head gasket thickness and compression ratio's

[tscott9330]

I just got my new power max cam from crane, and I have been crunching some numbers. I have been figuring up my dynamic compression ratio.

I have 2 questions for all you seasoned veterins out there

1. What kind of dynamic compression ration should I run for best performance with a good set of aluminum heads,(I have heard I should try and stay between 8.5:1-9:1)?

2. If the correct answer is indeed 8.5:1 or so, then in order to accomplish this number with the 64cc heads i plan to get, I will have to run a pretty thin head gasket. I know its not good to run an extra thick gasket, but is it bad to run a thin gasket?


Tom

 

[biltogo]

I'll answer part of your question, but I also have one.

Normally with aluminum heads you go with a 9.5 to 10 easy because of the heat disapation. A thin gasket is ok. You want to have a quench dimension of about 040". Your static compression with 64 cc heads should be higher than 8.5?

But the cam is what determines the dynamic compression ratio because of where in the stroke the valves open and close relative to the piston position? Soooooooo...that being the case, the dynamic can only be checked with a compression test, or a close call can be made by someone who has a similar setup. Or am I using the wrong terminology?

 

[MaineShark]

I think you're doing this a bit backwards, Tom. You should choose your heads, pistons, gasket thickness, and quench distance, first. Then get a cam to work with it. There is a lot more variation available in cams than in the others.

If you know your stroke, bore, rod length, chamber size (64cc, you say), and what piston brand you want, then I can help you figure out what shape and size piston will put you are the compression ratio that you want. Then, you should choose a cam to give you the right dynamic compression and power curve.

Joe

 

[tscott9330]

I realize I am doing this a little backwards but unfortunatly this is my first engine build and I'm learning as I go.

heres what I have.
3.75 stroke crank
5.7 connecting rods
flat top forged pistons with valve reliefs ( not sure how big the reliefs are).
bore=4.030
power max hydraulic roller cam:
cam timing @ .004
OPENS CLOSES ADV. DURATION
intake 33 BTDC 75 ABDC 288
exhaust 81 BBDC 35 ATDC 296

Cam timing @ .050
OPENS CLOSES DURATION MAX LIFT
intake 4 BTDC 38 ABDC 222 107
exhaust 52 BBDC 2 BTDC 230 117

Well thats what I have.

I need a set of cylinder heads and I was using the DYNAMIC COMPRESSION RATIO CALCULATOR I downloaded, to calculate what size combustion chambers I need in my heads.

So here is my dilema, I have done these calculations and if i use a really thin (.019) steal gasket then it puts me at 8.5822063 DCR butthat only gives me a quench of .19. Is this a problem?

If I run a thicker gasket (.039) it gives me a DCR of 8.14929.

This is all based on 0 deck height and a 64cc head

So I guess i messed up a bit but I think I can still make it all work.

So I guess my question is, With what I got how can I make this work?

 

[MaineShark]

First thing, figure out what size the valve reliefs are. Either look it up with the manufacturer, or measure it with a cc-ing kit. This will have a huge impact on both the static and dynamic compression ratios, and any calculations done without taking the reliefs properly into account won't mean anything in the finished engine.

Second thing, quench should really be in the .040-.050 range, on a street motor (particularly if this is your first). If the quench is too low, then when things stretch and flex at high rpms, the pistons can contact the head. I don't think I need to tell you what happens then. I would err a bit on the side of caution.

Third thing, is check out what the crushed thickness of your composition head gasket is. It might start at 0.039, but it will probably crush to 0.036 or something similar.

Deck height does not have to be zero, and probably shouldn't, with a 0.036 gasket. 0.005 under would work better. That would give you a 0.041 quench.

And, again, you may end up having to buy a new cam, and sell the one you have, if you can't get an acceptable DCR with it. Cams are a lot cheaper than heads.

Joe

 

[tscott9330]

As of yet i have not bought a set of heads, so could i just go with a smaller cc heads. Maybe a set of ported corvett heads.

I really dont think i can afford to buy another cam, my budget is stretched thin right now as is. and it was a custom grind small base circle cam and i dont think i could get as much for it as i paid.


So the bottom line is, If it can be done without buying a new cam i would appreciate any suggestions you may have.

I know i'm being difficult but I am only a poor college student and i dont havce much cash to throw at this.

Thanks, Tom

 

[MaineShark]

Well, heads are only available in certain sizes. And low-cost heads are available in even fewer sizes. So, it might end up being cheaper, in the long run, to get a new cam.

But, in any case, the valve reliefs are extremely important. A couple cc's different will totally change the CR. Do you have the manufacturer and model number for the pistons? Or even just the manufacturer, and look through their catalog to find something that matches your stroke and bore, and looks the same as your pistons?

