I'm rebuilding a 200 inline six to the " Building a street motor" article put out on "Classic inlines" web site . The engine builder is using stock dish piston with a combustion chamber vol. of 64cc and swept vol. of 599 cc giving a 9.7 to 1 compression ratio. There is a .050 gap at top dead center, if I zero gap the compression ratio would go to 11.3 to 1 comp. Is this going to be too much comp.? If there is such a thing . I don't want to run Av gas in this or put additive in. Concerned that motor will "ping" if it is not zero decked. Can someone give me pro's and con's of this scenario if I do nothing and keep 9.7 comp. or zero deck and jump to 11.7 comp..
Compression Quandry
- Thread starter Spankys66
- Start date
Howdy Spanky:
A performance profile cam will bleed off cylinder pressure, but 11.3 is too high for a street engine on available pump octane gas. IMHO, zero decking the block would only really matter if you were going for an enth degree, every last detail engine. The elevation of Olathe adds to the argument to limit CR at about 9.5:1, again depending on the cam.
what is the rest of the engine/vehicle?
Adios, David
A performance profile cam will bleed off cylinder pressure, but 11.3 is too high for a street engine on available pump octane gas. IMHO, zero decking the block would only really matter if you were going for an enth degree, every last detail engine. The elevation of Olathe adds to the argument to limit CR at about 9.5:1, again depending on the cam.
what is the rest of the engine/vehicle?
Adios, David
Spanky, redo your math, if using dished pistons & "0" decking the block with a combustion chamber 50 cc's it should be close to 9.5.
Thats the combo i am running. Flat top pistons would bring it up to 10.4.
Those #'s are using a .045" victor head gasket.
Thats the combo i am running. Flat top pistons would bring it up to 10.4.
Those #'s are using a .045" victor head gasket.
Hey David thanks for your reply, the cam was replaced with a 264/264 12 lobe with largest valves that would fit, port and polished balanced bored .040 , Arp rod and head bolts with honey comb springs. The machinist made the head as light as possible for more HP. Installing port divertor headers and a progressive weber 32-36 carb. Elev. of Olathe KS is 1070 ASL. Head gasket is 8.9 cc. don't know thickness, this is the number I got from machinist. OK so vol. combustion chamber is - piston is .050 below deck with piston dish equals 13cc. head gasket is 8.9cc and 42cc combustion chamber for total combustion vol. of 64cc. Swept vol. is bore x bore x storke x 12.9 = swept vol. in cc which is 3.720 x 3.720 x3.125 x 12.9 = 558cc. Compression ratio = swept vol. plus compressed vol. total then divide by compress vol. that came to 9.7 to 1 compression. Total all of this i'm just learning from machinist. Hope this is right if not let me know if it is wrong.
Thanks Mark
Thanks Mark
spanky, you got good advice from two of the best here, let me give you a bit more. as wsa111 said rerun your numbers, in fact run them a few times carefully. you do want to keep your static compression ratio right around 9.5:1 for the street, but you also have to be careful not to run your dynamic compression ratio to high either. with the 112 lobe separation angle, your dynamic compression ratio is going to go up. somewhere there is a calculator that does the dynamic compression ratio as well. you want to try and keep that below 9:1 for best detonation resistance. again though, run the numbers carefully.
Here's what I get running the numbers you've quoted. The big hitter is the combustion chamber volume of the head. The deck height number also has me a little confused. Did the block already get zero-decked? Could the machinist have been quoting the 0.050" crush height of the gasket? I just left in the standard deck height value quoted on the CI website. CI also quotes the gasket diameter as 3.75 but I got 3.81 somewhere. Maybe from a spec sheet on a gasket that is currently available. The 2.75"/0.025" is the stock gasket. Has the head already been milled and is this a measured value of 42 cc?
===================================
Cylinder Bore = 3.72 in
Deck Height = 0.019 in
Gasket Crush Height = 0.05 in
Gasket Diameter = 3.81 in
Combustion Chamber Volume = 42 cc
Piston Dish Volume = 6.5 cc
Advertised Cam Duration = 264 deg
Lobe Separation Angle = 112 deg
Camshaft Advance = 0 deg
Intake Valve Closing Angle = 64 deg
===================================
Static Compression Ratio = 10.0:1
Dynamic Compression Ratio = 8.16:1
===================================
===================================
Cylinder Bore = 3.72 in
Deck Height = 0.019 in
Gasket Crush Height = 0.05 in
Gasket Diameter = 3.81 in
Combustion Chamber Volume = 42 cc
Piston Dish Volume = 6.5 cc
Advertised Cam Duration = 264 deg
Lobe Separation Angle = 112 deg
Camshaft Advance = 0 deg
Intake Valve Closing Angle = 64 deg
===================================
Static Compression Ratio = 10.0:1
Dynamic Compression Ratio = 8.16:1
===================================
Thanks for the input you all. You guys know your stuff! I on the other hand i'm learning a lot, thank you! Yes Cr Bobcat the head has been milled so that is where the 42cc comes from. So I believe that the .05 in is space not gasket crush weight. I ask the machinist if we should take away the gap by milling the block so we could zero deck but that would jump up compression to 11.3. Since head has been milled and he was worried it would start encroaching something in the head if we milled the head, can't remember what I want to say spark plugs regardless could not be done on top side. If zero decking is not as important as CR we will leave as is, which should be according to you all about 10 :1.
Thanks Mark
Thanks Mark
Lap the intake to the seat, then backcut the intake at a 30 degree to the lap line. This will pick up the intake flow close to 10%.
You would get more chamber quench if you zero decked the block & get the combustion chamber to at least 48-50 cc's.
You would get more chamber quench if you zero decked the block & get the combustion chamber to at least 48-50 cc's.
