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Distributor advance curve
Posted: Thu Aug 02, 2001 8:06 pm
by Peter Pleitner
Hi Whitworth Heads,
According to Chip Old in The T Series Handbook advance is approximately 30
degrees at 5000 rpm. According to Mike Sherril, its 15 degrees at 2500 rpm.
The progression of advance seems linear from no degrees at TDC instead of a
curve. Any more information out there to add to this?
I've just installed a Pertronix unit for positive ground (a bit weird the
wiring hook-up as compared to a unit for negative ground). It was a bit
down on power and found the points to be pitted.
Cheers, Peter
Re: Distributor advance curve
Posted: Fri Aug 03, 2001 2:31 am
by Eric Worpe
Hi Peter,
The degree of advance depends on whether one
measures the advance at the distributor or crank-
shaft. As the distibutor is geared to run at half the
crankshaft speed, 15 degrees at the distributor is
equal to 30 degrees at the crankshaft.
The original set up starts to advance at 200 RPM
and progressively advances some 14 degrees by the
time it reaches 2000 RPM, the advance "curve" then
"tails" off so that the maximum advance (15 degrees)
is not reached until some 2600 RPM.
Hope this helps, Eric.
Re: Distributor advance curve
Posted: Fri Aug 03, 2001 5:56 am
by Chip Old
On Thu, 2 Aug 2001, Peter Pleitner wrote:
> According to Chip Old in The T Series Handbook advance is
> approximately 30 degrees at 5000 rpm. According to Mike Sherril, its
> 15 degrees at 2500 rpm. The progression of advance seems linear from
> no degrees at TDC instead of a curve. Any more information out there
> to add to this?
The difference is that I specified engine degrees and engine RPM, as you
would measure it with the distributor on the engine. Mike specified
distributor degrees and distributor RPM, as you would measure on a
distributor testing machine. The distributor rotates at half engine
speed, so 30 engine degrees @ 5000 engine RPM is the same as 15
distributor degrees @ 2500 distributor RPM.
The advance "curve" is actually two linear progressions. I don't have the
specs here at work, but if I remember correctly the advance on the
TC/early TD distributor is 0 degrees up to about 300 distributor RPM,
increases fairly rapidly to around 12 distributor degrees at about 1500
distributor RPM, then advances more slowly to 15 distributor degrees at
about 2500 distributor RPM.
If nobody comes back with the exact specs, I'll try to remember to look
them up at home this evening.
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net
Re: Distributor advance curve
Posted: Fri Aug 03, 2001 11:01 am
by Peter Pleitner
Oops, here goes again, first time clicked on reply instead of reply to all.
Hi Chip,
I hope the data I quoted was not confused by the difference in the rate of
rotation between the dizzy and crank. Sherrell equates 15 degrees advance
with respect to the crank pulley by stating "the mark should climb up the
pulley about half an inch or so" at 2500 rpm (page 149). Later when I have
the time I'll dig out a spare pulley and overlay a degree wheel then
measure.
Yes, please do look up you data on advance curves. In the past ten years
timing guns with dial in advance feature have become affordable (distributor
machines are getting scarce), and I've been stung before receiving two light
tension springs as replacements for both in the centripetal advance
mechanism. I hope this hasn't happened to other XPAG and XPEG dizzys.
For these reasons I believe this is a worthy thread. This LIST needs
definitive data on ignition timing.
Cheers, Peter
-----Original Message-----
From:
fold@mail.bcpl.net [mailto:
fold@mail.bcpl.net]On Behalf Of Chip
Old
Sent: Friday, August 03, 2001 8:56 AM
To: MG-TABC
Subject: Re: [mg-tabc] Distributor advance curve
On Thu, 2 Aug 2001, Peter Pleitner wrote:
> According to Chip Old in The T Series Handbook advance is
> approximately 30 degrees at 5000 rpm. According to Mike Sherril, its
> 15 degrees at 2500 rpm. The progression of advance seems linear from
> no degrees at TDC instead of a curve. Any more information out there
> to add to this?
The difference is that I specified engine degrees and engine RPM, as you
would measure it with the distributor on the engine. Mike specified
distributor degrees and distributor RPM, as you would measure on a
distributor testing machine. The distributor rotates at half engine
speed, so 30 engine degrees @ 5000 engine RPM is the same as 15
distributor degrees @ 2500 distributor RPM.
The advance "curve" is actually two linear progressions. I don't have the
specs here at work, but if I remember correctly the advance on the
TC/early TD distributor is 0 degrees up to about 300 distributor RPM,
increases fairly rapidly to around 12 distributor degrees at about 1500
distributor RPM, then advances more slowly to 15 distributor degrees at
about 2500 distributor RPM.
If nobody comes back with the exact specs, I'll try to remember to look
them up at home this evening.
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net
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