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TR6 Ignition Timing with Triple Webers


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I've just bought a TR6 (CP) with triple webers (40s), fast road cam, phoenix sport manifold+exhaust and lumenition optronic performance ignition. I think the compression ratio is around 10:1. It seems to go well (the webers appear to be well tuned) but I am not sure if the timing is optimised for maximum power from the modified engine. I would appreciate your thoughts and recommendations for a distributor timing curve - i.e. the static timing, the maximum timing degrees at high rpm and roughly what rpm this maximum should be reached?

Edited by TFTR6
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6 hours ago, TFTR6 said:

I've just bought a TR6 (CP) with triple webers (40s), fast road cam, phoenix sport manifold+exhaust and lumenition optronic performance ignition. I think the compression ratio is around 10:1. It seems to go well (the webers appear to be well tuned) but I am not sure if the timing is optimised for maximum power from the modified engine. I would appreciate your thoughts and recommendations for a distributor timing curve - i.e. the static timing, the maximum timing degrees at high rpm and roughly what rpm this maximum should be reached?

11 strobe at 800 rpm then up to 36 at 5000 rpm 

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14 hours ago, InfinityJon said:

What thickness is the head?

just about to do the same mods to my US spec car and need to know how much to skim off the head. At the moment it’s due to be skimmed to 3.400” which I think gives 9.5:1 CR. However it’s no hardship to make another pass to increase the CR. 

According to my BL ST tuning book if the stock compression is 8.5 to 1 the head s/b 3.460" thick. To achieve a compression of 10.25 it will be necessary to mill .085" from the head.The head should now be 3.375" thick, this 10.25 CR is now correct if zero deck clearance has been done. If not done the CR will be around 10.0 to 1

Bruce.

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Thanks very much for all the comments. The distributor looks original (Lucas 41219) and has weights and springs that have been checked out fairly recently. I am about to try to remeasure the advance but I think it is about 10 degrees in the distributor so 20 degrees at the crank. The static is currently set to 10 degrees so that would make 30 total advance at high rpm. Thank you NTC for suggesting 35 - I may try increasing the static advance to see if I can get some pinking (I can't hear any pinking at the moment but then the car is pretty noisy!).

The 10:1 compression ratio comment came from a garage that tuned the car last year for the previous owner. It was based on cylinder compression test results of around 180psi. So this may not be accurate. Perhaps I should try to measure the thickness of the head and use Bruce's numbers to check - can this be done with the engine in the car?

Tom

Edited by TFTR6
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The 41219 distributor is a 7 degree nominal centrifugal advance type. With 11 degrees static time this gives ~ 25 degrees crank advance maximum and reaches this value at 2600 rpm crankshaft.

I have these on both of my Webered TR250s. The driver has ~ 9.5:1 c/r and CP cam. It will ping if I dial in as much as 30 degrees total advance so I reckon TRIUMPH set them below this value for good reason. The other one has very high lift, high duration cam and 10.7:1 c/r and it can take somewhat more advance before it pings.

Frankly, I haven't found much power difference at all within a range of +/- 5 degrees advance; only pinging and fuel economy. For the latter it would be nice to have vacuum advance for highway cruising but I haven't found a way to get it.

 

Tom

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On 6/30/2020 at 8:21 PM, Tom Fremont said:

The 41219 distributor is a 7 degree nominal centrifugal advance type. With 11 degrees static time this gives ~ 25 degrees crank advance maximum and reaches this value at 2600 rpm crankshaft.

I have these on both of my Webered TR250s. The driver has ~ 9.5:1 c/r and CP cam. It will ping if I dial in as much as 30 degrees total advance so I reckon TRIUMPH set them below this value for good reason. The other one has very high lift, high duration cam and 10.7:1 c/r and it can take somewhat more advance before it pings.

Frankly, I haven't found much power difference at all within a range of +/- 5 degrees advance; only pinging and fuel economy. For the latter it would be nice to have vacuum advance for highway cruising but I haven't found a way to get it.

 

Tom

Thanks Tom - really useful data.

I've just measured my head thickness (my engine not my brain!) and get around 3.39" which combined with 0.020" bores gives 10.1 compression ratio - so my engine may need a timing in between your two engines. I've also measured my advance at around 8 degrees at 500rpm and 22 degrees above 2500rpm which seems low but does fit with your 7 degree distributor advance comment. So it seems my timing needs to be advanced a little and I may need to increase the centrifugal advance. I will experiment and see if I can advance to the point of pinging then back off a couple of degrees.

