Distributor advance curve

[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

[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.

[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

[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 Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/