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PostPosted: Mon May 06, 2019 6:57 pm 
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Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 2862
Location: Indianapolis
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Doc

do your 'My Experience' rules apply to the UEM calculator when following their instruction of using IVC @ .050 + 15
or to your practice of using observed seat timing?

asking as
when I follow the UEM instructions on a build with over 10.6:1 compression and a 256 @ .050 duration cam I get an effective compression of
8.4 which seems reasonable with a moderate high static compression and a medium sized cam. The medium size cam will not bleed off cylinder
pressure like a really large cam

when I follow your instructions and use seat IVC on the build mentioned above I get an effective compression of 6.8, which is off the scale on the low side.
That does not seem right.


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PostPosted: Tue May 07, 2019 3:37 am 
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Board Sponsor & Moderator
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Joined: Mon Oct 21, 2002 11:08 am
Posts: 13722
Location: Blacksburg, VA
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Nice to see these points of reference, Doc.

Coincidence and observations: My 64 Dart engine I just pulled out (built in 2013) has 10.7:1 and a 257 @ 0.050" cam degreed at 100. On 91 octane (driving in Canada and out West) it was detonating a little so I had to retard timing a bit. On 93 it was just fine. I'm sure with computerized ign control there would be no problem.

Lou

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"You mean you still have a Slant 6 in that thing?"


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PostPosted: Tue May 07, 2019 8:00 am 
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Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 2862
Location: Indianapolis
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Quote:
10.7:1 and a 257 @ 0.050"


my similar build looks promising on DynoSim


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PostPosted: Wed May 08, 2019 3:10 pm 
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Guru
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Joined: Sat Oct 12, 2002 11:22 am
Posts: 3583
Location: Sonoma, Calif.
Car Model*: Many Darts and a Dacuda
DadTruck wrote:
Doc

do your 'My Experience' rules apply to the UEM calculator when following their instruction of using IVC @ .050 + 15
or to your practice of using observed seat timing?

asking as
when I follow the UEM instructions on a build with over 10.6:1 compression and a 256 @ .050 duration cam I get an effective compression of
8.4 which seems reasonable with a moderate high static compression and a medium sized cam. The medium size cam will not bleed off cylinder
pressure like a really large cam

when I follow your instructions and use seat IVC on the build mentioned above I get an effective compression of 6.8, which is off the scale on the low side.
That does not seem right.


Agreed.. that a a large difference that does not make much sense. :?

I base my calculations on "what the engine will see" and that is the point where the intake valve is on the seat... solid on the seat.

There is some judgement on when the true on the seat IVC "moment" occurs because some lobe ramps set the valve down slowly so try increasing the clearance a bit, to get off the "lazy" lash ramp area, then take a new reading.

Use a dial indicator to get a better idea on where the cam's lobe flank movement transitions into the lash ramp. (the lobe flank moves a lot faster then the clearance ramp area)
My guess is that the lash (clearance) you are using for testing may not be correct for the lash ramp ground onto the cam.
DD


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