4 pot SU tuning

[Peter Cobbold]

- with a wild high overlap camshaft.

Has anyone found that tuning the SUs at full throttle for best power leaves the cruise mixture too lean?

Am just curious...

Peter

[TriumphV8]

We ended up opposite on a TR4 with 280 degree cam.

Tried a lot with the needle compare-o-rama

but at last had to accept that even for the HS6

with a lot of needles none was acceptable and we

took the best fitment and gave the needles a grind

on the lathe what was quite a bit more work to get two

both suitable and absoliutely twins.

[Peter Cobbold]

We ended up opposite on a TR4 with 280 degree cam.

Tried a lot with the needle compare-o-rama

but at last had to accept that even for the HS6

with a lot of needles none was acceptable and we

took the best fitment and gave the needles a grind

on the lathe what was quite a bit more work to get two

both suitable and absoliutely twins.

Andreas,

So you made needles that were richer than any standard needle?

Did you need to make them richer along the whole length, or just at the thicker end, or just towards the tip?

Peter

[TriumphV8]

Hi Peter,

I worked on a modified TR4 with some needles fitted I forgot what type in the beginning.

After setting the idle I fitted a wideband AFR controller and we drove and noted

what the result was at different heights of needle.

 

We looked for lean/rich/okay.

 

After that we used a German needle finder to enter the existing needle and entering

bigger/smaller values of diametre where needed.

 

http://www.su-stromberg.de/HTML/content/pages/needleoptimizer_su.php

 

The system offered new needles based on these data.

This needles where put into the compare-o-rama to see how they look.

 

http://www.mintylamb.co.uk/suneedle/

 

After some testing we found a needle that fitted properly at part throttle

but was too lean on the top end.

 

As I can make a lean area richer by grinding but not the other way round

this was the way to go.

 

There are many other needles that are richer but they are also richer in areas

where we did not want that.

 

What I remember is that I made the needle 0.15mm smaller about 20mm from the tip.

 

Reading about the problem I would bet the piston of the SU is on top and airspeed

is too high. Normally the venturi would get bigger but can not. As the SU has no provision

to control the mixture it will make the mixture richer with rising airspeed.

The remedy would be bigger carbs or harder springs and new setup.

[Peter Cobbold]

Hi Peter,

I worked on a modified TR4 with some needles fitted I forgot what type in the beginning.

After setting the idle I fitted a wideband AFR controller and we drove and noted

what the result was at different heights of needle.

 

We looked for lean/rich/okay.

 

After that we used a German needle finder to enter the existing needle and entering

bigger/smaller values of diametre where needed.

 

http://www.su-stromberg.de/HTML/content/pages/needleoptimizer_su.php

 

The system offered new needles based on these data.

This needles where put into the compare-o-rama to see how they look.

 

http://www.mintylamb.co.uk/suneedle/

 

After some testing we found a needle that fitted properly at part throttle

but was too lean on the top end.

 

As I can make a lean area richer by grinding but not the other way round

this was the way to go.

 

There are many other needles that are richer but they are also richer in areas

where we did not want that.

 

What I remember is that I made the needle 0.15mm smaller about 20mm from the tip.

 

Reading about the problem I would bet the piston of the SU is on top and airspeed

is too high. Normally the venturi would get bigger but can not. As the SU has no provision

to control the mixture it will make the mixture richer with rising airspeed.

The remedy would be bigger carbs or harder springs and new setup.

Thanks Andreas,

I have not yet done calculations for the H6 carbs. But a single 2" SU needs a huge air flow, enough for 160hp, to fully lift the piston ( with 4.5oz spring). So I'm not so sure the pistons in each of the twin H6s will be fully lifted.

What I'm trying to find is whether the pulsing air flow from a wild camshaft alters the mixture delivered by the same point on the needle.PGG Knight outlined the effect. At wot air flow pulsations with each iv opening give brief extra depression, and more fuel. The pulsations are lost ( at the jet) in part-throttle operation. Overall, tuning the needle at full throttle leaves the part throttle mixture lean. But your measurements show the opposite.

Peter

[oldtuckunder]

- with a wild high overlap camshaft.

Has anyone found that tuning the SUs at full throttle for best power leaves the cruise mixture too lean?

Am just curious...

Peter

 

At a guess if you really mean best power at WOT, then I suspect you are probably a bit lean all the way up. At prolonged WOT at the top of the rev range CD type carbs have a tendency to start pulling rich and richer limiting the power a bit, but way safer for the engine. So only an idle conjecture but if your not seeing that you may be lean all the way up, what AFR's are you seeing at WOT?

