Don't know if you don't ask, right?

I have a SBC 350, .030 over. I now know what cam I am going with and my heads are 76cc. So this leads me to the compression.........

I know I should be able to run .125 pistons, but can I get away with .275?

Rockers are 1.5
The deck height is stock ( No I do not want to get the block decked)

Comp XE 274
Cam lift @.050
-.501 intake
-.510 exhaust

I have only used flat tops in the past and know to putty check it. But before I order pistons I would like some opinions of some that may have built something similar.

Heads with a low plug location need smaller dome to stay most effective.

How much compression are you trying to make with a .275 dome piston. Your looking at 12.0 or so.

No I was looking at Speed pros' and getting 10.1:1

Since these heads have a low plug location, is it safe to assume that .125 would be alot better than the .275's ?

I would really like 10.1:1 , I think the .125 would knock it down around 9.2 or so according to them.

Ok, so I was reading a book from David Vizard last night and it pretty much said the same thing. .125 dome would be acceptable and do not go over .150
That means .275 is out of the question.

So do I stick with .125 pistons or change my rods to 6.0, would changing the rod help and is there that much of a difference with going from 9.2 - 10.1 compression?
I want to get the recommended compression for the cam.

The cam I want is the XE274.

Should I drop down to the next smaller cam?

You can make the cam you want work just fine with any of the compression you listed. I would do some actual calculations on compression ratio's because you don't know what the manufacturer has assumed and not told you with advertised CC's. Some include a 9" deck and others assume a 9.025 deck. Some assume a gasket others assume a shim etc...

What heads are you using?

6.0 rods won't change compression. Yes, the piston moves up further by .300 but it doesn't move down as far either so it becomes a wash.( I hate that word) Stroke is dictated by the crank and only the crank.

I am using 882 heads that I had rebuilt and ported some years back before I knew about head cc. The speed pro pistons were listed with a 9.025 deck.

Actually if it was just as you stated the 6" rod would greatly increase compression because the compresed volume would be greatly decreased. However if you also MOVE THE PIN LOCATION down by .300" then you'll be looking at the same static compression. The dynamic compression would still be affected though.

Your not listening. This will the last time I say this. I chose not to argue with you.

Crankshaft stroke moves the piston.

I don't care if the rod is 5.7 inches or 6 miles long.

When I built my 350 I used some 882 heads also. When talking to my machinist, I too was considering the .125 domed pistons. I also had my block decked to 9.000 give or take .002. Their figuring was the .125 would push me into the premium gas range and even then pinging could be a big problem in the summers here in Phoenix. I elected to use the Speed Pro coated hypers with a flat top. I did a compression test 2 weeks ago and all were running very close to 165 pounds cranking compression. Most charts show that piston (flat top) yeilding a static CR of 9.4 to 1. Not sure what deck height they used, but for me, it worked out great. I can still run 87 octane in the summer and don't have any pinging or overheating problems.

Mark

[QUOTE=Jmark]When I built my 350 I used some 882 heads also. When talking to my machinist, I too was considering the .125 domed pistons. I also had my block decked to 9.000 give or take .002. Their figuring was the .125 would push me into the premium gas range and even then pinging could be a big problem in the summers here in Phoenix. I elected to use the Speed Pro coated hypers with a flat top. I did a compression test 2 weeks ago and all were running very close to 165 pounds cranking compression. Most charts show that piston (flat top) yeilding a static CR of 9.4 to 1. Not sure what deck height they used, but for me, it worked out great. I can still run 87 octane in the summer and don't have any pinging or overheating problems.

Perfect. I'd be interested in knowing the piston to head clearance you used and also the intake closing point of the cam.

To Techinspector1...

I don't know the piston/head clearance, the machine shop didn't see a need to putty check it with the cam I used, the Crane 00052. Here is the link to the specs

Ok now you guys got me a little confused.

Can I run this combo with the .125 and be alright with these heads and keep the stock deck height?

Or do I really need to spend more $ for different heads or to get the block decked?

I'm listening to all this advice because I don't want to waste time and money on something that won't run worth a dam*

I'm sticking with a .125 dome and 882's headswith a deck of 9.020 to square it up. The plug isn't that low on those and you should still net at least 9.5ish.

Thanks very much I appreciate the patience and everyones comments!

According to the Sealed Power site, the .125 dome piston (H618cp) installed in a block with a 9.025 deck height with 76cc heads, they claim the CR will be 9.38 to 1.
That should be fine for 87 octane gas. If you decide to have the block milled to "0" deck height, (piston flush with the top of the block), the CR will be higher.

According to them, the .275 dome (H617cp) with the same considerations will yeild 10.3 CR.

For the flat top version, (H631cp), again, all the same as above, they claim 8.63

If there are any math wizzzs out there, i'd be curious to know what the numbers change to after bringing the deck down to 9" even for proper quinch.

Mark