Running a 1969 302 with heads milled to 50cc, TRW forged .068 dished pistons, roller rockers and 1.94/1.60 stainles valves. I fgure the compression is around 10 to 1 or 10.5 to 1. Can anyone confirm this? I keep burning up starters and may need to go to a higher hp stater.
i doubt with dished pistons you would be that high...maybe you need block height ...how far down in the bore is piston at TDC compressed head gasket thickness cc of dished area there is approx 1 cc of area above the rings
They were 0 decked - not sure on the head gaskets though - 0.068 dished on the pistons Thanks for the input.
Like Racefab I'm doubting a compression ratio that high. A flat top piston would yield approx. 10 :1 maybe a little higher. With a dish I'm thinking no more than 9.8-10 on the high side. You've got to have all of the specs that Racefab listed to compute the EXACT ratio.
I'd guess around that too, actually I figure it could be anywhere from there 9.2:1 to 9.8:1 because like RACEFAB suggested, you havent figured in all the actual nooks & crannys. There's the actual cc's of the dish, the cc's of the gap around the piston down to the first ring, the dish or lack of, of the valves, the depth of the valves, the depth of the spark plug even! An .022" copper of steel head gasket can make a .5:1 ratio difference compared to a composition gasket thats, say .056" thick. (headgasket, adds 5-15cc's) And the smaller the engine displacement, the more criticle all the "hidden" cc's become in the equasion. Cruise night bragging? you could say "I built it for a 10:1 ratio! (doesn't say it is 10:1, just you were trying to make it that... in politician-speak...) Reality, probably what Fat Hack said... That's a good thing too, it just might run on pump gas!
You could use a graduated cylinder to find the cc volume at tdc and use that to compute compression ratio, couldn't you ?
My biggest concern is that I am burning up starters - not that interested in bragging rights. I'll figure 9.5to1 Thanks
I am ***uming its a L2305F Sealed power piston. They are forged pistons with 68 thou dish. I will try to pull some numbers, but to be truely accurate - you need the numbers racefab has ask for to be measured. We can get you in the ballpak however. There are two difference deck height 302 blocks. 8.206" and 8.229" Only 73-76 blocks have the oddball 8.229" deck height. The L2305F has a compression height of 1.605" with a 7.4cc dish. I will also ***ume the motor has a common 30 thou overbore and uses a common head gasket like a felpro which has a compressed height of about 0.041" Lets also ***ume you are using stock 5.09" 302 rods and the much more common 8.206" block that has a blueprint deck height and everything is blueprinted to the exact factory specs. We can figure out what the deck clearance of the pistons are by taking the deck height of the block, the stroke, the pistons compression height, and the rod length. We add the compression height, rod lenth, then half the stroke, and subtract that from the deck height of the block. so - 5.09+1.605+1.5 = 8.195" Minus this from the block and you have the piston to deck clearance 8.206-8.195 = 0.011" deck clearance. EDIT - I saw that you are zero decked, so I will leave figuring out the cc's of the deck clearance. You do it the same as any other cylinder. We have everything else, so lets figure out your compression ratio if everything is as it should be. I am lazy and will just plug it into one of the many online calculators. But to figure it out properly, you need to figure out the total swept volume of the cylinder. Use this formula Cylinder volume = 0.7853982 x bore2 x stroke = 0.7853982 x 4.03(4.03)x3 =38.179ci displacement per cylinder Multiply this by the number of cylinders to get total displacement if you want. Since everything else is measured in cc's lets convert. Multiply the total in cubic inches by 16.387064 to get cubic centimeters 38.179 x 16.387064 = 625.64cc per cylinder This is the total swept volume. Now lets figure out the total chamber volume. The chamber consists of all the volume above the piston rings once it is at TDC. This includes the piston valve reliefs or dish, chamber volume of the head, the compressed volume of the gasket, and the volume above the rings. Head chamber is listed at 50cc, the piston dish is 7.4cc, the area above the rings is usually about 1cc. You have to figure out the gasket compressed volume. Its the same as figureing out the swept volume but you are doing it to the gasket. Typical felpros are 4.100" bore for a 302 with a compressed height of about 0.041". gasketr volume = 0.7853982 x gasket bore2 x compressed height = 0.7853982 x 4.10(4.10)x0.041 =0.541 ci Lets convert that to cc's like everything else =0.541 x 16.387064 =8.87cc's So the total chamber volume is 8.87+50+1+7.4 = 67.27cc's and the swept volume is 625.64cc's Now we cam figure out the compression ratio. Remember its swept volume divided by chamber volume 625.64/67.27 = 9.300 In a blueprinted engine this combo would give exactly 9.3:1 compress ratio with a 50 cc head.
