Ok.. i've been playing around with compression calculators alot lately, cuz i'm worried about it.. lol.

So, i've tried several different calculators.. and for some reason, entering the same information, they come up with different compressions.

I like the one i downloaded, one of you smart guys on here made it. thx btw, i really do like it. But.. it differs from the others, and the others differ from the others out there...

& yes, i'm putting the same information in it.. just some ask for more than others do, that's one reason why i like the one i got from here, it asks for alot. so i'm guessing it's more accurate?..

i d/l that desktop dyno stuff, but it won't let me run a test on the engine stuff, to see how much power i'd get.. so i'm stuck in the mud w/ that.

SO.. ya.. :-/

What compression calculator would you say is the most accurate?

Thanks.

 

None of them are really all that good; because every piston is different, and there has to be some sort of fudge factor built into them to account for missing material in various places that doesn't appear in specs such as around the edges of the piston above the ring lands, a chamfer in the block if there is one, the non-circular bore shape that head gaskets often have, the spark plug you use (there's the better part of a full cc difference between the coldest possible extended-nose tip and the hottest possible short tip), how far into the heads the valves are sunk from seat wear and valve jobs, whether the valves you use are truck valves with a ¼" thick head or thin light performance valves with tuliped faces; etc. etc. etc., all of which will introduce unpredictable errors into your calculations anyway. So beyond a certain point, it doesn't matter what the calculators say, they're only approximations at best.

The actual act of writing a compression calculator is trivial. It's a mathematical equation that has no choice but to spit out a certain number if you plug in certain other numbers. It's all those other hidden assumptions that make them appear to give different answers... assuming of course that they were all correctly written, which as anyone who uses it regularly knows, is a problem on the Internet in a general way ("It must be true, I saw it on the Internet!!! After all, you can't post lies on it, can you?" yeah right)

 

good point, so what's a guy to do, just throw stuff together & hope that the estimates of the calculator is correct... bleh.

or, take it to machine shop & check everything over & over... finding out you have too high of compression, wasting your money, & having to buy other parts to get it lower.

not cool.

what would you say is best then? just hope the calculations are right & check when you are putting the engine together?

 

Unfortunately, not a whole lot you can do, except take the numbers with a grain of salt....

Avoid attempting to build a motor with such a tight tolerance on the CR that the little cumulative uncertainty errors like that really matter, until you have enough experience with different combinations to be able to estimate the results.

Fortunately, nearly all of the errors are in the direction of making the calculator estimate high compared to reality; which is safer than the other direction of error.

If all you're doing is plugging CR numbers into Desktop Dyno or something, as opposed to actually building a physical motor, then it's mostly an exercise in futility no matter what the 3rd significant figure of the CR is. There's ALOT more opportunities for oversight, overestimation, and padding in there, than in a CR calculator.

 

i wanna build a motor, but i dont wanna start buying stuff until i know what imma need in it.. ya know?

plus i need the money, working for it now.

so you said.. the calculators typically do it on the high side?...

lets say i'm getting about a 10.2:1 10.8:1 compression on calculator... would that be streetable.... or should i really try to get it down to 9.

 

There's a HUGE difference between 10.2 and 10.8. Different calculators shouldn't be giving that kind of difference. If they are, then at least one of the 2 is defective, or at least one isn't asking for some critical piece of info without which the figure it gives is worthless anyway.

What's the combo you're working up? What pistons, heads (or head cc's), head gasket, deck height, motor CID?

 

10.2 to 10.8 is pretty high for pump gas even if your running aluminum heads. That doesnt mean it wont work. One of the factors thats gonna change your dynamic(actual) compression is your cam. If it was me i wouldnt risk going that high no matter what the cam was thou.

P.S.- The 3 or 4 times ive used desktop dyno its been just about dead on. Its just a garbage in and garbage out program. If you dont know the flow numbers on your heads and the cam numbers you may be close and you maybe be across the bay.

 

no.. that was one calculator.. just messed w/ piston size i think...

but.. no.. with diff calculators.. i went from 12.77 to 10.8... same exact stuff... i was like.. wtfs...

what i'm looking at is..

400 sbc
bored .30 over to make it 4.155 bore
4.155(or i could do 4.190[if it's possible for engine]) copper head gaskets and .043 or .062, i think i ran #'s w/ .043 and got 10.2.. mighta been w/ the 4.190 tho, both in 10 compressions
3.750 crank
5.7 rods
dished 4 relief pistons
64 cc aluminum heads
what else.... um...
deck height i duno if it'd be .025 or .020... cuz.. i dun have the block yet, i've done both #'s tho. both in 10 compression area
to deck clearance i think was .020 or something, i'd have to look at my info(at home, & not there now).

