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MG MGA - Extra Power at zilch cost!!

I have been absorbing David Vizard's excellent A Series engine tome and came across a section covering methods of getting more air molecules into the engine by working on the guts of the SU ..plus a fig.showing the results of the actual airflow experiments. I attach his fig 7.20.

The most basic one is the familiar rounding the SU entry port with a 1/4 in radius ..gaining about 6% more air for the same head loss compared to the basic.. ie the stub stack. This seems to be consistent with the results from the work of the two Steves . There are more additional interesting gains to be had with mods. on the butterfly spindle, the butterfly itself and the other SU flow passage internals..the most interesting for me was the shaving down of the spindle and removal of the split heads. These gave an extra 10% air ( line 7 on the fig)) IN ADDITION to the stub stack increase. All of these gains will not be realized because of other head loss restraints in the system as airflow increases but they should lead to greatly improved throttle response as well as much extrapower at the top end at near zilch cost.
The fig relates to a 1 1/4 SU H4 and I am considering for a HIF44 ( Vizard states the mods should get 25% extra air for same loss for this larger SU).

Would appreciate comments?. Has anyone done the spindle mods...or similar...or...???

Neil Ferguson

Neil, Interesting.
Are the various improvements in the list supposed to be additive, or does each line include the mods in the previous line?
I thought that the SU design was supposed to create a constant pressure depression throughout the flow range, so isn't a lot of this self-defeating?
Pressure drop is necessary to suck the fuel from the jet and to atomize it for efficient combustion.
Putting a bigger carb on will give you lower pressure drop, but I don't think that does any good in practice?
As you say also, the rest of the flow path has to be considered, so you can't just claim the flow increase measured across one component in isolation.
Art Pearse

I have read all that stuff many years ago and am always sceptical.

In my opinion the "cleaning" up of the carburettor throat can only be of benefit on absolute full throttle, I ask you the question how often are we at full throttle?

When racing more times than not I suspect when normal driving? The claims even at full throttle seem rather exaggerated to say the least although he has tested on a RR I have not. So he hould know. :)
Robert (Bob) Midget Turbo

From my reading of the book each one is relative to the previous...ie they build up.
The pressure drop through the carb.can be the same after each mod.--but is created by more air passing through which in turn will mean a needle and mix change to get more fuel into the stream .
I have put my bigger carb ( HIF44) on in combination with a Judson so I probably will already be stuffing about 20 to 30 % more air in....
I can imagine that the spindle slimming and butterfly changes will only be beneficial in terms of Hp when the throttle is significantly opened ( the slimmer profile creates less turbulence in the air stream ) but the change could improve throttle response just like the inlet stub stack.
Neil Ferguson

As per Bob, waste of time except at full throttle. Not likely to give any response benefit, since once downstream of the air piston you are not affecting mixture response. Throttle plate screws coming loose is not good. It is probably worth the radiusing of the bore directly aft of the piston, at the transition.

FRM
FR Millmore

Fletcher...downstream changes in a gas stream affect upstream conditions so do not agree with your comment re 'downstream of air piston'. I believe that if a small change in butterfly orientation ...once it is significantly open ... induces less friction loss ( than a butterfly with the standard spindle diameter etc ) then then more air mols. will be pulled into the system and more fuel pulled in from a properly contoured needle...ie quicker response...but not at low revs..
Vizard has actual torque and HP before and after performance curves for twin HS4 SUs which show both improving significantly about 3500rpm.
Agree totally re spindle screws but it is not beyond the wit of man to fix them without doing the 'splits'
Neil Ferguson

Neil-
"downstream changes in a gas stream affect upstream conditions so do not agree with your comment re 'downstream of air piston'. I believe that if a small change in butterfly orientation ...once it is significantly open ... induces less friction loss ( than a butterfly with the standard spindle diameter etc ) then then more air mols. will be pulled into the system and more fuel pulled in from a properly contoured needle...ie quicker response...but not at low revs.."

>>Airflow changes beyond the air piston are simply quantity, which the carb compensates for if correctly set up, so unless you use the extra, it will have no effect. How much driving do you do at full throttle? Turbulence near the jet will have an effect of some, possibly variable, sort, which is why smoothing the transition might be good.

"Vizard has actual torque and HP before and after performance curves for twin HS4 SUs which show both improving significantly about 3500rpm."