Did you check the starter gear interference? Bolt the starter up with the solenoid taken off of it so you can manually engage the starter gear into the ring gear and check that it isn't binding up. it may need shimming. Or you may need a starter ***embled by residents of America. (or not? anymore?)
Here's a little goodie from Hop Up mag, March, 1952. pigpen COMPRESSION RATIO Would you like to know the actual compression ratio of your engine? Simply stated, compression ratio is the volume of the cylinder, plus the total volume of the cylinder head combustion chamber, divided by the total volume of the cylinder head combustion chamber. Compression ratio may be changed by milling the head, using thicker (or thinner) head gaskets, by "stroking" the crank shaft or by boring the cylinders to a greater diameter. The new ratio may be found by the following procedure, in which a 51 Ford V-8 engine is used as an example: 1st step...Find the bore and stroke of your engine. Example: Bore, 3.1875": stroke, 3.75". 2nd step...Calculate the volume of a single cylinder (area of the bore times stroke). 1. To find the area of the bore--square the radius and multiply by pi (radius squared is 1/2 of bore times 1/2 of bore). Example: 1/2 of 3.1875 = 1.5937" (radius). 1.5937 times 1.5937 = 2.54" (radius squared). Then multiply the square by pi (3.14), this area gives you the area of the bore. Example: 2.54 X 3.14 = 7.9756 sq. in. (area). 2. Multiply the area of the bore by the stroke, this gives the volume of the cylinder. Example: 7.9756 X 3.75 = 29.908 cu. in. (volume). 3. Convert cu. in. to cu. centimeters to facilitate measurements (1 cu. in. = 16.39 cu. centimeters. Example: 29.908 X 16.39 = 490.19 cc (volume). 3rd step...Measure the volume of the cylinder head combustion chamber. 1. Remove the head from the engine and thoroughly clean the combustion chamber to be measured. 2. Insert spark plug in chamber. 3. Place gasket (used) on head and seal, with light grease, to the chamber to be measured. 4. Be certain that the head is absolutely level. 5. Using SAE-10 oil or hydraulic brake fluid, fill graduated beaker to 50 cc or other even mark. Pour into chamber until exactly level with top of gasket. Record total amount necessary to fill chamber.* Example: 85cc. 4th step...Add the volume of the cylinder to the volume of the cylinder head combustion chamber. Example: 490.19 + 85.0 = 575.19 cc (total volume). 5th step...Divide the total volume (as found by the 4th step) by the volume of the cylinder head combustion chamber. The answer will be the compression ratio. Example: 575.19 divided by 85.0 = 6.76. Therefore, the compression ratio of a 1951 Ford V8 engine is 6.7621.* a. If piston does not go to top of cylinder at top dead center, fill space remaining and add amount to chamber volume. b. If piston p***es top of cylinder at t.d.c., measure amount of "pop-up" multiplied by bore area, convert to cubic centimeters, and subtract from the chamber volume. c. If amount of piston pop up is equal to head gasket thickness (and the engine is ohv), just measure chamber volume without using gasket. Ref. Hop Up, March 1952
The Jalopy Journal
