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Compression Testing


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It has come to my attention the way Compression was checked at the last race was not normally the way STS has done it in the past. I am reviewing the procedure for checking compression. As soon as I have time to review all the facts and procedures I will place the procedure on this site. I should have it up by Saturday at the latest.

 

Ken Hobbs

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Not sure about the IMCA but that's the way I've heard of compression checking for as long as I remember. (One plug and the bumps) but we'll see what the track officials come up with. Sorry to hear about the dq, it was a hell of a race and clean.

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Update:

 

I am finding this to be much more interesting then I previously thought. A few items I have found are below. These will be apart of my final findings but may or may not be the final decision.

 

The reason you check compression has a lot to do with the way you check compression. If you are looking to see if you might have a bad motor or cylinder then it does not matter if the engine is warm or cold, air filter is on or off, if all or just one spark plug is removed or if you open or close the throttle. The only thing you look for is drop and or a change in compression from one cylinder to the next.

 

However, if you are testing the motor to get a true compression ratio all engine builders and true technicians I believe agree you must follow different steps. The compression rate is determined by how many molecules are in cylinder compared to the pressure generated by the piston moving up and down. Any physical or environmental element that affect how many molecules or movement of the molecules will affect the compression. So if you were to test a motor with the throttle closed then the size of the carb will play a role in the compression rating. All carb's stay open to some degree even when closed so there will be a variance in the reading. This means that also how much fuel is applied during testing or right before will also play a role in the compression test. (Flooding the engine before testing). Other items that can affect the reading are humidity and temperature. I am unsure if removing one versus all of the spark plugs play a role in the rating. I am also unsure if the speed at which the cylinder moves plays a role. More testing is needed.

 

I am still reviewing data and will make a decision this weekend. Thank you all for the patience.

 

Ken Hobbs

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Ford Motor Company procedure copied out of service manual.

 

Compression Test — Compression Gauge Check

  1. Make sure the oil in the crankcase is of the correct viscosity and at the correct level and that the battery is correctly charged. Operate the vehicle until the engine is at normal operating temperature. Turn the ignition switch to the OFF position, then remove all the spark plugs.
  1. Set the throttle plates in the wide-open position.
  1. Install a compression gauge in the No. 1 cylinder.
  1. Install an auxiliary starter switch in the starting circuit. With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of 5 compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.
  1. Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.
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This may be true for Ford.. It still isn't the way the majority of the mechanics, engine builders, and all tracks that I've spoken to do it.. That what is the whole point here.. The common standard seems to be 1 plug, 5 hits, no throttle..hello why should sts do it different.. Jmo

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