[Fot] Valve overlap and compression test

[John Styduhar]

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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