 

 

Thanks Andreas,

I have not yet done calculations for the H6 carbs. But a single 2" SU needs a huge air flow, enough for 160hp, to fully lift the piston ( with 4.5oz spring). So I'm not so sure the pistons in each of the twin H6s will be fully lifted.

What I'm trying to find is whether the pulsing air flow from a wild camshaft alters the mixture delivered by the same point on the needle.PGG Knight outlined the effect. At wot air flow pulsations with each iv opening give brief extra depression, and more fuel. The pulsations are lost ( at the jet) in part-throttle operation. Overall, tuning the needle at full throttle leaves the part throttle mixture lean. But your measurements show the opposite.

Peter

 

My 2ltr 6 will on standard springs reach full lift on a pair of 175's at around 6K.

 

On the pulsing and only because I'm playing in that area at the moment and observing differences between how the same size carb behaves on a 4 pot to a six pot, I think the pulsing effect is more pronounced at med revs. eg. on a 6 pot each carb gets 50% more pulses than on a 4 pot of the same size engine, but the 4 pot makes bigger gulps (so I conjecture but could be wrong) that the carb sees a bigger pulse higher air flow, certainly trying to get the mixtures right on a 6 in the med rev range on larger carbs doesn't correlate to jetting needles on a 4 pot, but by the time the revs get above 5K that all seems to disappear and their behaviour becomes similar even though the 6 is still doing 50% more carb cycles.

 

Alan

[Peter Cobbold]

 

 

 

At a guess if you really mean best power at WOT, then I suspect you are probably a bit lean all the way up. At prolonged WOT at the top of the rev range CD type carbs have a tendency to start pulling rich and richer limiting the power a bit, but way safer for the engine. So only an idle conjecture but if your not seeing that you may be lean all the way up, what AFR's are you seeing at WOT?

 

 

 

My 2ltr 6 will on standard springs reach full lift on a pair of 175's at around 6K.

 

On the pulsing and only because I'm playing in that area at the moment and observing differences between how the same size carb behaves on a 4 pot to a six pot, I think the pulsing effect is more pronounced at med revs. eg. on a 6 pot each carb gets 50% more pulses than on a 4 pot of the same size engine, but the 4 pot makes bigger gulps (so I conjecture but could be wrong) that the carb sees a bigger pulse higher air flow, certainly trying to get the mixtures right on a 6 in the med rev range on larger carbs doesn't correlate to jetting needles on a 4 pot, but by the time the revs get above 5K that all seems to disappear and their behaviour becomes similar even though the 6 is still doing 50% more carb cycles.

 

Alan

Alan, Pulse size and frequency will indeed change between the 4 and 6.

My expectation is that the mean, time-averaged depression will stay constant as the piston cannot rise in time with the pulses, becasue of its inertia and damping. The peak air flow in each pulse - at throttle openings above about 30deg - will acclerate fuel out of the jet, but only briefly. When the carb is tuned in that condition and the needle modded to set say AFR13, the pulsations are embraced. However at butterfly opeings less than 30deg the air flow around it is choked ( aka sonic) and the pulsations cannot pass it. So the mixture given by the needle - at the same lift - now runs leaner than AFR13. In effect the throttle position , above vs below about 30deg , determines the mixture at the same point on the needle. Because of pulsing or not pulsing.

Knight gives a brief description of the above. He mentions that the balance pipe between half-manifolds has a role to play in adding in some pulsations form the other 2, or 3, cylinders. He suggests the effect of pulsing is not a great problem - but I doubt he was including wild cams.

The huge number of needle profiles for the same carb size suggest to me that pulses are important. I beleive SUs were tuned to the engine at the factory, on a dyno I guess. A flow bench would not replicate the pulsing.

 

Are you able to measure the pressure pulses ?

 

Peter

Edited May 22, 2017 by Peter Cobbold

[oldtuckunder]

. I beleive SUs were tuned to the engine at the factory, on a dyno I guess. A flow bench would not replicate the pulsing.

 

Are you able to measure the pressure pulses ?

 

 

 

Nothing shows logging Map but I suspect at 50 pulses a sec at 2K its all to fast for a pressure sensor or for the recording granularity so you just see a smoothed result.

 

Re sizing setting up carbs I did read an old article last year that Zenith/Stromberg used to do it for engine manufacturers, they sent the engine (and I guess some objectives) and Zenith used to do all the carb sizing, needle, spring selection. Fairly certain it was in reference to Triumph in particular.

 

Alan