Compression

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75dartsport

75dartsport

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Could someone help figure my compression ratio. I need to order a cam and would like to know what compression is gonna be. I haven't had any luck on a compression calculator. Here are the specs. 340 .040 bore. KB243 pistons.
The heads are a 68 cc closed chamber so im going to run a .050 head gasket to clear the pistons which come out of the block .018. That should give me .032 quench. Here is a link to the piston specs.

http://www.ebay.com/itm/Keith-Black...Parts_Accessories&hash=item5aec56d595&vxp=mtr
 
Kieth Black's piston catalog says that piston is 9.6 compression with 68cc heads. Being 040 it might be slightly more, but head gasket thickness is not specified. So if you roughly figure 9.6 it is at least in the ballpark.
 
I have heard people say that when running aluminum heads you should step up the compression.Is this too low for aluminum heads?
 
It was a 0.050" gasket but I guess at its diameter at 4.125 but I also guess at the valve pockets on the pistons at 5cc but I see there 6cc so your CR is 9.75:1
 
Is that too low for aluminum heads. the heads im looking at flow

282/200 at .500
299/211 at .600


SO its gonna take a pretty good cam to work those heads.
 
I haven't chose one yet. That's why im looking for some numbers. Do you have any suggestions? Car is 3200lbs 4.56 gear. 727 probably 4500 stall. I would like solid or solid roller.
 
75dartsport said:
I haven't chose one yet. That's why im looking for some numbers. Do you have any suggestions? Car is 3200lbs 4.56 gear. 727 probably 4500 stall. I would like solid or solid roller.
Click to expand...

Two quick questions and i'll run some random numbers to see how it looks...

Do you want to run pump fuel?

Are the heads limited to a certain lift? Meaning were the guide done so you can get clearance to .575/.600 or so?
 
Not worried about the race fuel. I run it now anyways. I usually mix 92 and 110 50/50. I don't believe they have been clearance. SO we will go with no to that answer.
 
To get a dynamic compression of 8.3:1 intake valve has close at 40 ABDC at 0.050" lift
 
With that compression and aluminum heads do you think ill need race fuel?
 
From I understand a dynamic CR above 8.5:1
 
I see they were flowed to .600" but that doesn't always mean the clearance is there for the springs via cut down guides or longer valves. You can obviously use some substantial lift with those flow numbers, but the intake timing will be crucial at high 9ish to 1 to keep some cylinder pressure. The easiest way to get big lift and conservative cam timing is with a roller, or 1.6 rockers, but let me run some numbers and i'll get back and give my opinion......which may only be worth what you paid for it..lol :D.
 
If you are running higher octane fuel a bump up in compression wouldn't hurt. Is this a street car or strip car? Aluminum heads can tolerate more compression, 1 point more is usually no problem but if you already have the pistons, it gets more complicated.
 
Already have the pistons. SHortblock is pretty much done.
 
Should have asked earlier and you may have disclosed this in another thread, but what alum. heads do you have with a 68cc chamber? Are they 340 Eddy's with some chamber work?
 
I would mill the heads to reduce chamber CC's to around 63.

I believe somewhere in KB site it calls for a .040 thick head gasket for the compression calculator, so ..050 is going to cut the compression, a big cam and aluminum heads are going to want more then 9.5 compression, IMO
 
75dartsport said:
Let me know what you come up with.
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I'm still with you. This is some of the hardest research if done in a while. Good stall, good gear, great head flow and moderate compression. I do believe you want some limited street use also, and that add's a little to the difficulty factor. Mostly i hate leaving too much of that headflow on the table. I'm looking at off shelf stuff as if i'd use it myself, just so many options...........
 
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