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wisconsin compression data

tew45

Registered
I am starting a rebuild on a TFD and would like to know compression ratio before and after break in. I always like compression data to help analyze engines. Any info would be appreciated.
 

I like oldstuff

Registered
Last Subscription Date
11/09/2015
The compression ratio by design will remain the same. As to the psi generated, have a look at the manual for the number.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
I likke oldstuff is quite correct. The compression ratio won't change as the engine breaks in. It typically is around 6:1 though. As for compression pressure, there should be a spec in the owners service manual. i would think about 80 - 90 PSI, if there is no spec, and your engine hhas an electric starter, A manual cranked engine needs about 6 compression strokes minimum for a close reading. As for a differential between new and broke in, I would not expect more than a 10 PSI diferential. Pressure loss between new and broke in should not really change unless there is excessive depth honing present.
 

tew45

Registered
There is a difference from new to break in usually a lot due to rings not seated. I used to rebuild some two cycle engines with a new top end from India. Some had to have the end gap adjusted more than usual. The compression varied 10 t0 15 psi but after a tank of fuel all was up to standard. I tried to figure the compression PSI by using the ratio and it is usually fairly close but that can vary a bit and is theoretical and not actual. Thank you for the input, it was very helpful.
 

I like oldstuff

Registered
Last Subscription Date
11/09/2015
Yes the ratio theoretically can provide an approximate psi. But it's highly variable by valve timing.
If we had three engines in good condition of different makes with 8:1 compression they all will return a different number.
 

tew45

Registered
If my engine is not too expensive to rebuild, I will post compression readings for those folks that want them.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
C-R is the amount of swept volume (cylinder diameter and stroke), divided by the head space at the top of the combustion chamber. 8:1 can be made by any combination, pressure will increase by bore and stroke changes, as well as piston top and combustion chamber shape and design. In most small 4 stroke engines, the piston is flat topped, and the 'crush space' made by the combustion chamber (head) determines the ratio of compression. Valve timing will affect pressure. The more overlap, the greater the loss, however at higher speeds, this loss is not as great as to affect performance.
 

eddie bedwell

Registered
Hi Team,
just saying is all and offered simply as an example, my old Dyke's book gives an average for any auto engine of the era (most likely flat head--side valve) as a guide being 65-70 PSI.

Remember that the throttle plate must be full open for meaningful tests over about 4-6 compressions, or until a steady maximum reading is achieved, as pointed out above.

On these governed engines the throttle plate should be fully open anyway.

For manual speed controlled engines the throttle plate will likely be normally closed--idle position, so ensure it is fully open or the inlet air is restricted and results in lower compression pressure readings.
Cheers,
Eddie B.
 
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