Flat Top vs. Concave Pistons

NextGen · Post by **NextGen** » Fri Nov 09, 2001 9:36 pm

I been having some deep discussions with serval T-4 bug guys on the pros and cons on the two. First what would the difference in CR between two equally built engines using the two differents pistons. That dish is deep.
Also is it true the lower compression engine would have more performance and higher top end then the high compression engine. I personally would think the lower compression would due to the advantage ofhaving more room to take in more fuel and air. While the High CR engine would have faster accel at bottom end. Notice my Serious Tech Terms.
Feedback please.
Joe Next Generation

Shad Laws · Post by **Shad Laws** » Fri Nov 09, 2001 11:25 pm

Hey Joe-

I been having some deep discussions with serval T-4 bug guys on the pros and cons on the two. First what would the difference in CR between two equally built engines using the two differents pistons. That dish is deep.

A 914 Kolbenschmidt 94mm piston has about a 5cc dish (I'm looking at one right now). The exact CR difference depends on a LOT. If you know this figure, you should be able to tell the diff... if you want a formula, lemme know.

Here's the plus of dished pistons: two equally built engines with the SAME CR (i.e. you make the deckheight different) can see better performance and gas mileage with the dish. The reason why is because you can decrease the deckheight... and if you get it low enough, you make part of the bore a quench area, which gives a better burn.

One example: my 2.0 runs Mahle 94mm 914 pistons ( about same dish volume as Kolbenschmidt, but different shape) with 55cc heads (stock 2.0 914, flycut a bit due to rebuild) at 8.5:1 CR. That means that the deckheight is 0.035". To get the same with a flat top, I'd need about 0.065" deckheight, essentially killing the mega-quench benefit. Plus, at the same CR and engine size, dished ones have more clearance for high lift valves...

Take care,
Shad

Also is it true the lower compression engine would have more performance and higher top end then the high compression engine. I personally would think the lower compression would due to the advantage ofhaving more room to take in more fuel and air. While the High CR engine would have faster accel at bottom end.

High CR is always better for both power and fuel economy. However, the peak pressures and temperatures go up... so, if you get too high, you can get pinging and/or engine self-destruction.

Take care,
Shad

Rmcelroy · Post by **Rmcelroy** » Sat Nov 10, 2001 7:46 am

I'm going to use 1.8L heads on a 2056-I've been planning on using dished pistons because of the lower cc size of the heads. So far I havn't been able to find dished 96mm pistons. Are they available?

From what I've seen here lately about quality issues, I've started having doubts about going to 96's anyway.

Do I just need to go with dished 94's and be done with it?

Thanks

Shad Laws · Post by **Shad Laws** » Sat Nov 10, 2001 10:46 am

Hello-

I'm going to use 1.8L heads on a 2056-I've been planning on using dished pistons because of the lower cc size of the heads. So far I havn't been able to find dished 96mm pistons. Are they available?

Any piston configured any way your heart desires is available from JE. It just costs a bit, tis all... (you may have to get them custom as I don't *think* any retailer has JE pistons like these in stock)

From what I've seen here lately about quality issues, I've started having doubts about going to 96's anyway.

JE Pistons are EXCELLENT quality. It's the cylinders you'd have to scratch your head on. The best ones are actually well-reworked stock 94's. I believe that both our sponsor and Jake have these sorts of things in stock.

Take care,
Shad

ray greenwood · Post by **ray greenwood** » Sat Nov 10, 2001 1:21 pm

The domed version of 1.7 liter piston is nearly the mirror opposite of the dished version. It is usually taking up about 5-7cc of combustion chamber shape. Bearing in mind that the 1.7 heads have a considerable quench area, the difference in the three pistons (dish, flat, dome) in the same engine...seem to be about 7.8:1, 8.0:1 and 8.2:1 in the same order as above. I would assume that having about the same ratio of change from the dish and the flat on your pistons (the dishes are about the same size respective to piston diameter), they should make about the same amount of difference, the combustion chamber size being constant. The one caveat, being that with the quench area of the 1.7L taking up more of the combustion chamber than the quench areas in the 1.8 and 2.0...small changes such as the differences in flat to dome may have a little less effect on the 1.8 and 2.0 than they do on the 1.7. Ray

Post by **Tom Notch** » Sat Nov 10, 2001 1:25 pm

Give FAT a jingle, they had some JE pistons for me in a 4.+ bore that came with a 6cc dish. Maybe they stock a 96mm setup with a dished piston.

------------------
Tom Notch
Tom's Old VW Home

turboteener · Post by **turboteener** » Sat Nov 10, 2001 5:50 pm

What is it with the dished pistons. Flat tops are the way to go. They allow better flow and quench than any other design. I am running .040 quench on my engine with flat top 96mm JE pistons without trouble one. I don't believe the factory dish is acceptable for high performance use. There is no reson you can't run 9.0-9.75:1 compression maybe even 10.0:1 on pump gas with carbs and stock cooling. Detonation is a factor of poor mixture ditribution throughout the combustion chambers. We don't run enough compression to discuss the limits of pump gas.

