1 full comp point increase is worth what? Dyno results insyd

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

Yes as I have stated many times over I am a BIG believer in quench. Because Mopars suffer so badly from basement level piston to deck heights from the factory this serves as a prime example of getting it "right" and can and does reap excellent results. Also in play is the extreme reduction of "crevice volume" I believe quench and crevice volume share an extremely close relationship. Yes the gasket is a true .039" compressed thickness--I reused them BTW.

The BSFC's did improve by a remarkable amount. They are low to mid .4's as of now. I also believe that the intial "suck" on the intake port is a touch better with the closer relationship with respect to the intake valve/valve pocket--This is why minimum valve to piston clearance can be a power maker. J.Rob

This is interesting. I would like to have expanded on this even further.

What if I had preserved the comp ratio @ 9.4 with a -.055" deck vs. a 9.4 comp ratio @ zero deck? I believe the difference would have been more on the order of 1.5-2%--maybe less!

To answer your question--first I don't have an answer I have never done this. Second--I doubt it. Aluminum heads are not some magical engine parts. I'm not going to explain it here but --think of excessive advance as "negative torque" or not getting the job done as "fast burn" stuff. J.Rob

Although positive, just too many changes to equate the gain to CR change alone - which renders the info - info.

That's all that I am saying. I increased through machine work-the compression ratio exactly 1 full point--these are the results. Make of them what you may. I offered my thoughts on why and how--I do not know for sure--I only know what I know and have observed. J.Rob

I for one thank-you for sharing info. keep up the good work.

The quench helped more than the compression.Try a steel shim gasket and get a perfect quench. Cometic .027 would make an ideal .037

Thanks for the info Mr P !!!! Porky great chart thanks for posting !! Ramm great topic !!! Thanks for all the info !!! Marty

Thanks for taking the time to do it and post it.Quench not withstanding that is a fairly healthy cam so I can see it liking the extra compression.

What fuel where you using on both test? Any idea of how much the weather changes where? One of the local Mopar races did some testing on one of his pump gas street motors and had a issue with ignition timing on that motor, he says he verified the TDC on the motor and tried 34 or 36, CRS and it pinged noticably enough to stop the pull. We talked about the plug heat range, carb. jetting, timing light accuracy and jetting, the AFR where fat and the symtoms he was having. He ended up retarding the timing to 25 BTDC for the best results, which where substanstial I'm having troubles with my computer and ISP opening PDF files so I can't open and read the dyno sheets to help illustrate thhis results Test, test and test some more

X 2

Thanks that was a very intresting test!

Fuel was Esso 91. I can tell you the weather was similar both times. In fact I can tell you that density altitude was 1800+ feet both times. Weather aside the Correction Factor was almost identical both times. Raw power was up same amount as Raw power before. J.Rob

Nice job!
My opinion is that you created a more efficient combustion camber.
plain and simple. Using more of the mix for power. Matt

Sorry if my reply came off a little harsh, but your thread title leads one to believe that the HP gain could be attributed to a CR change alone, and my point was that that was not the case ( although stated poorly by me ). As stated in this thread, I think a quench change had partly to do with the improvement, and maybe piston top reshaping, etc. I to, appreciate all your hard work and sharing the results. Just wanna clarify to all that adding a point to your CR my not automatically have the same positive results as you did. Making multiple changes is my personnel nemesis, as I don't have easy access to a dyno - and 2 - 3 trips down the track once a month can give pretty limited info.

What does that prove?
That there was much more taking place than the CR increase.

The piston tops were reshaped for the initial build. I did not do anything different other than decking the block .045" This was the ONLY change. Pistons were in the same configuration both times. J.Rob

x 3

Rod

Not how I read your OP - "CNC machined them with a slight conical dish". No matter, interesting post anyway. Regarding timing, had a similar experience when going from cast iron heads to aluminum heads. Timing requirements changed considerably. Wanted lots more. Besides changing the CR and in turn chamber design, could be material also ( cast iron to aluminum ). Think you were pretty much in the "sweet spot" as far as getting as much as you did with the CR change. I remember when the 426 max wedge motor first came out in 63. two versions - 11:1 and 13.5:1. In 64 they were only offering 12.5:1 on the high side. Diminishing returns?

EXACTAMUNDO..........found similar results.

X4

Nice comparison.

so talking about quench.
can i say he went from not good with .055 plus gasket to .050ish with gasket?
just thinking on this to try and save my block as some say to lower my compression.
as far as quench i am either 0 deck or -.005 in the hole.my indys are 49cc chambers and am high 13s to 14 compression ratio.
i am wondering what i would loose going to 12.5 or so if i do next redo

Being from Bend, you don't count in the popularity contest. -1.

Would be nice to have known the cranking compression before and after. Again thanks for sharing.

I've never worried about being poplar, as long as I'm faster than most with similar parts

I understand the diminishing benefits of CR increase as you get into race gas territory but wild CRs will always be linked in my head with wild cams. Nobody with 15:1 compression is running the same cam intake or headers as the guy with 10.5 CR.

Which I guess gets into a dynamic compression discussion which will get in the weeds fast!

For sure the effect a part change has on a specific combo mostly only applies to that specific combination of parts and clearances. At the same time it is always nice to hear about this kind of stuff- especially on a fairly common motor/heads- thanks and I enjoyed the bench racing in the comments too!