Figure my compression

-
63dartman

63dartman

Well-Known Member
Joined
Dec 11, 2005
Messages
2,230
Reaction score
24
Location
rio rancho, nm
I figured my compression with a calculator and need a sanity check. Can someone double check my math?

360 engine
4.06 bore
4.108 head gasket .042 thick
49cc heads
stock stroke
pistons at zero deck with -5cc valve relief.

I'm coming up with 12.5-13.:1
 
LOL cheif. Long story but My machinist milled them just like I asked and did not check them prior to milling soooo I now have 49cc cast iron 360 heads. My guess is they were milled once before. I have been running them for almost a year now with no issues.

oh, almost forgot to thank you for taking the time to do the sanity check for me. I think I can live with the 12:1 better than 13:1
 
Not to throw anyone off but I come up with 12.86:1, so the consenses is that your definitely going to be over 12:1. But then these things are all different as to how they figure compression ratios. So go with one that you feel comfortable with and be satisified.
 
There shouldn't be any variation in the math. The only difference I have seen with the various calculators are some don't take into account the ID of the head gasket and others do take into account the space between the piston and the top ring.

The one I mentioned above takes into account the head gasket ID but not the area above the top ring.
 
13.03 witht he calculator I use. Some places want (-) numbers for piston releifs too. it doesnt take much to mess up a website calculator...lol.

49cc 360 heads? holy crap what a milling event that was...lol.
 
I ran it on Desktop dyno and it came up with 13.03 also.

That sure is a ton to mill off to get them down to 49 cc's. Mine are milled .020 and are 66 cc's. Hope your intake sides were milled too or the ports will not line up very well.
 
49 is low they must of had like .060 to .085 or so to gt that low!

I have took .035 from stock and went from an average of 68cc per chamber to now right at about 60cc

man that is high for the street. I think you will need a cam with a fairly large overlap to reduce low speed compression for pump gas?

What is the altitude where you live? I am at 5200 base to 6000 feet and my static is like 10.8:1.
 
LOL everyone. Thanks for all the help. DWJ I am same as you right at 5500-6000 ft. Do you guys think I will need to run a mix of fuels? I can only get 91 octane but there is a station that sells trick fuel at 109-110 octane range. Yes the intake was milled to fit perfectly.
 
You can safely say about 1/2- 3/4 point more in static compression for mile high use, but man that is ont he high side!

You need a good head gasket, at least a cometic gasket and you are on the edge for o-ring type sealing.

What are you using for cam and gears? Is this an auto or man trans car?
 
I am currently running 3.23 gears auto trans with stock stall, and a voodoo cam. I am looking at a cam change and new performance tranny this spring.

Advertised Duration : 262°/268°
Duration @ .050 IN/EX : 220°/226°
Gross Valve lift IN/EX .475''/.494''
LSA and ICL 112/108
RPM Range: 1400-5800
 
63dartman said:
I am currently running 3.23 gears auto trans with stock stall, and a voodoo cam. I am looking at a cam change and new performance tranny this spring.

Advertised Duration : 262°/268°
Duration @ .050 IN/EX : 220°/226°
Gross Valve lift IN/EX .475''/.494''
LSA and ICL 112/108
RPM Range: 1400-5800
Click to expand...

I think you will need at least a 3.91 screw. That will help with the high compression. I also think that you may need a cam that is more in the:
Duration @ .050 IN/EX : 236°/242° ranges to reduce the low end compression. Also the gears will help losen the load on that much compression.

Ansother thought, maybe you could have someone good to machine lightly and polish the combustion chambers on the heads to get you at around the 53cc-55 cc range?

my 2 cents for a street car on todays gas. I know in mist mountain states 91 octane is the best one can find. I think you would need at least about 96-98 octane. if not you are going to have to back the spark off a buch and the low end is going to feel mushy off ide.

Also what about exhaust and inteake what are you running and what is the cars total weight? About 3000 I would guess?
 
I am running spitfire headers into 2 1/2" out the back. Eddie rpm intake, eddie 750. I never thought about polishing and cleaning up the heads to increase the volume. I sure wish I could turn back time and make the heads about 60cc. Who knows, I may spring for some eddie heads this spring if things don't work out. Especially if I have to run special fuel just to get down the road.
 
13:1 on 91? Good luck...lol. You need a MUCH larger cam, even at 6K feet. I'mnot a pro with altitude correction although it is in my future..lol. but it seems like onaverage 1/3pt for every 2K feet is in the ball park. So if you are at six, and never go down lower, 13:1 might be ok with a decent cam. But 240° @ .050 I dont think will be enough. Not with iron heads too. I'd be looking in the 260°[email protected] stuff, and keep a close eye on the plugs' ceramic for signs of detonation. Have a caht with a few saavy custom cam grinders. The one you may need probably is not a catalog item. Engle has some nice lobes, and a full custom cam isnt very pricey.
 
Sounds like you should look for another set of heads. A set of magnum heads will run you about $100 and will have around 64cc chambers. I am running 10.6:1 with magnum heads and with the closed chambers and zero deck pistons I have a very detonation resistant engine. I run 89 octane with no issues.
 
Well, I got her fired up today and took her for a little spin to seat the rings and such. Looks like I need to replace a rear main that I thought was the pan gasket, not leaking real bad but does make it a little messy over time. Other than that she seems to run pretty good. She went a little sidways on me at 25mph when I got on her a little. I think she will be alot of fun once I get everything ironed out. It seems really wierd to me that I had no signs of pinging on the test run. I haven't checked my timing but I think I'm somewhere in the 6 or 7degrees btd. I also want to upgrade the distributor and box, maybe a cam change too. Redo the exhaust and mufflers, new tranny this spring and converter.
 
You must log in or register to reply here.
-

Back
Top