>> Yes, but I've yet to see any dyno curves at other than full throttle. It was one of the things I was musing about in Steve's first thread on all this. I'd love to see part throttle curves, and transition curves if they could be extracted on a modern dyno.

"Agree totally re spindle screws but it is not beyond the wit of man to fix them without doing the 'splits'"

>> True, but such can be tricky, and for little to no useful gain. Were I building a race car, I'd do it, and have, but it hardly is "extra power at zilch cost" for street use.

FRM
FR Millmore

As I mentioned in Steve's thread, I did some of the modifications on the single SU H2 on my Y-type; I modified the carb body around the piston area, streamline the protuding areas of the piston and cut and loctited the butterfly screws. I've run my now sold MGB with HIF4's with cut and loctited screws for years, without disasters happening.

I (have to) 'floor' the Y-type more often than you would do with an MGA. It climbs faster in revs and runs better at high speeds (60 mph is fast in a Y...), the throttle response is markedly better (especially between my ears?), so I do believe that there is truth in Vizards claims.

If we are all convinced that Steve's stacks work, then these modifications must do good, too?

Willem vd Veer

Willem..thanks for your comments on your own experience and I believe you are spot on ......removing unnecessary restrictions in the airflow path whether upstream or downstream of the SU will improve both response and HP. Reduced turbulence created by the restrictions means the air ( and air/fuel ) can accelerate more rapidly into the engine when the throttle is opened ......seems obvious to my fair self.
I have to remove the HIF44 from the Judson I have just installed to drill a distributor vac port through the body just upstream of the closed butterfly valve and when out I will inspect the internals and do at least two mods.....
Remove the intrusive run on valve set through the butterfly and installt a blank butterfly.
Remove the split threads and fix the spindle screws with locktite etc..

Plus maybe a little body profiling and spindle reduction ( but not around screw holes).
Neil Ferguson

Your probably right Neil except what is your comment about the following please.

The throttle response of an SU is purely down to the rate of damping on the piston isn't it? When the throttle is opened the piston rises and allows more air into the engine, this as we are all aware is designed to SLOW the throttle response to make sure we do not get a weak mixture. Changing the consistancy of the oil in the SU damper has a far greater effect on pure throttle response than anything else I would suggest?

The edges of the damper and the throttle disk only become a restriction at full throttle?

Perhaps I don't understand how an SU functions?
Bob Turbo Midget England

Bob..the SU is just a part of an overall air intake ( plus fuel ) system. It has to be tuned to take allowance of the cylinder configuration, the intake manifold, the filter etc etc. EVEN the exhaust system . When the throttle is opened and more fuel ingested the engine gets the umph and it is up to the system tuner to get the right mix, needle profile, the right spring and the right oil to keep up with the airflow. Reducing the turbulence and hence losses caused during rate of change of the angle of the butterfly ( i.e. hoof to floor) by smoothing its contour means air mol. intake can increase more rapidly ..this leads to quicker acceleration. The profiling of the throttle disc spindle and screws will begin to be effective as the disc rotates significantly open into the air/fuel stream but well before full open.( i was not proposing disc 'knife edging'..the improvement looks minimal ). In summary the SU characteristics have to be optimised to the overfall more responsive air handling and head loss system. One clear example of this is the affect of the upstream stub stack..which also reduces turbulence and hence allows easier rate of change of airflow ( drivability !) as reported by the folk who bought and installed such.
..You asked the question but I know conversion of sceptics is difficult .
Neil Ferguson

This is how I understand it.

The piston damping doesn't slow the throttle response, it speeds up the airspeed in the carburettor.

The damping in the piston momentarily causes the piston to rise less rapidly and that causes the air steaming into the carburettor to flow quicker (the venturi effect) and that will draw more fuel from the jet. The whole effect will be the momentarily richer mixture which is needed on acceleration. This is why the SU doesn't need an acceleration pump.

Thinner or no oil is less 'acceleration pump effect', thicker oil more 'acceleration effect' (but not to be overdone of course).

Streamlining the parts will be beneficial to the way the air can react to changes, thus in my mind at least will benefit the throttle response.

Fletcher; is above about right or not? I trust your knowledge more than mine...
Willem vd Veer

Willem..
Your explanation is spot on.... and very clearly written.
Neil Ferguson

This thread was discussed between 06/05/2012 and 09/05/2012

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