Like the title implies I had the chance to test a 440 RB @ 9.4 static compression ratio and then again (due to an oil leak) @ 10.4 static compression.

This is a 446 I recently built for a customer using '915 heads and flat top Wiseco forged pistons. It ran all right during the initial dyno session but the rear main was leaking a little and that was not acceptable so I took it off the dyno and couldn't leave it alone.

When I was assembling the engine the pistons measured .055" in the hole. Also understand the customer really wanted to save these pistons so I cleaned them up and CNC machined them with a slight conical dish netting a little more volume. Basically I knew there was plenty of room to play with here so I decked the block exactly .045" per side which put it @ 10.4 comp. I had the chance to re-dyno with NO other changes. The power increase is somewhat expected but the timing requirements changed dramatically. Previous timing for max power was 35-36 total--Now 30-31 total is all that it likes. Interesting to say the least. So +30 HP +38 ft/lbs at the peaks was observed--the graph tells a much better story. J.Rob

Very coo ! Thanks for the info !

Very cool ! Thanks for the info !

Wow, thanks for the info. What cam is he running?

I was contemplating shaving my heads for a bump of a half point to a point, this is good info and I think I'll pull those suckers off at the end of the season and shave them.

EDIT: Noticed you shaved the block and now it likes less timing. What was the before and after piston to head clearance?

Good info... from the testing I did.. the lower the starting
compression the bigger the gain... as in going from 9.1 to 10.1..
its a deminising return thing

Mr P body when does it start dropping off ?? 14.1 15.1 ?? Thxs !!!

Lunati cam Lunati

Yes the block was decked .045" so the pistons are .010" in the hole now. J.Rob

Peak power wound up like this 537 hp @ 5600 rpm and 564 ft/lbs @ 4300-4400 rpm. J.Rob

Thanks, I thought this was a pretty cool test. I have never had the opportunity to make only one change like this and re test. J.Rob

This is what I have come to understand as well. Also noteworthy is idle vacuum increased 2.5" as well from baseline. J.Rob

Did it go from having no quench to a real quench area with the milling? If so, I'd think that would have something to do with it needing less ignition lead.

Did the BSFC numbers improve, too?

It drops all along when the compression is increased.. the higher
the compression the less the increase... its more like a bell curve

This is what I was getting at. .040 compressed head gasket?

David Vizard chart for estimating power increase % due to CR increase.

Just looked and saw RAMM got a 6% increase, much higher than the Vizard chart I posted predicted. I suspect he found some "bonus points" by building quench into it, too.

Well, RAMM?

And according to his chart its the same thing I said..
look at the higher compression points and it shows like 1% increase
and even lower

You also said it was like a bell curve, but it's not. It's just a simple case of diminishing returns.

I think that being he cut the deck to gain the point he made the
chamber more efficient over what it was.. that might give him the
higher % increase AND the lack of timing.. which the lack of timing
is efficiency

Would a switch to aluminum heads allowed you to keep running the 38 degrees of timing and resulted in even more of a power jump?

JMO but if you ran higher timing on the alum head VS less timing
on a iron head that it would be the same.. if both heads were the
same thing and just different material... again... JMO