Which is better: Dish a taller piston, or deck the block?

Hi guys,

Just as the question says, is one approach to increasing compression ratio better than the other, or is pretty much equal either way?

Here is my situation. I have a 250ci. The head was milled in prior work (plus port & polish and 2v conversion). From the stock compression chamber volume of 62cc's the head is now around 51cc's. Up until now the bottom end was untouched. With a Felpro gasket the head milling brought my CR up to around 8.8 from a stock of around 8.2. Better, but not by much. However at the time I didn't want to take the engine out so it was as good as I was going to get.

Now the whole thing is coming out. I'd like to get my compression ratio up to 9.5 or so which seems to be the agreed-upon sweet spot for a mild street vehicle. I have the option of installing the 255ci V8 pistons which are taller than stock pistons (by 0.085"), and that removes a good deal of the deck clearance. However it's actually too much combined with the prior head mill: I calculate if I put the taller pistons in there and do nothing else, my static CR tips the scales at over 11:1, which is too high.

In this post by CZLN6 (David), the expert himself discusses dishing these taller pistons: Click here.

The guy he's talking to has a slightly different situation than me but the principle would still work. I figure I'd need a dish of about 12-14cc's milled into the top of the pistons, depending on the overbore we end up with. Total deck clearance in the end would be about 0.040" (0.125" total deck clearance minus the 0.085" taller height of the new pistons).

Alternatively I could retain the stock pistons and deck the block by some amount to arrive at my desired CR. I couldn't go to Zero Deck because again with the milled head CR would be too high. But I could bring it down some amount, I think about .040" would get me in the range (and would leave a deck clearance of about .085").

Either way I can get to my desired CR. One method involves milling a dish into the piston, another involves machining the surface of the block. One method (dished pistons) ostensibly leaves me with a lower deck clearance, which I think is a good thing. However this is maybe somewhat misleading since the surface of the piston is also dished, which may sort of act like the equivalent of extra deck clearance.

Well, I got to pick one or the other!


Luke

They would be close to same in results that is if block deck could be cut that much. IMOP the ideal would be a small CC head with a piston (dished) block decked to zero or even more (depending on head gasket thickness) so that there is a quench of about .035 to .040. Another way might be to see if there are some longer rods that can be swapped in and or off set grinding the crank to give a little more stroke.

I'm not sure if this will help but may add to your choices:

The US 250 performance engine I'm running, uses TRW/SealedPower # 470NP's pistons. The application is 79-89 AMC 4.2(258).

Ford 250: bore = 3.68 / comp distance = 1.5 / dish volume = 13cc
AMC 258: bore = 3.75 / comp distance = 1.633 / D-Sump = 21cc's

This utilizes the bore at @ .070 over and raise the piston toward the deck .133 . The specs list the pin diameter of the ford rods at .9122 and the AMC's at .9310 which the rod can easily accomodate.

The cc volume of the recess is 21ccs. With the .070" overbore the bore is 3.75", stroke of 3.91, a Felpro head gasket at .050", a zero deck height, chamber volume of 60ccs and a piston recess of 21 ccs, gives a street compression ratio of 8.8:1 with @ OEM deck/head before any milling. milled head chamber volume , gasket crush variable will raise CR.

Powerband

Howdy Luke and all:

The question of which is better needs a little refining. The goal in reducing the deck clearance (Deck clearance = deck height and head gasket thickness) is to increase combustion efficiency by increasing the quench effect on the combustion stroke of the pistons. The goal is to achieve a total deck clearance of about .030". The clearances on a typical 250 are usually near .150" on stock deck height. The commonly available head gaskets are in the .045" - .055" range for a total deck clearance of near .200". So, you can see that doing one, or the other will not get you to an ideal quench effect height.

You will also note in my previous article it is suggested that the head be milled last and possible reshape the chambers to increase volume to achieve the CR goal. Since you've already milled it, you can't unmill.

So, IIWIYS, I'd probably deck the top of the block, simply because it's a good idea to do on a rebuild anyway to achieve a flat and true mating surface for a tight seal with the head. And, stock type, overbore pistons are cheaper than 255 pistons. At least they were the last time I checked. I didn't recall what you said the stock deck height is on your engine?

That's my two cents for what it's worth.

Adios, David

8) let me throw one more option in there for you. you can mill the tops of the V8 pistons to achieve the deck clearance you need,as long as you go no more than .025-.030". there is enough meat in the tops of the pistons to do just this. if you do mill the piston tops, be sure to take a small file and eliminate the sharp edges that will result from the milling.

Hi everybody and thanks for the helpful replies.

Powerband: The 470NP pistons are an interesting choice which I had not heard of before. I have to say that the .070" overbore worries me a bit though and I think for that reason alone I will skip that route.

rbohm: One thing I have wondered about is how much could really be taken off the top of the piston - either milling the whole face of the piston or dishing it. If as you say the depth limit is somewhere around .030" I think it might not be enough for my situation.

David: You ask a good question which I realize I need to answer first before I can make any definitive statement. Even though I've had the head off several times I've never actually measured my deck height and in all my maths I have just been assuming 0.125" which I think is the published figure. But I have also read this can vary quite a bit so I do need to measure it. I have a dial indicator on the way and I should be able to do that soon. I can speculate about this or that but without an accurate measurement I could be far off.

But if my deck height is even in the ballpark of 0.125" and if the milling constraint listed by rbohm is also in the ballpark, I think there is really only one practical answer, which is as you say David, just to deck the block by the appropriate amount.

Here are some calculations - with the caveat of course that the deck height and final over-bore are as of yet unknown.

Assume:
Bore = 3.70" (assume .020" over)
Stroke = 3.91"
Gasket Thickness = 0.050"
Gasket Bore = 3.81"
Cylinder head chamber = 50.8 cc's (actual measurement)
Deck Height = 0.125" (assumption at this point)

Now, if I go with the V8 pistons everything above stays the same except the deck height is reduced to 0.040" (0.125" - 0.085" extra piston height).
Resulting static compression ratio: 11.25

Now assuming for simplicity's sake I mill the entire face of the piston instead of just making a dish. If I can only mill up to 0.030" as rbohm says, this is the same thing as reducing the extra height of the pistons by 0.030". So instead of being 0.085" taller the V8 pistons are now only 0.055" taller. Deck height is then reduced to 0.070" (0.125" - 0.055")
Resulting static compression ratio: 10.51

That's probably still too high. In fact, I pretty much have to mill the entire piston down so that it is only 0.005" taller than the stock piston to get a compression ratio of 9.5. Aside from the fact that this requires taking 0.080" off the top of the piston, which it probably can't handle, I guess it would also be an expensive way to end up where we started with the stock pistons.

Basically the prior head mill reduces my options considerably. But I can get any compression ratio I want by choosing the amount of block to mill so since that is a good idea anyway, I think that is where I am headed. Of course my total deck clearance will still be higher than optimal but that is a result of my prior decision to mill the head and so I can just live with that.

To conclude, I'll still take some measurements and see what I'm actually dealing with, but this discussion has really helped me clarify where I'm probably sitting at, so again, thanks to all.


Luke

So if milling some off the the top of the 255 pistons isn't enough what about grinding a few cc's out of the head? I saw them do it on tv car show once actually ( I know not the best source) also on an inline six but this was a jeep 4.0, You would also have the advantage of being able to clean up the combustion chambers a little and unshroud valves.

One other option you have, since it appears that you don't need THAT much more compression is to look at offset grinding the crank. It can easily be done to use standard undersized bearings on the rod journals and will increase stroke slightly. Between that and whatever overbore you need on the block you should get pretty close to your target.

Another thing to look at is dynamic compression. What cam you chose makes a pretty big differencein what you're allowed for compression as well ad what advance rates you plan to have with it.

Gerald

Why don't you use stock Tempo pistons? They have the same specs as the stock dished pistons, only they're flat tops. Then deck the block accordingly. If I don't have the right size in stock, I can get them in a few days.

Very interesting Mike, I had not realized that was an option but that would also work well. Essentially I would be decking the block less by using flat-top pistons. In other words, trading piston dishes for extra deck height.

Since I'll be machining the block anyway, and buying new pistons anyway (if they need overbored), and since I can achieve the same CR either way, is one option better than the other? In other words, is there an advantage to a flat-top piston with a taller deck clearance or dished pistons with lower total deck clearance?


Luke

I used the Tempo pistons and decked the block on my 250. Simple and cheaper than some other options.

Steve

A Flat top piston is Always better if the Compression wanted can be achieved without adding piston weight , a true flattop is the bottom portion of the Wedge chamber design , and anything that helps flame propagation ( Does NOT hinder the burning movement across the chamber ) it also induces Squish ( the kicking out of stagnate air fuel in a tight area into the center where it can help make power, A dish or Dome is Never a help in a Wedge chamber unless compression is too high or too low

or maybe you can unshroud the valves a bit, or at least the intake. that will add volume to your compression chamber, and maybe let it breathe a little easier too.

Ok, sounds like the flat-top pistons really will be the simplest way to go. When I know what my overbore will be I'll order them from Mike.

Depending on the actual measurement of my deck height I may not even need to mill the block very much at all. Assuming my deck height is 0.125" (it may not be) and an 0.020" overbore, my calculations show that with flat-top, stock height pistons and the other specs listed above, I will get:

Static compression ratio: 9.40
Dynamic compression ratio: 7.60

This is just about perfect. Of course it depends on the actual overbore and the actual measurement of my deck height but there is plenty of room for adjustment by varying the amount taken off the block. It will be milled in any case just to get it flat.

Guys, thanks for all the help. I feel good now about proceeding.



Luke