Compression ratio with 049 heads
 

Will I be ok with 9.4CR?

More info needed? The heads are worked and have the larger valves (2.19/1.88) so those flow numbers are off a little.

I've already got the parts so I'm sticking with it... the real question should be if 9.4 is a tad too high should I use a thicker head gasket to bring it down a few tenths?

The boat will live on a trailer so I should have regular access to 91 octane.

What Octane are you planning on running?

I'd like stay with 91.

The answers I'm getting from the machine shop and valve train guys is a reserved, "I wouldn't go any higher than that". The guys at strokermotor.com said I shouldn't go to 9.4... they wanted me to stick the 3cc flat top and run 87 octane (or get aluminum heads).

I would not with iron heads.
If you do be conservative with the timing.
You need to ask yourself is it worth the chance?:eek:

I can play with timing at the dyno... couple degrees on the safe side then. I can also choose a little more gasket volume when I know the exact deck height. I can probably get it down to 9.1. We also have 93 octane here, but no telling how much longer it will be at the pumps.

Here's a damn good article which really helped me understand compression ratio.

http://www.carcraft.com/techarticles...ression_ratio/

I've never seen it put in as simple terms.

"Compression ratio is the relationship of cylinder volume (or displacement) with the piston at BDC to cylinder volume with the piston at TDC. If the volume of the cylinder with the piston at BDC is 10 times greater than the volume of the combustion area with the piston at TDC, then 10 units of volume get squeezed into 1 unit of space, and the compression ratio is 10.0:1"

That is until I used this calculator on the KB site. I understand how cam timing effects CR (intake valve not closed at exactly BDC), what I don't understand is how a rod length can effect stroke!? In my mind a 4.25" stroke is a 4.25" stroke... even if you put a 10 foot rod on it the stroke will still be exactly 4.25". I may have to call KB to get this one explained.

I had the exact same concerns last year building my motors using the "990" heads. My compression worked out to be 9.3/9.4. The same size valves were also used. For whats it worth, I have had no problems running 91-93 octane at 36* timing. Go with it!

JackHammer,
What H.P. are you looking for?
and,
Where do you want the torque range?
What cubic inch motors are you starting with? 454's?

IMHO, You may want to mirror the cam specs for the HP500 carbed motor. Also, was a head flowed to your intake (dual/single plane?) to decide on cam choice?

I've made a few mistakes in the past :rolleyes: and don't mind sharing what not to do!

Rod length changes the angle of the rod vs crank. It really changes piston speed from the 11-1 oclock postion. A shorter rod tends to stay at TDC a little longer and is lazy approching or leaving TDC. A longer rod keeps the piston moving faster in that area. Keep in mind that is the spark area on the compression stroke. On the intake stroke. a longer rod starts sucking faster as the piston moves quicker, which is needed with a longer stoke. Hope that helps.

I think I get it... but how does the faster piston have a dynamic effect of shortening the stroke? the accelerated piston physically cannot suck as much air?

mtgbiker, I pretty much did exactly what you suggested. I backed off the crane 731 a notch. If I achieve a reliable 475hp at 5000rpm I'll be happy. I plan to spend most of my time in the upper mid-range where I hope to stay above 600 torque.

I really struggled with the intake, but in the end I chose the Performer RPM dual plane (port matched is all). If someday I want more top end out of the engines I can get more flow with intake and heads. Probably not on this boat though. It's an old heavy scarab.

Maybe the longer rod affects dynamic comp. ratio,but it doesn't affect static compression. That is simply a fraction. Volume of cyl. and head, piston down over volume of cyl. and head, piston up. Merc. always used 8.8 to 1 with 89 octane, 9.4 with 91 octane seems ok but Merc. was always conservative with fuel/ air mix and timing. Don't use a thick head gasket. Keep the piston to head clearence at .040 for best performance and knock control. Using thicker head gaskets to cut compression will actually make knocking worse.

here is a web site that has some cool calculations.
http://www.golenengineservice.com/html/calculators.html
it will figure all kinds of things if you have the basic info.