[Fot] Valve overlap and compression test

Sun May 3 05:44:59 MDT 2020

Marcel:
There are many variables that influence the absolute value of the maximum pressure you record in a compression test. The purpose of the test is not to determine your compression ratio, but to detect differences in pressure between cylinders. Significant differences are an indication of an incipient failure or worn parts. The best practice would be to do an initial compression test after engine break in and then periodically after that, making sure to duplicate as much as possible the conditions of your initial test. That way you can track the relative health of the cylinder components.
Phil Gott 

Sent from my iPhone

> On May 3, 2020, at 2:51 AM, van.mulders.marcel--- via Fot <fot at autox.team.net> wrote:
> 
> 
> The lobe separation angle is 105° and valve timing is 57-83 inlet and 87-53 exhaust. So I think it is  110° (inlet opens at 57° before TDC and outlet closes at 53° after TDC)
> Marcel
> 
> Van: "John Styduhar" <johnstydo at gmail.com>
> Aan: "Scott Janzen" <sjanzen at me.com>
> Cc: "van mulders marcel" <van.mulders.marcel at telenet.be>, "fot" <fot at autox.team.net>
> Verzonden: Zondag 3 mei 2020 02:38:14
> Onderwerp: Re: [Fot] Valve overlap and compression test
> 
> I think Marcel is talking about the lobe separation angle (LSA).  The Isky TR 777 cam I'm running has the same LSA.  LSA is not effected by valve lash adjustment.
> 
> 
>> On Sat, May 2, 2020 at 5:36 PM Scott Janzen via Fot <fot at autox.team.net> wrote:
>> Maercel, I’m puzzled. Are you referring to the lobe separation angle, or to the overlap between the intake valve opening before TDC and the exhaust valve closing after TDC on the same revolution?  Unless I’m reading them wrong, I’ve looked at some pretty radical cam specs and the opening/closing overlap does not approach 110 degrees, more like 50 degrees on a really aggressive cam, but the lobe separation angle is about 104 degrees on the cam I have in the car and 107 on the cam I’m putting in another engine.
>> 
>> On May 2, 2020, at 4:30 PM, van.mulders.marcel--- via Fot <fot at autox.team.net> wrote:
>> 
>> Hallo Chris,
>> of course the cylinderhead has to be repaired. I think the man who has put the big valves in it, has taken too much risks : the valve seats (the rings that are pressed into the cyl head) are small where they do meet, and maybe the seats are distorted a bit because of this, but I don't know for sure, I will see when pulling the head.  I know one cannot change the valve clearance too much, but maybe 0.05mm more valve lash is ok and it will reduce the valve overlap. I believe 110° overlap is a lot, even for a racecamshaft. Any idea's someone about that? The compression test with increased valve lash is much higher than with the recommended valve lash : I wonder if it is making any sense at all to check the compression of a race engine without increasing the valve lash : with the normal clearances, the inlet valve is  closing when the piston has already travelled a lot upwards, hence the big difference between the compression test results with normal and with increased valve lash. So I think, if you want to test the compression, you have to increase the valve lash firstly(at least when the engine has a race camshaft).
>> Marcel
>> 
>> Van: "fot" <fot at autox.team.net>
>> Aan: "fot" <fot at autox.team.net>
>> Verzonden: Zaterdag 2 mei 2020 21:29:59
>> Onderwerp: Re: [Fot] Valve overlap and compression test
>> 
>> Hello Marcel,
>>  
>> Your engine has leaking valves.
>> It won’t get better from changing measurement procedures.
>> Maybe the seat is rough, or the valves bent, or offset .
>> Something to make sure about.
>>  
>> Enlarging the valve leash could result in hammering everything dead in the valve train.
>>  
>> Cheers
>> Chris
>>  
>> Von: Fot <fot-bounces at autox.team.net> Im Auftrag von van.mulders.marcel--- via Fot
>> Gesendet: Samstag, 2. Mai 2020 14:43
>> An: fot <fot at autox.team.net>
>> Betreff: [Fot] Valve overlap and compression test
>>  
>> A leaking test on my TR6 engine is showing a leaking exhaust valve on cyl no 1 (48% on the gauge of the device) and a leaking inlet valve on cyl no 2 (75%). Compression on cyl no1 is 9.5 bar and on cyl 2 it is 8 bar. The compression on the other cylinders is between 12 and 13.5 bar. This is with a race cam with a valve overlap of 110° (320-F grind). Because of the high overlap, the piston has risen a lot before the inlet valve is closed, so I did the compression test again, but with a valve lash of 2.5mm/0.1" : compression went from 9.5 bar to 14.5 on cyl no 1 and from 8 bar to 15 on cyl no 2. With this valve lash, the compression on the other cylinders went up to 16 - 18.5 bar. Of course, when the engine is running at high rpm, a lot of valve overlap is necessary for best cylinder filling, but I wonder if 110° isn't too much?? The recommended valve lash is 0.40mm/0.016" when the rocker ratio is 1.45(0.43mm/ with my 1.55 roller rockers). Would it be risky to set the valve lash at some 0.05mm/0.002" higher?
>> PS : is it common practice to set the valve lash at a couple of mm to perform a compression test, because it seems that the valve lash has a lot of influence on the result when testing a race camshaft?
>> Marcel
>> 
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