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MG MGA - Supercharger Question

I have a 1950 MG TD and someone on our T series BBS suggested I ask this question here. He also mention that Dominic Clancy might be able to help. If anyone can hook me up with him I'd appreciate it. E-mail???

I recently finished the rebuild of my Judson supercharger. I've got a few hundred miles on it now and the car is running well.
I am a little concerned by the lack of boost pressure. When cruising locally at anywhere from 45 to 60 mph and I stomp on the gas I can only get a little over 2 pounds of boost. I thought it should be more like 4 or 5 pounds.
I thought it might be my old gauge but I hooked up a new one and got the same thing. My gauge line comes of the intake manifold and is new.
Any suggestions would be most appreciated.
Thanks,
Mort


Mort (50 TD "Mobius")

Mort, Dominic will probably be along shortly and there are several others on this forum with some Judson experience who may be able to help.

My first thought is a vacuum/pressure leak somewhere. Are you using a combination vacuum/pressure gauge and, if so, what vacuum does it show when the engine is idling?
Andy Bounsall

First of all it is a supercharger and unlike a turbo "stomping on the gas" will do nothing to the boost pressure

The boost pressure of a supercharger is determined by gear ratio to the engine. Unlike Dominic I am not an expert on the Judson but how did you work out the gearing? Is the boost pressure consistant throughout the RPM rev range?
Robert (Bob) Midget Turbo

"First of all it is a supercharger and unlike a turbo "stomping on the gas" will do nothing to the boost pressure"

Not true. Closed throttle stops air from getting to the blower, so it has nothing to compress. Opening throttle lets air into the already spinning blower, and pressure rise should be immediate, to the limit of the current blower speed. Boost pressure will then rise as engine speed, as (blower delivery minus engine consumption). On a well balanced system this is generally set to maintain a constant boost above some set design limit.

Turbo acts the same, except the increasing exhaust flow/energy when on boost causes compressor speed and therefore boost to increase rapidly in a positive feedback loop. System is ultimately only limited by flow restrictions in the induction & exhaust, resulting in the need for regulators like waste gates.

FRM
FR Millmore

Yes sound right the point I was considering is exactly described with a turbo stomping on the throttle will quickly wind up the boost pressure where as the same action with a supercharger will display he boost pressure steadily until the revs change by a fair amount. Then as you alluded to if the consumption is equal to production the pressure ill remain constant.

It is all to do with gearing :)
Robert (Bob) Midget Turbo

Are there any other symptoms of a vacuum leak?

Prior to rebuilding the Judson I was not able to get a reliable idle below 1400 RPM. Since the rebuild I can idle reliable (although still with a bit of engine shake) at 800 RPM.

I am using what I believe is the original Judson vacuum/pressure gauge. I've cleaned it and replaced the gauge face. At idle it reads about 5-7 inches of mercury.

My gauge picks up the vacuum from the intake manifold and I just put on a new hose.

The car is running quite well. That is why I ask about other symptoms of a leak.

Tomorrow I will test further for leaks with a propane cylinder.

BTW I tightened up the idler on the Judson and now get about 3 psi.

Any further thoughts would be appreciated.
Mort
Mort (50 TD "Mobius")

Bob has it. Mort, you do understand that we are saying that boost pressure is determined by engine speed and drive ratio for a positive displacement blower on a fixed engine/carb setup at full throttle, while a turbo has a time dependent factor even at fixed engine speed? So you can expect a jump as you open the throttle, followed by a slow rise to max boost, and a possible drop off at high rpm. The slope and peak of those curves would depend on drive ratio, all else being constant.

I do not have any recent experience with measuring manifold vac/pressure at idle on a blower motor. It may be that low idle vac (manifold) readings are normal. If you tee into the oiler vac line, you should be showing around 18"Hg carb vac. If the low vac reading is normal for blower motors, then it also could be expected to vary by drive ratio.

There is or was a restrictor at the center point of the inlet manifold, which may be having some effect on measurements at idle, depending on what was done there in fitting the gauge line. This is directly related to the recent detailed thread on this board re carb balancing. Still up as:
carb synch vacuum port adapters AJ Mail 16 June 2012

FRM
FR Millmore

Mort...I see from the photo that your Judson is fitted with a SU Carb.Is this a standard arrangemnt for the TD? I have just fitted a Judson to my MGA and changed the standard fixed jet Holley carb for a SU HIF44 ( 1 3/4in )and am progressively tuning with various needles etc.. . What SU is fitted to your Judson and if a 1 3/4in what needle to you use?
For info so far I have achieved about 4 psi boost.
Neil Ferguson

I've contacted Mort and put him in touch with George. I don't think it will be a leak because things very quickly get quite exciting if there is a leak in a joint and the available air supply is no longer under control through the carb!
dominic clancy

Neil

The SU is a standard fit to the T-Series Judson - see the relevant section on my site here

http://clancy.ch/Judson_T-Series_1.html#6
dominic clancy

Neil,
My Judson has a SU H4 carb with 1-1/2" throat.
The standard TD's come with twin carb's with 1-1/4" throats.
Mort
Mort (50 TD

Dominic and Pete...thanks for advice..on another Judson query do you use the Marvel Oil or another oil ..or have you tried latter etc.. I can get it here in Oz ( from Aviation suppliers ) but a little costly....worthwhile if it gives better protection.
Neil Ferguson

Neil,
I use the Marvel Mystery Oil.
Mort
Mort (50 TD "Mobius")

I use a standard single weight SAE10 oil for machine shops. I now pay 75 US for a 20 litre canister
MMO is not available here and I can't get anyone to ship it from the USA to Europe
dominic clancy

Dominic,
I thought you found a local supplier??
Here is the link for Marvel Mystery Oil's MSDS sheet.
http://whatsinproducts.com/msds.php?brandId=11052
It appears to be Stoddard Solvent, Hydrotreated Heavy Napthenic distillate etc., and an extremely small amount of Benzene.
Here is the US government site with the ingredients listed below.
http://hpd.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=13005003
If you go to the bottom of the sheet, you'll see the ingredients and an associated CAS number. This links to that ingredient and shows everything it's used in. Maybe you can mix your own.
Mike
Mike Parker

I had tried to revise that, but it doesn't show it.
The CAS number used for the Naptha is the same as that used in 3 in 1 oil, 97% for 3 in 1 and 99% in MMM. Also, MMM shows an older formula that didn't have the Benzene in it.
Good luck.
Mike
Mike Parker

Dominic ....I could not find any car parts/service etc suppliers of Marvel oil online or anywhere in Australia ..nor could I get it sent from overseas...I kept on looking and found it was still used in small private aviation engines ..and found a few Shops in this market that supplied it. I have just ordered a gallon for A$45... Try this source.
Neil Ferguson

One additional re Marvel...I did ask George Folchi whether using this would give the valve seats protection from using non leaded fuel. I do not have hardened or coated seats and have had the car for 23 years without problems ( I recollect leaded ceased about 8 years ago approx here) . I squirt an additive into the tank but George advised that the Mystery would do this job.
Neil Ferguson

This thread was discussed between 25/06/2012 and 28/06/2012

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