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MG MGB Technical - MGB tuning

I read the replys on this site and i am amazed at the knowledge thats shared and the differant views expressed. I have a 73 BGT which after a lot of work now runs really well, touch wood. Its never going to be a show winner but i would like it to be a bit quicker off the mark. My question is given that the car is completely standard what would be the most effective modifications to improve acceleration and what would be the best order to do them in.
John darby

Conventional wisdom is that improved breathing is the quickest way to bhp with your B. My order would be: K&N air filters, freeflow exhaust and manifold, and for the really noticeable gain but bigger money, a fast road cylinder head. You will most likely need to change the carb needles after any of the changes to allow more fuel in to mix with all the extra air you are flowing. After these modifications, things get more expensive and invasive.
Steve Postins

I agree with Steve, except about the exhaust manifold. The original is quite satisfactory for a road car, and makes the engine quieter too. If you are serious about this, you should talk to Peter Burgess, whose ads appear at the top of some of these pages. You won't buy a better cylinder head, and he won't sell you stuff you don't need. The head will cost about £400 and will give you 20 - 25 more BHP. PB will tune your car on the rolling road as part of the sale - well worth it. He is in Alfreton which isn't so far from you.

Mike H
Mike Howlett

You might be satisfied with just the results of getting the distributor and carbs sorted out on a rolling road, if you haven't done that yet. Odds are that your distributor curve is wrong due to tired springs and modern fuel, and the metering needles may not be spot on....
Rob Edwards

John. The first question would be how much money do you have available and how quickly do you want to see results?

The basis for any "improvements" you wish to make is a good, sound basic engine. If you have an engine with 100K on it without a rebuild, any performance upgrades, which actually work, will only tend to cause it to go out much sooner. Stressing a worn lower end by installing a high performance cylinder head would be a lesson in disappointment. So, if you do not already have it, the first step in "upgrading" the engine is a quality rebuild using quality parts. My personal choice is a Peter Burgess "basic big block" style engine--.060" over sized, high compression pistons, a Piper 270 cam, duplex timing chain and sprockets, a rebuilt rocker arm assembly and a good balance job on the reciprocating and revolving parts. Some basic cylinder head work, new valve guides, if needed, unleaded conversion and a three angle valve job would be a good starting place if money is tight. If you have the funds available, a Peter Burgess cylinder head is an excellent idea. However, with a basic head rebuild, the engine you have build will be much improved over the worn, stock engine. The modified cylinder head will not hurt to have to wait a year or two. I would, however, install it within 20K miles of the rebuild for best benefits.

The factory exhaust manifold is excellent and should be retained. You might have a small horsepower gain by having a custom exhaust header specifically built for your engine specification, but I doubt you would be able to see such a gain except on a rolling road. The factory system was of excellent design and is very hard to beat in general usage.

Rob's comments about the benefits of a carb tuning/rebuild and a dizzy tuning/rebuild are spot on. They can either be done now or in conjunction with an engine rebuild. But, a properly functioning distributor and carbs will show a significant improvement over old, worn assemblies and can be a part of a "improvement over time" program.

Steve and Mike also have some good points, but they are mentioning items which offer a "potential" for improvement rather than an actual improvement, at least in my experience. The K&N filters are a "free flowing" filter. However, the original Cooper's filters work well on the stock engine and with the stock cylinder head. When I installed K&Ns, on two cars, there was not noticable difference. Both cars had good, stock engines in good tune. However, the K&N filters offered increased performance when combined with modified cylinder heads. Thus, it is another "potential" gain that is not realized until the modified cylinder head is installed. Les
Les Bengtson

Les
There was a web site created by a guy who tested various air filters. He tested for filtering and for flow on his mazda miata. He did a good job making standardized measurements. The site is probably still out there with even more test results. He found that the K&N had some better flow than a paper filter, but it did a poorer job filtering. After some dirt built up on the K&N it did a better job filtering and was equal to a good paper filter, but, of course its flow was reduced. In the end a clean paper filter changed periodically was the best practical compromise for performance and engine protection. The pressure drop was very small through the filter.

With an early MGB without the power brake booster there is room for a little taller air cleaner. The K&N web site allows you to select filters by dimensions. The stock two carb filter set up has more filter flow capacity than permitted by the two snorkels. If you take the cans off and replace the top with a paint can lid the engine will flow enough extra air that it runs too lean on the stock jets and needs the richer needles. The snorkels are important for air cleaning. When the air changes direction and speeds up to enter the snorkel, the heavier dust particles will drop out of the flow.

I don't think changing the type of air filter, all by itself will make any measurable difference without other changes to the intake system.

Barry

Barry Parkinson

Not to qibble, but I've seen this "flow enough extra air that it runs too lean on the stock jets" a few times. It doesn't make sense to me, the SU is a variable venturi carb and should handle extra air flow. My guess would be there's lower vacuum in the venturi thanks to the free flow filter and the paint can mod, so the carb needs a richer needle for that air flow. Same difference I suppose.

Interesting points on the K&N - if true, as it gets dirty you might need to go back to the original needle.

Has anyone dyno'd the difference between the K&N and the stock filters? The K&N with the paint can mods effectively shortens the runner length doesn't it? While top end HP might increase by a bit, I wonder what happens to low end torque?

It sure seems like the head is the place to start - with only 1 3/8" siamesed intake ports, opening up the carbs, filters or exhaust are barely noticeable if that (I've tried all 3). Refurbished stock parts and proper tuning made the most difference.
Mike Polan

I think this may be the test Barry mentioned. Clifton
http://www.bobistheoilguy.com/airfilter/airtest1.htm
Clifton Gordon

Thanks you all for the information. Next step save my pennies for the 1, overhaul dizzie. 2, reworked cylinder head, 3, air filters and needles 4, exhaust system.
John

John,

I don't think any of the others have said this yet but your first expenditure should be on a copy of Peter Burgess's book "How to power tune the MGB" published by Veloce. One you have read it you can then decide how far to go.

If you are on a budget you can get some worth while air flow improvement for free by minor reshaping of the backs of the valves. The book explains what to do.

As a 73 your car probably has the carbs with the anti run on valves in the butterfly. You can remove these and block up the holes with solder or arildite. You can also modify the shaft. Both of these increase full throttle airflow.

Rob Edwards above makes a good point. Make sure what you have is properly set up.

David
David Witham

This thread was discussed between 29/01/2005 and 01/02/2005

MG MGB Technical index

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