12:1 piston questions/compression

[headache]

does anyone know if you can take the 12:1 je piston and machine the dome down to around 11:1 compression. would like to build a stock bore motor. i've already built a 366 on one of my other warriors and really don't want the .08" overbore again. would like to build an 11:1 motor without taking out or adding any gaskets. I've thought about contacting Arias about building a piston for me since they built the old white brothers pistons a few years ago. I just don't know what they would charge (prob wouldn't be cheap). any thoughts on this would be appreciated.

 

[beastlywarrior228]

Ya you can face it on cnc if you can figure out the compression cc to piston height or you could have je or cp make a custom piston

 

['00 WO-YA!]

I'm gonna have to agree with Beastly in this one, the best easy to accomplish what you're after will be to mill the piston dome enough to get the compression you're after.

 

[headache]

well thats what i was thinkn too. i am a machinist and can machine the dome no problem myself. I wish i wouldve measured the height of the dome on my old 10.5:1 piston and compare it to the 12:1 and would be easy to figure out an 11:1 ratio. does anyone have a piston layn around to measure?

 

[Gen1Pat]

Talk to weston. He just got his 12:1 and still has an old 10.25:1 wiseco.

 

[duneracerweston]

Ive got more then a few wiseco pistons, im in the process of getting my 83mm 12:1 to 11.5:1 so we shall see!

 

[beastlywarrior228]

Ive got more then a few wiseco pistons, im in the process of getting my 83mm 12:1 to 11.5:1 so we shall see!

Id still say thats too much for pump especially since you have the timing advance

 

[Gen1Pat]

Btw headache, if you are wanting to keep the gaskets but raise compression on a 10.25 or 10.5:1, you can do what I did and take .015" off the head and run a thinner head gasket. I'm a total .022" decked. Don't go more then .020" on the head though, after that you start screwing with the valve seats. But .015" works well.

 

[bones]

Id still say thats too much for pump especially since you have the timing advance

i run 11.5:1 on pump gas .. ultra 94 from sunoco with no problems what so ever

 

[duneracerweston]

Id still say thats too much for pump especially since you have the timing advance

How many times do I haveto say this?
Knock caused by timing advice is a spark knock not predetination.

 

[beastlywarrior228]

Its still building more heat in your motor

 

[duneracerweston]

So? The about of heat gain your talking is minimal, and with a advance spark you can run it richer due to the 4 degrees of extra burn time. So over all your talking 15* maybe.

 

[headache]

Btw headache, if you are wanting to keep the gaskets but raise compression on a 10.25 or 10.5:1, you can do what I did and take .015" off the head and run a thinner head gasket. I'm a total .022" decked. Don't go more then .020" on the head though, after that you start screwing with the valve seats. But .015" works well.

thanks man that was my next question. do you know how much the compression raises by machining .015" off the head?

 

[headache]

I was just thinking that it might be better to run the wiseco piston and machine the deck of the cylinder some if the piston is below the deck surface. motor would respond better to having 0 deck clearance if possible. gonna see if i can find a formula for figuring compression and see what i come up with.

 

[headache]

hey weston post a picture of the 12:1 piston that you have if you can. would like to see how large the dome is on it.

 

[bones]

COMPRESSION RATIO FORMULA


1. Determine the displacement of your engine. Displacement formula is:

BORE X BORE X STROKE X .0031416 = DISPLACEMENT

Example: 92 x 92 x 82 x .0031416 = 2180cc

2. Determine the swept volume of one cylinder:

Example: 2180cc divided by 4 = 545cc

3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.

Example: on cylinder #1 you measure and find you have .020" deck height.

BORE X BORE X DECK HEIGHT X .01996 = CC"s

Example: 92 x 92 x .020" x .01996 = 3.378cc's

Measure each cylinder.

4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.

5. You now have all the measurements to determine your compression ratio.

one cylinder swept volume + deck cc + head cc

deck cc + head cc

Example: 545 + 3.378 + 48 = 596.378

3.378 + 48 = 51.378 = 11.6:1

In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems.

from: http://www.greatplainsas.com/icompratio.html

 

[headache]

thanks bones. thats what i was looking for.

 

[jasonp]

How many times do I haveto say this?
Knock caused by timing advice is a spark knock not predetination.

spark knock is detonation. detonation can occur from advanced timing.
Pre ignition happens before the spark.

 

[jasonp]

So? The about of heat gain your talking is minimal, and with a advance spark you can run it richer due to the 4 degrees of extra burn time. So over all your talking 15* maybe.

No you don't get 4 degrees of extra burn time and no it really fuel mixture doesn't have anything to do with it. There is a point where if you start to run to rich (same goes for to lean) is your heat goes up. You start the ignition cycle 4 degrees more advanced and basically when cylinder pressure peaks. Which can cause some problems if you are raising cylinder pressure and starting the ignition closer to cranks downward stroke. Something to think about. As you add compression and speed up burn time and the burn cycle is more violently because of this you need less and less ignition advance.

Per the manual. ignition timing peaks at 5000 rpm at 33 degrees advanced. That's a pretty good amount already. You add 4 degrees to that 37-38 total advance. That would most likely be ok for a stock piston but man you are really getting at a level where can loose power.

The most I would want to see or tune for is 33-34 total max and hold that on through the rpm on anything above 11:1.

 

[jasonp]

BTW: stock stroke/stock bore you need to have 33.2cc combustion chamber volume to achive 11:5 compression.
34.8cc for 11.1
36.7cc for 10.5

considering the stock head is 36cc + the deck and head gasket height or volume you may want to think about how much you take off that piston....

85mm piston.
34.8cc for 11.5
36.6cc for 11.1
38.5cc for 10.5

see how bore effects compression ratio? Taking the base gasket .015" on a stock bore isn't going to do much.