ET: Engine (Twin) ET35 to valve guide clearance

[bmetcalf]

My sidevalve 1931 Ford has almost flat top pistons and 7:1 compression. I assume the Sunbeam is OHV, I wonder why the high peak?

Regarding the Sunbeam, imagine what a mold of the combustion chamber would look like, an orange peel?


Edit: I meant 6:1 compression

 

[davidd]

That's an interesting piston David. Is that a ceramic coating on the crown and skirt? The compression ratio used is also interesting. Another tuner, I know, used a different shape of squish/quench band and found that 10.5:1 was too high and the engine was happier on 10:1. I take it that you are using petrol/gas intended for road use rather than alcohol?

It is ceramic on top and Teflon on the side.

The cam timing brings the compression ratio down to 8.1:1. The compression ratio on the box just tells you how tall the box is. It does make the combustion chamber quite small.

Bruce,

The last time I wrote about the rockers I used the correct spelling but looked it up. I now remember it was not the correct terminology and I rewrote it. But, I can't remember what I wrote!

David

 

[timetraveller]

Hi David, I'm wondering about the statement about your compression ratio. To me compression ratio is a geometrical thing, Displacement plus combustion chamber volume above the piston crown divided by the combustion chamber volume. I do realise that cam timing can release pressure, hence the need for small overlap values on supercharged engines, but I have never come across an affective compression ratio being expressed before. Interesting. I wonder how this is determined?

 

[BigEd]

Hi David, I'm wondering about the statement about your compression ratio. To me compression ratio is a geometrical thing, Displacement plus combustion chamber volume above the piston crown divided by the combustion chamber volume. I do realise that cam timing can release pressure, hence the need for small overlap values on supercharged engines, but I have never come across an affective compression ratio being expressed before. Interesting. I wonder how this is determined?

I seem to remember the Japanese two-stroke makers used to show quite low compression ratios in their specification sheets for engines but I think these were calculated from when the piston closed the (exhaust?) port. Geometrically the ratio was much higher.

 

[davidd]

Norman,

You are of course correct, but the advertised or static compression ratio (CR) calculates the ratio based on the intake valve being closed at BDC. As we know, this is not the case but it is the industry standard. We know what the stroke is 3.543" and that the advertised CR is based on the intake valve being closed for the entire stroke.

What the dynamic calculation does is it relates the stroke to the compression. If the intake closes at 51 degrees ABC the stroke in the formula is changed from the stock 3.543" to 3.02" because that is when the valve closes and the compression starts to build in the cylinder. They call this "Effective Stroke."

I often use this site for the calculators:

Wallace Racing - Automotive Calculators

www.wallaceracing.com

The dynamic CR calculator is under "D."

Here is an example with the Megacycle Mk2:

When you push "Calculate" the info disappears and a new screen pops up:

It's best to capture a screengrab of the first screen because it will disappear with the new calculation.

The calculator is using the same formula that we are familiar with, but it is changing the stroke to match the valve opening.

The value is using this information to match the fuel to your CR. You don't want to use fuel with a higher octane than necessary because its volatility is suppressed the more the octane is boosted. It is also important as general information as someone who is running a 9:1 piston with a Mk2 is getting somewhere around 7.5:1 DCR and not the high performance that they may be expecting.

David

 

[timetraveller]

Thank you David. Always good to learn something new.

 

[Bill Thomas]

So if I tell my 9 to 1 s they are not that high, Will it be more easy to start ?? .

 

[LoneStar]

Interesting information! But, doesn't the scheme using the inlet close just substitute one arguably meaningless figure for another?

After all, what you care about is the maximum pressure of the compressed mixture. The inlet valve is kept open after BDC for the purpose of allowing in extra mixture at speed, from the momentum of gas flow. Rather than reducing compressed pressure, this cam timing is designed to increase it.

So, the "effective stroke" calculator gives a lower CR that's not reflected in compression pressure while the engine is running. Not very useful. Or am I missing something?

 

[bmetcalf]

Good topic to discuss over a pint!!

 

[stu spalding]

Ta very much Bruce, mines Theakston's Old Peculier. Cheers, Stu.