Inlet Manifold to Fit SU Carburettor to a Ruby

[Jamie]

would this exhaust manifold suit your needs??? http://www.southernsevens.co.uk/austin-7...gearboxes/

Thank you, but I think that I have one sorted: just waiting to confirm.

Ian.
I am hoping that the carburettor will be all right for the engine. The engine has a high compression head, a modified camshaft, shortened cam followers, smoothed ports and the John Barlow inlet manifold. The head and block have been very lightly skimmed to ensure that they are flat, which I imagine has increased the compression ratio a smidgen as well.

The carburettor cost me £9:00 and I have fully refurbished it, including a new butterfly valve spindle: oddly, the old spindle had worn badly, yet the bushes were fine. The new spindle went in snugly.

I hope to run it on a dynamometer, so I should be able to see how it goes when I have it fully assembled it. If the carburettor does prove to be too large, I shall change it.

Regards,

James.

[Austin in the Shed]

I would advise against using a 1 1/4" Su, unless you have a reasonably well tuned engine it will be too big so metering will be even more of a compromise. You would be far better off with a 1" or 1 1/8" instrument. To clarify the reasons; for an SU to work manifold depression causes the piston to rise and fall depending upon throttle opening and engine speed. If you use an oversize Carburettor the piston will only move a small distance rather than fully open when required, therefore you are trying to meter the fuel on a very narrow band of the needle. The consequence of this is lower performance than optimum in which case you may do better with a refurbished Zenith, they work well in good condition. Although most that have been through my hands need new throttle spindles at the very least.

I have a couple of cars running on 1 1/8 Su's and both run really well from low down.I also have one fitted with the sports type log manifold and 1 1/4 SU and no matter what Alterations I've done the best I have got is power from now 1500 rpm. Not managed yet to swap carb and manifold to a smaller one to see how they compare however.

[Steve Jones]

I ran my Trials Chummy for 20 years on a 1 1/8 SU. I also changed the 1 1/8 on my hillclimb Ulster for a 1 1/4 for last season but it was never as good as it'd been before. Changed back for this year and it's back to it's best. At the level of tune the OP is reporting I'm afraid he might be disappointed with the 1 1/4.

Steve

[Austin in the Shed]

I picked up a late box ruby manifold recently with the intention of fitting it and the 1 1/8 SU purchased at spring Beaulieu to the car previously mentioned.it took about 2 1/2 hours cutting the inlet out of the exhaust and machining it up to fit on to a separate exhaust.
It takes 5-10 minutes to saw a crankcase in half.

[Ian Williams]

Jamie Even if you get it running and tuned on a dyno I still in my experience believe that you will not attain the performance you would with a smaller instrument. I would hazard to say the the Dyno money would be far better spent on setting up an 1 1/8" carb. I would also suggest that you ensure your dyno operator knows a thing or two about SU's and has a full range of springs and needles at his disposal, otherwise you are rather wasting your money.

[Jamie]

Good evening.

Thank you all for the advice.

It sounds like I might be on the lookout for a fresh 1 1/8" carburettor body soon as well.

The Dyno operator is familiar with sevens, but I don't know whether he holds spare jets etc
Regards,

Jamie

[Ian Williams]

Without a full selection of needles he will NOT be able to set the carburettor up correctly, the mistake most people make with an SU is adjusting the jet height to solve mixture problems. This should only be done for the IDLE mixture, correction of mixture anywhere else should only be done with the needle, if you don't follow this advice the result WILL be a compromise and ultimate performance WILL suffer.

[Hugh Barnes]

In my amateurish view on the world, I have always thought that a larger carburettor may not give better performance. I have always imagined that a smaller 'throat' will generate a higher speed of airflow, thus giving a greater charge to each cylinder. There may be other advantages as well as a result of greater mix of air and petrol. I run 1" OM's (or are they 7/8", I'm never quite sure which bit you measure) and they run fine with good performance and econony. Noting Ian's comments, yes a selection of needles is required. The Minty Lamb web site is helpful here (http://www.mintylamb.co.uk/suneedle/) for assessing needle choice and comparison. If you wnat to get it spot on, however, the only real answer is a session on a rolling road. I had a Riley Nine once that when I first put it on the Rolling road, gave 31bhp at the rear wheels. An hour and a half later, it was giving 42bhp with careful needle choice etc... Best £150 I ever spent...

[Ian Williams]

Hugh is, based on my understanding and experience, essentially correct. The speed of air flow helps pickup fuel droplets and improve improve atomisation, if any or all of the carburettor, manifold and ports are oversized, you can get areas of slow moving air and subsequent fuel dropout. (highly polished ports cause the same, its far better to have a smooth but unpolished surface) If they are to small you restrict the volume of air that can be packed into a cylinder during an induction cycle. Hugh also alludes to inertia of the mix which can assist volumetric efficiency particularly during the early and late parts of a valve opening. The same is true of exhaust systems, getting them sized and designed correctly improves drag and can help scavenge cylinders particularly during the late overlap period of the exhaust cycle, smaller primary's 20 to 25mm are often far better than the 32mm+ that you regularly see with after market A7 exhaust manifolds. The hard part about precisely tuning an SU is knowing which measuring points are rich and which are weak, yes you can have both on the same needle. I have a large selection of needles and made up a calibrated rod that fits into the piston, once on a rolling road I can measure the fuel/air ratio at any given measuring point and adjust needle selection accordingly. Of course all of the above means needle selection dependant on numerous factors; alterations to valve size, Cam design, C/R etc and a reason why simply saying and AN or NO6 or whatever will rarely be the perfect solution. I agree that the advice often given here about a specific needle can provide a good starting point, but it seems many are still somewhat confused by the often touted AN advice and end up applying it to the wrong engine set up with disappointing results. Do as Hugh has done, find a Dyno operator with SU experience and good needle selection then the money will be well spent.