 

the reason i say 64 cc heads, is cuz that's what i can get cheaper.... and i'm wanting to save as much money here & there as i can...

or should i just not worry about that.. and go for safer.. and get some 74 cc heads and then have compression easily down. (dun wanna do that)

 

What size dish do the pistons have?

 

-4cc i think it was. could be wrong.

 

According to my calculator, with 64cc heads, 4cc valve reliefs in flat tops, pistons .020" in the hole, and a .039" head gasket (1014), I get 11.3:1. Same motor except 74cc heads, 10.15:1.

Like I said, the actual true compression will come out a bit lower than that, in reality, when the motor is actually built.

If I was building that motor, I would use flat-tops and 74cc heads if they're aluminum with the block zero-decked to them for a calculated CR of 10.6:1; or 64cc heads and 22cc dish pistons again with the block zero-decked to them if the heads are iron, for a calculated CR of about 9.8:1. Same head gasket in all of them.

 

4 cc sounds kinda like flat tops with valve relieves. With those numbers i got a low 11:1 depending on how far down the piston will eventuallly end up in the hole.

 

cool, so then, you think then if i did basically same thing, but with 74cc's... and.. flat tops, running 10.6:1 compression would be good? i mean.. you think that it can handle pump gas then?

see.. i'm wanting to get the most power, from the best compression, w/o killing anything, ya know? i'm not planning on blower. so, that's why i'm thinking of 9-10 compressions.

*thinks, why can't there be leaded gas at pump still, so i could run 12 compression like my father did 40 years ago*

 

Very important....

Maybe I should have used the mathemetical & formal logical "iff", which is short for "if and only if"

 

ya, aluminum. what about 72cc heads? that's all i can find that's under a grand, rofl... and then just do really big dished pistons...?

 

Get pistons with 4-eyebrow valve reliefs instead of 2-relief ones.

 

RB,

All I can ask is; What calculator are you using?

I used three different calculators and the highest I got was 9.995:1

Here's one... http://members.uia.net/pkelley2/crc.htm

Here's another... http://kb-silvolite.com/calc.php?action=comp

and the other is one I downloaded from the first website.

Ironically, those two came out the same..... 9.992:1

That was using the specs you gave, with 4.166" as the gasket diameter. I don't know the specs on that gasket, but I'm sure it's at least 4.100" diameter.

Even then I get 10.023:1

With the version I have of DD2000, you can't use that compression calculator unless you actually measure the volume above the cylinders. You have to have the piston at least .100" in the hole then measure the volume in cc's with a burea (spelling).

There is a way to cheat that program. If you have a piston with 4cc's because of the valve reliefs, you simply use the "Flat top pistons with no Valve Reliefs", then add the 4 cc's to the combustion chamber. So instead of entering 64 cc's for the combustion chamber, you'd enter 68 cc's.

When doing that, I got 9.99:1

All those calculators were pretty consistant. Without sounding rude, I'd have to call "user error" on this one. You need to know how to figure these things out in order to use them. None of them are idiot proof.

 

lol.. your the only one that didn't get #'s in the 10s', rofl... me & RB83L69 got near the same #'s... and.. then...dankhound got alittle higher.

so.. we have 4 ppl here.. two have near same compression.. and then two others, one high, one low.. i'll just take it in the middle & call it that. lol.

 

In AJ's defence, I think he was using 350 specs to get those CR numbers.

 

Damn. I sure was. Assumptions are bad (but in your case it worked).

I get 11.13:1 , 11.13:1, and 11.13:1

Sorry. I'll go crawl back in my hole now.

 

Top ring depth, deck height, ring end gap, gasket bore diameter, gasket compressed thickness, and actual piston dish/dome volume are factors often overlooked in common calculators. I wrote my own spreadsheet for the purpose.

Incidentally, I run 10.8:1 on iron heads and pump gas, no problems. Actualy dynamic compression is a factor of both static compression ratio and induction efficiency, which is greatly affected by the cam profile.

 

lol, gj w/ the 350 stuff, lol. np tho, lols. thx for looking into it,

& 10.8:1 eh? hmm.. interesting..

well... i decided to find some -6 cc pistons, and 72 cc heads... i got a 10.4 compression.. w/ those .062 4.155 gaskets, and everything else the same. even if it's somewhere in the 10's.. i'm happy...

thanks guys for all your help, you'r letting me rest easier at night, & have helped me figure out which parts better suits the engine i want to build.

thanks.