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Old 02-25-2008, 10:36 PM   #1 (permalink)
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I'm going to be running the Edelbrock Performer RPM set-up (367HP) and was wanting to know what pistons I should get. Forged flat or dish. What's the difference? Let me know fellas. Thanks again.
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Old 02-26-2008, 05:19 AM   #2 (permalink)
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The Edel heads make about 9.5:1 compression with flat top pistons. Dished pistons would lower the compression to 8-8.5:1
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Old 02-26-2008, 07:22 AM   #3 (permalink)
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It really depends on what type of compression you want to run. Compression depends on the size of the cylinder head combustion chambers, the compressed thickness of your head gaskets, piston-to-deck clearance, and the dish or dome volume of your pistons.

What differs between a flat top and a dish is the extra volume they create. A dish pistion adds more volume to the combustion chamber, therefore lowering compression. Dish pistons have a dish, or bowl, shape in the top of them. There are different volume sizes for dish pistons as well as flat tops (some flat tops are not completely flat). Dome pistons reduce the combustion chamber volume, therefore increasing compression.

Many pistons have a different compression distance; which is the distance from the top of the piston to the center of the wrist pin. That will affect your piston-to-deck height. You need to first establish what compression you want to run, then you need to start looking at all the other factors and how to get to that compression.
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Old 02-26-2008, 09:33 AM   #4 (permalink)
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I have been wanting to know this too. How do you determine what compression you want to run? Is a higher compression always better? What are the pros and cons? I hope I am not stealing your thread, but have wondered the same thing.
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Old 02-26-2008, 02:30 PM   #5 (permalink)
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Quote:
Originally Posted by rsalway
I have been wanting to know this too. How do you determine what compression you want to run? Is a higher compression always better? What are the pros and cons? I hope I am not stealing your thread, but have wondered the same thing.
You want the highest compression you can run without running into detonation problems IMO..You have to decide what type of octane level you want to run and go from there...The camshaft plays a very large part in this as well..You can increase or decrease cylinder pressure by altering the valve timing events.
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Old 02-26-2008, 04:27 PM   #6 (permalink)
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A set of -5cc flat tops should put you around 9.2-9.3:1, spot on for a street engine with aluminum heads.
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Old 02-26-2008, 10:23 PM   #7 (permalink)
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Quote:
Originally Posted by BaronVonAutomatic
A set of -5cc flat tops should put you around 9.2-9.3:1, spot on for a street engine with aluminum heads.
Thats a good number for a very mild engine with cast iron heads but a little low for an aluminium headed engine in my opinion..Thats one of the bonus's of aluminium heads that you can run roughly an extra point of compression or so on the same octane.
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Old 02-26-2008, 11:12 PM   #8 (permalink)
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^^^ Agreed.

I've built and dyno'd naturally aspirated engines that have run upwards of 10:1 w/ aluminum heads and 94 octane. You will also have to be aware of your total ignition timing. Typically we would back timing out a couple degrees or so when running a higher compression ratio on pumped gas. We would normally dyno w/race fuel (105 octane minimum) at lets say for example 34 degrees total timing. Same engine now running on pumped gas, we'd back timing down to about 32 degrees to be safe and go from there. We would also jet the engine a little on the phat side (~12.6 - 12.8:1)to help keep things cool too. Not to mention, a lot of our engines usually made good power at about a 12.8:1 a/f ratio. Some customers noted that they picked up a few tenths at the track after leaning it out a bit, but your results may vary. Too much timing and a lean fuel mixture can destroy an engine real quick if you're not careful. Personally, I like to stay on the conservative side.

Also, make sure to check your Piston to Valve clearance. Usually a flat top with a mild to moderate cam won't get you into trouble, but again you want to be safe.

That said, naturally aspirated, w/aluminum heads and mainly street use, I'd run (and am running) a set of Forged flat tops at about 9.5:1, but that's just me. I'm sure there are a lot of other guys on the boards that will recommend otherwise. Everyone has their own ideas as well as their own engine builders. No two are the same!

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Old 02-28-2008, 09:13 AM   #9 (permalink)
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Yes, a good time to do the homework is before anything gets bought.
I've got dished pistons with aluminum heads with a compression ratio of about 9:6 to 1.
I did that because the car is a cruiser that I want to at least run faster. I run 87 octane all day without any knock. When I know I'm going to the track, then I'll get some high octane juice.
I used a mild roller cam.
If all I was building was a track or drag car, believe me, it would have a higher compression ratio, and a bigger cam.
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Old 03-01-2008, 08:05 PM   #10 (permalink)
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What size are the CC's are the aluminum heads your running? I just built a motor with stock dish pistons, casting # stated with a C8, and 58 CC cast iron heads. I was hoping I would be around 9.5 compression. Also what lift/duration cam did you go with? Please let me know. Jim
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Old 03-01-2008, 08:15 PM   #11 (permalink)
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One thing to think about, is what octane gas is even available in your area. You cannot get higher than 91 in California, so out here, you pretty much want to stay at around 10:1 or lower with aluminum heads and less with iron heads.

Lots of great advice in this thread. I particularly like the response by tophe7d ... REALLY good info in that response.

I'm running 10.2:1 compression in my 408, with AFR 185 heads. I use nothing but 91 octane in it. This car is a weekend warrior and a car to take to shows and the track, so I don't mind paying for 91 octane in it.

I'm running 9.2:1 in my 289, with iron heads ('69 351w heads with KB Flat top pistons). I can run just fine with 87 octane in it. It's my daily driver, so I didn't want to be strapped into the higher priced gas.
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