Compression ratios? 4

[gtrack]

Sorry to open up a much debated question. Please bear with me.

I underatand how to obtain a ratio, but I am stuck on finding the piston dome volume. Easy if you have a dish or a dome only using the 1 inch down method BUT........

I have a piston which has a dome, different depth valve reliefs and both flat `lands` are at different heights (its out a m/cycle motor) so where do I take the 1 inch measurement from?? (obviously this will also alter the total burette cc......

If I take it from the crown (highest point) I dont think this will measure the dome, only the arear around the dome but I cant find the lowest point as valve releifs are rounded and slanted....

Any help, much appreciated.

 

[NormPeterson]

Which surface is the piston's compression height measurement taken to?


Norm

 

[evelrod]

The pistons I just had made for our Lotus are similar and I had the same problem.
One, I put one in a cylinder and did the 'one inch down'.
Next, I wrapped the piston with a sheet of plastic and took direct measure. Lastly, I took an assembled combustion chamber volume through a spark plug hole.
Averaged the three and called it "good".

If you ask the piston maker, they can usually tell you the approximate volume from experience...even they often do not know exactly. With large valve sizes and extra deep cut outs in a hemi, the volume of a large pop up can be cut in half, easily.

Rod

 

[gtrack]

Guys, thanks.

its a 4 valve jobby.
The compression height is I think taken from the left hand flat, it is closer to the top of the barrell by about 1mm.

Thanks

 

[ivymike]

does it matter where you measure "1 inch down," as long as you measure it in the same spot and move the piston an inch from TDC?

 

[NormPeterson]

1" down is a position rather than a movement.

 

[ivymike]

okay, so what's this 1" down method, and why does it care?

If it's "cc the piston with it positioned exactly one inch down in the cylinder bore. Fill the area above the piston with fluid, then subtract this volume from the calculated cylinder volume at one inch and you get the piston dome displacement in ccs. " then I'm going to stick with my original comment - pick any point on the piston at TDC, and move that point down an inch. (all the other points will be down an inch from their TDC positions too! Friggen amazing! It's almost like picking a datum or something!)

 

[patprimmer]

I'm with Isaac

Go to TDC, move 1" down an measure volume compared to the displacment from 1" of travel.

That gives the volume displaced from the chamber at TDC

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gtrack]

Basically thats what I need to know. Does it matter where I take the `1 inch` measurement from as long as all the piston is below the deck?

But then again, if you take it from the crown aren`t you just measuring the area around the dome and not what the dome itself displaces? Arent I looking for the dome displacement to take from the head chamber cc, not the area around it as at TDC you have already measured a portion of that in the head cc??

I`ve tried the piston 1 inch down from the crown and filled the area with liquid from burette = 73cc.
Now if I leave the piston where it is and make a calculation from 1 inch from crown and then same but for distance from flat lands I get different results, i.e. 4cc???

Too much math is making my brain hurt!

 

[NormPeterson]

That's why I asked about compression height. As far as I know, any dish or dome volume is relative to the plane of that surface. It is possible to have some of each, but for CR purposes you only need to know the net. Anyway, set that plane 1" down the bore and measure the volume. Compute what 1" of cylinder bore should be giving you. Dome/dish volume is the difference.

Maybe it's just a matter of semantics, but I prefer to think of setting to a position relative to the deck surface rather than TDC, because the two don't necessarily coincide once you've assembled the short block.


Norm

 

[patprimmer]

By measuring from TDC you get the dome and deck clearance in one go. Then you only need gasket and chamber volumes.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

it's important that you do it the other way, if you want the right answer. If you're just picking a particular point on the piston and putting it 1" below the deck height, without regard to the distance it must travel from TDC to get to 1" below, then you have no way to calculate the TDC volume of the cylinder. The answer in that case is (volume in the head) + (volume in the cylinder with the piston wherever you put it below TDC) - (cylinder area * that unknown distance from TDC) + (cylinder area * head gasket crushed thickness)

If you start at TDC and move down an inch, then you can always calculate the volume at TDC as (volume in the head) + (volume in the cylinder at 1" below TDC) - (cylinder area * 1") + (fire ring area * head gasket crushed thickness)

Depending on design, the head gasket crushed thickness can sometimes be the hardest part... but it's probably more important for getting piston-to-valve clearance right than for getting compression ratio.

 

[evelrod]

Good grief guys. What is this, an eighth grade remedial math.

It's obvious Pat has done this before and Isaac IS a math whiz, but, come on. This is really not rocket science. I've been doing this since I was 18 and I learned it from an engine builder that never even graduated primary school.

The one inch down deal is just figurative. If one inch down clears the dome, fine. If it does not, as in the case of many motorcycle pistons I have built, then two inch or whatever. My math sucks, that is why I always use three methods to find the end CR.

I tried a computer program once, but it was so convoluted that I got lost half way through. The easiest way on a hemi or pentroof is to just fill it from the plug hole (provided you already know bore/stroke/cylinder head volume)

Yes, Isaac. Valve clearance is most important...especially if, like me, you are using camshafts with radically different timeing and lift. But, it's pretty difficult to get the CR correct on the first try if you have nothing to go on, a previous piston or such. Valve clearance cuts just throw the CR calcs into a 'cocked hat'. Trial and error, very expensive T & E !

Rod

 

[ivymike]

hey, five sets of pistons for an I6 still cost less than a week in a test cell... can always measure running (hot) clearances and gaps with some porcupine pistons (roll pins all over)... but I still forget that 90% of the threads in here are garage projects.

 

[gtrack]

Guys, thanks for the responses. Think i`ll try all of them in one way or another and see what happens just out of interest.
I guess static compresion means little anyway in the grand scheme of things once the motor is hot and running at xxxx amount of revs, but I couldn`t get that damn 1 inch thing outta my head!
As it happens I used over an inch last night to get the dome in, all clearances seem to be ok, i have .080 on the valve to piston.

Ivymike, that last post, your right, I had enough trouble with this let alone something of that nature, i`ll leave that to you NASA types!!

 

[SomptingGuy]

NASA types??

Running engines on beds is not rocket science. All OEMs do it. It does cost a lot, but then so does type approval.

- Steve

 

[evelrod]

Thanks, Isaac. That's a good idea...Roll pins...I'm sick of modeling clay cause half the time I have to many 'do overs'.
Clay sucks!

Rod

 

[gtrack]

Steve, the NASA thing was just a euphemisn. Bearing in mind Im working in a 14ft square wooden shed!

Incedently, what does everyone use as fluid to measure the cc? I use an old barrell of red x thin fuel additive purely because I have boxes of the stuff and it seems to give me the same result as dyed alc.

 

[patprimmer]

Anything that flows freely, is easy to see and does not tend to froth and bubble or evaporate to fast. Kerosene with a dash of ATF for colour works fine.

Heck, beer would work except for the froth and the wicked waste.

I have used petrol, but it evaporates a bit fast.

I have used cleaning solvent from the parts washer.

Use clay or plasticine for the piston to valve and piston to head clearance.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

I use Marvel The Mystery Oil...I use it as a thread lube...I sometimes use it as a substitute gun oil...I use it as a water proofer for my work boots...it's a great weed killer...;-)

I've used ATF and water with Redline Water Wetter once in a pinch (dish soap 'bubbles') but I do not recommend it.

Pat, "clay sucks"!!!! I drilled an old piston yesterday for a generic roll pin just to see how it would work...I like it!

Rod