How To Test Your Compression - Page 2 - LotusTalk - The Lotus Cars Community
Reply
 
LinkBack Thread Tools Display Modes
post #21 of 33 (permalink) Old 07-11-2008, 12:41 PM
Registered User
 
Join Date: Oct 2005
Posts: 1,162
Quote:
Originally Posted by Dave View Post
140 LBS? Try a different gauge, seems too coincidental for it to be that low and so linear on all 4.
thats 100 psi lower than my car or Joe's we used 3 different gauges all within 5psi. i think the range is 200-250. carl

User formerly known as 4carl
carl is offline  
Sponsored Links
Advertisement
 
post #22 of 33 (permalink) Old 04-20-2009, 12:46 PM
Registered User
 
robains's Avatar
 
Join Date: Jul 2007
Location: CA, Bay Area
Posts: 3,738
As per my other engine thread. I'll add my compression test results to this one:

190 - 185 - 180 - 190

This is with Mahle 9:1 pistons.

Also noted that with all plugs OUT, throttle open/closed made no difference in the readings. Clutch in/out also made no difference.

I think what could cause the biggest variance in readings is how well one secures the compression tube in the spark plug hole. Small dab of oil on the rubber seal is pretty good at ensuring a good seal and more accurate readings.

5 or 6 cranks worked well for me.

Rob.

"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan
robains is offline  
post #23 of 33 (permalink) Old 04-20-2009, 01:51 PM
Registered User
 
goldengatehornet's Avatar
 
Join Date: Oct 2004
Posts: 155
Most of the old manuals say to stop at three pulses and use that reading. A concern would be if the first pulse is low, even though it gets up to pressure by the third or fourth. I'm lucky to have a compressor and leak down guage. The only way we measure for leaks in the chip manufacturing industry is by the leak rate over time. Did anyone mention to have the engine at running temp first?
goldengatehornet is offline  
 
post #24 of 33 (permalink) Old 04-20-2009, 02:14 PM
Registered User
 
robains's Avatar
 
Join Date: Jul 2007
Location: CA, Bay Area
Posts: 3,738
Interesting, the procedures I've read and follow for compression testing are those provided by the manufacturer of the testing tool and/or service manual -- they suggest to never remove spark plugs when the engine is hot because of potential thread damage to the head -- hence why cold torque specs for spark plugs is only 13 ft/lbs.

Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.

"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan
robains is offline  
post #25 of 33 (permalink) Old 04-20-2009, 04:59 PM
Registered User
 
goldengatehornet's Avatar
 
Join Date: Oct 2004
Posts: 155
Quote:
Originally Posted by robains View Post
Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.
A Viton O-ring is good for over 200 degrees C. A Teflon one is good for over 500. On my race car, I start with a hotter plug, warm the engine to temp, then change to the colder plug for the track. This eliminates the problems of fuel fouling. This is a common practice. Antisieze on the plug threads is good for even street engines.

P.S. work hard in Cupertino. I need my Apple stock back up to $150. Typed on my Powerbook, as my HP is half the speed.
goldengatehornet is offline  
post #26 of 33 (permalink) Old 04-21-2009, 08:40 PM
Registered User
 
robains's Avatar
 
Join Date: Jul 2007
Location: CA, Bay Area
Posts: 3,738
Golden,

Thanks for the info, much appreciated -- I didn't know.

Cheers, Rob.

"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan
robains is offline  
post #27 of 33 (permalink) Old 04-22-2009, 07:10 PM
Registered User
 
goldengatehornet's Avatar
 
Join Date: Oct 2004
Posts: 155
Welcome. Apple reported well today. Much of the money for my Elise came from Apple stock options.....................
goldengatehornet is offline  
post #28 of 33 (permalink) Old 09-26-2010, 06:10 AM
Registered User
 
Big Top Gt's Avatar
 
Join Date: Sep 2010
Location: Florida 33071
Posts: 905
Quote:
Originally Posted by jim-clayton View Post
My Milton has a value screwed into the end of the hose. It has to be a special valve. A normal tire valve will give you low readings.

could be, no way it's 140
Agreed.

