Well, I've got more rational numbers, I guess. I redid the centerline
settings to 105 on the intake and then read everything with a .020 checking
clearance as tony suggested. I got:
intake
39.5 BTDC open
74 ABDC close
105 measured centerline 107.5 calculated 293.5 duration
exhaust
81 BBDC open
34 ATDC close
114 measured CL 113.5 calculated 295 duration
Overlap is 73.5 degrees.
That looks in the ballpark. Still looks like a fairly weird cam, but it
worked well last year.
I redid the numbers at .050 just to see:
intake
open 26 BTDC
close 55 ABDC
cl 105 measured 104.5 calc 261 duration
exhaust
open 66 BBDC
close 15 ATDC
cl 115 measured 115.5 calc 261 Duration
overlap is 41 degrees
Basically it's about 11 degrees advanced. I might trim that back to 3 or 4.
Be nice to have a dyno to play with this. I bet there's a pony in there
somewhere.
-----Original Message-----
From: kas kastner
To: Bill Babcock
Sent: 2/26/2005 8:24 PM
Subject: Re: Yikes and cam problem:: HELP!!!
If you are measuring from the cam, as you should, why is the rocker
ratio even part of this? You don't care what is happening at the valve
at this point, just is the cam in right!
If you are checking at .050" and this is a RACE cam I expect it will
have at least 300 degrees duration therefore the opening should be
something like 22/62 for the inlet. Something like 50/80 = 310 degrees
(This would be a cam of about 310 degrees duration) I haven't bothered
to do the math, just an estimate, so something in about those numbers.
On that engine I would expect a centerline of 104-5 degrees.
You could check it at say .015" checking clearance and that will give
you a pretty darn good idea what you have though it is better to degree
from a bigger clearance in the end for accuracy. At the .015" checking
expect to have the timing to be about 50+ opening and around 90 degrees
closing.
----- Original Message -----
From: Bill Babcock <mailto:BillB@bnj.com>
To: 'fot@autox.team.net ' <mailto:'fot@autox.team.net '>
Sent: Saturday, February 26, 2005 5:37 PM
Subject: RE: Yikes and cam problem:: HELP!!!
I think you're having a lot more fun than me. I'm struggling with cam
timing on my motor and I need a sanity check.
Here's the problem. I've got a kind of unknown cam in Peyote--the guy
who
could give me some hints on the specs is incommunicado for a few weeks.
I'm
compounding my problem by installing a vernier cam wheel, so there's no
marks to start from. I set up a degree wheel and dial indicator, and
started
measuring. I assumed that the timing should be set with .050" lift, and
since I'm coming straight from the cam follower and assume a 1.4 rocker
ratio, I'm using .036" lift.
I tried setting it up with 50 degree BTDC inlet open and got some very
screwy numbers. So I checked the centerline of the intake and exhaust
cam
lobes and decided to start with 110 degrees ATDC. After a bit of
fiddling I
measure:
Intake opens 33 degree btdc
centerline 112 degrees ATDC
Intake closes 65 degress ABDC
which gave me
exhaust open 72 degrees BBDC
Exhaust CL 115 degrees BTDC
Exhaust closes 25 degrees ATDC
and that calculates to:
Overlap 58 degrees
Intake duration 278
Exhaust duration 277
calculated centerline intake: 106 ATDC
Calculated exhaust CL: 113.5 BTDC
Weird. there's no way the exhaust should close that early and the intake
opening is about good for a street engine.
So I tried again, setting the centerline of the intake cam to 100
degrees
I got:
Intake
open 37 degres BTDC
CL 98 degrees ATDC
closes 57 degrees BBDC
exhaust
opens 79 degrees BBDC
CL 120 degrees
closes 17 degrees ATDC
which calculates to
Overlap 54 degrees
Intake dur 274
exhaust dur 276
Inlet cl 100 ATDC
exhause cl 121 BTDC
I'm flummoxed. Can't figure out what I'm doing wrong. Any ideas?
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