I'd be interested to know what jet sizes you run in your webers - do both cars use DCOE40s or is your higher CR engine on 45s? My DCOE40s currently run well with the following setup: Venturi=30, main jet=115, air corrector=180, emulsion=F11, idle jet=50F6, pump=40.

Tom

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37 minutes ago, TFTR6 said:

Thanks Tom - really useful data.

I've just measured my head thickness (my engine not my brain!) and get around 3.39" which combined with 0.020" bores gives 10.1 compression ratio - so my engine may need a timing in between your two engines. I've also measured my advance at around 8 degrees at 500rpm and 22 degrees above 2500rpm which seems low but does fit with your 7 degree distributor advance comment. So it seems my timing needs to be advanced a little and I may need to increase the centrifugal advance. I will experiment and see if I can advance to the point of pinging then back off a couple of degrees.

I'd be interested to know what jet sizes you run in your webers - do both cars use DCOE40s or is your higher CR engine on 45s? My DCOE40s currently run well with the following setup: Venturi=30, main jet=115, air corrector=180, emulsion=F11, idle jet=50F6, pump=40.

Tom

My SAH book advises  the following as a starting point for 40 DCOE Jetting: Bear in mind that I have no experience with these carbs.

                               Road    Fast Road      Sprint

Choke                       28           30                   33

AUX.Vent                4.5          4.5                  4.5

Emulsion Tube      F16        F16                  F16

Main Jet              120           120                  125

Air Jet                 175            180                  185

Idler Jet             50F9           50F9                50F9

Pump                 35                35                     35

Back bleed?      0                  0                       0  

 

I don't know if this info helps? They recommend that to get the best out of weber DCOE'S, that their kit B is fitted first. This consists of Extractor manifold& exhaust system' Modified cyl. head, camshaft, L/W cam followers.

45 DCOE's are not recommended for general road use?

Bruce.  

 

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9 hours ago, TFTR6 said:

Thanks Tom - really useful data.

I've just measured my head thickness (my engine not my brain!) and get around 3.39" which combined with 0.020" bores gives 10.1 compression ratio - so my engine may need a timing in between your two engines. I've also measured my advance at around 8 degrees at 500rpm and 22 degrees above 2500rpm which seems low but does fit with your 7 degree distributor advance comment. So it seems my timing needs to be advanced a little and I may need to increase the centrifugal advance. I will experiment and see if I can advance to the point of pinging then back off a couple of degrees.

I'd be interested to know what jet sizes you run in your webers - do both cars use DCOE40s or is your higher CR engine on 45s? My DCOE40s currently run well with the following setup: Venturi=30, main jet=115, air corrector=180, emulsion=F11, idle jet=50F6, pump=40.

Tom

Hi Tom,

I have 40s on both my engines, and concur with the perception that 45s are for larger displacement TR5/6 engines, e.g. 2.7 liter etc.

If your settings give satisfaction across the range ( no low speed - mid range gasp )  you ought to stay with them. Keep a record should you venture into the wilderness so you can find your way back. They say there are over 4 billion permutations.

I have used F2 and now F7 emulsion tubes and have tried F11, F15 and F16 without success - each failed in the low-mid range because the main system doesn't come into play until 2500-3000 rpm with them on my engines. The F16 was the worst, and F11 in 3rd place behind the F7 and F2. Main jets are 125/175 air in the driver and 130/190 air in the concours / higher mod engine. Exhaust manifolds matter a lot, and a true extractor like the GOODPARTS makes for smaller jets and/or  larger air correctors. I have one of these and it was a must for the latter with its really extreme cam in order to get the mid-range up to snuff. Fitting it led to 1 size smaller main and (7) sizes larger air corrector to restore the top end. Chokes are 30mm in the driver and 32 in the concours one. Pump jets are 45 in the driver, 40 in the other with closed bypass valves. A critical factor is float level. I have 40DCOE18 on both engines but the driver's are very old with brass throttle shafts and the same setting in both cases results in a fuel height discrepancy of 3.5mm! The pertinent measurement therefore is 28mm below the top of the well. Below this level ( higher number ) will make it difficult to bring the mid range in low enough; in my case 2000 rpm but others hold out for 1500. Some pundits urge setting the level to 25mm, leaving a scant 1mm below the pipe into the venturi.

I've never had my engines on dynos and I had a brief fling with a a/f meter system when I was tearing my hair out with the wild one; it was a piece of !@#$ and didn't tell me anything I didn't already guess before it stopped working altogether.