The only way you can get a 140 pound reading that's correct is to have some serious blow by.

The car would be consuming oil.

Also, the instructions in this thread are for a "dry" compression test. One way to see if you're getting an accurate reading (if it;s really low) is to squirt a little oil into the cylinder through the spark plug hole. (not a whole lot)

The oil will help seal the rings.

If you really do have a blow by condition, as would be indicated by a 140 PSI reading, putting oil in the combustion chamber will bump up the reading by a considerable amount.

The funny thing is it's probably just the gauge.

I recently bought a 96 Cobra Mustang as a parts car. (needed to give the motor away. Long story for another thread.) The car was in a light front end accident, but the motor was totally fine. I paid a local "Performance" shop to perform a compression test prior to purchasing the car.

They came back and told me it was 85-90 PSI across the board.

If your car is below, or even too close to, 100 PSI, it generally won't want to start. It'll have to crank and crank and crank until it builds enough chamber pressure to actually start. This car started and ran just fine.

Knowing this couldn't be right, I talked the seller down a few hundred more bucks and brought the car home.

When I got home, I went to AutoZone and rented a compression tester. That tester gave me a higher reading (115, max), but it still wasn't right. One thing I noticed was that the fitting spun at the base where it should have been fixed in position.

On a lark, and taking the ever desirable opportunity to buy a new tool for the collection, I went to Sears and bought a brand new compression tester for like 70 bucks.

I dropped it in, screwed it down and cranked.

Every single cylinder was ABOVE 195 PSI.

The moral of the story: if the car starts on the first hit of the key and idles/drives fine, the compression should be okay. Make sure your gauge is working properly.

Hope this helps.
Big Top Gt is offline  
post #29 of 33 (permalink) Old 04-02-2012, 10:31 AM
Registered User
 
Join Date: Feb 2012
Location: Austin
Posts: 173
rcuser is offline  
post #30 of 33 (permalink) Old 04-02-2012, 12:42 PM
Registered User
 
holmz's Avatar
 
Join Date: Sep 2006
Location: Antipodes
Posts: 2,309
Quote:
Originally Posted by robains View Post
As per my other engine thread. I'll add my compression test results to this one:

190 - 185 - 180 - 190

This is with Mahle 9:1 pistons.
...

How does that work?
9:1 CR would be 9X15 PSI so ~135.
Then assuming that the intake valve closes after BDC you would have an even lower pressure.
What am I doing wrong with the math?
holmz is offline  
post #31 of 33 (permalink) Old 05-20-2013, 11:23 PM
Registered User
 
Lotus F1's Avatar
 
Join Date: Aug 2003
Location: Fantasy Island New York
Posts: 5,411
Quote:
Originally Posted by holmz View Post
How does that work?
9:1 CR would be 9X15 PSI so ~135.
Then assuming that the intake valve closes after BDC you would have an even lower pressure.
What am I doing wrong with the math?
My 9:1 got similar to his.. 195-195-200-195
Lotus F1 is offline  
post #32 of 33 (permalink) Old 05-21-2013, 05:02 AM
Registered User
 
addertooth's Avatar
 
Join Date: Nov 2011
Location: Arizona
Posts: 2,630
Do not forget your choice of cams/cam timing/cam overlap all make a big difference in compression figures.
I built one engine with a static 10.5:1 compression, but with the high overlap cams, it measured at 8.5:1 at cranking RPMs, and with overstuffing, was 12.5:1 at peak RPMs.

Storm titanium 2005 Elise with carbon fiber spoiler, BWR sway bar, BOE toe links, gPan2, LidBone, sector 111 wheel studs, Grey RAC Monolite wheels with toyo R1R, Team Dynamics 1.4 with Hoosier A6/R6s, Katana2 Supercharger, ECU with 265 flash. PPE headers w/decat, CEL angle eliminator, Silent touch catback, Rear Panel Eliminator, HKS intake, V2 fuel tank, Walbro255 fuel pump. HID lights.
addertooth is offline  
post #33 of 33 (permalink) Old 02-13-2016, 11:06 PM
Registered User
 
ChrisH's Avatar
 
Join Date: Mar 2004
Location: Melbourne, FL
Posts: 2,771
Theoretical pressure and test results

Quote:
Originally Posted by holmz View Post
How does that work?
9:1 CR would be 9X15 PSI so ~135.
Then assuming that the intake valve closes after BDC you would have an even lower pressure.
What am I doing wrong with the math?
I just did a compression and leakdown test today (to see if my engine is in good enough shape to add a supercharger). I read this thread and saw the quoted question.

