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Triumph TR6 - Re-curving distributor
|Looks like the list has been down for some time, or is just me?|
Well anyway, I have been thinkin of my distributor and weather it is up to its task or not... There seem to be some sort of opinion that a "carb distributor" is not the optimal solution for a modified engine, and perhaps not even for a stock one!
The early PI distributor advance seem to be something to aim for.
The carb/US distributors apparently give too much advance, and too late (=at too high rpm's). Of course I dont have my notes with my right now but as I remember my '70 US spec distributor gives a max advancve (22 deg) at 5000 rpm. The early PI ditributor gives its max advance at appr. 3000 rpm, and olny 16 deg. (Please remember I am quoting these figures out of my head, I did look into it last night but they migt not be accurate, but I hope they still serves the puropse of this exercise). Thus my distributor will give a total advance of 22+12=34 @ 5000 rpm and the early PI 16+11= 27 @ 3000 rpm.
Assuming that the early PI adfvance curve is something to aim at for my modified engine (PI head, cam, exhaust, SU HS6's, pertronix & sport coil) I have thought of two options to proceed:
1. Buy a set of distributor springs and try it out on a rolling road with someone who actually knows what to do.
2. Buy the early PI distributor cam, springs and weights. install them and hope all is well.
I suppose "1" is the way to go if you want to do it right, but I am still tmepted by "2"...
What do you guys think, am I way off here with these thoughts? And do you know if the "PI internals" of a distributor is simply a bolt-on, or might there be problems on the way?
Opinions, thoughts, info etc are very welcome!
|I had an advance problem with one of my TR3's. I think it was not advancing fast enough. Amazingly, I got it working well by scrounging a weaker spring for one of the weights. Hardly scientific, just lucky, I guess. It took three tries using different combinations if I remember right. The cost was minimal, and the gratification immense. If you want to get a bit techie use a fish scale to see how much weight it takes to pull different springs. Remember where you started out! Maybe dabs of different colors to keep 'em all in order?|
If you don't fix it, give it to a pro.
|A good place to start is to limit the cent. advance to 12-16 deg. This can be done by making a bushing to fit over the spring post that acts as a stop for the cam arm. The od of the bushing will vary depending on the year of the car. Then set the static timing to 11 deg and check the total advance and at what rpms it happens. It maybe close enough to the PI curve without changing the springs. I don't think the PI springs,weights, and cam arm are available from the usual sources. The other alternatives might be a used PI dist. or a rebuilt unit from the UK.|
|SR--Suggest you read Marcel Chichaks' article here:|
|Thanks for the comments! Berry, that is a good idea. In the document Rick O. mentioned (that I have studied before, but thanks anyway!) there is a suggestion to weld some more onto the cam, your idea seems a lot easier! If my cam has a max advance of 22 degs and I want to reduce it to, say, 15, then there is a difference of 7 degs, wouldnt that then equal (7/2=)3.5 degs of the cams/distributors circumference, given that distributor cam advance = 0.5 of crank advance? Or am I way off here??|
BTW, the distributors I was referring to before are:
1. Lucas part no 41306 (1970 US spec), max centrifugal advance 22 crank degs at 5000rpm (total 34 degs if timing set as per manual at 12 BTDC)
2. Lucas part no 41219 (early PI), max centrifugal advance 16 crank degs at 2600 rpm (total 27 degs if timing set as per manual at 11 BTDC)
27 degrees of total advance is not very much, I've understood that 30 (or perhaps even a little bit more) could be something to aim for, given the 9.5 CR head?
I would really appreciate your comments and thoughts on this matter!
|It looks like the 41306 dist actually reaches max adv. at 3000 rpm vs 2600 for the PI dist-quite close. The bushing method of limiting the adv. is easier to work with and to return to stock than welding the cam arm, especially if a lathe is available. I used a degree wheel and pointer mounted on the shaft to determine the size of bushing required. Some trial&error is necessary. 8 deg. movement of the cam will give 16 deg of crankshaft adv. Final checking was done with a timing light after the dist. was installed. I agree that 27 deg. total adv. doesn't seem like much and Kastner's comp manual reccommends 30 deg up to 10.5 cr.|
The dist on my 71, 41352, reaches 30 deg. cent. adv. at 5600 rpm and is used with a 4 deg static setting. Not exactly ideal for use with the PI cam.
|My speculation on the 27 degrees of total advance on the PI engine is due to higher compression ratio relative to the carb cars. This is not to say that it cannot be set to more advance without problem, but this would have been a nice, conservative setting for day in, day out use.|
There was a thread from the Feb/March time frame last year that partially addressed the subject. The thread title was "Mechanical Advance and Vacuum Retard." So Signal Red, if haven't membered up, now is time so you can go archive digging. If you are like me and membered up, but are usually too slack to sign in, time to switch into member mode, log in and go searching.
Brent B inidicated in that thread that he had some spring and weight info to share, but I either failed to get it or got it and lost it in the ether. Another option if you will be running an MSD spark box is to add one of their programmable advance boxes into the mix. Lock down the action plate in the distributor and program/adjust away for the advance curve of your heart's desire.
|Hey Steve. I haven't looked back, but I think all I did was post the link to the Lucas dist tuning article that I'd come across that discussed the weights & springs - that was the sharing. I've never dug into the guts of one yet, nor do I plan to. Hopefully. But hey - who knows....|
As far as MSD, I hope everyone knows that the effect diminishes as 3000 RPM is approached, and there is no multiple after that due to timing.
|Sorry, thought it was something in addition to the link. As for digging into the guts of the distributor, they are not complicated at all. In a way it is somewhat amazing that something so rudmintary does such a good job of controlling the ignition curve. |
True enough, the MSD folks will tell you that (it's one of the reasons I included all three URLs). It's really more of a lower RPM, burn all the fuel, keep the plugs clean sort of thing. At higher RPMs, you run out of time for multiple sparks to provide any benefit.
This thread was discussed between 04/05/2004 and 05/05/2004
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