Thread: Making Combustion Chamber Bigger

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I need to pick up about 10cc at each chamber,is this possible? This is a 1984 SBC production head.

Originally Posted by FAYLUR

I need to pick up about 10cc at each chamber,is this possible? This is a 1984 SBC production head.

You can, but you might hit water.

How about getting some 10 cc dished pistons, they effectively give you more combustion chamber volume

Denny,I can't change head gaskets to a thicker set and maintain decent quench.
Southerner,I already have bought domed.
I want to keep these heads if at all possible but the small chambers are making a 10.2:1 dynamic compression ratio and cyl.pressure will be to high. My mind is made up on the motor parts combination such as pistons,cam,etc,and I would like to use these heads if I can get about 10cc more out of the chambers. I havn't bought heads for this build yet and would prefer not to if possible.

Hmmmm, well, there are other ways. How about getting some valves with receased valve heads ? they concave into the center face of the valve,. They are also lighter as well. You could also try flycutting out around the valve are of the combustion chamber to get more flow as well as physically making the chamber area bigger.
Those SBC heads are probably 76cc open chamber. What kind of cam are you running again ? And what is the cc measure of the dome ?

Southerner they are around 59. Those valve shapes sound good I'll check into it. Facing around the sides of the valve area is where I was hopeing,because these heads will take a .020" bigger than original intake valve and unshrouding without breaking thru but I have no idea how much would have to be removed with unshrouding a .020" bigger valve.
Denny thanks for those figures they give me a better understanding of what material has to be removed seeing that I havn't a clue of how much 10cc would be.

Have you got the casting number for these heads ? 59 cc sounds awfully small. have these heads been planed ? One way to check is to see how thick the head deck thickness is.

Oh !, and we are talking a 350 SBchevy here are we ????

Posted by Denny:
"Heres a 10cc domed piston."
Did you mean to say reversed-dome?
Take another look at your photo.

Faylur, first off, don't assume they are 59 cc's until you've poured them yourself. You can find a burrette and sealing plate at summit for about $100.

If you are going to enlarge the chambers, bolt the block to the engine stand. Clean the heads of oil and coat them with Prussian Blue (machinist's dye). Bolt the heads to the block with a couple of bolts and without gaskets. Turn the block upside down and use a sharp-point scribe to run around the top of the bore and show where the inner diameter of the cylinder will be on the heads. Remove the heads and you will get a good idea of how much unshrouding you can do. Personally, I'd prefer to cut the bowl around the valves with a cutter indexed in the valve guide bores than to sink the valve seats. Take one head to the machine shop and have the machinist cut just one chamber with a round-tipped cutter so you have a nice curve from the seat to the edge of the chamber right up to the line you scribed. Then pour that chamber. If it comes out on the money, have him do the other 7. If not, plan B.

If you can, use a valve that will leave 0.200" between the edge of the valve and the bore of the cylinder. Done this way, the head will probably flow more than if you put larger valves in and shrouded them all over again.

Thanks guys. Denny I thought you blurred that pic on purpose and wrote in very small letters,,,,don't tell me my eyes made it look like a 13:1 dome
yes I knew,,,I said I was 'some' dumm,not 'plum' dumm
but I wasn't smart enough to know if anywhere near 10cc was possible.

Thanks again everyone,the replys have been very helpful. One more question while on this subject if you will please.
Is there a certain cranking pressure that you guys try to stay close to? Say 190 psi or lower? And what would be to high of cranking pressure using race gas of 112 or so octane,240 psi or higher? I know some race engine builders seem to pay more attention to cranking pressure than a calculated dynamic compression ratio,but with a naturally aspirated engine on race fuel what would be considered safe cranking pressure?

Air is a pretty elastic substance, it is sensitive to heat and pressure. The reason most racers go of a pressure reading is they are checking the local conditions. Like the altertude of the track they are racing at (air pressure). And also the local air temperature at the time, this affects the density of the air, also you can figure into this the amount of humidity (moisture) in the air. They all combined have an effect on engine performance through it's compression ratio (air pressure) and the amount of oxygen in the air by it's density or mass for the people that want to split hairs. So you may have to change your jets to suit the local conditions.

For these reasons (and others) that is why automotive producers have such lazy profiles on thier cams to suit a vast array of local conditions and the habits of ALL the driving public over a very large continent from sea level New York to mile high Denver. For example suppouse mr car driver lives in New York and decides he has to drive all the way to Denver...(it's a long drive I know but this is hypothetical..) He expects his car to run well all the way there and back.

I probably should leave this for the professional engine builders, but I'll put in my 2 cents worth so you'll have a starting point. Crane Cams site says 160 is about the limit on a street motor, pump gas. But I've built motors with a little more c.p. than that and many others have as well. I remember someone on here who was saying he built them with somewhere around 220, then he also said that everything had better be perfect. No egr, no pcv, tight squish, polished chambers, perfect afr, optimum timing, etc., etc.

180 to 190 ought to be acceptable with all the above, keeping squish at 0.030" to 0.035" with a tight piston to wall clearance to prevent piston rock on the pin. If doing a c.p. in that range, I'd make sure all sharp edges in the chamber and top of the bore had a little chamfer on them as well as all the polishing. I'd even take a little 320 paper and work on the spark plug electrodes.

There has been a lot of hoopla lately about Singh grooves. I haven't heard anybody speak negatives about it, even though everybody has his or her own opinions about the number of grooves and placement of them. I have my own ideas of how they should be cut, but will have to wait for dyno results to see how it all shakes out. I don't build enough motors myself to do the research. Some guys point them at the spark plug, others at the far corners of the chamber to pick up some turbulence. Do some research and determine if you'd like to copy someone else's patterns or determine them yourself, or if you even want to mess with it at all.

Posted by Denny:
"That reply was for my friend Tech. He has a habit of doing little things on occasion like that, and I still don't know why exactly."

Denny, I don't know exactly what you're sayin' here. If you mean I have a habit of correcting things that aren't exactly right, then yeah, I'm guilty as charged. It's just my nature. On the flip side, I expect others to correct things that I say or do that aren't just quite right. I wasn't trying to put you down or anything. It's just that maybe some newbie will see that photo and think that really is a dome piston and that just doesn't seem right to me, not when they come on here to learn.

Thanks guys. So there are many variables,both externally and internally that come into play here when talking dynamic compression ratio and combustion pressures. Sounds like an area that just one spec out of a dozen or so could be out and cost an engine.
This is a tough one to find alot of info. on (I'm trying to learn) because of evidently, those many variables. As one motor with 9:1 SCR and 200+ cranking pressure may require a certain high octane, another motor built the exact way except with a different cam,may have 9:1 SCR and 180- cranking pressure because of it's intake valve closing point being higher than the first motors,and run fine on a lower octane,yet be faster?
I was hopeing for a 'rule of thumb' figure that would link Dynamic compression ratio with a certain or 'required' octane but it looks like an "inexact" science.
Mr. Ericson and Mr. McCarthy have quite a bit of experience in this I would suppose, would you guys,or anyone else chime in?