The issue with the math is that you have to take into account that the air is heating as it compresses, which raises the pressure more than just the ratio of volumes. Assuming that there isn't enough time to transfer significant heat from the heated vapor to the walls of the cylinder, then this is called adiabatic heating in physics lingo. You can read all about it in terms of engine compression ratios on Wikipedia:
Compression Ratio
The formula is: P = P0 * CR^gamma
where P0 is the pressure at the bottom of the stroke, P is the pressure at the top of the stroke, CR is the compression ratio, and gamma is the specific heat ratio (a.k.a., adiabatic index, heat capacity ratio) for the working fluid. For air, gamma is about 1.4

If valve timing, actual pressure at the bottom of the stroke (air can't flow in fast enough to reach 1 atmosphere by the bottom of the stroke in a normally aspirated engine), leakage, and heat transfer weren't an issue, and the pressure in the cylinder at the bottom of the stroke was 1 atmosphere (at sea level =14.7psi), and for the 11.5:1 compression ratio of a 2ZZ-GE engine, then at the top of the stroke, the pressure would be 449 psi. For 9:1, it would be 319 psi theoretical maximum.

But, all those other factors do come into play, as addertooth pointed out for valve timing, which is why we end up with less pressure than theoretically possible just based on 1 atmosphere and simple compression ratio.

For dynamic compression estimates, see the “Dynamic Compression Ratio” section of the above Wikipedia webpage. With dynamic compression ratio about 25% lower, and an effective specific heat ratio of 1.3 due to lost heat during compression, you get
P = 14.7 * ( (0.75*11.5)^1.3 ) = 242 psi
which is about what we see in the 11.5:1 2ZZ-GE engine.
For a 9:1 compression ratio, this computes to 176 psi.
(Keep in mind that the 25% and 1.3 derating values are just rough rule-of-thumb values, and it varies by engine type and modifications.)

So, my results for compression today were
Trial 1 (about 170 deg F engine temperature) 244 240 235 220 psi
Trial 2 (a little cooler) 242 235 229 220 psi
It was pretty close to these values three years ago using the same compression gauge. (By-the-way, I used a high accuracy gauge and an air compressor to calibrate my compression gauge and corrected the readings to the above values. It was just a straight 8 psi offset for calibration, regardless of pressure to the gauge.) I think these compression readings are fine (the Toyota manual simply says it should be >203 psi), but I will continue to keep watch on cylinder 4. I think these are pretty good for an engine with 125,000 miles on it.

I also did a leakdown test, and the values at 90 psi regulator pressure were:
5.9% 4.7% 5.9% 7.1% (and less percent leakage at lower regulator pressures), although the engine had cooled off to 100 deg F by then, so these numbers are higher than they would be if I had reheated the engine (I was too lazy to put the spark plugs back in and reheat the engine). But, I think they are still acceptable.

By-the-way, I use a leakdown tester with two pressure gauges like the OTC 5609



I strongly recommend against the type where the second gauge tries to produce a leakdown value. This second type is very error prone, and often only works properly, if at all, at low pressures. I threw away the one I had. You want to work at higher pressures - close to 100 psi if you can. The good kind does require you to compute a ratio of two numbers and subtract 1 to get the answer, but I think everyone here can do that.

Last edited by ChrisH; 02-28-2016 at 09:40 AM. Reason: Added adiabatic estimate for 9:1 compression ratio.
ChrisH is offline  
Sponsored Links
Advertisement
 
Reply

  LotusTalk - The Lotus Cars Community > Lotus Discussions > Maintenance and Repairs.

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the LotusTalk - The Lotus Cars Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome