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Triumph TR6 - Vacuum Advance

A post over on the 6-Pack group about recurving the distributor reminded me about the vacuum advance mod. I have that and it has worked REALLY well. The dist drive gear had to be rotated a "cog" on the initial installation to get the module away from the head, but that was pretty easy. I think Peter Gooch in Hawaii also had it done. Any comments, Peter?

BB
Brent B

Hi Brent,
Can you please post the details of the recurving mod on the 6 pack list. I dont have access to that but am very interested.
Thanks
Michael

Go here

http://www.telusplanet.net/~chichm/tech.htm

and there is a section on "Tuning the Lucas Distributor" that involves modifying the springs & weights. The springs/weights we not adjusted with the vacuum advance mod.

BB
Brent B

Yes Brent, was extremely happy with the Vac Advance modification, easily the cheapest horsepower per dollar.
Exceeded expectations, after moving drive gear one tooth ccw to get dist back where it should be. Pulls smooth and strong across the range. Recommend it. Peter
Peter Gooch

Ok, whats the "vacuum advance mod"?? Do you mean removing the vacuum advance gear or fitting it to an engine that does not have it?

As I understand it the vacuum advance&retard should not affect the engine power under load, merely improve economy&emissions....?

Could someone pleas explain what you are talking about?

Thanks!

SR
SignalRed

Sure - Richard Good (GoodParts) will machine the distributor and relocate the vacuum retard module to the other side of the breaker plate so it functions as a vacuum advance module. Acceleration performance is very good with vacuum advance.

BB
Brent B

My understanding of the function of vacuum advance is to allow a longer combustion time when the engine is on the over-run to reduce emmissions. When the throttle is wide open it will have no influence on timing- as there is no vacuum to operate it. UK cars have the capsule but it was never connected up and i have not heard of anyone claiming that connecting it up improved performance. It seems to me an even cheaper route is to disconnect whatever capsule is on the car and retime it to UK spec.
P H Cobbold

Hate to disagree Mr C, but vac holds timing advanced for good cruising performance and when you hit the gas (dumping the vacuum), timing drops back for that initial burn so you don't detonate under load and advances the timing again as the throttle (vacuum) stabilises, in combination with centrifugal advance dependant on RPM. Vac retard may???? help emissions, but definately not performance. Peter
Peter Gooch

The effect of vacuum advance IS quite noticable. The module won't be on-off, but will influence the centrifugal advance over the modules 'working range' - but you're right about full effect at idle and little effect at full throttle. It's the in-betweens and how it modifies the centrifugal advance curve that make it useful. It might not benefit or could burn-up a supercharged engine, though.

I didn't like the throttle response using vacuum retard or the straight centrifugal set-up in all situations, and I felt that playing with the advance springs would be a pain ITA to me. This was easy and I found worthwhile.

Cheers,
Brent

Brent B

This must be something to do with the different specs of the UK versus exported engines.
The need to vac-advance the ignition at cruise must be to compensate for poor fuel octane in USA and to give longer burn time to compensate for the low CR of USA engines. The retard of the igntion upon opening the throttle would indeed reduce the the tendency of poor fuel to deonate.

My point is that UK cars never had either vac advance or retard implememted- but we had access to 98 octane fuel and standard 9.5 CR. Maybe the extra expense of fitting vac-retard was necessary to meet USA limitations regrading fuel octane and the low CR. (the lowered CR was the cheapest way to meet NOxide emissions.
So anyone who raises the CR too around 9.5 or more should be able to run happily without any vacuum actuator as long as the fule is good enough to avoid pinking under load.
P H Cobbold

Peter&Brent-I was looking through the Bentley manual and noticed how different the advance curve is for the PI and carb cars. My car, a 71, is really different. The static&idle setting is 4 deg. btdc and doesn't reach max adv. until 5600 rpm. The PI cars use a static setting of 11 deg. and reaches max adv of 16 deg at 2600 rpm. I have a regrind of the 150 hp cam and 9.5 cr and would like to re-curve the dist to PI specs.
Berry
Berry Price

Berry
You could put sleeves on the limit stops on the centrifugal advance so that max swing of the weights ends at 2600 rather than 5600. That would also reduce the max advance also, but if the carb cars had more max centrifugal advance then that might not be a problem- is there a curve in the manual?- you might be lucky.
If that wont work then you could add weight to the centrifugal weights so they swing outmore at lower rpm. As 'centrifugal force' is proportional to the square of velocity it is 4 fold larger at 5600rpm than at 2600rpm so I suspect that you would need to increase the weight 4-fold to get all the advnace in by 2600rpm. That would be difficult so the springs will need altering.
I have never in UK been able to find springs off the shelf- and getting one of the right size (several parameters influence them- wire diameter. temper, radius,number of turns) will be dificult as UK suppliers only sell them as part of reconditioneded distributors. You need weaker springs- approx four fold less stiff.
You would also need to leave the vacuum capsule hose off as in the UK cars.
That said, the engine will still work well with timing that is not spot on so you can play around with limit stops and thicker ( heavier) weights etc until you get something that works with the fuel you have available, which is probbaly lower coatne than the PI engines wee designed to run on. So having less overall advance than a PI set up might be quite useful!
Peter
P H Cobbold

Peter-I think my first step will be to make a sleeve to limit the total adv. to 16 deg., better than lengthening the cam arm. I believe that the vac adv was only used on the TR250 and very early TR6s, the later cars having only a vac retard, which most people disconnect unless it is necessary to pass an emission test. If the stock springs are too strong, I may order a packet of springs offered by Moss UK that contains an assortment of light, med., and heavy springs. The last resort would be buy a spring winding tool and one to form the ends.
Berry

Berry,
That should work nicely. It will be a bit retarded in mid-range rpm compared with PI engines but that will be good for reducing pinking upon opening the throttle wide.
My blown motor has boost-retard device that instantly takes around 12 degrees off the timing when it hits 6psi boost at about 2500 rpm- there is no change in engine sound or performance that I can detect. So there's a lot of latitude in timing it seems.
Peter
P H Cobbold

This thread was discussed between 17/12/2003 and 24/12/2003

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