458 - Compression Test Results - how to read? | FerrariChat

458 Compression Test Results - how to read?

Discussion in '458 Italia/488/F8' started by lamontlawyer921, Apr 4, 2020.

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  1. lamontlawyer921

    lamontlawyer921 Formula Junior
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    Jul 1, 2010
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    Close to purchasing this 2010 Italia, 2-year power warranty included. Attached are the results of the compression test. No idea how to read/interpret? Anyone able to lend a hand? Thanks!
    This is the red one at southbay ferrari currently.
     

    Attached Files:

  2. klinkman

    klinkman Formula Junior
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    #2 klinkman, Apr 4, 2020
    Last edited: Apr 4, 2020
    Basically this chart is showing all 8 cylinders with between 7.1 and 8.25 bar or so. 7 bar according to google is 101PSI. you can convert it yourself here: https://www.google.com/search?client=firefox-b-1-d&q=convert+7+bar+to+psi

    The key info is the black trace on the motometer chart for each cylinder moving to the right, where it stops is the high value.

    Ideally, you want all cylinders showing similar compression. Here is a blurb from Popular Mechanics "Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings." https://www.popularmechanics.com/cars/how-to/a8520/cars-101-how-to-do-a-compression-test-14912158/ This car is close, if a motor has a problem cylinder it's generally on the low side. If there is more specific Ferrari or 458 detail, someone else please chime in, my info is more general than specific.
     
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  3. Yellowgallardo1

    Yellowgallardo1 Karting

    Sep 19, 2018
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    marcus
    A better depiction of an engines condition is a leak down test.
     
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  4. Dontbethatguy

    Dontbethatguy Karting

    Nov 25, 2017
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    ATX
    I agree that if you’re doing a compression test you also want to do a leak down. It will show how well the rings are dealing. Was the engine hot or cold when the test was done? You typically want it at operating temp. Cold numbers are more likely to have larger variances.
     
  5. lamontlawyer921

    lamontlawyer921 Formula Junior
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    Not sure of the details. I will inquire regarding if leak down was also performed.
     
  6. RayJohns

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    A leak down is always a good idea, but a compression test also tells you a lot. The two tests can tell you slightly different things. A leak down says more about valve and ring sealing, whereas a straight compression test might be more about showing up issues such as a failing head gasket, broken pistons, spun bearings, etc. They both give a picture into motor health.

    Also, keep in mind that the test is only as good as the person doing it. For example, holding open the throttle body butterfly valves (or not) can have a big impact on things. Also keep in mind that removing all the plugs vs, one at a time can impact things. The other thing is that oil pressure can increase a lot between testing cylinder 1 and cylinder 8, which might impact things. It's not an exact science sometimes. Also, the tools used can impact things.

    I'm attaching a video showing the device used for your test.



    Not a device I personally would use. Myself, I prefer just a standard ol' dial type pressure gauge with a check valve. This gives a peak reading and more of a cumulative pressure due to the check valve. I'm not sure how big a difference this makes over the crazy German Motometer system, however.

    I just ran a compression check on my Toyota motor and the readings were all within 1 or 2 PSI of each other. Here are the readings for my pickup, run with all spark plugs removed and the throttle box fixed open:

    Cylinder #1 - 175 PSI
    Cylinder #2 - 175 PSI
    Cylinder #3 - 177 PSI
    Cylinder #4 - 175 PSI

    So you can see the variation between readings is only 2 PSI, which is slightly over 1%. Don't forget this is a 22RE motor that is over 5 years old and was originally bought from a local wrecking yard :) Gotta love Toyota!

    Here is your info below, with my notes. As you can see the difference between the highest (A) and lowest (B) pressure reading is about 15% - that's pretty bad, if it's an accurate reading. My guess, however, is that maybe they ran the test without manually holding open the throttle linkage in order to allow enough air in during testing. This can really throw off the numbers, because you are working against a vacuum. The other thing suspicious is that the readings are rather low, given how high a compression ratio (12.5:1) the 458 motor has. Why would my lower (9.0:1) compression ratio Toyota motor generate 175 PSI, yet the Ferrari motor only generate barely over 100 PSI? Seems a bit off to me, unless something else is afoot here.

