cylinder head question?

[STONEYH]

Wouldnt a 64cc head be better than a 76cc. I know it depends on what you are looking to get out of the motor or the intended use of the vehicle.I bought a motor that I was able to hear run, I want to change the cam, I ran the head numbers and found out they are 1.72 1.50 valves and 76cc. Shouldnt I change to 64cc?

 

[82355supreme]

All cc size does is change your compression ratio. Going with a 64cc head if it's an iron head, you'll HAVE TO run a .041-.045 thick head gasket or your compression ratio will be over 10.5:1 which is too high for pump gas with an iron head. This is assuming flat top pistons.

Now, the valve size & runner size of your current heads is what sucks worse than the chamber size. Most aftermarket heads are offered in 64 & 72cc chamber designs. 72cc with flat top pistons & a .039 gasket will give you 9.5:1 If you'd go with a 64cc, a .041-.045 gasket will net about 10.2:1 which is still fine with 93 octane gas.

I had the exact same heads on my engine when first built. They make good torque, but that's about it. I quickly ditched them in favor of some DART Iron Eagles. The power difference is HUGE!!! Do yourself a favor & if money allows, just buy different heads. Changing cams & intake will do NOTHING if you don't have the heads to flow the numbers the cam & intake are capable of.

 

[megaladon6]

actually you can run 10.5:1 compression on high octane pump gas. but if you have a stock chevy motor which is usually 8-8.5:1 then changing to a 64cc head brings you to 9-9.6:1 which is a decent to good ratio for street power.
gasket thickness won't make much of a difference to compression. your head gasket thickness choice should be based on quench distance, not compression. you want .032-.035in from the top of the piston to the head. (ideally)
you definately want better heads. the darts look good, if your cam has .480 or smaller lift you can get a set of vortecs (or go higher after some machine work), or look into the patriots.
what do you want out of the engine? usage? what kind of engine is it exactly? what kind of budget?
the heads need to be matched to the cam and the intake for the best efficiency.

 

[megaladon6]

http://kb-silvolite.com/calc.php

 

[STONEYH]

It is a 350 Chevy .040 over. It has a Performer intake and Performer 600cfm carb. I can see that the pistons are not flat top. I want a motor for my 78 Cutlass that has low speed torque. It will be primarily a street car.

 

[megaladon6]

try and find a way to find out how much volume the piston has. you'll need this to figure out your compression figures. you also want to find out what material and height. is it a d-dish, or have valve reliefs? if you can, post some pics. rotate the piston to TDC and measure the distance from the deck to the top of the piston. this way we can figure out your quench. since you already have an intake i wouldn't use the vortecs. if i remember correctly the darts flow very well at low lift numbers (.100-.500) and are pretty cheap. you'll probably want something like a compcams xe262 cam. if you can afford it, get a roller setup.

 

[beeterolds]

with flattops, 76cc chambers, .45 thick gasket and the piston .020 in the hole...you should have around 9.4-1 compression (rough guess here) if you want to add 64cc chamber heads... youd have 10.7ish...somewhere mid to high 10s....

perfect for a 224/230 duration cam!

 

[85 Cutlass Brougham]

Umm...if they are 1.72 intake valve heads, they are probably worthless for any kind of performance use. Look for a set of new or used 165-180cc intake port heads instead. The Vortec heads are good, so are the S/R torquers, Patriot heads, Dart Iron Eagle, Trick Flow, Edelbrock, etc. Just be sure you stay away from any head with a port larger than 180cc's if low to mid range torque is what you want. I run a XE 262 cam with a Performer intake, Q-Jet, S/R torquer heads (2.02 intake, 1.6 exhaust), 9.5:1 compression, etc and it has lots of torque. It also idles well and is somewhat decent on gas. The acceleration feels like a very strong stock V8 and is not at all peaky or high strung. If that is what you are aiming for, try a combination similar to mine. There are things I would do differently if I did it again, but they involve using a factory roller cam block and more money.

Also, what do you mean that they are not flat tops? Are they domed or dished? If dished, you really want a smaller chamber size to make up for the dish. Hopefully they are a "D" dish and not a complete dish as this would allow you to have some quench. If the whole piston crown is dished, the engine will be detonation prone and inefficient as it will be without any quench.

 

[82355supreme]

You're right 85 brougham, they ARE worthless for performance. Like I stated above, they do make great torque, but sign off by 4,000-4,500. What those heads are, are truck heads. The donor for my build was a '76 GMC 3/4 ton pickup, & these are the same heads I had (actually still have them in my shop)

I had new valve guides & seals put in them just to get me going for budget purposes, but once I had enough coin saved up I ditched those pieces of $hit & got the Iron Eagles.

He says they are dished, I would bet they are "D" shaped, that's what my '76 had in it for stock slugs. So between that & 76cc chambers, it was maybe a lousy 7.5:1-8:1???

 

[bigdan]

those 76cc heads are good as a boat anchor....
any 64cc heads that have 1.94intake valves will give you cheap power gain... ( around 9.5cr + 30 hp)

and maybe you have what we call flat top pistons but with valve recess
flat top means no ridge on the outer portion of the top of the piston ( a dish)

dont think that a flat top piston is necessarely flat all over ....