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Saturday Night Racer >> RACE TECH >> Compression Rules
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Message started by Missle Motorsports on 11/02/11 at 11:35pm

Title: Compression Rules
Post by Missle Motorsports on 11/02/11 at 11:35pm

After a Race,in TECH, what is the appropriate way to test a Motor for it's actual Compression Ratio?

Title: Re: Compression Rules
Post by CHAD HAMMER on 11/03/11 at 12:41pm

That depends on the rules, If they are measuring the "static" compression or the "cranking" compression which is measuring cylinder pressure. Cranking compression or pressure will fluctuate based on cam timing and engine temp would also play a factor. Compression tests are usually based either hot or cold. To get a true static compression would usually require tearing down the engine and measuring piston to deck clearance, combustion chamber volume cc, compressed head gasket thickness, bore volume. Trying to tech compression without tear down would require some kind of range based on cranking cylinder pressure.

Title: Re: Compression Rules
Post by Dietzsch_Racing25 on 11/03/11 at 2:17pm

One alternatve to tearing down and measuring the appropriate components is using a piece of equipment call the "Whistler".

I don't have any idea how it works but it has been used on several of my engines and has always produced results that were very close to what I calculated.

It is very simple too. All you have to do is pull a plug, measure cylinder temp, put the probe in the plug hole and turn the motor over very slowly (I am sure there is more to it but that is what I observed).

I think the unit costs a few thousand dollars.

Title: Re: Compression Rules
Post by Brad33 on 11/03/11 at 3:07pm

We got checked for compression this year. They used the katech whistler. It was before the races so the car had not even been started. It was with a tenth of our calculated compression.

http://www.katechengines.com/motor-sports/services/whistler/

Not sure how it exactly works but I think its measures the volume of a cylinder at BDC and again at TDC by using some sort of gas. not 100% sure though

once the machine gets that info it can figure out the ratio.

Title: Re: Compression Rules
Post by CHAD HAMMER on 11/03/11 at 4:17pm

I would be very concerned with the accuracy of a machine that needs calibrating from time to time to determine my compression ratio. Also states that can only be effective engine temps up to 200 degrees. Is that the water temp they are looking to have input or do they need an infared temp gun taking the actual block material temp? I bet many of the local tracks using this device are not having it re-calibrated often enough to call it "reliable" data. If this machine is off by a tenth and your legal compression is 10.5-1 what happens if now machine reads 10.6 or 10.7-1?

Title: Re: Compression Rules
Post by Dietzsch_Racing25 on 11/03/11 at 5:02pm

I am pretty sure (not positive) it is cylinder temp. I think it comes with a thermometer and everything. I certainly appreciated pulling a spark plug vs pulling a head. You still have the option to pull everything apart and measure if it doesn't give you the number you expect (says you're illegal).

VV had that issue a few years back. One car measured high. He pulled the head off to prove he was legal.

My numbers always matched my calcs though.

Title: Re: Compression Rules
Post by TOOLCHUCKER on 11/03/11 at 6:08pm

No matter what tool they use is there a guidline as to how far off you can be? I am sure there is a variance in readings because of tools, temp, etc.

Title: Re: Compression Rules
Post by dirtmopar44 on 11/03/11 at 9:00pm

The whisler is accurate but only if you know the ci of the engine being tested, That was the issue with me at VV. It is measuring a known volume of air and as the piston gets closer and closer to TDC the air blowing into th e machine is measured when its at its smallest amount it converts it to comp ratio. My comp was mathamaticly done before asembly and its is 10.58 to 1. The wisler showed it to be 10.6 because it goes to the nearest 10th. On the other hand the ci pump showed me 385+ ci. There is no way the ci pump can tell you how big your engine is unless they are all turned over at the same RPM thats why I was tore down and measured.

Title: Re: Compression Rules
Post by Missle Motorsports on 11/04/11 at 12:45am

Thanks for the Replies.
Just looking ahead to next years IMCA SportMOD Rules,& wondering how the 9:1 Motor Rule might be enforced

Title: Re: Compression Rules
Post by 83v on 11/04/11 at 1:58pm

Mark you can call Or talk to Brad about how he will check

Title: Re: Compression Rules
Post by CaliforniaPickle14B on 11/04/11 at 4:07pm

is the katech whistler the same as the bubble ? cause i think chow uses a bubble heck i dont know lol

Title: Re: Compression Rules
Post by AW10 on 11/04/11 at 9:12pm

2 different tools. Compression/Cubic Inches. Got it Vlasic?

Title: Re: Compression Rules
Post by CaliforniaPickle14B on 11/04/11 at 10:13pm

AW10 wrote on 11/04/11 at 9:12pm:

2 different tools. Compression/Cubic Inches. Got it Vlasic?

ya i think i got it, katech whistler does compression and bubble does cubic inch, BTW anyone want some pickles? not going to lie i had to look that up on google lol