Anyone know about 1990-91 502 mag Difference
 

First off, I am no gear head.

When I bought my 270 Baja I was told that this year only GM didn't have the normal 502's to give Merc so they supplied them at no extra cost with a higher performance and horse power engine.

What is the difference? Higher compression?

The id# on the tag is d602746

The motor has had Stainless marine exhuast, a Dart High rise manifold and a holly 830 DP and a K&N filter added. What kind of horse power (ball park) should I expect.

Also the previous owner told me the engine need 93 octane fuel. Would this be correct?

Thanks,
Bryan

I believe the earlier gen IV 502's had GM bowtie blocks.

Yep. The first batch of them had Bowtie blocks. No other differences whatsoever. They were I believe, actually 1989 blocks, but found their way into boats thru mid 1991 as they sat in boat mfr warehouses until all sold.

The Bowtie block is recognizable by a bowtie emblem cast into the block down near the fuel pump mount. Is a bit beefier block.

Horsepower output would be not one smidge of difference.

Didn't come with dart intake.
Didn't come with Holley.
Didn't come with Stainless Marine headers.
Didn't come with high compression.

Bottom line, is that your motor has been messed with. You don't know what it does or doesn't have in it unless you know every guy who turned a bolt on it. You don't even know if that is the original motor in the boat.

Test Cranking compression. That will give you some idea of the static compression ratio. We can only assue that it is running a stock magnum cam, but it may not be.

Stock 1990/91 502 Mag will run wide open all day long on 87 octane. Don't know how far from stock you currently are, though.

Thanks, I believe except for the upgrades I mentioned the motor is original.