IMCA Sportmod compression check, need help!

[dirthammer]

QUESTION?

The 2009 IMCA rules for sportmod state under Rule #24 "ENGINE SPECIFICATIONS", second paragraph titled "BLOCK" ;

Maximum 175 lbs. per cylinder compression, no tolerance. Compression checked with one spark plug removed, turned five engine compression revolutions.

My question is, If the compression is checked with more than one plug removed , lets say three, and you only turn the engine over very slow with a low battery will you get an accurate reading? What's the reason for the "one plug removed" and the " five continuos revolutions"?

 

Thank you for your comments!

[Off The Wall Racing Services]

The one plug removed is for the installation of the compression gauge fitting. if other cylinders plugs effect your compression on the cylinder your checking, you've got worse problems like bad rings, or head gaskets. a stroke is a stroke whether it be fast or slow, but it must be 5 compression strokes to get an accurate reading. atleast with all of the ones I've ever done.

[1-Crew]

In a perfect world, no.. The "five cycles" allows the gauge to read the highest pressure reached over an average number of cycles. If you have a lot of blow by on the rings or a bad valve, turning slower will reduce the measured compression. Removing three spark plugs would potentially increase the cranking speed and under the same conditions, raise measured compression pressure. In either case, the measured compression should be maximum (and accurate for the test) at minimum blow by or leak down and maximum cranking RPM.

[thumper]

I cant speak for IMCA,Im sure they would answer your question if you called them.

 

About compression tests,they are not always accurate,different guages read differently.But there must be something done to try to get the readings as uniform as possible.The speed at which an engine is cranked will influence the reading hence the 1 plug removed to make the test more equal for all cars.As far as trying to equate a low battery condition with more plugs removed thats impossible to regulate or measure.Its similar to testing engine displacement with a P&G meter on a hot engine.The tempreture must be calcuated.On an air cooled engine thats impossible so all the P&G tests I have had done with air cooled engines were on cooled off engines.

 

Whats intresting is no mention of throttle position,a closed throttle will create resistance to air flow so all tests done in my shop are done at WOT.Also unless the test is done on random cyls,a smart engine builder can create a given cyl with a different reading.I know engine builders that will build one bank with flat tops and the other side with pop ups.

 

IMCA used vacuum at one point but cam builders easly got around that rule with special profiles.What is comforting is the fact that on round tracks and especially dirt a small percentage of horsepower advantage means little because circle track races are won or lost in the turns and theres no such thing as equal cars whose lap times are influnced by a few horsepower.

 

In my racing I love when people are working on engines and building more HP,tuning and adjusting engines while I check tire temps,tune around with tire pressures,stagger,weight transfer,etc and seldom even open my hood all nite.

 

IMCA is dilligent with rules and is not afraid to make changes,To me thats not necessarly always positive.But someone has to regulate somehow.

[LMR21]

compression is measured in two different terms, static , and dynamic. An engine that is being cranked very slowly by mechanical means is usually considered static. After an engine is cranked and is running on its own potential then is becomes dynamic. This is where it gets interesting, a good camshaft designer can use his grinding skills to increase or decrease where your dynamic compression peaks and thus where the motor starts developing horsepower(which is only a calculated number based off of torque) Your question of does it matter how fast the engine is cranked, and the answer is yes. the person who pointed out whether the throttle blades are open and closed has a valid point as well. Since an engine is just a pump, if it has nothing to suck it cant pump, just like putting a piece of cardboard over a box fan it speeds up because there is no load. Its also note worthy that when doing a compression test and wondering about cranking speed, you must also consider that air is a compressible gas, and the amount of moisture and air density is relative...and we have only scratched the surface of this subject. Just so you know compression ratio is not the same thing, it is a ratio of the cylinder volume at BDC vs the cylinder volume at TDC. Physics blows dont it.....lol just mash the gas and have fun!