Piston Design: Dished vs Brow?

[colnago]

Long story short, I decided to rebuild a 390 engine I have. I brought the heads in to my machinist for rebuilding, and we got talking about piston designs. He said that he favored a piston with a "dish" on top, over a piston with the "brows" for valve clearance, because it lowers the compression ratio to 9:1. From a quick look on eBay, the dish piston seem to be marketed more for car engines, and the brow more for truck engines. So, does anybody have any real-world experience with both designs, and/or input into this?

Joseph

[6Delta]

Its been a while since I have been on the forum but in stopping by I caught your post. It really is not a one size fits all thing. As far as lowering the compression, what is the bore and how far down are the pistons? I have seen where a "stock" type rebuild will use round dish pistons when the engine is bored to keep the compression down. Nothing fancy you could get them anywhere. Flat top with valve reliefs on both sides are usually so that the factory can slap in the piston on either side. Also usually stock bore. Plus there are other considerations. If you are going close to flush to the deck you will want to (probably) look at a dish that mimics the cylinder head to keep your compression down and get away from detonation problems. The heads will need to be cc'd and you can calculate the calculate the compression (you will also need the ring height and a few other things but you get the general idea). The last engine I built was bored 30 over, the pistons were 3 thousandths down, I used flat top pistons with 3 cc valve reliefs and a 62cc head. I also used a stroker crank so valve clearance was a consideration when setting it up. The first one was like you are doing. Flat pistons, bored 40 over, moderate cam from Ford, adjustable rocker arms, stock crank. Nothing fancy but it ran alright.

You need to discuss a lot of things with the builder along with the type of pistons. Like what camshaft are you using, what are you doing to the heads, what about the deck and stroke? Where do you want you power band to be? What are you really going to be doing with it? There are some good books on rebuilding for horsepower and you can read a lot of guy's builds online. If you are just doing a stock rebuild with a standard overbore to clean up the cylinders your machinist has probably done tons just like he is saying and they have probably worked out just fine.

[colnago]

Are there "general rules?"

"Dished piston are for towing, and flat for racing." Or vice-versa? Why would I want one or the other?

Joseph

[6Delta]

Not really. Like I said the piston is just part of the build. You kind of have to have a whole engine in mind when you start picking out parts. Towing, road racing, drag racing, hot street with highway use, whatever. Then what are you doing for a transmission (automatic, manual, 4, 5, 6 speed of either?) and rear end (3:00-1, 4.10-1?) you get the picture. Then look at the motor. If you get it machined and have the block decked you will have an idea where your piston height will be. (If you know also which crankshaft you are using so you can determine the stroke) Down in the hole, flush or proud. Then you can match the piston to the compression you want. Sometimes you will go with flat top or shaped. Either dished or extruded both will probably try to either match or mirror the head’s valve ports.

Honestly though if you just by a “kit” from the shop it will probably be based on the bore. Like I said a round dish is usually what I have found in a stock type rebuild with a standard overbore. To my way of thinking saying a dished piston gets the compression down to 9-1 is not very scientific and probably not very correct. If you shave your heads, and deck the block just to make sure everything is flat and bore the block to get rid of a scoring you still need to measure everything to know what the compression is. Check out an online engine compression calculator. There are several that are free to play with.

[DuckRyder]

On an FE a flat top piston with 68cc heads is going to put it probably over 10:1 static (SCR) after the block is decked and the heads are done. With careful camshaft selection you can get the dynamic compression (DCR) down and it’ll run on premium without too much timing.

Where FE pistons are concerned you also need to watch the compression height “truck pistons” are actually 410 pistons and will put the piston down in the bore and you’ll lose the quench.

Depending on how bad your heads are, you might be better off with a set of Edelbrock heads.

I’ve said this a before, but if I had to to do over I’d have built at least a 410 and Edelbrock or Blue Thunder heads.

[colnago]

I don't think he was giving a specific compression ratio, so much as saying that it would give a lower CR. Yes, he did say 9:1, but that would be lower than a flat-top piston would give. With today's gas, for a street vehicle, a lower CR could help prevent pinging. I read today that this could also be controlled by the choice of cams (lobe separation angle, valve duration, and cam timing relative to ignition).

I downloaded the Silv-o-Lite catalog today. Man, there are about seven zillion different piston designs. An art unto itself.

Joseph

[DuckRyder]

Read this.

http://cochise.uia.net/pkelley2/DynamicCR.html

[Jacksdad]

Good article, but as Robert has already pointed out, quench is also an extremely important factor and should be taken into consideration when choosing pistons. It allows you to run a higher compression ratio without detonation by better mixing the charge in the combustion chamber. When manufacturers started to reduce compression ratios in the seventies by dropping the pistons further down in the hole, they actually made them more prone to detonation because there was no longer any quench (or squish).

[colnago]

Hmmm, looks like I need to re-think the question. How am I going to use the truck, what cam, etc. From there, I select the piston design that will meet the requirements. Okay, I can go with that.

Thanks for the realignment. Not what I wanted to hear, but it is what I needed to hear.

Joseph

[DuckRyder]

It is all about the combination... I’d worry not at all about the piston design and quite a bit about how the actual numbers work out as far as quench, SCR and DCR. I mean actual numbers too, measured on your parts, not theoretical or catalog numbers...

It is a lot of work, cc’ing heads, pistons, deck height, degreeing cam...and so forth and so on. Even then if your machine shop deviates at all from your request (maybe because they had to in order to clean something up) you may end up with something less than ideal (like 10.6:1 in an iron head FE with a not extremely radical ICA)

If you just want to bolt it together then my advice is pick a factory engine spec and build it or go follow Edelbrocks instructions on a power package.