Compression ratio with 049 heads
#11
Off the Radar
Platinum Member
Join Date: May 2001
Location: Short Pump or Colonial Beach, VA
Posts: 1,097
Likes: 0
Received 1 Like
on
1 Post
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.
#12
Registered
Thread Starter
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?
#13
Registered
Thread Starter
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.
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.
#14
Registered
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.
Last edited by PatriYacht; 06-12-2007 at 07:59 AM.
#15
VIP Member
VIP Member
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.
http://www.golenengineservice.com/html/calculators.html
it will figure all kinds of things if you have the basic info.