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MG MGB Technical - 76 MGB and increased air flow

I have a 76 B and I have been looking for a solution to replace the restrictive air filter system. I guess I was really hoping there would be a way to get a K&N on this baby and get rid of that black cylinder all together. I wasn't looking to switch out the carb just yet though. Any tips would be great.

Thanks in advance,

Josh
Josh Pointer

Josh
K&N do a kit to fit the b...i would suggest that you contact your local/internet specialist to find out the costs etc. one other change you will more than likely need is a change of needles. ask about this when ordering the airfilters.
ian holliday

Order the conical K&N air filters, they just fit (albeit a close fit) near the brake servo I have them in 76B..order them from APT at http://www.aptfast.com

rn
RN Lipow

With regard to conical filters, I have read that they reduced power on a A series engine, anyone have any before and after bhp numbers for B engine?

Paul
Paul

Josh-
You'll find that simply replacing the airfilter on your single Zenith-Stromberg carburetor won't make much of a difference. The problem with getting more power out of your engine is that horrible OE one-piece cast iron combination intake/exhaust manifold that strangles the engine.
Steve S.

Josh,

I think Steve S. is correct. What type of emissions testing do you have in your state? Visual inspections would be difficult to get around. However a "B-Series" engine in good condition can usually be set up to pass the 1976 standards without the catalyst. We have a few car club members in this area that are doing just that.

So, if you can dispense with the catalyst manifold and switch this out for say the down draft weber with a three into one header you should see the power you're looking for. I say the down draft weber because the brake booster on the 1976 would restrict your air filter options vs. the dual S.U. setup.

Regards,

L.C.
Larry C.

In Colorado they just hook it up the the hose and rpm meter and let it run. No visual inspection. If I had to really get into it and do it right do you think I should get the dual carb config going or just stick with replacing the existing one with the weber?
Josh

Most people who do the downdraft weber swap love it. It is said to be trouble free and reliable. I had one on my 71BGT and hated it enough to replace it with the original SU setup. The Weber started great but performance was just not up to my wishes. No matter what conversion you do you will need to replace the exhaust manifold with the earlier style. There have also been people who replaced the stock ZS with a single 1 3/4" SU on the original intake but I don't know any details. Maybe someone else can supply them.If it were me, I would start serching for a stock SU setup complete with intake and exaust manifolds, all linkage and add the conical K&N airfilters. Of the SUs, my favorite is the HIF4s used on the 1972 thru 1974 1/2 cars. If you do not want the original style then go with the Weber downdraft. Either setup can be found from time to time on Ebay
gerry masterman

i recently read somewhere (MG Enthusiast Jan 2002?) that Burlen is now making (or marketing) a HIF44 to replace the z/s carb on a B..

rn
RN Lipow

Has anyone heard of mounting a K&N directly on the Zenith for the 76B?
Josh

Here's a couple of photos of a K&N that bolts directly to the stock ZS carb. I got this one from Moss. There is plenty of space to clear the brakes. I even modified mine by adding a 1/2" thick plate between the carb and filter to create better airflow. The plate has a large radius flare to smooth the transition of the airflow into the carb.

http://www.geocities.com/geraldius/MGBfilter1.JPG
http://www.geocities.com/geraldius/MGBfilter2.JPG
Gerald O'Docharty

Sorry, I guess geocities wont allow direct access to the photos that way. Try this link and look at the bottom of the page:

http://www.geocities.com/geraldius/mgb.htm
Gerald O'Docharty

First off great looking car. I am far shot from being that clean in the engine compartment but I will eventually get there.

One quick question so you removed the cat converter and old air filter stuff and mounted this right on the Zenith?
Josh

I would like to add that, without the homemade radius spacer mentioned by Gerald, that air filter is little better than the stock set up. In fact, unless the radius is very large and outside the element it self, the distance between the inlet "stack" and the outer cover will be too small and the filter will choke off at about 3,500 RPM. We have experienced this recently on one of our custom setups similar to Barry Kindig's.

This may or may not be a concern with the stock cast iron inlet manifold incidentally, but it(the spacer)is still worth doing.

I would also like to add that substituting an SU in place of the Zenith will only make rebuilding and re-jetting simpler. Your performance won't be all that much different. All of the SU mod's generally apply to the Zenith as well.

Sean
Sean Brown

Josh,
Something not already mentioned but worth thinking about is to run a cold air pipe from ahead of the radiator to the carbs to get as much cold air in there as possible.
This is certainly well worth doing here in the Australian summer heat and I would think Colorado gets pretty hot as well.
HTH Pete, Sydney, Australia.
Peter Thomas

Josh,

As mentioned by others, the flow problem is related less to the ZS carb and more with the manifold setup. The strange stock air cleaner/filter setup is only the first weak link in a very weak chain. What you would find is pretty much what Sean described...no power beyond about 3500 rpm. In fact, you might just find that you lose it closer to 3000. Want to know how to find out how a "bolt-on" K&N solution would work? Try running a couple of miles (in a non-dusty environment) without the air filter. I can assure you that performance (or lack thereof) with the K&N filter (and no other changes) would be the same.

