Tweaking your MG for the road  

Paul’s tips on how to go about it!

(Part 1)

NOTE: While the following is written mainly around the MGB, it also applies equally to the MGA and Midget other than remembering that Midgets are a size smaller in the carburettors. (Even though the standard heads for 1100 and 1275 Midget breathe relatively much better than B series heads, useful gains can be made through modifying them, and the cam profile mentioned works equally well in all engines.)

So, you want to make your MG a bit livelier on the road and a bit nimbler round the corners. The good news is that it is relatively easy to do so, and end up with a car which has very respectable performance even by today’s standards and which can comfortably surprise many modern cars.

The main areas requiring attention to achieve this are the engine and suspension, the brakes having sufficient reserve in standard spec to cope with the additional performance where the car is used on the road - even for rally use I found they needed nothing more than uprated linings.

Before going any further I’d like to stress that the following is written in the context of putting together a `fast road’ car - one that performs nicely more than standard, yet which remains very drivable, retains reliability and doesn’t cost a fortune. It is NOT a cheap guide to producing a race car, which things need a bit more work to overcome adrenalin-induced reliability problems even if you think the power might be enough to start with!

 Part 1:   More Go from your engine

Before you start tweaking an engine for more power, give some thought to the difference between tired and new engines. I’ve never ceased to be amazed at just how much more go a newly rebuilt motor has. This is very noticeable in the hilly west where we live - hills that cause an average B to think about or grab for third gear are accelerated up easily in top with a new engine.

This brings us to rule number one in modifying: before you start, check that your car is properly tuned and in good condition - once that’s done, you might decide it is so much better, it’s enough!!...not to mention the fact that tweaking a sick engine can achieve little other than killing it properly.

Now if that’s not enough power to keep you happy (never any such thing as too much!) you’ll then find a confusing plethora of modifications and views about which are best - particularly for a limited budget, road modified engine (full race types are more likely to have an open cheque book, `if in doubt, have it too’ approach!). The only way to sort out what’s best for you is to look at the facts, not the sales brochures, and also to beware of adopting uncritically mods others have done.  I recall one trader  who commented `I think it (the mod) makes it worse, but people keep on asking for it so I’ll give them what they want’!   

In order to see what works and what doesn’t, and hence where our money is best spent, we need to look at just how much difference the various mods make to the way an engine breathes and how much power it produces. Well known A-Series expert David Vizard conducted an extensive series of flowbench and dyno tests on the MGB engine a number of years ago, and although since then there has been significant evolution in the various means of giving the B more power, the basic principles of his findings remain unchanged.   

These were:

• The cylinder head provides the single greatest resistance to air flow into the engine - flow gains of up to 22% were made by modifying ports and chambers alone,  increasing to 34% with bigger valves
  
  

• There is very little gain (<1%) from simply polishing the ports and chambers without reshaping
  
  

• Airflow is very responsive to higher valve lifts, particularly when the head has been modified - at lower lifts, gains from head mods were typically 10 - 15%, or about half those at higher lifts. 
  
  

• With an unmodified head, very little is to be gained by increased carburation - a 45mm Weber with no air filter flowed only 2.1% more air than the std SU’s and filters. Dyno tests were comparable. However, the gains by the bigger carburettor were much better with the modified head - from 7% upwards, depending on choke size. 
  
  

• Capacity increases from overboring the engine gave higher than expected results.

 Applying these principles, we see  

1. It is not cost effective to change carburettors or filters as a first modification step. They may look impressive, but won’t really do much until the head is opened up as well.  
   
   

2. The cylinder head has to be the first step in modification - a  properly reshaped head, not merely a polish. Best value for money lies in retaining the standard valves, with worthwhile gains available from large valves if your budget stretches that far.
   
   

3. High valve lifts are a very effective way of increasing power. This can be achieved by high lift cams or higher ratio rockers - available commercially, but expensively, as the roller type - or (even better) both. Best value for money lies in a suitable cam reprofile on its own.  
   
   

4. If your engine needs overhauling and you want more power, a 1950cc big bore provides good bhp/$ as well as more torque everywhere and is well worth considering.  
   

A point to watch about high lift cams is the length of time the valves are open at the larger openings. There is a large difference in effectiveness between two cams of the same lift when one opens slowly and spends very little time fully open, while the other opens rapidly and spends much more time open at the higher lifts. (This latter is easily achieved by overhead cams, due to lower valve train inertia,but that’s getting a bit too high tech for B’s!)   Race cams spend lots of time at high lifts, but the penalty is long duration and accompanying low speed uncouthness due to the need to start opening the valves much earlier to get them open far enough at the right time.  

It was with this thought in mind, not to mention feeling a bit past late night cam-changing parties, that I developed a cam profile which utilised higher acceleration and lift rates in order to get a respectable amount of high lift time whilst retaining short duration and therefore tractability (very impotant for keen motorkhana types.... and day to day use). The result was a cam which has ‘1/4 race’ timing yet spends as much time at higher lifts as a ‘3/4 race’ example....and lots more torque than standard in the lower and middle rev range as well as the usual increase at the upper end.  It has proved a good all round cam in B’s, while its use in conjunction with nothing other than an extractor in our trusty BMC1300 has found it virtually another gear yet retained good idle and tractability for Titirangi hill starts. This profile the one used for the fast road camshafts listed in this catalogue and can be recognised by a 734 suffix to the part no.