Joe

 

[tscott9330]

alright here are the numbers i'm using for my claculation, Tell me what you think.

Bore diameter: 4.030
Stroke (dynamic): 2.65566942162317
Combustion chamber cc: 64
head Gasket thickness: .036
head gasket bore: 4.15
piston to deck clearence: .005
valve relief in cc: 6

which yields
quench: 041
DCR: 8.02443238157326

What do you think? Is that alright.

p.s. My machinest has the block right now so i'm not real sure what the piston to deck clearence is gonna be. Is .005 pretty Standard for a performance street engine?

Sorry I seem so clueless

Tom

 

[MaineShark]

You should call your machinist and ask about the deck height. There isn't a standard height - it should be matched to your other parts. So, I'd give him a call ASAP and discuss it with him.

I ran your numbers, and I think you're really going to need to consider a cam change. With 64cc heads, you're close to 11:1 static compression, and only seeing 8:1 dynamic. I think you're going to have serious problems with running all your vacuum accessories, with that sort of bleed-back.

But, again, you need to find out what the actual size of the valve reliefs is, since this can play a large role in the ratio.

Joe

 

[tscott9330]

alright I just calle dthe shop.

heres what I got,

The block has been 0 decked
and the piston relief is 6cc exactly


I am in sort of a dilema right now. My mechanic is telling one thing and you all are tellying me another and i'm not sure who is right.

he says to run a 64 cc iron head on pump gas. I thought that was risky.

he says 0 deck height is what I need, to get to the compression he wants.

he says hes built these before and had no problems.

Are you mybe just being cautious so as to not give bad advice or are these sort of things open to debate?

I am very confused right now, I cant believe my friend would give me bad advice but on the other hand I trust the opinions here.

Tom

 

[MaineShark]

I don't think he's giving you bad advice. I just think he's giving you advice that doesn't necessarily match your needs. That cam is going to have very low vacuum (the difference between static and dynamic CR's should give you and idea of how much of the intake charge is being blown back into the manifold), and C3's require lots of vacuum to operate everything.

I think you're going to run into troubles with low vacuum.

And, like you said, he's planning this for iron heads, but it seems that you want to run aluminum.

Joe

 

[Jack]

Tom:
I think we chatted about these issues not so long ago. Without running the numbers, I'd estimate that thing's gonna make about 11:1 - 11.5:1 SCR w / zero deck & iron 64cc heads & -6cc flat tops. Maybe that cam'll kill off a bunch off CR? ... and leave you with no vacuum. I agree w/ Joe's last post ... and ... I wouldn't want that combo for my purposes ... there's only a few places around here to get 110 race gas & it's $5/gal.
JACK:gap

 

[tscott9330]

well guys, you have given me alotto think about and i think i agree with you. I just bought a house over on my side of town and now have a place to keep my shark. So I think i'm gonna Get theshort block from him and do the rest myself.

I am definetly going with aluminum heads.
I think i'm gonna try this cam fo now just because if i sell is now or 6 months from now, its gonna sell for about the same price.

I do have 1 more question though.

I'm planning on converting the lights to electric and I was giving serious consideration to converting to power brakes (hydratech).

What else besides the cruise control, which i think i'll get rid of anyway, what else uses vacuum? I think the ac does but that's shot anyways, so i need to trade that out for a vintage air system eventually.

Tom

 

[MaineShark]

The only problem with that shortblock, is you're going to need large-chamber heads to keep the compression workable. 72cc heads will put you at approximately 10:1

Heads with larger chambers tend to be more expensive, from what I've seen. So, what you save by using this setup may end up getting dumped right into the heads.

I honestly don't think this engine will produce enough vacuum to work even the critical systems like the power brakes.

True, you can remove the vacuum systems (I plan on doing this), but that needs to be done before putting that engine in, not after. So it opens up a whole other can of worms, if you figure that you're probably looking at a grand or so, by the time you get the brakes and headlights done.

Joe

 

[Jack]

So I think i'm gonna Get theshort block from him and do the rest myself. I am definetly going with aluminum heads.
I think i'm gonna try this cam fo now Tom

Suggest having builder assemble short block with cam ... w/guarantee of adequate cam-clearance w/your rods. Typical street performance/HEI ignition requires decent vacuum ... so do the doors & vents in dash for HVAC.

Speedway is now offering free cap w/ $170 Aluminum "UNIVERSAL FIT" rad ... the 31 x 19 that I'd suggested is P/N 9173471 ... it'll cool anything you're likely to come up with. http://www.speedwaymotors.com/xq/as....60/cont.1/intPgNo.4/redirect./qx/product.htm
JACK:gap