Air/fuel ratio and timing for peak power

[MikeParker]

Hi All,

I have been running with a wide-band air fuel ratio (AFR) gauge for a while now. I also have an ignition controller that allows control over timing versus both revs and vacuum. I am using the standard ported vacuum off the stroms at the moment. What I find it that for peak economy at light throttles (so cruising along the flat at 50-60 MPH) an AFR of 16.5 works really well with very advanced ignition(45 degrees total at 2000 rpm). My problem is, when when I open the throttle, the AFR actually goes up to about 17. The car runs fine and feels powerful, but cannot be generating maximum torque and power for the revs. Most references state that the AFR should be less than 14.7 (stoic value) for peak power. So I was going to re-shape the Strom needles to get richer mixtures for wide open throttles. So my question is, has anyone checked this on a rolling road? i.e. varied the mixture and timing until peak power was reached. If so what are the optimum AFRs and timings for peak torque? I am mainly interested in "normal" driving revs, say around the peak quoted torque value of 3500 rpm. I would then at least know what to aim for!

Thanks

Mike

[bullstarz]

No-doubt that you’ve done a lot of research on the subject with regards to AFR, rpm, load and throttle position. As these all have a deciding factor on how well the engine runs. An air fuel ratio of 17 is extremely weak and will damage your engine in seconds. Every engine is slightly different to where it would produce optimum power but usually this is around AFR of around 11.5 to 12.5 for Max Power. Now if your engine is leaning off then it could be that your carbs are not completely synchronised. Tuning is very difficult with the carbs, especially Triumph stag, as each carb provides mixture to both sides of the engine. Realistically it be good to have two sensors, one measuring each carb. In may be a good idea to use a Exhaust gas temperature sensor, EGT, as these also show what’s happening.
You mention that the car seems to run well at quite a high AFR, weak mixture, but it will be having a very lean burn and possibly very high temperatures, which could cause damage to the valves. Realistically you have to make sure that every cylinder is getting the same spark and the same airfuel mixture, so the whole ignition and fuel side of things must be 100%. Otherwise you’ll just be chasing errors and this could be leading to incorrect AFR readings which could in turn cause catastrophic damage to your engine.

[trunt]

I also have a wideband but with a normal distributor so standard advance curve. It runs around 15.5 AFR at cruise and 13 or so on constant acceleration but does dip lower for a very short time.

The normal fix to enrich mixture during acceleration/WOT is to use thicker dashpot oil I assume you tried that but I found that even a 40wt oil did not move mine much so went back to atf. It’s not truly in that ideal power band but there is absolutely no sign of leanoff there and the engine feels lively so I didn’t think it worth the huge effort to polish needles.

One question is what Total advance you are seeing during acceleration? My Des Hammill “bible” says that a total advance of 36-38 at 3500rpm is suitable for the wedge type head chambers. He also says 10-15 degrees vacuum advance on top of that is usual so you are in that range

Also what needle? Did it get to a nice idle mixture at the middle of the adjustment range? I found the B1AQ on the edge there, so used the “richer” B1BF.


Terry

[trunt]

Also beware of polishing for WOT. I found that when just accelerating it was quite a few seconds until the dashpot oil allowed the tip of the needle to present itself I was usually changing gear before it got there. The WOT part of the needle is only seen if you hold the throttle open for quite a while, so find a hill so that you can hold it there.. or use very thin oil to assess it.

[MikeParker]

Thanks for your inputs.

Terry, Yes, I am doing my WOT testing on a long fairly constant gradient hill. I have the option of adjusting the timing with a pot (rotary control) , so I can tweak it as I drive. The fixed settings I program in are just a bit retarded from peak torque timing value, to try and lessen the load on big-ends etc.

