Piston choice question

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540slant6

540slant6

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Gents,

I have a stock bottom end 318 with stock cam. It has a set of .040 decked j-heads. I need to do an over bore and wanted some piston advice. I'm going .060 and was lookin at the speed pro pistons from Mancini. Would like to bump the compression but am concerned about clearance. The only future engine mod would be a stock 69 340 4-speed cam.

No juice or forced induction, just a snappy street/strip car. Any advice is much appreciated.
 
I put Wiseco flat tops in the last one I built with J heads and it works great.
 
540slant6 said:
Gents,

I have a stock bottom end 318 with stock cam. It has a set of .040 decked j-heads. I need to do an over bore and wanted some piston advice. I'm going .060 and was lookin at the speed pro pistons from Mancini. Would like to bump the compression but am concerned about clearance. The only future engine mod would be a stock 69 340 4-speed cam.
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Did you sonic check to see if the block will take .60 over?
 
KB 167's would be another choice. cheap alternative if you like hyper's.
 
Yeah, I've heard that some blocks get a little thin when going. 060. I may go .040 depending on the machine shops recommendation.

I'm just not sure if a high compression piston would contact the valves? ?
 
Most of the flat top valves with a higher compression height have valve reliefs that can accommodate a decent amount of added lift. No sure if anyone will tell you it if OK for sure unless they built the same combo, but you'll very, very likely be just fine with any of the above choices and a cam in that lift range. You just have to get them and mock up a cylinder with the head gasket of choice, and put some clay in there and see how much it gets compressed.

The Speed Pros shown on Mancini are down in the hole by about .065" so don't do a whole lot for compression over stock. The KB167's come up .055" higher in the hole, so will have better CR, by at least 1/2 point. The Wiseco's are a we bit lower than the KB's, so they drop maybe 0.2 points...not much difference. Either of the last 2 wil have SCR around 9:1.
 
KB 167's will give you a decent bump, just check for clearance before ordering a cam, and after installing the cam.
You should get away with .500" lift easy with no issues, since the J-heads are open chamber.
 
Hensley Motorsports bores 318s to 4" and beyond. I think yours will be fine at .060.
 
Hensley sonic checks the blocks before boring that much as so should you no matter how
much you intend to bore the engine.

Aftermarket pistons are designed to accept a high lift cam. .500 is not high. When you start getting to .600, that is the area high lift starts to come into play. Pay attention to the cams timing since a few degrees may cause contact and a little bit of clay becomes your best friend and (engine) savior.
 
540slant6 said:
Thanks guys, I'm going with the KB's. Should be a pretty peppy teener with my combination.
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As far as .030 (or .040) or .060, it will not make any noticeable difference in CR or displacement... 5 ci and 0.1 on CR from .030 to .060 . That might be noticeable on a circle track but not on the street. I tend to go as small as I can to get more 'rebuild cycles' out of the block. Be sure to use the Felpro 1008 head gaksets that compress to .039". Sounds like a good path you are on! You can go with a wide range of cams from torquey to higher RPM's to get it where you want for your intended use and TC, etc., and not worry on DCR.

BTW, we just put some flat top KB's in a 340 with a .500" lift cam and with the piston tops about .017 higher than yours will be, and had gobs of clearance, even with 5 degrees cam advance. But check on yours to be sure...
 
Thanks, I'll clay the pistons before finally assembly. Good advice and insurance.

Next question, is a 600 CFM Eddy carb with vacuum secondaries enough carb for my setup?
 
rumblefish360 said:
Hensley sonic checks the blocks before boring that much as so should you no matter how
much you intend to bore the engine.

Aftermarket pistons are designed to accept a high lift cam. .500 is not high. When you start getting to .600, that is the area high lift starts to come into play. Pay attention to the cams timing since a few degrees may cause contact and a little bit of clay becomes your best friend and (engine) savior.
Click to expand...

Call Hensley. I bet you a whole dollar that they would laugh at the prospect of sonic testing a block for a .060 overbore.

All the rest is just blah blah blah.
 
540slant6 said:
Thanks, I'll clay the pistons before finally assembly. Good advice and insurance.

Next question, is a 600 CFM Eddy carb with vacuum secondaries enough carb for my setup?
Click to expand...

I would really not use an Eddy if you do not have one.
Look at a 600 vac secondary Holley or 625 street demon.
Way more tune able.
 
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