Deck Height

First you measure deck height as if they were flat top pistons. Then you measure the piston dish separately. You add the dish volume to the combustion chamber volume in your calculations.

if you had domed pistons, it would get a bit more complicated. I use play dough: make a thick, flat plate out of the play dough, and press the piston crown against it firmly. The dome will leave an imprint, a "negative" of the dome. You would then measure the volume of that "dish" in the playdough. And deduct it from the combustion chamber volume in your calculation.

Jan

Hi Jan, Dished and domed pistons (called "bolas de fuego" (fireballs) around here) are very popular in the DR. Can you or someone else tell me why? do they provide more power? less heat? lower CR? what?

TIA, Karls

Dished pistons LOWER the compression ratio, to make the engine run cooler. Often found in buses, where the engine has to work harder. Came that way from the factory.

Domed pictons are the opposite, they raise compression ratio to produce more power. Used especially in 1300 dual port engines in Europe, factory installed.

Jan

On Fri, 02 Jul 2004 22:31:55 +0300, Jan Andersson scribbled this interesting note:

Didn't do too well in calculus, did you? (That's ok, neither did I!:~)

-- John Willis (Remove the Primes before e-mailing me)

Huh?

Don't know what sparked that comment, but all through highschool I royally sucked at calculus. Wasn't until college when it started making sense and I was in teh top 5 of my class. :) (I still didn't bother to study)

Jan

You are right here, but there have always been a number of people who believe there is a 'special' high compression 1300 dual port engine, which came with domed pistons. The increase in compression on the 1300 d/p was in comparison with the 1300 single port. All 1300 d/p engines have a CR of

7.5:1. I can't say for sure, but I imagine the only reason VW put in the dome, rather than change the pin height, was to maintain a certain amount of deck height. This might be the same reason they put in the recess on the lower CR engines. When they later reduced the volume of the head chamber, they also reduced the height of the dome, which suggests that the deck height is the important factor. Again, I am not sure, just guessing.

Area under a curve. Only way to measure it accurately without actually doing a physical measurement, so far as I know, is by using calculus.

I didn't have it in high school and really sucked at it in college. Haven't bothered to go back and actually learn it yet. Haven't had any real application in which I need it...yet. When I do I'll learn it!:~)

Of course, as you recently pointed out, experience sometimes wipes its ass with theory!:~) (So there may be differences if figuring the area using calculus and actually measuring the volume. It is always best, I think, to see what is actually there instead of assuming the part was manufactured correctly!:~)

-- John Willis (Remove the Primes before e-mailing me)

John Willis wrote in news: snipped-for-privacy@4ax.com:

When I was in Algebra I in junior high, I asked my teacher if I should take Algebra II the next year and his reply was try poison. Yep, nice guy, but I really did suck at math.

Amazing what a few years of the real world will do for ya. I had to take several math courses in Electronics School and something just clicked and now can solve some pretty ugly equations. But figuring volume is still a mystery. The only volume I am good at is the radio or TV. LOL

Now for another question, Why would you choose dished over flat top pistons? If you are changing them out, you could just as easily put barrel shims in to set the height and use flat pistons.

...but you would lose quence area and have a much more inefficient combustion process.

...Gareth

---------------------------------------------------

The most common reason in production engines is that it allows you to alter the volume of the combustion chamber without upsetting the valve train geometry.

VW de Mexico assembles its 'export' engines with flat-topped pistons that give a compression ratio of about 7.3:1 Aircooled engines for domestic consumption are derated to 45bhp through the use of dished pistons giving a CR of about

6.6:1 and allowing the engine to run reliably on Pemex regular gasoline, having an octane rating of about 73 (which used to be the world's standard for 'regular.')

In understanding why this works so well for Mexico it helps if you know that about 90% of all private vehicles in Mexico are registered in the Federal District (ie, Mexico City & environs) which has an average elevation of about

7,500'.

-Bob Hoover

Ah, right. What I remember from highschool advanced math was back row in teh class and Vodka&lemonade. Oops :)

That's what I thought, "I'll never ever in my life need these skills, so why bother". And I still haven't. (Highschool was like.. 14 years ago. Or so.

heh heh

If I had chosen the calculus method, I'd still be on it, trying to figure it out.

Jan

Don't know of any "special" 1300, the ones with domed pistons all came from cars that had nothing special in them. One or two of them were my own.

Jan

Potential leaks... maybe.

Or too confusing for the backyard mechanic ;)

Also increasing deck increases quench area (volume, actually), and results in an inefficient engine that has a tendency to ping easier.

Jan

I didn't mean to suggest that there were "special" 1300's. Only that some people 'think' that they exist, whenever someone mentions 'domed' 1300's. It seems that there are many people who just assume that all 1300 pistons are flat, presumably because that was how the single ports were, and how the aftermarket pistons for the dual ports came. So when the subject of 'domed' comes up, there is an assumption that this is rare. I think it all started with the issue of a 1972 1300 special edition which looked sportier, but ran just the same as the rest.

Jan Andersson wrote in news:2l0ueqF79l0gU4 @uni-berlin.de:

I thought shims were the standard for setting the CR?

...things get standardized for many reasons. It is a a lot cheaper to slap a shim under the cylinder than to increase the combustion chambvr size..and/or dish the piston. And for the majority of low rev engines that the CR is being lowered on for street usuage with normal gas...the loss of efficiency does not much matter.

..Gareth

Standard, because it's easy.

Doesn't mean it's always the best way. ;)

Jan

On Thu, 08 Jul 2004 23:41:55 +0300, Jan Andersson ran around screaming and yelling:

yep...a good example of this very same practice would be the "hardstart relay"..... JT(flame me Speedy)

....................Jim doesn't do flames and I'm busy.