Shad Laws · Post by **Shad Laws** » Sat Nov 10, 2001 11:29 pm

Hello-

What is it with the dished pistons. Flat tops are the way to go.

It depends on your application... least deckheight possible is definately the way to go. It depends what gets you there...

They allow better flow and quench than any other design.

Not really... dishes tend to center the charge a little better, especially if you get the dish to be a mirror image of the combustion chamber.

But, no matter how you figure it, it comes to the same thing: it depends on the application. It's all about the deckheight... if you want a certain CR that, with flat tops, needs a deckheight of 0.100", then you're better off with dishes.

I am running .040 quench on my engine with flat top 96mm JE pistons without trouble one.

That's a good deckheight!

I don't believe the factory dish is acceptable for high performance use.

It all depends on the application... if I'd used flat-tops in my little 2.0, I'd have a 9.1:1 compression with a really short cam (only 224 duration at 0.050" lift), which *may* tend to hurt the components it has.

There is no reson you can't run 9.0-9.75:1 compression maybe even 10.0:1 on pump gas with carbs and stock cooling.

I can think of a quite a few pros AND cons to it... it all goes back to the same thing - it depends on the application.

Detonation is a factor of poor mixture ditribution throughout the combustion chambers.

Not quite...

We don't run enough compression to discuss the limits of pump gas.

So long as you don't get too hot...

Take care,
Shad

Post by **Tom Notch** » Sun Nov 11, 2001 9:16 am

As Shad pointed out, application drives requirements. Do the Math on head CCs on a 2.3L and bigger engine. If you want huge chambers, then a dish may not be needed. A small dish in the piston crown, like in my 2.7, got me down to a .060 deck at my desired compression. Or I could have opened up the heads more which requires a major amount of work and starts killing the chamber shape.

Trade-offs are sometimes required.

------------------
Tom Notch
Tom's Old VW Home

Trebor · Post by **Trebor** » Sun Nov 11, 2001 12:38 pm

What do you guys mean by Quench?

Robert Martinez

Shad Laws · Post by **Shad Laws** » Mon Nov 12, 2001 2:22 am

Hello-

What do you guys mean by Quench?

Put the piston up at TDC. Then, look at the resulting volume distribution over the bore of the cylinder. Consider the parts of the bore that have very little height to them(under ~0.060" or so... less is better). The part of the bore that this area occupies is defined as the squish, or quench, area.

More quench tends to promote a better burn... for more fuel economy AND more power. While making considerations for such things in the VW world may seem "rare" or "new", it is common as pie for modern engines. Sometimes you even see the squish area (typically given in actual area or percent of total bore area) listed right in a list of engine specs...

Bogus considerations for "performance through ultra low compression" that the VW world has mistakenly followed have, for the most part, left such considerations out... a pity...

Take care,
Shad

P.S. I do not mean to point to anyone here with that remark, but the idea that the best CR for your normally-aspirated engine is 6.6:1 is laughably ludicrous.

builtbychevy · Post by **builtbychevy** » Sun Mar 24, 2013 5:12 pm

You guys are mind banging it too much. Pick out your heads and cylinders Have them prepped and machined to perfect specs. Figure out the best deck height & have it machined true. Figure out the biggest cam you intend to run (need to know your cam timing straight up or advanced 4* etc) (some circuit guys swap cams due to weather, track etc.) assemble the heads to the cylinders in a jig (split case would be best), wd40 and bondo your combustion chamber, scribe a line a crank relationship line. send your chamber mold to Ross or your favorite piston shop. And get your custom pistons made forged or hypereutectic's for your engine with your specs. Keep it simple stupid = KISS

I typically have each individual piston numbered and made to match each cylinder.

Keep in mind if you run coated pistons and heads that changes your compression ratio allot.

NextGen · Post by **NextGen** » Sun Mar 24, 2013 8:37 pm

WOW !!! Builtbychevy, it took you 12 years to come back with a replay, I think I am the only guy still around, ha.
Serious good to hear from you, but funny, it was like a lost letter from back in 2001 and the post man just found it.
Actually I like the Bondo idea, good information, hey and it is 12 years later we know alot more now.

benito26 · Post by **benito26** » Sun Mar 24, 2013 9:04 pm

Theres also a lot to be said about dynamic compression ratio taking into account the specs of cam and headwork for application.
I would love to build a zero deck dish piston 1700/1800 with 250*/254 cam, nice heads, 10:1 static and lightened assembly.

poop, I didn't realize the thread was so old..............

builtbychevy · Post by **builtbychevy** » Sun Mar 24, 2013 9:06 pm

I knew someone would look at it. I do realize it is old but it was the first thing that came up in my search. I use the bondo concept on a ton of engines literally. Allot of people don't know you can have custom pistons made with excellent results and little time spent screwing with it.

Shoptalkforums.com