PISTONS- Flat top vs. Dished


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I'm making preparations for the installation of a D8BE head ( big log) on my 200. My questions is regarding raising CR. Is it possible to mill the head enough to attain 10 - 10.5 CR with the stock dished pistons; or would valve clearance be an issue ?????
BTW I'm running a Clifford 280H with 1.5 rocker ratio


Typically you can mill .060" off to raise compression one pointor rebuild engine using the 2.3L HSC pistons which are flattops. But at 10-10.5:1CR its kinda pointless running that high compression on cast pistons. I would not be comfortable with that. With cast pistons I would not go over 9.5:1
8) for the street 9.5:1 is about as high as you want to go normaly, however with the cam you are going to run 10:1 will work. with cast pistons though you need to limit rpm to 6000, and you need to run 93 octane and cut back timing to prevent detonation.
Yeah, Doug was up to 11 CR!

Of course, he had to mix rocket fuel in his basement just to drive to work... :shock:
Mark P said:

Yeah, Doug was up to 11 CR!

Of course, he had to mix rocket fuel in his basement just to drive to work...

Yeah, yeah MORE POWER

The fuel part I don't really mind, I don't plan on driving the car daily. I can get AvGas and/or Race Gas here. But is High Octane gas not enough??? Would I need the water injection to keep my engine from melting????

Howdy Alex:

If you were to mill this head the max, .090", assuming that you had 62cc chambers to begin with, and assuming you're going from the stock .025" steel shim gasket for a composite at .050" compressed thickness, you'd get 9.7:1 CR. A mill cut of .090" would reduce chamber volume aproximately 18ccs. Be sure to measure all deminsions yourself.

I doubt that valve clearance would be and issue with dished pistons, but you better check.

The best solution to higher compression is to deck the top of the block to zero and use one of MarkP's thinner gaskets. Stick with the small dish pistons for several reasons, if at all possible.

Be sure to use washers on the head bolts and chase and clean the holes.

Adios, David
Hi Alex,

Here's my setup run through Davids calculator;

Number Of Cylinders 6
Bore Diameter [inches] 3.74
Stroke Length [inches] 3.126
Combustion Chamber Volume [cubic centimeters] 58
Head Gasket Compressed Thickness [inches] .025
Head Gasket Bore Diameter [inches] 3.75
Piston To Deck Clearance [inches] 0
Select Piston Type Flat Top
Dish/Valve Relief/Dome Volume [positive cubic centimeters] 0
Volumetric Efficiency: 85%
RPM: 6000

Displacement, [cubic inches] 206
Displacement, [liters] 3.4
Static compression ratio 10:1
Cubic Feet per Minute required @ 6000 rpm, [cfm] 304
Estimated Horsepower @ 6000 rpm 195
Assumes altitude of sealevel, barometric pressure of 14.696 and 60° air supply to carburator 195

Now I had .040 milled off my head and when I was all finished unshrouding the valves it only decreasced it from 62 cc to 58 cc. One thing that will affect this is how much work are you going to do as far as unshrouding the valves??

I unshrouded mine as far as you can.

You will add cc's when you do this and you can only make up so much by milling the head.

You might want to unshroud the valves 1st then go from there. With a 280 cam the minumim I would go is 10:1 compression.

The general consensus (here) has been that I've had such good luck running a 272H cam that was only meant to be run in a stick shift car in my car which is equipped with a automatic is that I've been running a lot more than normal compression and it helps a bunch on the bottom end as far as torque and hp goes.

I still have my water injection hooked up but I can unhook it and it still runs all right on 91-92 octane with 38 degrees total advance. It is a bit cooler here in Indiana then what it is in Costa Rica though ;)

I dont know why everyone is so scared to run water injection here....Just check and fill it once a week....


hell it makes sense doug but not alot of people are running on a built motor that would need it. although a washer pump and tank would prob make a OK low buck setup.