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help desk message archives (one-lunger)
« on: October 31, 2008, 12:10:59 AM »

As they build up, then after a month or so of no posted we'll archive them here and http://www.teshio.com/oldski-doosleds/rr.htm

Engine Compression Tables
Engine Compression Tables

kingdavid--Anyone know a good source for compression level charts for 69-73 doos?  Specifically 299, 318, 335 singles and 340, 399 and 440 twins.  Thanks.
Jimbo Jessup--
Singles (I don't have the numbers for the 318)
299- 6.5:1
335- 7.5:1

299- 7:1
335- 8:1

299- 7:1
335- 9:1

1973- No changes

399- 8.75:1
399(TNT)- 10:1

399- 8.75:1
440(TNT)- 10.5:1

399- 9:1
340(TNT)- 11.8:1
440(Nordic)- 9:1
440(TNT)- 11.8:1

340(Olypique)- 9:1
399- 10:1
440(Olypique)- 10:1
440(TNT)- 10.5:1
gspaulding--Hello!  Just a few added notes.  If your altering compression ratio along with any exhaust port timing changes, these compression values are calculated as full stroke ratios not effective ratios.  Just wanted to mention that.
kingdavid--Great info!  Thanks.  I just picked up a compression tester and I have been testing all of my engines.......that turn over.  I am getting readings around 150 psi on all my one lungers.  Is that pretty standard and within effective specs?  I have one 400 with one cylinder at 150 and the other at 140....any concerns?  Thanks again.
lowtekrednek--Greg,  I would love that info- I think a lot of other guys would too.  if you email it to me I will post it- I think I can make it permanent
gspaulding--Hello!  I think I can fit it in right here.
Basically compression ratio is how many times the trapped cylinder and head volume at top dead center, will go into the cylinder and head volume at bottom dead center.  This is full stroke ratio and will be much higher than effective ratio because the entire cylinder volume (engine stroke) is being used for the ratio calculation.

But a 2 stroke doesn't start compressing mixture until the piston closes the exhaust port, and changing exhaust port timing (raising, lowering) can have a big effect on performance and of course compression ratio.  So if you raised your exhaust port 1mm, true (effective) compression ratio has changed but using the full stroke calculation (like a 4stroke) you will always show the same compression ratio.

So in the calculation, stroke (full stroke) should be used for a 4 stroke, and port height which is the distance from top of cylinder to top of exhaust port opening should be used (effective).

So the ratio calculation is pie (3.1416) times bore, times bore, times stroke, (full stroke) or times port height, (effective) divided by 4, divided by the combustion chamber volume at TDC not counting the plug threads equals ratio. (Move the decimal point to the left 3 places)

Example would be an engine with a 66.5mm bore, and a 63mm stroke, with an exhaust port height of 30mm and a combustion chamber volume of 17cc. using the calculation, this configuration would have full stroke compression ratio of 12.87-1 regardless of any exhaust port timing changes whereas the effective ratio would be 6.13-1 and would change with port timing changes.

The combustion chamber volume needs to be known of course and I can go over that also if anybody wants to know. But I think I've used up enough space for now.

luckyketch--Greg, I would be very interested in how to figure figure out the combustion area volume. We used to fill fill the head chamber with liquid (water), after removing it of course,  and approximate the volume that way,  but I am sure there must be a more accurate way.

I found this site and his method is exactly the way we used to do it when working on race engines. Did not know about the food coloring or the alcohol then. Wish I would have.

Here is the site:  http://www.merkurxr4ti.com/chambervolume.html

luckyketch--Not a problem. You can also find the squish area of a domed or stepped piston by using the same method only slightly modified.

Drop the piston down exactly 1" or 2.54cm from top dead center. Calculate the cc's at this distance. Then fill with liquid and and measure the amount of liquid required to fill cylinder up to plexiglass plate. Subtract this from the cc's calculated in previous step and you now have the volume filled by any irregular shaped piston, measure the thickness of a previously install gasket and calculate the volume this takes up. You now have all the info required in Gregs formulas.

The idea was to find out what the difference between all cylinders was. I can't remember for sure but I think was 1 to 1.5 % was acceptable.

This should all be calculated prior to final assembly cause it could get a little messy.
gspaulding--Hello!  Here's how I have always found combustion chamber installed volume to use with the compression ratio calculation.  You'll need a cc buret which is an accurate liquid measuring device,  dial indicator for finding TDC, measuring liquid for the buret, and a little grease.
A 50/50 mix of marvel mystery oil and parts washing solvent or gas works well for the liquid. It shouldn't be too thick or the liquid clinging to the sides of the buret after filling the combustion chamber can give you an inaccurate reading of volume.

First remove the cylinder head, wipe a thin film of grease around the upper area of the cylinder bore. Rotate the piston to TDC, then wipe any excess grease from around the piston.  This grease seals the ring end gap so your volume remains stable. Install the dial indicator and get exactly TDC. Lock the crankshaft/flywheel etc. in place somehow so the piston stays at TDC.

Install the cylinder with gaskets if any, and torque correctly. Use the buret to fill the combustion chamber with liquid up to the bottom of the spark plug threads.
Let the buret settle for a few minutes, read the volume used from the buret, and you have your combustion chamber volume needed for the compression ratio calculation.

Filling the combustion chamber to the bottom threads is fairy accurate, but to be dead on you need to measure the actual volume the plug displaces and then fill the liquid to the top of the plug threads and subtract the value the plug displaces.  That's another process called flat plate cylinder head volume measurement. But the process above will be very close.

Thanks!  Greg
luckyketch--1971 TNT 640 10:1 and 1970 Nordic Alpine 9:1
1970TNT--FYI  Compression Ratio Compression (psi) Range (psi)
6.5 96-114 18
6.6 98-116 18
6.7 100-119 19
6.8 102-121 19
6.9 104-124 20
7.0 106-126 20
7.1 107-129 22
7.2 109-131 22
7.3 111-134 23
7.4 113-136 23
7.5 115-139 24
7.6 117-142 25
7.7 119-144 25
7.8 121-147 26
7.9 123-149 26
8.0 125-152 27

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