2 litre engine and 5 speed mazda gear box

[Derfnalla]

I am considering upgrading to a 2 litre lead free engine and mazda 5 speed gear box on my 1968 mgb roadster and would be interested in peoples opinions / thoughts.

[Ian F]

Without knowing anything about your '68 B it's difficult to comment.
If it's in good original condition I would say that you may reduce it's appeal and value if you move too far away from that condition?
You could always buy a Mazda MX-5……....

Ian F

[Paul Hollingworth]

Much better to get an overdrive box (if you haven't got one already). Why settle for 5 gears when you can have 6. Do you need to go as far as 2 litres. I'm sure a mildly tuned 1868cc (+0.060") would suffice. First step, get Peter Burgess to work his magic on your head, second fit K&N filters plus richer needles. Think about an electric fan (+3hp). This is assuming you haven't done this already. I personally would stick with the standard cam else you will change the character of the engine.
As Ian has said going as far as you have proposed there is a danger you will make your car less valuable

[Peter Cresswell]

I'm not sure I agree that a 5-speed conversion with make the car less valuable. Adverts I have seen for cars with both the Ford Type 9 conversion and the Mazda conversion seem to suggest the opposite - at least that is what the adverts are suggesting!
The problem with the MGB gearbox is the low ratio of 1st and 2nd gears, and this is more obvious with a tuned engine with more power and torque. When used with the standard rear axle ratio, the Mazda ratios are closer to the MGB close ratio gear cluster available, and thus more usable in modern traffic especially for overtaking. Unfortunately the close ratio gear set for the 4 synchro box costs around £2,300 (about the same as the Mazda based kit) and you still have to get the gears built into your gearbox. This will then give you a 60mph first gear, and if you drive in built up areas, a clutch life that can be measured in days rather than years! So the diff ratio needs lowering to around a 4.3 or lower, which is another £500 plus rebuild costs! Hence the popularity of the 5 speed conversions.
I have also heard stories of high oil consumption on 2.0 litre and larger engines due to bore flex. By sticking at the max oversize of 60 thou you don't get this.
Having said all the above my Stage 3 engine is 1860cc, with standard 4 synchro gearbox with overdrive and standard 3.9 diff ratio. So for cost reasons I am putting up with the low 1st and 2nd gear ratios.
Incidentally, the MGC had a higher 1st and 2nd gear and the MGB GT V8 had even better ratios - if you are luck enough these days find a first motion shaft and lay gear for either!

[Paul Hollingworth]

I have some experience of how the factory decide the gear ratios. 1st is about gradeability, being able to start on a 1:4 (or was it 1:3 - cant remember). 2nd is decided so you hit 60 mph at the red line. I agree with Peter that there seems too big a gap 2nd to 3rd on the MGB, but this may have been decided looking at in gear acceleration. 4th will be decided to give the maximum top speed. If there is a 5th then this will be an overdrive for fuel economy. I remember colleagues plotting the gear cascades (not for the MGB - before my time), this is a graph of tractive effort against road speed. Ideally you want to drop onto max torque when you change up.
I don't pretend to be an expert in this discipline but here is a link if you want to know more about it :-

http://www.thecartech.com/subjects/auto ... Effort.htm

I remember on the Midland Centre Welsh Rallye we replicated a special test (just for fun) on Bwlch Y Groes also know as gearbox hill because the factory used to test gearbox durability there. One entrant (who will remain nameless) in a rally prepared MGA couldn't get off the line because his diff ratio was too tall (low numerical value). My profile picture is taken at the top of Bwlch y Groes on that event. Also I drove my standard MGB up Shelsley Walsh and acceleration was fine in 1st and 2nd but almost ceased when I went to third. I had to change back down to 2nd again for the S's anyway. Ever likely my time was 60+ secs. Mind you I did have the wife and picnic furniture on board.

