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Thu Dec 11, 2008 8:55 pm

What is the purpose of a dished piston? What setup would this benifit?

Thu Dec 11, 2008 8:56 pm

They lower the compression.

A good idea for a Bay.

Thu Dec 11, 2008 9:16 pm

Why is a lower compression better for a bay?

Thu Dec 11, 2008 10:07 pm

1600 dual port right?

dished pistons & the resulting lower CR will generally lower temps compared to stock if you stay within 2000 ft of sea level. be moderate with lowering, 7:1 is the limit IMO. but having done it both ways, i would recommend lowering CR in combination with dual carb conversion.

if you live above 2000 ft. do not lower CR. i have seen what happens- it runs much hotter. plus the science shows that cylinder pressure at TDC is determined by CR, altitude and intake system among other possibilities.

CR lowering requires a balance of these factors. in other words- regarding engine temps, as you go lower and lower with compression, there is a point of "diminishing returns" (if thats not too techy for ya). that point is where efficiency is lost.

But that is not the last word on CR. Jake Raby has found much higher efficiency and lower temp in ratios around 9:1 or higher, not that i want to put words in the mouths of others.

Thu Dec 11, 2008 10:10 pm

Lower compression = less heat Less heat = longer engine life longer engine life = more trips in the bus

Thu Dec 11, 2008 11:15 pm

Lower compression = lower thermal efficiency = less power and less gas mileage.

Higher compression = higher potential for knocking and preignition.

Fri Dec 12, 2008 7:00 am

Too low is bad and too high is also. There is a nice mid-point. You want the engine to be producing power efficiently. And you don't want it creating excess heat or damage from pre-ignition. Two of the top pro builders here had different recommendations for a T1 bay bus. But the compression ratio range was 7.0:1 to 8.0. Both were considering durability, performance, and heat issues. It doesn't get any hotter elsewhere than here. A puny low compression, low power engine is going to have to work harder to push the bus and because of that it will run hot. I just isn't efficient. It just doesn't burn gas efficiently. Not much different than a heavily worn engine. In considering the whole thing, I went 7.5:1 and have been completely pleased. It runs cool and strong. With the single stock carb and also the dual carbs. It runs cooler and stronger than my bug which has a lower CR. I'll have to dig through the old records to find out what that C/R was. Regardless both have flat pistons. Dished pistons would have been totally out of the question. It's really the other factors involved that are just as meaningful in establishing the C/R. The cylinder mating surfaces on the case and the head chamber volumes. The measurements and calculations once you know the deck heights determine what compression ratio you have. If it's too high you can use barrel spacers to lower it. But a good head rebuilder will flycut and step cut the heads to give you the ratio that you choose. With ignoring all that and just switching from flat to dished pistons could easily end up in real problems.

Fri Dec 12, 2008 7:17 am

Wildthings wrote: Lower compression = lower thermal efficiency = less power and less gas mileage.

Higher compression = higher potential for knocking and preignition.

Blanket statements on this topic can get people in trouble.

I won't say more, because it'll start a war that I don't have time for.

Fri Dec 12, 2008 7:45 am

Glenn wrote: They lower the compression.

A good idea for a Bay.

Jake Raby wrote:
Blanket statements on this topic can get people in trouble.

Fri Dec 12, 2008 8:19 am

if you lack the skills to lower the cr by shimming or chamber reshaping or semi-hemicutting, I would think they are a good 'bolt in' solution. if you keep everything else stock I believe it drops the cr to 7.25-7.35. Someone had the actual number listed recently, a search should turn it up.

Fri Dec 12, 2008 8:21 am

Desertbusman wrote: Glenn wrote: They lower the compression.

A good idea for a Bay.

Jake Raby wrote:
Blanket statements on this topic can get people in trouble.
VW did it from the factory.

I stand by my statement for stock applications. Hi-po is a totally different stock. Last year i put a 2276 with 9:1 in a Bay SC. The owner does not expect to get 100,000 from it and doesn't plan on loading it with 1 ton of cargo and haul it through the mountains.

Fri Dec 12, 2008 9:49 am

fusername wrote: if you lack the skills to lower the cr by shimming or chamber reshaping or semi-hemicutting, I would think they are a good 'bolt in' solution.

yes thats a good way of thinking about dished pistons. the exception being that dished pistons may lower too far with new heads. so to raise it, just flycut the right amount and you are there

my belief is that dished pistons are the way to go on the low CR application for the following reasons:

1) with chamber shaping its too difficult to remove enough in the right places. in addition, it amounts to hit-or-miss experimentation with chamber science.

2) cylinder shimming leaves pockets of unburned fuel behind (in theory), which only hemi cutting would solve. but hemi cutting just isnt preferable to having the quench.

therefore it seems to me that dished pistons are the only way to have: low deck quench, lowered CR and not be messing with the chamber science.

Fri Dec 12, 2008 10:38 am

and to translate for the less technical minded, check the inside of your heads, ie the chamber. if there is a raised step all around the edge where the cylinder sits, you have an after market head which already has a slightly lowered CR. so dished pistons may be a bad idea unless you get the step flycutt out.
check that your heads have not been hemi cut, meaning a semi circle has been cut into the head. a little harder to explain, but if the head has a small chamber in the middle surrounded by two big flat areas, it has not been hemi cut. if the center has been dished out, and basicly the only flat surface is right around the edge where the cylinder will sit, then it has been hemi cut. if it has been hemi cut, you don't want the dished pistons. if it has not been, the dishes are not a bad idea.

if anyone sees an error in what I just posted, or has pics of the heads, it would be a nice addition.

Fri Dec 12, 2008 11:32 am

Just an FYI,

There are more than one type of VW T1 Engine dished Pistons, one I believe was VW Germany OEM in the 70’s, and there is a VW of Mexico version with a bigger & deeper dish in the pistons 12-13cc, I do not have the cc for the other but from the pictures I have seen they are smaller…

It would be interesting to know how many cc where taken out of the other type of OE VW dished pistons, just to get an idea of what VW Engineers thought was good for T2 busses with higher quality fuels… :0)

PS I belie that Mahle Also made Dished Pistons; on their advertising they where promoting better fuel mixture(=efficiency) as a result of the dish pistons creating a swirl affect looking somewhat like “00” in the combustion chamber…