whats the diff between 7.5 and a 9.8 merc?

T

thriller

Cadet
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Apr 7, 2009
Messages
17
I was just looking at the engine specs for a early 80's 7.5 and see that the 9.8 has the same displacement. What is the diff if the engines have the same size? thanks
 
L

Laddies

Banned
Joined
Sep 10, 2004
Messages
12,218
Re: whats the diff between 7.5 and a 9.8 merc?

Porting, timing, jetting and RPM.
 
S

Starppy

Petty Officer 3rd Class
Joined
Mar 23, 2009
Messages
84
Re: whats the diff between 7.5 and a 9.8 merc?

Yes and no. I've swapped blocks from a 7.5 to a 9.8. Basically making one good engine out of two bad ones. Timing is pretty much the same. The porting and jetting are what is really different. Basically, more fuel/air mix in and more exhaust out. RPMs go up as a function of the increased fuel/air volume.

If you think you are going to get something for nothing by converting your 7.5 to a 9.8, forget it. You'd be better off just trading in your 7.5 on a 9.8.
 
Chinewalker

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: whats the diff between 7.5 and a 9.8 merc?

Easiest way to convert a 7.5 to a 9.8 is to take the 7.5 cowling and put in on a 9.8.

Second easiest way is to drop a 9.8 powerhead, in its entirety, on your 7.5. As Starppy noted, it's not a "nothing" conversion, best done if your 7.5 is toast and you luck into a 9.8 parts motor....
- Scott
 
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