360 compression issues. pick a piston??

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moparmat2000

moparmat2000

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I have some issues. I have a set of speedmaster 65cc heads, and a 360 i intend to build. My machinest and i agreed to go with 9.8-1 compression to run it it on pump gas. He is recommending dished pistons, and a .040" MLS headgasket to keep the compression where it needs to be.

I also talked with a mopar buddy who said just use flattops with 5cc valve reliefs in them, and a .100" MLS because of the loss of quench with dished pistons. Then if i want to bump up compression later on, i can just drop on a thinner headgasket.

While both ideas have merit, i want to go with the most bang 4 the buck, and get some decent dyno numbers on this when its done.

Any thoughts on this
Thanks
Matt
 
I'm no guru when it comes to these technical aspects by any stretch.

I ended up choosing the H116CP Speed Pro pistons. Flat top with 2 valve reliefs. According to the reference it should put me in at 9.72 with a 65cc head (with standard bore).

Head gasket with a .039 compressed thickness.

http://www.campbellenterprises.com/mopar_speed_pro_trw_forged_pistons.htm
 
Depends on what the rest of the car is built, gears, tranny, gas availability etc.
You can run fairly high compression with the right cam.
Also I am not familiar with the heads you are using, are they iron or aluminum?
 
sireland67 said:
Depends on what the rest of the car is built, gears, tranny, gas availability etc.
You can run fairly high compression with the right cam.
Also I am not familiar with the heads you are using, are they iron or aluminum?
Click to expand...

A-833 OD , 3 55 gears. 91 octane gas.

Heads are 65cc closed chamber aluminum eddy copies.
 
I ran my combo thru a compression calculator with .039" headgasket, 5cc valve reliefs with flattops, and 65cc chambers with a .030 overbored 360 stock stroke i get 10.58-1

Thats pretty high. I am trying to keep it about 9.8-1 just in case i get a batch of bad gas.
 
I ran my combo thru a compression calculator with .039" headgasket, 5cc valve reliefs with flattops, and 65cc chambers with a .030 overbored 360 stock stroke i get 10.58-1

Thats pretty high. I am trying to keep it about 9.8-1 just in case i get a batch of bad gas.
 
This depends on the piston; you can get a dished piston WITH a quench pad. (That assumes your 65 cc heads have a closed off chamber above the valves, which they should if they are Edelbrock clones.)
Example: KB232

The Speed Pros as above are in the hole a bit without decking the block, and so won't get the quench at standard deck height. (I have found 2 different compression height spec's on them, so that exact number is still a bit murky.) Even the KB232's need a bit of decking or a thinner head gasket to get the quench gap down around .040".
 
KB107's seem to do well around here. Lighter than speed pros also. FWIW.

Without getting off of your original question, I would go with the most reliable set-up, because it sounds like you want to actually drive it somewhere. This includes head gasket material. Your only giving up 15-20HP/TQ per 1 full point of compression after 9:1, so it isn't that big of a trade-off.

With an OD you can step-up to the next biggest gears. That will make WAY more of a difference! Think 3.91+.

http://vexer.com/automotive-tools/speed-rpm-calculator

Big gears, STILL cruise at 2300!
 
moparmat2000 said:
I ran my combo thru a compression calculator with .039" headgasket, 5cc valve reliefs with flattops, and 65cc chambers with a .030 overbored 360 stock stroke i get 10.58-1

Thats pretty high. I am trying to keep it about 9.8-1 just in case i get a batch of bad gas.
Click to expand...
Alu heads you can run it higher , I'm running 11.3 and no problem with 94 , can even run 91 if I have to , you should be good with 10.5 8
 
moparmat2000 said:
I ran my combo thru a compression calculator with .039" headgasket, 5cc valve reliefs with flattops, and 65cc chambers with a .030 overbored 360 stock stroke i get 10.58-1

Thats pretty high. I am trying to keep it about 9.8-1 just in case i get a batch of bad gas.
Click to expand...

Your calculations look like they are based off of flattops with "0" deck. If the block you will use hasn't been decked yet, you will end up closer to the compression you are looking for. "As delivered" OEM deck heights are usually taller than spec calls for.
 
If you run the fel pro head gaskets from the mancini racing gasket kits, they are .065" and measure 12.4 cc if you need to go with the higher compression pistons and need to "take some compression out"...
 
I plan on getting the block decked. But only enough to make sure the sealing surface is flat.
 
Locomotion said:
Your calculations look like they are based off of flattops with "0" deck. If the block you will use hasn't been decked yet, you will end up closer to the compression you are looking for. "As delivered" OEM deck heights are usually taller than spec calls for.
Click to expand...

Roughly how far down in the hole on a 360 is a piston with stock deck height at TDC ?
 
Do you have a part number? I looked on Summit's site and none of them are dished. There are two valve relief pistons and four valve relief.
 
Yes the 2 valve relief flat top. I dont have a PN handy. Just remember thr cuts being 5cc

Sealed power H655CP a forged version of this one. I remember the dished volume being listed as 11.23cc
 
Ok. I am confused. First you said dish. Then you say flat top with 2 valve reliefs. Which piston are you talking about?
 
RustyRatRod said:
Ok. I am confused. First you said dish. Then you say flat top with 2 valve reliefs. Which piston are you talking about?
Click to expand...

Rusty, please reread my original post. It was 2 different schools of thought. I am looking for bang for the buck. This is why i listed 2 different pistons.
 
Then go with the piston that provides quench.
 
Bad Sport said:
I'm no guru when it comes to these technical aspects by any stretch.

I ended up choosing the H116CP Speed Pro pistons. Flat top with 2 valve reliefs. According to the reference it should put me in at 9.72 with a 65cc head (with standard bore).

Head gasket with a .039 compressed thickness.
Click to expand...

Those are the same pistons i have in my 30 over 360 with Edelbrock heads the comp is 10.5 to 1..
 
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