[Paul Hollingworth]

Just to contradict what I said above, I've just looked at an Autocar test on the MGBGT and 6000 rpm in second only gets you to 50 mph. Perhaps 0-60 wasn't seen as such a benchmark in the sixties. Roughly each gear change will cost you a second. That was a 3 synchro box where 2nd gear ratio was 2.21:1, the 4 synchro box had a taller 2.167 ratio, so 6000 rpm would have been 51 mph. not much different. Still a big jump to 3rd at 1.382:1 which will get you to 80 mph

[Peter Cresswell]

I used to have a Toyota Corolla GT (the AE86 car and rear wheel drive) which I used for sprints and Hillclimbs in the Standard Car Class. It featured a close ratio gearbox and a fairly low ratio back axle. In 2nd gear it would hit the rev limiter at 59 mph, so quite a poor 0-60 time of 8.7 sec compared to the MR2 with same engine but different gearing that could do 0-60 in 7.9sec all in 2nd gear. Overall though the Corolla was a much quicker car and was really only beaten on fast tracks like Goodwood or Castle Combe by Honda VTECs which had around 70bhp more at the wheels.
To get fast acceleration times, gear changes are much quicker than 1 sec, but I did break the Corolla's gearbox twice! Oddly I never had a problem with the standard gearbox in the MG TC I raced despite pushing over 120bhp through it and 12 years of racing it. Clearly they don't make them like they used to!

[David Witham]

If you are interested in a modified MGB engine it may be worth your while reading Peter Burgess's book on modifying MGB engines.

My car has been rebored to 1870cc and has a compression ratio of 10.6:1. I approximately followed Peter's standard class compliant recipe. This gives plenty of torque but is prone to pinking at wide open throttle between 2000rpm and 3000rpm. It is best not to exceed a compression ratio of 9.5:1 unless you use an aftermarket aluminium head.


I have an overdrive box in the B. In our other cars we have either 5 or 6 speed boxes. I like the overdrive.

[Peter Cresswell]

Hi David
It was the Corolla that I ran in a standard class.
My MGB sounds like it roughly the same as yours, so close to a Stage 3 engine. I used the Special Tuning Booklet, Peter Burgess' book and also the Huffaker Competition tuning manual plus a few of my own ideas to modify and build the engine. Capacity is 1868cc (+060 overbore) and compression ratio is around 10.5:1. Cam is Piper 285 Magnum. Ignition is by Aldon 101-BR2 distributor (with points and vacuum). Head is 2709 casting with 1.625 inlet race valves and standard size race valves for the exhaust. It doesn't pink at low revs which may be due to the different advance curve of the distributor, but I don't particularly drive in top gear from low revs. On the advice of Gerry Brown, carbs are still HS4, but with richer needles. It would give more top end power at the expense of bottom end power/torque with HS6s or a 45DCOE so it would be less easy to drive. I use it mainly for tours.

[Paul Hollingworth]

What distributor are you running with David ? I've been doing a bit of a study for a friend whose bought a 123 distributor which has a choice of 16 curves. On the advice of Peter Burgess's book we have set it up with the advance curve of the chrome bumper cars with the 25D4 distributor which uses curve 40897 (curve2 in the 123). The 45D distributor is no good at all. His engine is bored to 1868 cc and we think the comp ratio is about 9.5:1 as the head and block are both skimmed.
All the advance curves are available on Paul Hunts Hammer & Spanner site including the Aldon 101BR2 that Peter is using. 40897 has a static timing of 10 deg BTDC whereas the Aldon has 12 deg BTDC. The 40897 reaches maximum advance of 20 deg crank at 2200 rpm and the Aldon has 20 deg at 2600 rpm. Taking account of the difference in static timing, the Aldon is still a few degrees more retarded 1000- 1800 but then the curves merge. so nothing there to help you with your pinking problem in the mid rev range. What fuel are you running on ?
My friend hasn't had chance to thoroughly test his car yet but he is running on Super so I'm hoping it doesn't pink.