MGB Register Forum

I should have mentioned, that like Paul I run my B on Super Unleaded. The competition distributor specified in the old Special Tuning booklet is 40943 with no vacuum advance, which has slightly more advance through the rev range than the 101BR2. This may be due to the Aldon Distributor being advanced to cope with modern unleaded fuels.
I'm sure I have read somewhere that an early Cooper S distributor (no Vac advance) works well in the B-Series. I might have come from a Cars and Car Conversions magazine article - but I can't prove it!

My B has a 2709 head with the standard valves. I always use super unleaded. Tesco 99 octane if I can get it.

When I get around to it I will take the head off again to double check for sharp edges. I thought I had done them all.

On the dizzy front, my mechanical one is a 41288 which I use with 10 degrees static. I also have a locked one that I have used with an Aldon Amethyst. For the last year or more I have been back on the 41288 but I have just started trying to get to grips with the Amethyst again. Was using the Amethyst before it gave up on the way to the Goodwood Revival in 2017. Fortunately, I was able to bypass it and run at 10 degrees fixed for the rest of the journey. Good thing I had the mechanical dizzy in my spares locker for the return journey.

Peter your 285 cam will reduce the tendency to pink at low revs as the increased overlap will reduce the low rev cylinder filling efficiency. I run a standard cam and have considered a wider cam as part of the solution but I want to fiddle more with timing and mixture first.

Sorry if I have high jacked someone else's thread!

Hi David
I've plotted 40897 and 41288 and the curves lie virtually on top of one another and both use a static timing of 10 deg BTDC. I'm not sure which of these I've got in my 71 B, but who cares. The strobe settings are 14 deg BTDC for 40897 and 13 deg for 41288 both at 600 rpm. Its easier to set at 20 deg BTDC at 1000 rpm and all sources suggest this is the same for both. Maximum advance is 20 centrifugal plus 10 static = 30 deg BTDC at 2200 rpm.
40897 is curve 2 in the 123 distributor and 41288 is said to be curve 6, but there appears to be a mistake in their data sheet because is says the maximum advance is only 14 deg crank. Who knows if this is replicated in the electronics. I'm convinced that what we have done on my friends engine in selecting curve 2 is for the best.

Hi All, just joined today and finding my way round. Probably not to everybody's delight but I intend to fit both the mazda engine with gearbox and tweek the power upto 180/200bhp Mark to the gt that I bought. Would this sort of level of modification not enhance the character of the car and it's value?

It will enhance the character by giving it a lot of performance! You will need to consider serious upgrades to the suspension and brakes to cope with the extra performance. This is what Frontline have done with the MGB LE50 and the Abingdon Edition.
On the downside you risk having a Q plate and lose the Historic tax (VED) as the car should be classified as being substantially modified on account that the Mazda engine was not fitted to the MGB during its long production run.
A chat with Frontline might be useful before you start. See http://www.frontlinedevelopments.com/

Naturally I'd upgrade the brakes and suspension which I was thinking possibly gaz shocks all round, rear panhard rod and a vented disc set up on the front with goodridge or alternative hoses. Hopefully with a little trading on the old engine and overdrive box etc plus a new stainless exhaust that came with the car, I'll cover most of the cost!
When I fit the mx5 drive line, I'll fit new fuel lines and pump etc which look like they are past there best and refurb/change the clutch and brake master cylinders to hopefully give me some trouble free motoring. Car should stay on original plate acording to dvla guidelines on radically altered vehicles and there points system (only loses 3 with different engine and gearbox).

Peter
Interested to hear how you went to 10.5:1. what is your head chamber volume on the 12H2709? tight squish i assume and/or shallow dish pistons? ie shallower than stock?
G

Hi Graham
Firstly, the compression ratio I have ended up with is a bit of a 'guesstimate', but based on good reasoning. I followed the calculation set out in Peter Burgess' book and basically used his figures to calculate the 'Clearance Volume' - capacity for head gasket thickness (4.5cc), the dish in the pistons (6.5cc), the volume above the piston to the surface of the block (2cc).
As far as I know the 2709 head has the same size combustion chambers as he heads with smaller inlet valves, so for the combustion chamber volume I used the standard volume he gives (43cc) less (a 'Rule of Thumb') 1cc for every 10thou taken off the head thickness. My head has had 1/16th in taken off (so 0.0625in). This gave me a combustion chamber capacity of 37cc given I polished the chambers to the bottom of the casting marks. So my Clearance Volume works out at 50cc (4.5+6.5+2+37)
The swept volume is the engine capacity divided by 4, so for a standard bore this is 449.5cc and in my case for 1868cc it is 467cc.
Then the sums are:
Compression Ration = Swept volume + Clearance Volume divided by Clearance Volume. In other words the Total Volume of a cylinder is being squashed into the Clearance Volume.
So for my engine, the CR is 467+50/50 = 10.34:1. I did have to have a light skim of the head face when I had finished the porting and polishing, and I had the piston set a couple of thou down the bore by having the top of the block skimmed. So my guess is I have a CR around 10.5:1.
For a standard bore with a head skimmed by 1/16th in it will be 449.5+50/50 = 9.9:1 which is close to the figure (9.8:1) given in the Special Tuning Booklet given the estimates along the way.
I think around 10.5:1 CR is about as high as you can go on pump fuel (Super Unleaded) and not have to worry about detonation at high rpm and running on when the engine is switched off, but the timing needs careful adjustment to prevent too much pinking. I haven't had it on a rolling road yet so BHP and Torque remain unknown, but it sounds very crisp and with a B&G lightweight flywheel it revs very quickly. Suffice to say it is adequate!