"Operating limits of an SU carburettor"

[Peter Cobbold]

Article in pdf format here:

https://supertrarged.wordpress.com/2015/05/04/the-operating-limits-of-an-su-carburettor-2/

 

My motivation was understanding how to get the right mixture out of one 2" SU supplying 160 to 180hp feeding the blower.

The article shows how it can be done.

 

There's also interesting information on the effects of changing the mouth rounding. Such as when trumpets are fitted to a stock tuned SU.

Am very interested to know how much piston lift has been measured on carb'd TRs in standard tune at full horsepower. I think they will not be fully lifted.

If anyone can supply the measurements needed in step1 the calculations can be done for SUs on a TR.

 

Peter

[jerrytr5]

Excellent article Peter.

I've just bought a cheap boroscope to check piston lift on my Hurricane but haven't got around to it yet.

I was thinking of picking up a wideband AFR too but they're still a bit pricey for occasional use.

Jerry

[Peter Cobbold]

Thanks Jerry.

Can a passenger see the borescope while driving? as without loading the engine the piston will barely lift, eg if the car is stationary.

 

I bought a USB camera on 5m lead with led light for about £10:

http://www.amazon.co.uk/gp/product/B00A6K4DCK

- but it does need the laptop to come along for the ride...

 

Yes the Bosch sensor itself is around £65 but the kits are expensive. If you have electronics expertise this could be useful info:

http://www.waltech.com/open-source-designs/wbo2_report/

 

Peter

Edited May 7, 2015 by Peter Cobbold

[westaj]

Peter,

 

I,too,thought that your article was excellent and really enjoyed reading it.

 

I am thinking of fitting triple SUs to mildly tuned TR 250, mainly because they look nice!

 

Ideally, three HS4s rather than HS6s, but then I would need to fabricate adaptor plates at the HS4 carb outlets. The topic of piston lift is relevant to the choice of carb size.

 

A possible way of establishing the max lift at wot could be to smear a small amount of Vaseline or grease on the piston groove at the front of the carb.

You might then be able to establish how far the piston lifted when the engine was fully loaded.

 

Just a thought.

 

Jerry West

[Peter Cobbold]

Thanks Jerry,

I like the grease idea !! it would be easy to find max lift. Put the theory to the test.

 

There's a triple carb'd 6 with a billet, CNC-machined manifold, a one-off. Don't recall its builder's name. But it looks terrific.

I'm sure it will be possible to calculate lifts etc for HS4 or HS6 on the 250, given the measurements of piston mass and so forth.

Its best to avoid overcarburation, so HS4s would be my first choice for calculating lifts etc. And three 1 1/2" is what the chart shows.

 

There's a slight complication with tuning the needle on triple-carbed 6-cylinder engines, to do with the firing order.

153624

the first carb gets a pulse of air when iv is open at 1ooo2o ( the ooo indicates iv closed)

the second carb at 3oo4oo

the third carb at 5o6ooo

So the pulses of flow are equally separated in time in carb 2.

In carbs 1 and 3 the piston tends to sink a bit more than in carb 2 during in the three consecutive non-flow periods (ooo). Then they will tend to get a good lift with two iv opening in quick succession : 2o1 and 5o6. This will tend to make the damper work a bit to give slighly richer mixture. So the middle carb needs slightly richer needle profile than the outer carbs.( at least at lower hp ) .

 

Are there triple SU manifolds on the market or are you fabricating those?

 

There might be a small "exit loss" where the adaptor plate opens up into a bigger runner bore.

 

Peter

[jerrytr5]

Don't do it !!

Tried triple carbs on a mate's MGC many years ago. Took an awful lot of work (and that was with a Downton manifold) and we never got it running right. Reverted back to twin carbs in the end.

Admittedly, we were young and knew nothing, so I'm not saying it's not possible, but .....think very carefully.

Jerry V

[Peter Cobbold]

Don't do it !!

Tried triple carbs on a mate's MGC many years ago. Took an awful lot of work (and that was with a Downton manifold) and we never got it running right. Reverted back to twin carbs in the end.

Admittedly, we were young and knew nothing, so I'm not saying it's not possible, but .....think very carefully.

Jerry V

I suggest we do the calculations first, see what the lifts will be. Then work from there.

With what we now know there's several areas that can be explored to get the carbs matched to air flow.

What size SUs did you try on the MGC ?

Peter

Edited May 9, 2015 by Peter Cobbold

[jerrytr5]

Can't remember Peter, it was a long time ago. Probably HS6 as we would have had the two originals and only needed to source another.

[graeme]

Goodparts do a triple manifold. https://www.goodparts.com/shop/index.php?categoryID=2

 

I have heard various reports of difficulty in balancing three carbs on the six pot engine.

