Crank pulley and timing

[Peter Pleitner]

Hi Whitworth Heads, Thanks for advise and contributions from a few experts via this list and off-line last week. Next month I will run my spare distributor from a TD on a distributor machine to plot an advance curve as a function of rpm. However, this week I finished and verified my Pertronix Ignitor electronic ignition unit installation and marking of degrees on my TC's crank pulley. I used a stroboscopic timing light and an adjustable advance function to verify the following data: 1. Ten degrees on our crank pulley is equal to a cord equal to 5/16 of an inch. or more precisely 8.2 mm 2. The total advance of my installed distributor is about 18 or 20 degrees on the crank 3. Maximum advance is at approximately 2500 rpm 4. Maximum advance advised by list members is 30 stock, up to 36 degrees for a modified XPAG Use of 94 octane fuel is assumed only for modified and high compression engines. Mine is stock. I run 87 and sometimes 89 octane fuel. My timing at idle speed is now set at 10 (@ 800 rpm), this provides the full 30 degree advance at speed. I used to think static timing or idle speed timing is the accepted and proper practice. Since working with Italian engines, I have learned their practice is to set timing at high rpm when maximum advance has been achieved (adds to excitement under the bonnet). This now makes more sense to me, because that is where the engine is working hardest, where timing is more critical for good performance and reliability. The only downside is drivability and ease of starting if the distributor is not sweeping through enough advance to provide the necessary retardation back in the direction of TDC at low rpm. I also believe that lower octane fuel is less stable and burns faster, in other words it requires less ignition advance. If any of you know more about this please correct me and our readers. Cheers, Peter

[anngene@bellsouth.net]

So Peter, For all of us who don't understand exactly what you're saying below, what does this mean? Should we set our timing approximately 5/16th of an inch before the mark on the pulley or 5/16th of an inch after the mark to achieve the best at speed driveability and economy? And should we switch to 89 octane fuel? Thanks, Gene ============================================================ From: "Peter Pleitner" pleitner@dundee.net> Date: 2001/08/08 Wed AM 11:18:24 EDT To: "T-ABCs List" mg-tabc@yahoogroups.com> Subject: [mg-tabc] Crank pulley and timing Hi Whitworth Heads, Thanks for advise and contributions from a few experts via this list and off-line last week. Next month I will run my spare distributor from a TD on a distributor machine to plot an advance curve as a function of rpm. However, this week I finished and verified my Pertronix Ignitor electronic ignition unit installation and marking of degrees on my TC's crank pulley. I used a stroboscopic timing light and an adjustable advance function to verify the following data: 1. Ten degrees on our crank pulley is equal to a cord equal to 5/16 of an inch. or more precisely 8.2 mm 2. The total advance of my installed distributor is about 18 or 20 degrees on the crank 3. Maximum advance is at approximately 2500 rpm 4. Maximum advance advised by list members is 30 stock, up to 36 degrees for a modified XPAG Use of 94 octane fuel is assumed only for modified and high compression engines. Mine is stock. I run 87 and sometimes 89 octane fuel. My timing at idle speed is now set at 10 (@ 800 rpm), this provides the full 30 degree advance at speed. I used to think static timing or idle speed timing is the accepted and proper practice. Since working with Italian engines, I have learned their practice is to set timing at high rpm when maximum advance has been achieved (adds to excitement under the bonnet). This now makes more sense to me, because that is where the engine is working hardest, where timing is more critical for good performance and reliability. The only downside is drivability and ease of starting if the distributor is not sweeping through enough advance to provide the necessary retardation back in the direction of TDC at low rpm. I also believe that lower octane fuel is less stable and burns faster, in other words it requires less ignition advance. If any of you know more about this please correct me and our readers. Cheers, Peter Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ ============================================================

[Chip Old]

On Wed, 8 Aug 2001 anngene@bellsouth.net wrote:

> For all of us who don't understand exactly what you're saying below, > what does this mean? Should we set our timing approximately 5/16th of > an inch before the mark on the pulley or 5/16th of an inch after the > mark to achieve the best at speed driveability and economy?

Before the mark. The mark indicates top dead center. If you set it to 5/16 inch after the mark, your timing will be grossly retarded. Keep in mind that 5/16 inch before the mark (about 10 degrees before TDC) is what worked best for Peter's engine. It may or may not be best for yours. No two engines (even supposedly unmodified ones) are identical, but in my own experience about 5 degrees TDC static timing works out pretty well for most stock XPAGs with standard (and unworn) distributors running low-test fuel.

> And should we switch to 89 octane fuel?

Not unless your compression ratio has been increased to the point where you need higher octane, or unless you've advanced the ignition timing a great deal. It's a waste of money to switch to higher octane fuel if your engine doesn't ping on lower octane. -- Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271 Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E fold@bcpl.net

[Peter Pleitner]

