Comp. Ratio ?s

rod · 04-18-2015, 07:39 PM

I was figuring the Comp. ratio for a bracket engine. I have these question for you smart math guys.
1. when figuring the volume of the cylinder, shouldn't you add for valve notches and subtract for the dome when the piston is at BDC?

2. since the valve notches are in the cylinder at both BDC and TDC, then they should be added into to the total volume, right? or because they are there at BDC and TDC and thus cancel out.

3. since the dome is displacing at BDC, but not in the cylinder at TDC, how do you deal with that?

anything else you want to add, please do.
Thanks Rod in AZ

Rob Petrie E395 · 04-18-2015, 09:03 PM

Wallace Racing.com has some good calculators for this.

Bob Mulry · 04-18-2015, 09:06 PM

Hi,

It's the large total volume (BDC) forced into the small volume (TDC).....

The reliefs and the domes make up the total volume at BDC and TDC after that it's just division to come up with the CR...

Subtract the reliefs and add the dome to a flat top for the correct number and don't forget the gasket thickness and bore diameter of the gasket..

Bob

Larry Hill · 04-19-2015, 01:01 PM

The Eagle rod web site has a C/R feature that is fun to play with.

04-18-2015, 07:39 PM	#1
rod Member Join Date: Dec 2009 Location: Kingman, NW AZ. in the middle of the longest stretch of Route 66 Posts: 305 Likes: 0 Liked 17 Times in 13 Posts	Comp. Ratio ?s I was figuring the Comp. ratio for a bracket engine. I have these question for you smart math guys. 1. when figuring the volume of the cylinder, shouldn't you add for valve notches and subtract for the dome when the piston is at BDC? 2. since the valve notches are in the cylinder at both BDC and TDC, then they should be added into to the total volume, right? or because they are there at BDC and TDC and thus cancel out. 3. since the dome is displacing at BDC, but not in the cylinder at TDC, how do you deal with that? anything else you want to add, please do. Thanks Rod in AZ

04-18-2015, 09:03 PM	#2
Rob Petrie E395 VIP Member Join Date: Dec 2007 Location: Woodlawn IL Posts: 1,093 Likes: 162 Liked 219 Times in 79 Posts	Re: Comp. Ratio ?s Wallace Racing.com has some good calculators for this.

04-18-2015, 09:06 PM	#3
Bob Mulry VIP Member Join Date: Apr 2002 Location: Coarsegold, CA Posts: 1,017 Likes: 57 Liked 321 Times in 101 Posts	Re: Comp. Ratio ?s Hi, It's the large total volume (BDC) forced into the small volume (TDC)..... The reliefs and the domes make up the total volume at BDC and TDC after that it's just division to come up with the CR... Subtract the reliefs and add the dome to a flat top for the correct number and don't forget the gasket thickness and bore diameter of the gasket.. Bob __________________ Bob Mulry 7516 STK A & M Motorsports

04-19-2015, 01:01 PM	#4
Larry Hill Live Reporter Join Date: Oct 2004 Location: Hickory, Ky Posts: 10,272 Likes: 1,631 Liked 9,422 Times in 1,951 Posts	Re: Comp. Ratio ?s The Eagle rod web site has a C/R feature that is fun to play with. __________________ IHM Used Parts https://ihmusedparts.com 888-821-1817