Compression Ratio

[backformore]

How does one go about determing compression ratio of an assembled engine?

[Daryl DeArman]

The Katech whistler is the tool that most of the sanctioning bodies that do this kind of test use.

I don't know how accurate it is...maybe a .1 point or two...

I may be misinformed but I understand that you back off/remove all rockers on the cylinder to be tested to ensure that all valves stay closed as you cycle the motor from BDC to TDC. The tool is connected to an air source and to you motor through the spark plug hole. The motor is rotated slowly by hand and the device measures the maximum amount of air the cylinder could hold (volume at BDC) and the minimum (volume at TDC) the software calculates the CR and displays it digitally on the tool.

I don't think it could be used alone to DQ someone but certainly a minimum amount of work to see if you do need to look further.

------------------

[Joe Harlan]

In the car it is pretty hard. If you can get the plug hole pointed straight up you could try to CC it through the plug hole. This would assume good rings.

[backformore]

I was afraid it was difficult. I was looking at a car that claimed to have had the head shaved and I wanted to make sure it did not exceed the limits per the ITCS (.5 I think)

Thanks

[apr67]

Bull dukie. It can be done in the car, you just have to pull the head.

You do it the same way you would do it with the motor out of the car.

[MikeITA240SX]

The initial question was "with an assembled engine". That precludes pulling the head.

I have done it "in the car" by pulling and "cc-ing" the head and then "cc-ing" the block side with the piston at TDC. Of course, this only works with a dished or flat-top piston that sits below the deck. If you know the precise dimensions of the piston and where it sits in relation to the deck, you can calculate all of that and just "cc" the head. But we are talking about pulling the head.

I'd be real interested to know how the measurement device mentioned would work. Pretty hard to tell the remaining volume at TDC without injecting a fluid in.

------------------
Mike Conatore
ITA 1989 240SX
Olympia, WA

[Joe Harlan]

Originally posted by apr67:
Bull dukie.

Is this some new tech term I have not heard of before or some kind of new cc'ing device.

As far as buying the car goes you could do a holdback or and agreement that the seller pays to bring the engine to spec if you determine it not to be after the purchase.




[This message has been edited by Joe Harlan (edited March 13, 2005).]

[Daryl DeArman]

Maybe someone could find out how it works by doing a little digging on the web...I haven't had any luck

I could be speaking out my arse but I believe it has a very accurate air flow measuring device. Leakage around valve seats and rings may be what causes accuracy issues. I have only had one motor teched this way and with the piston at TDC the digital readout was fluctuating a tenth of a point (both under maximum rule spec). Maybe that means it was between the two readings displayed ???? Who knows, everyone was reasonably satisfied and it went no further.

These may be the folks:

http://www.katechengines.com
Click on "engineering" under corporate services.




[This message has been edited by Daryl DeArman (edited March 13, 2005).]

[Joe Harlan]

If I remember correct the K-tech device is designed around a known quantity....like 310 displacement and or a known compression figure like 9:1 I don't believe it is effective in a real world multi displacement/compression set of classes. I am pretty sure this is why club racing has not used it. kind of a go no-go gauge for pro or spec classes.

[ITANorm]

The "Whistler" works on the same principle as a trombone: resonant audio frequency of a sealed cavity. Leaking valves and rings would not be enough of a factor in determinination of the ratios to account for much - unless the engine was so leaky it ran like crap anyway.

------------------
Norm - #55 ITA, '86 MR2. [email protected]

Website: home.alltel.net/jberry

[Ron Earp]

I read a little bit about that thing a year or so ago and I think the description above is what I remember. If so, it is plently accurate as the resonant frequecy physics that govern it are well understood and measuring a frequency can be done extremely accurately. My feeling is that it would be plently accurate to detect a difference of a few percentage points change in compression. But, acceptace will be an entirely different matter.

Ron

------------------
Ron Earp
NC Region
Ford Lightning Tow Beast
RF GT40 Replica
Jensen-Healey ITS
1/2 a 260Z ITS - Zero

[Daryl DeArman]

So whatever device was used on my motor must not have been the "whistler" as it was connected to an air source or is an air source utilized to determine the frenquency (ala trombone)?

[This message has been edited by Daryl DeArman (edited March 14, 2005).]

[Knestis]

You know how a beer bottle goes "hooot" when you blow across the top? And how, as you drink more beer and the air volume gets larger, the "hooot" gets deeper? That's how the Whistler works, if I understand correctly.

It blows air across the top of a tube that threads into the plug hole and measures the frequency of the resulting note.

K