The stock setup on later B's was intended to suck air and fuel through a straw. Think about it...less fuel/air mix to burn, very conservative emissions profile cam, lot's of air injected through the emission ports in the head, a very poor flowing exhaust manifold, and an extremely restrictive cat converter, which finally burns off whatever couldn't be digested by the aforementioned low-flowing components...great for emissions, but very bad for anything resembling performance. The reason that many people find they enjoy the Weber conversion, is that they (out of necessity) replace the integrated intake/exhaust manifold. Moreover, removal of the cat converter also lends itself to much better flow.

There's nothing inherently wrong with the ZS carb (except perhaps it's auto-choke design). The setup it's employed with, however, is simply awful. I'm running a ZS with a custom setup (K&N filter, custom air cleaner (a "ram tube" in effect), custom intake manifold, Peco header, Mike & Sean Brown's cylinder head work, and no emissions stuff. I also have HC pistons and the earlier style cam (and the dual row timing chain, which evidently has an impact on cam timing over the later single row). The ZS carb with a better flowing setup also needs a much richer metering needle profile (I use the B1BT, otherwise used with Jaguar XJ6's of similar vintage). As an aside, I'm going to go with twin SUs for a while...my reasoning is that they look more "traditional"...I may get some performance boost, as Mike Brown is going through them and ensuring they're setup for optimal performance (using some of the tricks similar to those described in Dave Vizard's book on the A-series engine), but I'm mostly doing it for show (and because I love to tinker). My point...the ZS can be used in a performance application, but you need to have head work (as described in Peter Burgess's book), either the early exhaust system or a header, and a different intake manifold (Mike Brown designed and fabricated mine...it's excellent...I've also heard you can use an Austin Marina manifold, which ran a single 1 3/4" carb and was not integrated with the exhaust manifold).

So, you'll want to save the $$s you would otherwise use on a ZS K&N air cleaner and look at the system as a whole. Believe me, if you want performance, you'll need to spend more than $50.00, or whatever that particular filter now goes for. Again, because of the manifold changeout (not the carb change, as the Weber DGV, in and of itself, is not much of a performer), many have enjoyed the Weber change. Beware through, as opinions have varied...YMMV.

There's just no single magic bullet. The system is restrictive, therefore the system must be corrected...system-wide.

Have fun,

Barry
Barry Kindig

I definitely understand what you are saying. I am not actually all that upset about the performance and when I am I think I will probably change the whole thing out. For now I was just looking for some things to tinker with and this doesn't sound like a good place to start. What kind of gains do you get when you remove the air pump assembly?
Josh

Josh-
You'll find that removing or disconnecting the air pump has about the same effect as turning off the airconditioning on a car of similar power. Since Colorado has emissions testing, I'd just swap out the belt and leave it all in place for when you need to pass the emissions test.
Steve S.

I once tried the cold air induction with the ZS carb. I ran a 2" flexible duct from a fitting attached through a hole in the radiator support to the stock air filter box where the hot air 'flapper' attaches. This resulted in a quite noticeable increase in power especially when passing as there was actually some boost attained by a 'ram air' effect at the air inlet. Unfortunately the positive pressure created chaos with the fuel mixture and it ran lean most of the time. Coasting downhill at speed with the throttle closed would cause the engine to die. If any kind of cold air intake is to be considered it would have to be placed in an area where there was no pressure change caused by various speeds.

Disconnecting the air pump would probably yield a gain of about 1 HP or so. It would be advisable though to remove the air injection manifold from the head in that case and plug the inlets. Otherwise the air manifold will eventually fill with carbon from the exhaust and it's a real pain to clear it out if you need to reconnect it.

In my case I have made other extensive mods to my engine including port matching and polishing the exhaust manifold inside, it has a fast road cam, modified head, overbore 60 thou, and a big-bore exhaust. The stock cat was replaced with a high flowing unit under the car. Set up like this the K&N filter does help with better flow than the stock airfilter. It was necessary to richen the mixure to compensate just for the filter change. Also the ZS carb has been modified with thinned throttle plate shaft and the poppet valve removed for a few extra CFM.

My next step will be to install the Austin Marina manifold mentioned by Barry. I have one waiting to go in. I hope to still be able to pass the emissions test with it, but it will get swapped out for a pair of SUs when the smog police aren't looking.

Gerald O'
Gerald O'Docharty

Josh,

You might consider an attempt to "remove" the stock cast inlet from the log type cast exhaust manifold with a hacksaw or airsaw. You could then fit a Peco header and a free flowing exhaust, both of which would benefit you down the road regardless of future modifications.

This might cost a little more, but it would be fairly inexpensive compared to some of the alternatives. You could also do the inlet filter mods as discussed above and have a real go at it.

Incidentally, the Marina manifold is basically the same as the cast one piece, but without the integral log exhaust manifold. So performance should be similar if it's possible. Just be sure to not cut into the inlet, obviously if the exhaust side gets damaged, (ventilated) it won't matter.

Sean
Sean Brown

This thread was discussed between 24/02/2002 and 03/03/2002

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