and lower CC's to about 34 and you'll be fine !

Jay @ JSRE http://www.jaysmithracing.com

Bruce Washburn
05-09-2001, 08:30 PM
I agree with Jay 160 is too high for pump gas. 33 to 34 cc heads should put you in the ball park. I would suggest that you look at the pistons instead of trying to read the plugs.This is especially true if you are running perma gap plugs. Keep in mind to get a truely accurate plug/piston read you need to do a quick shut down at speed. No idle time etc. The best way to see what is going on is to install an EGT (exhaust gas temp.)guage.

CarolinaBurt
05-09-2001, 09:06 PM
Jay, that work you did for me last winter has been perfect. I checked compression yesterday and all 6 were 135 to 138 pounds and the Allison sure likes the improved holeshot and pull to top-end. Dan I know Merc sells .70/.55/and .42 thickness gaskets, I found out the hard way that .70 and .42 gaskets yield approx. 10lbs difference in compression after the local dealer mixed a set for me. Jay saved the day though. Good luck with the swap. CB/Gordon