pistons,dome or flat top??

[Bens69PlymouthSatellite]

I was curious what would be a better choice 4 me I have a 78 440 block, .030 over,wanting to run a decent size cam hydraulic flat tappet,think 540 lift and somewhere around 232 duration @.050 also says the sweet spot is 2000 to 5500 rpm, I found it from Hughes engines.I also am using 452 heads,2.14 in 1.81 ex. and 90cc 600 lift springs,so my question is would running a flat top piston make my compression too low to really utilize the potential of the cam, seeing as how I was told I would be at like 9.5.1 at that, or should I run a dome style to kick the compression up? Would I still be safe to run 93 octane? trying to stay 10.5.1 or 11.1 tops,no racing fuel just a beefy play toy,also if I did run a flat top should I run a bigger cam? Thanks alot! to elaborate even further tci 727,thinking about a spool in the rear,mopar dual plane manifold,holley 870 cfm street avenger,the pistons I was looking at are speed pro and say at 88cc heads they make 11.1 4 the dome pistons.

 

[Meep-Meep]

Not all flat tops are created equal. You need to specify the compression height. If you run the 78 spec piston you will have maybe 8.0:1 CR. I'm running KB237's, which have a 2.065" compression distance and I get 10.1:1 CR with 915's and a steel shim gasket. All this is on a 77 block. I personally think you can get away with a lot less cam and still get the results you want. Something in the HEMI grind area should be fine. For pump 93 you might want to shoot for 180-185 lbs of cylinder pressure. You might be able to go as high as 190 PSI reliably using 93 but I can say that PSI doesn't work with 91 on a hot day.

 

[Bens69PlymouthSatellite]

I want a hot street machine,would like to squeeze some power out of this engine what pistons would u recommend 4 the setup I mentioned I appreciate your opinion Meep,thanks dude.

 

[Cranky]

Yup, it's not so much of what your static compression ratio is but more of how much cylinder pressure you create at low rpm that dictates the octane rate you need and your cam plays a huge part in that. KB offers a reverse dome piston that puts the raised part of the piston on the quench side of the head. Don't know the part number but you can go to their site and do some shopping. The old style domed pistons actually interfere with the flame front as it travels from the quench side of the head over to the combustion chamber and the KB's helps promote it. The general consensus is .040 piston to head clearance on the quench side but I like them closer to .030 There's also a 'white paper' here that talks about quench and it's from the KB site.

 

[polkat]

Meep-Meep, with the KB237's and 915's and steel shims, I'm guessing you are getting pretty good quench. What cam are you running, and how is the valve clearance?

 

[Meep-Meep]

Meep-Meep, with the KB237's and 915's and steel shims, I'm guessing you are getting pretty good quench. What cam are you running, and how is the valve clearance?

My pistons are .015" in the hole so with a .020" gasket that puts me right at .035" quench distance. I'm running the MoPar .455" / 272 cam and with the generous valve reliefs I have a mile of room. The heads have 2.14" / 1.81" valves and a pocket port job and the exhaust port was opened up a bit to compensate for the larger valve but nothing crazy. Intake is an M1 (like stock cast iron except aluminum) with a 750 AFB and Hooker 2" headers. So far the car has run a 13.38 @ 109 on street tires and 3.55 gears. Better gears and slicks I should go 12.50. Also, I recently put on a stock cast iron intake just for fun.

It's interesting, but I drove the car today on straight pump 91 and it didn't ping at all. Must be because it's cooling down so the octane requirement isn't as high. I bet I'm knocking right on detonation's door (bad pun) with my cylinder pressure and available pump gas.

 

[polkat]

Sorry Ben, not trying to hijack your post, but Meep-Meep, did you have to rebalance much for the KB's?

 

[Meep-Meep]

Oh yes, I got a good sized hole in the front and rear counterweights. I'm running a 77 block but have a steel crank and stock rods.

 

[Bens69PlymouthSatellite]

Hey thanks Meep I just got the receipt from the guy I got them from,88cc which seems a little better,thanks alot man!

 

[SGTPaul]

Here is a simple compression calculator:

http://www.doverusa.com/compression-ratio-calculator.htm

You will need to know the piston volume to get an accurate calculation. I believe a tru flat top piston (no cups for the valves) value is 0? Please correct me if I'm wrong.

 

[Cranky]

Yup...flat tops with no valve reliefs would be '0'