can I go from using flat top with four valve relief pistons and use dish style with no valve relief instead? If someone could explain the valve relief and what it means i would appreciate it.

 

Quick answer: It depends.

Long answer: Flat top pistons will (generally) raise your compression by effectively making your combustion chambers smaller. Combine this with a high lift/long duration cam and you can run into problems with your pistons smacking your valves @ the top of the intake & exhaust strokes depending on how you cam is timed in relation to the crank. This is bad. Valve reliefs allow some additional clearance between the valve and piston to combat this. The 302 in my Mustang is running hypereutectic flat-tops (w/reliefs) with a set of warmed-up 289 heads. Compression is about 9.2:1 and it runs great. Extra compression almost always = more horse power. 9:1 seems to be a pretty safe level for the street without running premium fuel (if your AFR & timing are correct.) A bump in compression is a standard performance upgrade when working on these 70's vintage motors.

Dished pistons will (generally) lower your compression ratio. Ford did this in the 70's and 80's for emissions compliance. Lowering your compression ratio is not a performance upgrade (unless you are building a motor for a turbo...) and is not usually recommended.

I can't give you a straight answer because it all depends on what cam lift you are running, rocker arm ratio, piston compression height, length of your conn rod/stroke, block height, etc etc. There are a lot of variables in the equation to take into account. I don't know your level of engine tech knowledge, so I'm trying not to be a jerk-face here, but to be safe I would talk to an experienced engine builder (I am not) and go from there.

My question is this: Why are you looking to go from a flat-top to a dish?

Hope that helps!

 

It's a long story, I'll give the short version, I went to a cheap machine shop to machine my 352 block, the guy there said "lets bore it .050 over and make it a 360 (since they have the same stroke) and then your pistons will be $150 cheaper" I said sounds great, come to find out the pistons are cheaper because they have a lower compresion height, the 360 was a dog engine designed for emissions so i was gonna have 7.5 to 1 compression, I took it to a way better machine shop (that actually sonic checked it for me first) they reconditioned the rods and found me some pistons that weren't custom made ( I looked into that it was an arm and a leg) that had a slightly higher compression height than my stock pistons, they did all the math for me and told me once i build it i should have 9 to 9.5 to 1 compression, I was just curious as to why my old ones looked so different than the ones I have now, thats when i stumbled on all the valve relief stuff, I fully trust this engine shops math and I believe it will work, My mind just likes to understand stuff before i do it, lol. no offense taken by the way this is my very first engine build, i Just ordered the "how to rebuild your big block ford" book, lol. thanks for the response.

 

First off, your first machine shop guy is a moron and deserves to go out of business.

Second, the 360 was NEVER an emissions engine. It was a TRUCK engine and was low compression for that reason alone. Actually there was a high compression 360 back in 1958/59 in the Edsel. It was 10.5 to 1 in 1958 and 9.5 to 1 in 1959.

Believe your new shop guys.

 

lol, ya, he was. You get what you pay for was the lesson I learned there. Partly my fault though, I should have researched it before just going along with what he said. Oh well this is my first complete build and im learning a lot.