A previous thread discussed the new Prus head
http://www.mtfca.com/discus/messages/257047/274091.html.
It is advertised as an 8:1 compression ratio, but someone cc'd one and got about 210 ccs which gives it something like 5:1, i.e. similar to a Z head. Has anyone confirmed the 210 cc measurement? I have cc'd a lot of heads and they always have less compression than advertised. Why? The difference between 8:1 and 5:1 is ridiculous. Determining the compression ratio is not rocket science.
Are you accounting for the piston in the squish zone of the head. Compression ratio is not as simple as bore volume vs head volume. Piston displacement at TDC needs to be accounted for!
John
Good point. The piston can ride high into the chamber. Put a used 'Z' head on the '26 pickup and the pistons hit. Had to grind away some aluminum there for more clearance.
Did home garage work, without elaborate cc'd of each cyl, figured is was just a T. But wow, great compression, and this stock T, stock brakes too, ran pass 52mph, still asking for more, but I pushed up on the throttle lever at that pace
The piston displacement at TDC in the squish area was accounted for as can be read in the Tulsa club's description on how they determined compression ratio: http://mtfctulsa.com/Tech/head_design.htm
"The compression ratios can be determined from the measured volume. A stock high head has a volume of about 294 cc or 17.9 cu in. The volume is reduced by 2.8 cu in because the piston rises 5/16 inch above the deck. The volume is increased about 0.8 cu in due to the head gasket. For a stock head, the combustion chamber volume is 17.9 - 2.8 + 0.8 = 15.9 cu in. An engine with stock bore and stroke has 44.2 cu in displacement per cylinder, so the total volume when the piston is at the bottom of its stroke is 44.2 + 15.9, and the compression ratio is (44.2 + 15.9)/15.9 = 3.8."
Dean Yoder measured his early production Prus head to 225-230 cc: http://www.mtfca.com/discus/messages/257047/276192.html?1332042419 (wich gives 4.76:1 in compression ratio according to the Tulsa club's calculations) He's still satisfied with the head, though: http://www.mtfca.com/discus/messages/257047/281421.html?1334112289
The odd 8:1 claim seems to come from a compression pressure reading Mr Prus has made. (If a std 3.8:1 head usually gives 45-50 PSI, then a 100 PSI reading must mean 8:1 he thinks)
My Prus head looks good and the engine should be fast enough for the chassis I'm building for it
If you are measuring stuff in CC's (which is very accurate), The best way to measure piston dome volume for YOUR engine, is to do a 1/2" down fill method. Basically you need to figure your volume in CC's of your bore size at .500" (1/2") height. Once you have that volume figured mathematically, put your piston .500" down in the bore as measured on the edge where there isn't a dome. Then CC the bore and see what you come up with. Take that number and subtract it from your first number and that will give you your negative volume to subtract from your head CC number.
This is an old Chrysler method that is used on HEMI stuff to figure compression as they always had domed pistons, and it works very well. You may have to use a little grease pushed around the edge of the piston so the water or alcohol solution won't get past the rings.
I wrote the pages that Roger referenced at the Tulsa website. Over the years I have repeatedly asked if anyone disagrees with my calculations and no one ever has. According to Ford specs the piston is supposed to popup 5/16 inch. I calculated this volume using the formula for a cylinder, which worked out to 58 cc. I measured the bevel as 0.44 wide and 0.25 high and by formula its volume is about 10cc giving an overall volume of 48 cc. for the popup. The head gasket volume was determined by tracing one out and calculating the area numerically. The compressed thickness of about 0.045 gives a volume of 14 cc. The overall correction to the head volume is 14 - 48 = -34 cc. With an 0.030 overbore the cylinder displacement is 738. So, 210 cc head gives a compression ratio of (738 + 210 - 34/(210 - 34) = 5.2. With all the effort Mr Prus went to to make this head, it is too bad he didn't bother with this basic calculation. His combustion chamber looks similar to the Riley head which we tested, so I would expect it to perform much like the Z head with a similar compression ratio. I was hoping Mr. Prus had given us another alternative, but instead we now have three heads available with (4 if you count the new Rajo head) with little more than 5:1 CR, and all of them misadvertise the values for their heads.
Compression ratio is a ratio of two volumes and nothing more. A compression test cannot give you compression ratio. I have an engine with 6:1 compression ratio which gives 105 psi cranking pressure. No where close to 8:1.
Let me add a comment about Dean Yoder's posts. He is comparing it to a Waukesha-Ricardo, Giant and Haibe heads. The Waukesha-Ricardo and Haibe heads both have 4.1 CR. We tested the Wakesha-Ricardo and it was better than a stock head, but not as good as a Z head. His observations are consistent with the tests we have done.cc
Thanks, Larry. Of equal concern is how the designer has calculated the heat transfer from the combustion chamber to the water jacket.