So it's been seat of the pants pretty much all the way, with helpful advice here and there, and a very fortuitous set of F7 emulsion tubes in a used set of DCOEs I threw in 20 years ago. The driver's engine now has 135K miles on its last rebuild, runs terrific with scant oil consumption and excellent oil pressure - thanks in no small measure to the wonderful Webers metering fuel as well as can be for a carburetor.

Cheers,

Tom

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Stand by for pedantic comment:

45s aren’t necessarily for bigger engines but are more suitable for larger cylinder displacements (TR6 has a larger engine but smaller cylinders than a TR4)

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17 hours ago, Tom Fremont said:

Hi Tom,

I have 40s on both my engines, and concur with the perception that 45s are for larger displacement TR5/6 engines, e.g. 2.7 liter etc.

If your settings give satisfaction across the range ( no low speed - mid range gasp )  you ought to stay with them. Keep a record should you venture into the wilderness so you can find your way back. They say there are over 4 billion permutations.

I have used F2 and now F7 emulsion tubes and have tried F11, F15 and F16 without success - each failed in the low-mid range because the main system doesn't come into play until 2500-3000 rpm with them on my engines. The F16 was the worst, and F11 in 3rd place behind the F7 and F2. Main jets are 125/175 air in the driver and 130/190 air in the concours / higher mod engine. Exhaust manifolds matter a lot, and a true extractor like the GOODPARTS makes for smaller jets and/or  larger air correctors. I have one of these and it was a must for the latter with its really extreme cam in order to get the mid-range up to snuff. Fitting it led to 1 size smaller main and (7) sizes larger air corrector to restore the top end. Chokes are 30mm in the driver and 32 in the concours one. Pump jets are 45 in the driver, 40 in the other with closed bypass valves. A critical factor is float level. I have 40DCOE18 on both engines but the driver's are very old with brass throttle shafts and the same setting in both cases results in a fuel height discrepancy of 3.5mm! The pertinent measurement therefore is 28mm below the top of the well. Below this level ( higher number ) will make it difficult to bring the mid range in low enough; in my case 2000 rpm but others hold out for 1500. Some pundits urge setting the level to 25mm, leaving a scant 1mm below the pipe into the venturi.

I've never had my engines on dynos and I had a brief fling with a a/f meter system when I was tearing my hair out with the wild one; it was a piece of !@#$ and didn't tell me anything I didn't already guess before it stopped working altogether.

So it's been seat of the pants pretty much all the way, with helpful advice here and there, and a very fortuitous set of F7 emulsion tubes in a used set of DCOEs I threw in 20 years ago. The driver's engine now has 135K miles on its last rebuild, runs terrific with scant oil consumption and excellent oil pressure - thanks in no small measure to the wonderful Webers metering fuel as well as can be for a carburetor.

Cheers,

Tom

Hi Tom,

Thanks for the advice and detailed response all of which is very useful. I am tempted to experiment by increasing my main jet from 115 to 120 (and maybe even 125 as you have) as many sources seem to suggest 120 as a starting point (like the SAH data copied above - thanks Bruce). I might try the F7 emulsion tubes too. I can always return to my current settings.

Cheers, Tom

Edited by TFTR6
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  • 6 months later...
On 6/30/2020 at 9:21 PM, Tom Fremont said:

Frankly, I haven't found much power difference at all within a range of +/- 5 degrees advance; only pinging and fuel economy. For the latter it would be nice to have vacuum advance for highway cruising but I haven't found a way to get it.

 

Tom

You can do a somewhat override the mechanical advance.

leave it as it is but put the dizzy output into MegaSquirt or Megajolt or Alden or similar.

You now take into calculation what the dizzy already does mechanically and do two things:

1.) add advance due to manifold vacuum

2.) swap the dwell time from mechanical 55/45 to MegaSquirt controlled constant dwell time. This will give nice spark at high revs and not heat the coil, especially when parking with ignition on.

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On 7/2/2020 at 7:41 PM, TFTR6 said:

I've just measured my head thickness (my engine not my brain!) and get around 3.39" which combined with 0.020" bores gives 10.1 compression ratio - so my engine may need a timing in between your two engines.

If you want to know the CR you really need to burette the chambers and measure the piston sweep and deck clearances. Hard to put material back on the head once it's gone! The cylinders may well have varying CRs as well. Before I had the head machined on my 6 I had CRs which varied from 9.5-9.7:1. Pistons were 0.003-0.005" proud of the block, and chambers varied from 44.2cc to 45.6cc. Of course if your car has had professional machining done it's likely all variances have been evened out.

John

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