Piston shape and detonation.

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The Badger
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Piston shape and detonation.

Post by The Badger » Wed Dec 12, 2018 9:03 pm

So ive spent the last few days going over what i could find on the internet and what few papers i have on the stuff and ran across the "super squish" pistons that Larry Widmer( "oldone" on the fourm i believe) developed. From what i researched, it seems as if it started with nascar in the 80s and 90s and spun into the air cooled vw market. Now while reading up on this i came across alot of people saying that the cam and chamber had to be just right for the pistons to work. I also ran across a few pictures of what i imagine the pistons are. My questions are as follows.

1. If cam events matter what would it be? Is the cam required to bleed of the excess compression? So static is high but dynamic in the normal range. Or is this piston making it so both can be higher?

2.are we just aiming to make the most squish area by using intake dome to force charge over to exhaust?

3.is there a noticable amount of charge that gets stuck on the other side of dome and is unburned almost like in the top ring land?

hoffman900
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Re: Piston shape and detonation.

Post by hoffman900 » Wed Dec 12, 2018 9:18 pm

On a 4 valve engine, you need to look at what the sportbike manufacturers are doing. They're routinely selling street bikes from the factories with 12.5:1-13:1 compression for pump gas, on bores as large as 4.40" (Ducati).

Here is the Ducati:
Image
Image

A lot of it is in the mixture motion in the chamber. Ducati has been students of Keith Duckworth and were noticeably ahead of the Japanese in refining this technology (obviously the Japanese finally figured it out and refined it too). The other is modern EFI and engine management systems (being able to run the engines right on the edge of detonation). Also, lots of time spent on cooling passage design and removing heat helps quite a bit.

The rules do apply, lower smoother domes are always better. Squish is probably as much as you can get away with. The other is run the top ring as high as you can, safely. Look at a modern OEM piston design or a highly developed NA race engine (NASCAR). Very little crevice volume.

I know some people have a lot of respect for Larry, but some of his accomplishments were found to be false (like the SCCA MGB he told a story of on his site), the VW guy seems to have some of the same black magic thing going on too. Reading about either leaves me with the "that's kind of what's happening but not really" feeling...
-Bob

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Re: Piston shape and detonation.

Post by hoffman900 » Wed Dec 12, 2018 9:47 pm

I think another good example would be the NASCAR engines, for 2 valve applications. They're spec'ed at 12:1 compression ratio, and the valve angle of the heads affect the shape to some degree, but no doubt they've poured tens of millions of dollars in improving the combustion efficiency.

Image

and chambers like this:
Image
-Bob

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Re: Piston shape and detonation.

Post by Truckedup » Thu Dec 13, 2018 6:25 am

double post
Last edited by Truckedup on Thu Dec 13, 2018 6:28 am, edited 1 time in total.
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Re: Piston shape and detonation.

Post by Truckedup » Thu Dec 13, 2018 6:28 am

Truckedup wrote:
Thu Dec 13, 2018 6:25 am
Does NASCAR specify valve arrangement or combustion chamber shape?
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The Badger
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Re: Piston shape and detonation.

Post by The Badger » Thu Dec 13, 2018 7:44 am

Truckedup wrote:
Thu Dec 13, 2018 6:28 am
Truckedup wrote:
Thu Dec 13, 2018 6:25 am
Does NASCAR specify valve arrangement or combustion chamber shape?
The limit compression now, think that started in 90s. Used to be run as much as you want.

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Re: Piston shape and detonation.

Post by Newold1 » Thu Dec 13, 2018 8:15 am

My feeling about "SQUISH" is that the squish activity in the cylinder takes place when the flat top areas of the pistons come up and meet areas of the flat head deck surfaces that are not open to the chamber itself. The possible changes that can make differences here are the size of the gap in these areas as the piston approaches and reaches TDC and the remaining top shape of the top of the piston open to the chamber. Another thing to consider is that in some combustion chamber sizes and shapes is that domes depending on shape and size can actually interfere with the action of squish. I think this is the reason in newer head design chambers and piston top shapes the tendency now is to make the chamber smaller and more centered and reduce or eliminate domes as much as possible. JMHO
Last edited by Newold1 on Thu Dec 13, 2018 8:16 am, edited 1 time in total.
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hoffman900
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Re: Piston shape and detonation.

Post by hoffman900 » Thu Dec 13, 2018 8:15 am

The heads are homologated pieces, but are a blank slate circa about a decade ago. The chamber can be any shape, just that the valve included angles will dictate the shape. That said, being blank sheet designs, it’s all optimized.
-Bob

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Re: Piston shape and detonation.

Post by Truckedup » Thu Dec 13, 2018 8:58 am

I know you all hate bike stuff, but my vintage Triumphs are a perfect example of high dome pure hemi chambers....

This is a piston shape used by my competitors about 12-1

Image

This is what I use, 10.5 compression

Image

I'm just a hobbyist engine builder but it seems obvious that my pistons give up compression for a faster flame travel, quench and makes the same power with less compression ,milder cams and smaller carbs...Those tall domes are just heat collectors and I believe interfere with the way the air fuel mixture enters the cylinder
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Re: Piston shape and detonation.

Post by hoffman900 » Thu Dec 13, 2018 9:16 am

I agree with you 100%.

I think domes like that are just builders not seeing the forest (good combustion) for the trees (compression, more is better!!!).
-Bob

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