All the values I talk about are steady state. It goes all over the shop when accelerating and decelerating (12-20). I am using Miller 10 60 fully synthetic engine oil in the dashpots and that could do with being a thicker oil as it does lean further when I push the throttle down fast. It does get slightly richer when fully hot (after 30 minutes plus) and sits around 15.75 to 16.5 . It may be my AFR gauge is out, but I have re-calibrated it in fresh air a couple of times.

Bullstartz, So far the mixture settings are just the historic settings I have used for years now. I have not touched the mixture for more than 10K miles. I had one head off last year and the valves looked fine. Also no closing of the tappet clearances over the miles I have been running this mixture. Over that time I have been tweaking the ignition design and have much less timing jitter and the engine is much less prone to pinking now than it was at the outset. I do not thrash the engine for long periods (because it is running lean) and that might be why I have got away with it. My aim is now to try and get a safe and powerful mixture for WOT. I know this will be between 11.5 and 14. The question is what AFR is optimum and at what timing?

I have read up a lot about the theory and the precise optimum values will vary with the carb/manifold/engine design, so it is really someone who has tested this out on a Stag V8, preferably with Stroms, that I am after. Might be a bit of a tall ask, I know!

Mike

[MikeParker]

Terry,

I don't know which needle I am using and I don't generally adjust at idle because of the dominating effect of air leaks. I am running on 160K mile old Stroms but with relatively new needles/jets (about 80K miles old). The fact that you are getting 13 (WOT) and 15 (cruise) is a lot better than my carbs which get leaner with increasing throttle, so perhaps I should start by changing my jets and needles again. So I gather it is the B1BF needle you are using now? I guess there are no options for the jets if I use our regular suppliers, so I will see how that pans out. Thanks for your input!

Mike

[trunt]

It’s easy to check which needles you have without messing with the adjustment, just pull the carb tops and the needle number is on the needle under its holder, you just need to pull against the spring and it’s fairly easy to see. the B1BF is only richer on the first 2-3 stations so would not affect the WOT, in fact when you set it at idle it sits lower so leans off the rest.

160k on Strommies? I think my first move would be to rebuild them, especially as you feel that you have air leaks at idle.. I don’t think there is such a thing as different jets, just needles, old jets can be worn but yours are already newer. So what is your idle sitting at? I set mine around 13.5-14 as it idles nice there and when it gets hot doesn’t lean off too far.

Have you tried turning the needles a bit richer to see the effect? 1/2 turn should show some difference.

What type of timing adjustment are you using? Can you separately adjust vacuum advance and centrifugal? If not then take the vacuum advance off and see where it is.. I would say the 36-38 @ 3500 would be my aim point. TBH I’m not sure what you mean by “just a bit retarded from peak torque timing value” enlighten me!

[jbuckl]

Beware that some modern fuel is no longer 14.7:1 for stoichiometric a.f.r.
anyone fitted a closed loop knock sensing ignition yet ?

[jbuckl]

Trunt I recall in the engine design office there was an s.u. Needle guru still hand fettling s.u. Needles on a miniature lathe (10” long) on his desk!
the last s.u. Application was in 1993 for euro 19+5 emission legislation. I did the evaporation loss pipe work for that. It was £150 cheaper than the injection system iirc

[trunt]

Trunt I recall in the engine design office there was an s.u. Needle guru still hand fettling s.u. Needles on a miniature lathe (10” long) on his desk!
the last s.u. Application was in 1993 for euro 19+5 emission legislation. I did the evaporation loss pipe work for that. It was £150 cheaper than the injection system iirc

150 - Cheap, makes you wonder nowadays.

I used a bit of wet+dry and a stand drill, a tiny lathe would work, at least you would have a half chance of getting both needles the same.
one reason I don’t fancy it on the Stag is the cost of the stromberg needles, I remember blowing a heck of a lot of SU needles before I got it right..

Theres a question, anyone ever fitted SU carbs to a Stag? I did that on my TR and it gave me a little more hp. I really liked the HIF carb on the Mini as far as they go they finally got it right.

Terry