So - what does all this mean in terms of good old BHP?  

In a general sense,  my own dyno testing has shown trends consistent with the results of Vizard’s work. Setting aside all the race/rally spec engines, here are some results of the road tuned engines:(All power is measured at the wheels)  

Notes on dyno results:

1.   All runs done in full road trim, ie with fan and aircleaners in place.  Aircleaners for SU’s were standard MGB.

2.   Runs (a) to (c) were done back to back, ie same car, same day.

3.   Runs (d) and (e) were on the same car and day as each other, but were different to the first three, so the two sets of results are not directly comparable - hence the brackets. They do, however, give a good indication.

4.   Compression Ratios:  
(a) 8.8:1,  (b) and (c): 9.8:1,   (d) and (e): 11:1

5.   To get  an idea of BHP at the engine, increase the above by 30 - 35 %.  The actual amount varies, depending on the dyno and the day, and other factors, but it will give you a guide.

Now exactly what constitutes a road tuned engine is a matter of some elasticity as to how much lumpiness individual owners find acceptable, not to mention the subterfuge as racing types try to play down the extent of their tweaks!   However, all the above had very good tractability and no flat spots at all (other than when the Dellorto pump jets weren’t working!). Engine (e) felt very V8-ish, with the power pouring in at 1500 rpm.  

So what does all this mean?  Put simply, for a relatively modest outlay, it is possible to extract a useful amount of extra power out of an MGB.   Assuming your engine is in good order and up to the task, a good head job retaining standard valves along with a reprofiled high lift camshaft will leave you with useful change out of a thousand dollars if you fit the goodies yourself, and an increase in power of around 15%. Allow another $500 - $600 ea if you want to go for bigger valves or carbs.   However, bear in mind that all this is in the context of a fast road engine which spends most (though by no means all) its time under 5-5,500 RPM...if you want to go racing, or frequently spend time over 6000 revs, you’ll need to dig much deeper into your wallet!

On the topic of more carburation, other reserarch indicates the gains that can be made by modifying the standard 1.5 inch SU’s - half the gain at a fraction of the cost of the 1.75 inch version or the 45mm Weber. The largest single component of this gain comes from simply narrowing the throttle spindles (in the manner that a Weber’s is from the factory).

NOTE: Installing higher lifting cams requires careful checking of valve travel,  to make sure they still have at least 1mm spare travel on full lift. Results of mucking this one up: 100+%  budget blowout!!! If in doubt, ask!! Or get someone else to check it!!  

Don’t let this put you off a high lift cam - the effort is worth it.

Now for a comment about extractors. The standard MGB manifold and downpipe are together an efficient extractor configuration. They are sized towards the mid range as you might expect on a road car, and I really question the benefits of fitting a larger bore extractor (more suited to higher revs) on a road modified car. Again it comes down to how much you want to spend, and where value for your dollar lies.... and how much time you intend using 5000 RPM or more. Suffice to say that all the above tests were done on engines fitted with the standard cast iron extractor manifold and frontpipes, so they don’t seem to have been that detrimental, and you’d be better spending your dollars on more carburation or bigger valves first. (There are useful modifications which can be done to the standard exhaust manifold and we’d be pleased to advise you on them.) There appears, however, to be useful benefit in fitting a size bigger in the rest of the exhaust system,though for a road car noise is an issue to watch for both WoF and comfort.

On the other hand, the MGA and Midget (which have "all into one" standard manifolds) stand to gain significantly from the use of an extractor manifold.....in fact it is essential to use one if modifying the camshaft, even mildly. With increased cam duration, the standard manifold enables the exhaust gas from a just-opened valve to dump straight into its neibouring cylinder which is just about to close its exhaust valve, so filling that cylinder with exhaust gas instead of fresh fuel/air, and checking the flow in the manifold by virtue of the greater valve overlap. An extractor's separated pipes eliminate this effect and allow a much greater proportion of new fuel/air in the cylinder - hence, more power!

The type of silencer fitted can also have a major effect on power output. There are two types of perforated tube used through the centre of the straight-through type of silencer : louvred (with vanes which protrude into the gas stream) and plain (with neat, round holes of about 5mm diameter in the tube walls).   In my dyno testing I have found significant differences between the performance of the two types, with around 6 bhp loss (at the wheels) from the louvred type being not uncommon.   This could negate a big chunk of the gains you've just spent your hard earned cash on, so be careful about which type you fit!

The final word on engine tweaks has to be: don’t forget the importance of tuning!   When an engine has just been modified, the distributor advance rate and carburettor needles or jets usually require a different spec from standard. If you buy a package of mods, the seller ought to be able to advise you; alternatively seek help - it would be a shame to damage your engine due to a lean mixture or over advanced spark after all that effort!