    Anyway, that's my 2 cents. You might want to speak with the tech doing the work and see what they have to say. Even if the test wasn't performed exactly right, having a 15% difference is definitely enough to raise a few eyebrows.

    The fact that the numbers decline as the test moves on could indicate friction and heat played some role here.. When I do a compression test, I always do it twice (from start to finish, back to back) and then compare all the figures to make sure both tests are in sync.

    Ray



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  7. lamontlawyer921

    lamontlawyer921 Formula Junior
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    Ray, thanks so much for the detailed and comprehensive breakdown. I am going to talk to the dealer tomorrow, and now possibly have them take the car to Fastcars Ltd, and have Craig do a PPI for a 2nd opinion. The car is coming with the 2 year power warranty, but I still want to ensure there is not something wrong with the engine. i assume a low compression will also effect performance. Thanks again.
     
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  8. RayJohns

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    You're most welcome.

    I definitely would speak with the person who actually did the test, if possible, and find out if whatever butterfly valves Ferrari uses in their intake system were held in the open position during the test. Given the high precision of the Ferrari 458 motor, I would expect to see only a difference of 2-3% maximum between cylinders, assuming the motor is in good shape and there isn't some other overriding factor at work here; something which might throw the readings off (e.g. something related to actuators or variable valve timing or whatever the case could be).

    I've never personally worked on a 458 motor, but those numbers look way off to me - especially given the sky high 12.5:1 compression ratio. Even if you just loosely apply the Ideal Gas Law, using a multiplication factor of 12.5-ish, it seems to me you'd expect to see a number more in the range of 150+ (see chart below). The only way I can see readings of 100 PSI would be either something terrible like burned valves or if the test was being run where a vacuum is being created in the motor's induction system due to a throttle plate being closed. It could be related to variable valve timing (and the motor not being on), but it's hard to say. It's possible very late closing of the intake valve on the compression stroke (e.g. due to some cam actuator not being fully in play with the motor being off) could affect the pressure, but again I've never rebuilt a 458 motor so I couldn't tell you either way.

    If it were me, I'd want to be present during testing and/or make sure the test is being run by someone who knows engine building and diagnostics beyond just a casual level. Either that or get a definitive answer as to why the readings are so low and also why there is such a huge percentage difference between the highest and lowest reading among cylinders.

    Ray

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  9. RayJohns

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    And yes, compression ratio issues can have a dramatic impact on performance. I once blew a motor by over revving it and spinning a rod bearing. The bearing halves stacked on the bottom side of the connecting rod and this allowed the piston to move down in the cylinder by the thickness of the rod bearing. I barely made it home. When I checked the compression ratio of the motor, it was something crazy like 150,150,80,150.

    Even just dropping the position down maybe .040" in the hole ruined the compression.

    Another time, I ran across a chainsaw where someone else had built it and used the wrong crankshaft. I think the stroke was only 2mm different, which caused the position to sit 1mm lower than it should have - but power was way off.

    Let me put it like this, I'd venture a guess that the majority of the power gains between the 458 Italia and the Speciale come largely from Ferrari bumping the compression ratio up from 12.5 to 14:1 - if you run the math, displacement and pressure are where you see the largest HP gains.

    So yes, that's not an area where you want to see substandard figures if you're interested in making maximum horse power.

    Ray
     
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  10. Shadowfax

    Shadowfax F1 Rookie
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    Usually you get an analysis summary with these types of tests - unusual not to supply one - but other than that nothing appears untoward which would warrant further investigation. There can be inaccuracies between cylinders depending upon the skill of the tester and the equipment used too. A test like this really just gives a basic snapshot on the engine anyway and should not be relied upon as a true indicator of how the engine has been used. That situation can only be revealed and more accurately established upon strip down.
     
  11. lamontlawyer921

    lamontlawyer921 Formula Junior
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    update. Had a PPI and Compression Test done at my regular shop, trusted Fastcars Ltd, everything checked out as it should. PSI was 145-150 on all cylinders. Moving forward with purchase. Getting Xpel on full front and then ceramic coating premium on it before I even bring it home. The car is super clean, so want to keep that front end without blemishes. Thanks again guys, for the very detailed input!
     
  12. RayJohns

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    Yeah, those readings sound more believable.

    Ray
     

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