I have a set of new 84 cc Edelbrock RPM heads I planned on using on my 400 stroker build. My engine builder had planned on using flat top pistons but told me today that he found out that in order to use flat tops, the heads need to be milled down to about 72 cc in order to get the compression to around 10.8. Otherwise the compression will only be about mid 9's. The other option he said is to use dish pistons if I don't want to get the heads milled but he favors flat tops over dished so I think he would rather I have the heads milled. I am no engine expert so I would like to know if this sounds right and would also appreciate any input on this subject. Thanks.

I can't tell you exact compression ratios, but dished pistons are gonna put you way lower than a flat top will.

Me thanks some thing has gotten lost in the translation between your engine builder, piston maker and you and us
Is he wanting to use a quench dome piston instead of a flat top with valve releifs
Your not saying what C.I. your motor will be but my first pump gas 505 C.I. 400 block stroker motor had to have 22.0 CC dish piston at .025 down in the cylinders to have 9.25 to 1 compression ratio with 84.0 CC heads
I had a set of large valve ported and polished 906 iron heads on that motor originally and later swap them to a set of Eddy RPM with 84.0 CC heads, the compression ratio stayed the same so I ended up swapping out the 4.25 stroke crankshaft to a 4.300 stroke crankshaft the next winter to raised those dished pistons up to zero deck to get 10.3 to 1 compression ratio.
Most of the pump gas stroker motors I build now have a reverse dome (half a dish) pistons in them with around 10 to 14.0 CC to get 10.4 to 11.0 to 1 compression ratio to run with aluminum heads on the street on pump swill

This.

Any 500 stroker with 84 cc heads and zero deck flat tops will put you over 11:1.

If it is a 451 motor, you should be close to what you are looking for.

The initial plan my engine builder and I had discussed was to build a 400/512 using a 440 Source kit with flat top pistons. We intended to use Edelbrock RPM 84 cc heads (which I already have) that will be ported. The plan was to have a compression of somewhere between 10.5 and 10.8 in order to be able to run on pump gas. He told me after talking to someone at 440 Source that we wouldn't be able to use flat tops without milling the heads in order to achieve the targeted compression.

I recommend you find a new engine builder, as BSB67 said, a 4.25 stroke engine with flat tops and 84cc chambers will be over 11:1 and a decent builder would not rely on 440 source to figure out such a basic concept like compression calculation

With a 4.25 stroke, I would just use a flat top piston and aim for 11 to 1. Use a bigger cam with some overlap to bleed off some cylinder pressure and you'll have a great running pump gas engine.

Agreed, either something is lost in translation, or engine builder is not really a builder.

But even if they were relying 100% on 440 source for that information, the charts on 440 source's website are actually pretty accurate.

OP, take a look at the spreadsheet/chart for "400-512" on their website to give you an idea of what is required and what ballpark CR you will arrive at http://store.440source.com/Stroker-Kits/products/3/

A 512 with rpm heads and flat tops, even .015 or so in the hole, is going to be close to 11.5:1.

Attempting to "read between the lines", maybe this is what they're talking about:

440 source offers only one dish volume-24cc. If that doesn't work for you, then you'll be milling heads or buying new heads to arrive at the target CR. Maybe the real question is, should it be left alone at 9.7:1 or will it be worth it to mill the heads to 75cc and arrive at 10.6:1? Was that really the question?

Everything else being equal, more overlap increases cylinder pressure.

Don't need a dish and mine are milled to 72cc'a and at .002 below the deck I'm at 12.1.1 comp w/a JE flattop..........

Yes, that is what I would like to know. After looking at the 440 Source chart, I think I misstated what I was told. I believe what he said was using the 84 cc heads with flat tops was going to result in a comp ratio too high to run well on pump gas and using dish pistons with the heads milled down to 72-75 cc's would give the desired comp ratio. I believe he said the cc's would have to be in the low 90's in order to use flat tops to get the desired comp ratio. This is all Greek to me but given this information, is there any reason that flat top pistons would be preferred over dish? Thanks.

Sell the Edelbrocks, buy the TF 240. You'll probably be out $500 in the end, but way ahead in the log run. You'll have a better head, and better CR without milling stuff or compromising stuff.

To gain about a full point of compression my slightly milling the heads is likely the cheapest hp you can get. Only reason for a dish is to lower the compression.

If getting it over 10.5:1 is very important to your and your builder, the easy solutions are either mill the heads, replace the heads with smaller chambers + get some that flow better at the same time, or use custom pistons with less dish.

The last option is going to be the most expensive, the first option is the least expensive, the middle option....if you sell your existing parts like bsb suggests, will probably get you the biggest overall gain for the money spent.

I think lowering compression is his goal.

Give Ron Beaubien at Diamond Pistons a call. We reduced the dish on these pistons to give me 10.8 CR with 84 cc E heads on my 505cI

Then you need to get different parts or talk to someone that has a clue as to what they are talking about .

What stroke crank are you using and what is the CH of the piston in the kit?

The most common piston is 1.320 and it works with multiple stroke cranks, and it's close to zero deck in most builds.

you might actuially need a D shape dish piston to keep the compression ratio in check ... 12cc's out the heads

This is probably the best solution because he said he was going to have the head ported, adding all the porting and head machining in it would probably be a wash , no porting required on the TF and they don't need to be milled either ...

That is assuming he uses the Dish piston and it's a D dish to retain the quench effect that will be needs to run pump gas ...

Diamond will change the dish volume for about $35
for the set.. and make them to what volume you want

"Give Ron Beaubien at Diamond Pistons a call."

Tried calling him a couple of days ago and apparently he doesn't return voice mails as I never heard back from him.

Another suggestion that had come up is to increase the heads cc's to 92 in order to increase the compression. I'm not sure I would want to do this (or milling for that matter) in the event I ever wanted to use the heads on another motor. I'm thinking of going back to my original plan of building a 470 and using the RPM heads as-is with flat tops to avoid having to alter the heads in any way (other than porting).

Increasing chamber cc's reduces the compression ratio. If this "builder" is telling you otherwise, run do not walk away.

The simple solution is to pick a compression ratio and then find a piston with a reverse dome/dish volume that will deliver that number at zero deck with your chamber CC and .039" head gasket.

Like MR_P_BODY said, if you can't find a shelf piston, a $35 upcharge gets you exactly what you need.

I don't know if 440Source is still a Ross dealer but Ross did up a 34cc reverse dome for my 4.15 493 Source kit that became a shelf stock item until Brandon changed suppliers. I was looking for no more than 9:1 for 87 regular gas and a 34cc dish put me at 8.97:1 with zero deck with RPM heads.

Kevin