Critique my MGB Plan

[adamwatson]

I'm doing an MGB with a friend for hill climbs and track days. He is insisting on 300bhp and sub 10 minute Nurburgring laps.

Our plan is B234 Saab turbo engineand BMW E46 330i gearbox. As it we can get this for a lot cheaper than the SR20DET set up we originally planned. The big question is suspension and diff. I know nothing about MGBs but used to do fabrication on historic race cars. The front subframe looks usable to me with a bit of strengthening on the wishbones. Are the stub axles strong enough or are we better binning the lot and using something else?

Rear suspension looks like we need to go full custom and thinking we'll use the diff to match the gearbox out of a BMW but will have to check the ratios unless anyone has any suggestions?



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[surprisingskoda]

The 330i getrag box is the same as the e36 328, e46 328, e34 525 and e39 528 and 530 for ease of finding.
Standard ratios therefore vary. A 3.34 ratio comes to mind for the 330. The e36 328 uses a 2.93 as standard. E34 might be 3.23

[sonus]

Isn't the SAAB engine very tall? It might get a bit tight between the front crossmember and bonnet (?). Maybe you could loose the heater shelf to move the engine back? Front suspension is hardy stuff, but you might want to fit coil overs. Look at Hoyle Engineering or Moss' units. Rear suspension is usually upgraded to 4-linked, but you'd need a MGC or a MGB V8 diff to come down to something that is close to BMW ratios I believe. Hoyle does a IRS set up but it is big bucks.

[RetroWarwicK]

You can do sub 10 with 100bhp but depends on the driver :-)

[Ian]

Feb 8, 2016 0:53:21 GMT surprisingskoda said:

The 330i getrag box is the same as the e36 328, e46 328, e34 525 and e39 528 and 530 for ease of finding.
Standard ratios therefore vary. A 3.34 ratio comes to mind for the 330. The e36 328 uses a 2.93 as standard. E34 might be 3.23

Do you know which is the stronger of the two gearboxes the ZF or the Getrag, just for road use?

[surprisingskoda]

Feb 8, 2016 10:28:52 GMT Ian said:

Feb 8, 2016 0:53:21 GMT surprisingskoda said:

The 330i getrag box is the same as the e36 328, e46 328, e34 525 and e39 528 and 530 for ease of finding.
Standard ratios therefore vary. A 3.34 ratio comes to mind for the 330. The e36 328 uses a 2.93 as standard. E34 might be 3.23

Do you know which is the stronger of the two gearboxes the ZF or the Getrag, just for road use?

My bad. I meant ZF. The Getrag is the weaker box from a 323/320.

[docjohn]

Feb 7, 2016 23:13:18 GMT adamwatson said:

I'm doing an MGB with a friend for hill climbs and track days. He is insisting on 300bhp and sub 10 minute Nurburgring laps.

Our plan is B234 Saab turbo engineand BMW E46 330i gearbox. As it we can get this for a lot cheaper than the SR20DET set up we originally planned. The big question is suspension and diff. I know nothing about MGBs but used to do fabrication on historic race cars. The front subframe looks usable to me with a bit of strengthening on the wishbones. Are the stub axles strong enough or are we better binning the lot and using something else?

Rear suspension looks like we need to go full custom and thinking we'll use the diff to match the gearbox out of a BMW but will have to check the ratios unless anyone has any suggestions?

There's a lot of experience of 300+ bhp MGBs for racing. The front suspension can be made to work OK, the issue is the low roll centre when the car is on short springs. The fix is either re-locating the wishbone pick up points or dropped spindles. It's worth remembering that the crossmember from the chrome bumper cars gives a 1" lower ride height than the later rubber bumper one. The rest is all the usual stuff you are probably familar with.

The rear is usually converted to 4 links with Watt's linkage, or Panhard rod, or Mumford Link, and coilovers. Lowering the roll centre at the back is a good thing.

The Hoyle front and rear suspension kits are very good; a proper coilover double wishbone set up at both ends with a Granada/Sierra LSD diff and discs. I've driven a B roadster at Castle Combe and Donington and it was excellent.

If you are building a hillclimb and track car, then you should bear in mind that it's easy to over gear them. The MGB final drive is 3.909:1 which gives you 108mph at 6000rpm in direct top on 195/50/16 tyres. The 6 speed box has a 0.85 top which gives you 127mph @6000. I just hit the 6200 rev limiter in fourth (direct) in my GTV8 at Blyton Park which is one of the quicker sprint venues in the UK. There's a 3.07 final drive for the BV8 and MGC, which will lift the gearing from 18mph/1000 rpm to 23mph/1000 in direct top.

In the States it is common to put the 8.8" Ford axle in which is very strong and has a big choice of ratios and LSDs.

Sounds like a fun project, good luck!

[adamwatson]

Thanks for the info guys. The Hoyle stuff is out of price range but I think I'll end up fabbing up a rear suspesion system based on the Hoyle stuff with a BMW diff

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[gib]

There may be some useful info in here.

www.saabrally.com/forums/viewtopic.php?f=18&t=3586

Or look at some of Luke's threads as they will give info on Saab engine / RWD application with fully fabricated rear end, as they are Saab based (Obv) but the layout and pickup points may be of use as I presume you are going to basically space frame the rear end ? or completely re-panel?

Around page 5 is where it gets to the current setup.

[bjornagn]

This does bolt in to the MGB and takes care of a lot of your concerns including weight. Might seem expensive until you realize that it includes everything to do the job compared to piecing out your own and doing the development work.


Is it perfect? Probably not, but if Sabine can get a Transit van around the Ring in around 10 minutes, you should be embarrassed to not be able to so it in a stock MGB!



The 4 link out back is easy enough to fab or buy from Classic Conversions.

[Rob M]

Feb 8, 2016 7:20:03 GMT RetroWarwicK said:

You can do sub 10 with 100bhp but depends on the driver :-)

Exactly.
Having the most BHP has taken over from the obsession some men have with having the biggest willy.
Both can be trumped with technique and skill.

[ChasR]

Feb 8, 2016 7:20:03 GMT RetroWarwicK said:

You can do sub 10 with 100bhp but depends on the driver :-)

The best advice here IMHO.

If your friend is unfamiliar with trackdays and has never been to the 'ring it's most likely to be a tall order.


FWIW the only mods that 1.4D AX has are bar less weight are as follows:

Citroën AX 14D « Entreprise » -- 1993 -- 685.100 km -- 1,360 litre diesel engine 52 hp -- 720 kg
The car is stock except: xenon lights, front tyres (A048), increased fuel pressure, strutbar (because of the mileage of the body) and an aluminum intake from 1.5D engine (because of the strutbar).

Sure, he was a little brutal on gearchanges but with the right driving a sub 10 minute lap is possible. With the 'ring it's the knowledge that will get you the time above most things, probably even almost a minute back in time I reckon.

[daslandroverman]

If you're going with a 330i gearbox then why not put the M54 up in front of it?

They're cheap, light enough, have a good torque curve, and if anything like the M52 will fit in the same hole as a Rover V8, which has been proven to fit in an MGB, not least by the factory.

[xbl]

Why the BMW gearbox? The R28 from the Omega will bolt straight on to the B204 & B235 block. Failing that www.gearboxman.co.uk does a bell housing for the Borg Warner T5 to the Vauxhall XE pattern.