Hi Gene et. al.., Use lowest octane fuel (US conditions) if your XPAG head and pistons are close to standard compression. Higher octane is required to prevent spontaneous ignition due to compression stroke of piston and resultant heat. Think of it as built in stability of fuel. This attribute also decreases the speed of the flame front generated at the spark plug, hence the need for more ignition advance when running high octane fuel. As to the comment about 5/16 of an inch (8.2 mm), it is simply my basic contribution to our general knowledge. Mike Sherrell states in his book that 3/16 of an inch represents 5 degrees on our XPAG's crank pulley. When I used this measurement things just didn't quite line up as I expected so I decided to check this figure. As to where on the pulley to mark this (typing white-out paint is handy), think of the pulley's TDC mark as 12 o'clock. Then 10 degrees advance will be at 20 minutes past 12 (all engines turn clockwise (but not most Hondas, some boat and airplane engines). I marked my pulley with a 30 degree advance mark, and set my distributor at this mark when the engine was turning 2500 rpm. At idle (800 rpm) timing went back to 10 degrees. I then dialed in thirty degrees advance on my timing light and verified that my pulley's TDC mark lined up with the pointer when turning 2500 rpm. This is the part that made me question Mike's 3/16 inch measurement in the first place. Bottom line, use lowest octane your engine is happy with, and time your engine at 2500 rpm and 30 degrees advance for a starting point. Most of our distributors are old enough to question the validity of proper advance function for timing static or at idle. Advance this timing a little if you have to burn high octane. It not so important what the timing is at low rpm because the engine typically is not working hard or shouldn't be (down shift). However, an engine is easier to start with timing closer to TDC (remember the manual ignition advance on steering wheels, all the way back to retard was for starting only. And retarded timing causes lots of heat, as my friend with a 1927 Bentley just found out. If you time the engine's ignition static or at idle you will not know whether or not your ignition timing is retarded at high rpm, except by the temperature of your engine and its performance. Cheers, Peter -----Original Message----- From: anngene@bellsouth.net [mailto:anngene@bellsouth.net] Sent: Wednesday, August 08, 2001 11:44 AM To: Peter Pleitner; T-ABCs List Subject: Re: [mg-tabc] Crank pulley and timing Sensitivity: Confidential So Peter, For all of us who don't understand exactly what you're saying below, what does this mean? Should we set our timing approximately 5/16th of an inch before the mark on the pulley or 5/16th of an inch after the mark to achieve the best at speed driveability and economy? And should we switch to 89 octane fuel? Thanks, Gene ============================================================ From: "Peter Pleitner" pleitner@dundee.net> Date: 2001/08/08 Wed AM 11:18:24 EDT To: "T-ABCs List" mg-tabc@yahoogroups.com> Subject: [mg-tabc] Crank pulley and timing Hi Whitworth Heads, Thanks for advise and contributions from a few experts via this list and off-line last week. Next month I will run my spare distributor from a TD on a distributor machine to plot an advance curve as a function of rpm. However, this week I finished and verified my Pertronix Ignitor electronic ignition unit installation and marking of degrees on my TC's crank pulley. I used a stroboscopic timing light and an adjustable advance function to verify the following data: 1. Ten degrees on our crank pulley is equal to a cord equal to 5/16 of an inch. or more precisely 8.2 mm 2. The total advance of my installed distributor is about 18 or 20 degrees on the crank 3. Maximum advance is at approximately 2500 rpm 4. Maximum advance advised by list members is 30 stock, up to 36 degrees for a modified XPAG Use of 94 octane fuel is assumed only for modified and high compression engines. Mine is stock. I run 87 and sometimes 89 octane fuel. My timing at idle speed is now set at 10 (@ 800 rpm), this provides the full 30 degree advance at speed. I used to think static timing or idle speed timing is the accepted and proper practice. Since working with Italian engines, I have learned their practice is to set timing at high rpm when maximum advance has been achieved (adds to excitement under the bonnet). This now makes more sense to me, because that is where the engine is working hardest, where timing is more critical for good performance and reliability. The only downside is drivability and ease of starting if the distributor is not sweeping through enough advance to provide the necessary retardation back in the direction of TDC at low rpm. I also believe that lower octane fuel is less stable and burns faster, in other words it requires less ignition advance. If any of you know more about this please correct me and our readers. Cheers, Peter Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ ============================================================ Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

[Steve Guttormsson]

I have been following the discussion on distributor timing with great interest. Chip Old published an excellent review of this, with distributor advance curves in the October 1981 issue of TSO on page 47. I have enclosed a copy of that diagram. It covers the four common distributors that show up in our T types. Using this info along with a timing light with adjustable advance (I got mine at Sears) should allow the most accurate timing of your dizzies. Steve Guttormsson TC 3995

[Peter Pleitner]

Thank you Steve and Chip!! You've just made up for a lot of trite traffic. 1981 might just pre-date my membership and TSO archive. Excellent diagram, should be included on our web site under technical data. Good work Chip, is there a story how these curves were obtained or derived? Now I'm wondering why my 40367 dizzy behaves like a 40089, and which one my spare dizzy might be. Looks to me like the good old 40048 & 40162 have the best curve. Or is the 40089 better? Experts? And while we are on this tricky subject, does anyone know of a source for proper fly-weight springs?? A lot have rusted over the years. Like I said before, I tried two well known sources and got two identical springs each time, both were light weight springs without the long "eye on one end", didn't use them, but wouldn't that be the way to create a single slope advance curve? Cheers, Peter -----Original Message----- From: Steve Guttormsson [mailto:xpag@chartermi.net] Sent: Wednesday, August 08, 2001 3:40 PM To: mg-tabc@yahoogroups.com Subject: [mg-tabc] Crank pulley and timing I have been following the discussion on distributor timing with great interest. Chip Old published an excellent review of this, with distributor advance curves in the October 1981 issue of TSO on page 47. I have enclosed a copy of that diagram. It covers the four common distributors that show up in our T types. Using this info along with a timing light with adjustable advance (I got mine at Sears) should allow the most accurate timing of your dizzies. Steve Guttormsson TC 3995 Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/