 

Cheers

 

Graeme

[Peter Cobbold]

The attraction of triple carbs is the symmetry of the runners in the manifold. That's got be good for equalising mixture distribution between cylinders compared with the twin manifold The downside is the air has to be shared with three carbs, so each needs to be smaller to ensure the air velocity keeps acceptably high to ensure decent atomisation. So the trick with SUs I think is not to let the air velocity go too low. Big droplets of fuel dont burn well ! So HS4s might work better than 6s. But it needs a few numbers to check.

And dont forget the Dragon's teeth

 

Peter

[Peter Cobbold]

Graeme,

Yes I've heard too.

 

 

I think its related to a combination the unequally timed air flow surges with iv opening. Air speed can drop off in carbs 1 and 3 giving bigger droplets longer to wet the manifold walls.( post#5). Lenz' book shows that during iv opening small droplets of around10um will keep up with the surge of air flow, while droplets of 50um almost do. But 250um droplets fail to accelerate to even half the peak air speed at max iv opening. So they have plenty of time to wet the walls in that long stagnant period in carbs 1 and 3. But less time to fall out of the flow at higher rpm

At part throttle the extreme turbulence around the butterlfy should correct the mixture quality. My expectation is that it will be at wot and the lower rpm range where poor atomisation might be an issue.

 

Peter

[westaj]

Hi Peter,

 

Not sure whether you had a chance to check the piston lift at wot with the method suggested in my earlier note.

 

I did; both HS6 pistons had fully lifted during a wot test run. I haven't had a rolling road bhp measurement yet, but I suspect around 126 hp with my current set up.

 

Using your estimated 80 bhp per 2" carb with air velocity of around 75 ft/s ( from the Healey reference in your supertrarged article) and ratio-ing my powers (63 bhp/carb) and areas (1.3 multiplier for HS8 to HS6) gives around 76 ft/s.

 

As mentioned before, In due course I intend to modify the engine and fit 3 x HS4s.

If the new power approached, say 144 bhp, then each carb delivers 48 bhp.

 

Ratio-ing the powers (48/80) and areas (1.77 multiplier for HS4) from your Healey estimates, would give a per carb air velocity of 80 ft/s. This would drop to 58 ft/s for 3 x HS6, which, as you cautioned before, is probably too low.

 

On the Healey forum was a reference to measuring piston lift using a straightened paper clip through the dash pot vent hole. With masking tape the measuring rod (clip) will rise but then, in theory, not fall.

 

Seems like this maybe be a neater solution rather than using grease on the piston slide!

 

I haven't had a chance to try it yet and am off to Spa with the London Group tomorrow, so it will have to wait.

 

Jerry West

[Peter Cobbold]

Hi Peter,

 

Not sure whether you had a chance to check the piston lift at wot with the method suggested in my earlier note.

 

I did; both HS6 pistons had fully lifted during a wot test run. I haven't had a rolling road bhp measurement yet, but I suspect around 126 hp with my current set up.

 

Using your estimated 80 bhp per 2" carb with air velocity of around 75 ft/s ( from the Healey reference in your supertrarged article) and ratio-ing my powers (63 bhp/carb) and areas (1.3 multiplier for HS8 to HS6) gives around 76 ft/s.

 

As mentioned before, In due course I intend to modify the engine and fit 3 x HS4s.

If the new power approached, say 144 bhp, then each carb delivers 48 bhp.

 

Ratio-ing the powers (48/80) and areas (1.77 multiplier for HS4) from your Healey estimates, would give a per carb air velocity of 80 ft/s. This would drop to 58 ft/s for 3 x HS6, which, as you cautioned before, is probably too low.

 

On the Healey forum was a reference to measuring piston lift using a straightened paper clip through the dash pot vent hole. With masking tape the measuring rod (clip) will rise but then, in theory, not fall.

 

Seems like this maybe be a neater solution rather than using grease on the piston slide!

 

I haven't had a chance to try it yet and am off to Spa with the London Group tomorrow, so it will have to wait.

 

Jerry West

Jerry,

Many thanks - nothing like a measurement to test theory !

The grease will also report transient lift, while the article covers numbers that relate only to steady state balance between downforce and depression and lift. So it's possible the piston lifted maximally, and wiped the grease, during a sudden throttle opening, when the depression can briefly be higher than normal equilibrium. Hamill suggests a wood dowel to replace the damper, but that means cutting a hole in the bonnet - OK for rr though. The article doesn't cover numbers for transients at all, its way too difficult for me.

 

I have asked a real pro ic engine engineer to look the article over. So in few weeks I hope to be able to add more information. And later, some videos of piston lifts.

 

If you care to redo the grease test on a lap of Spa opening the throttle only very slowly up to max power.......

 

 

....maybe not !!

 

Peter