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MG MGB GT V8 Factory Originals Technical - Is it worth lightening an SD1 flywheel?

My 3.9 Range Rover engine was from an automatic transmission car, so I fitted an SD1 flywheel and clutch when I built my MGB GTV8. The car suffers a little from transmission snatch at low speeds, making crawling in a queue tricky. Could this be due to the heavy flywheel? Do folks lighten these flywheels for MGB use? If so down to what weight? If you lighten it by reducing its thickness, do you run into clutch issues? I'm going to strip the engine this autumn anyway, so now would be a good time to know.
Mike Howlett

Mike, actually lightening the flywheel would make the car more difficult to drive in heavy slow traffic as the heavy flywheel helps keep the rpms up at low engine speeds when a load is applied such as engaging the clutch. I suspect your "transmission snatch" is more likely caused by a glazed flywheel surface and clutch disc. Have the flywheel surfaced and install a new clutch disc and I think you'll have better luck. As far as lightening goes, don't try to remove thickness. Instead focus on removing weight as near the outside edge as you can, that makes the most difference. A couple of pounds removed from the center would have almost no effect, while a couple of pounds from the outer edge will have a profound effect.
About all you really gain from a lightened flywheel is the ability for the engine to gain and lose revs quicker which is fine for racing, but really not too necessary on the street.
Bill Young

Mike ,
I was under the impression the SD1 had a lightened flywheel from the factory.

In any case as per Bill's suggestion a surface grinding to get it all trued up would be a viable choice.
Making it lighter will accentuate the problem.

If the "snatch" is after the clutch is fully engaged but travelling at low speeds the cause may be drive line take up or movement in the mounts.

Pete.
Peter Thomas

Mike ,here's the specs from the MGB V8 maint man. I used a Range Rover flywheel & had to have a LOT of metal removed to bring it up or should I say down to spec. Barrie E

B Egerton

Mike,

If the flywheel is standard SD1 weight approx 33lb then the problem is elsewhere.

What is the spec of the engine. cam, heads, induction and igniton systems.

The problem is more likely to be overfuelling at cruise or incorrect timing especially if using a standard Lucas distributor.

Kevin.
Kevin Jackson

Interesting Kevin. The engine has Burgess Econotune heads with a Real Steel Cyclone camshaft, although I know now that the camshaft isn't timed properly. I set it to standard cam timing, but have now realised that the makers suggest advancing the cam timing by 3 degrees, so I will be doing that.

It has hot wire Efi with Optimax chip, an adjustable fuel pressure regulator and a speed sensor, and the standard dissy with an external amplifier from RPI. The ignition timing was set by Mr Burgess himself on his dyno, but I guess it will want tweaking when the cam timing has been changed.

The car runs really well - its just this snatching at crawling speed which makes me feather the clutch.
Mike Howlett

Mike,

Is the idle stable and at what RPM?

If not the idle valve? may need cleaning. I'm not an expert on the Hotwire system but will post up a link which will help.

Kevin.

Kevin Jackson

Mike,
Me again, just checked the cam specs of the cyclone cam and see it is ground on a lobe centre angle of 108 degrees, the hopwire does not like to run on less tham 112 degrees LCA so that is more than likely the cause of the shunting at low RPM, although I see that Real Steel state that the cam will work with Hotwire!

The problem is caused by exhaust gas reversion back through the airflow meter because of the increased overlap of the cam,the airflow meter then reads this as a demand for more fuel and supplys an over rich mixture.

Fixes,a modified chip for the ECU from Mark Adams RPI sell versions of his chips but if I was going that route I would prefer to get a chip specked specifically for your particular engine and cam.

i would be cautious of RPI as thay are only too happy to tell you,you need this or that and sell you something that is less than perfect for your application.

Alternatively a change to a Megasquirt ECU which is more complex but does mean you can run fully programmable EFI and ignition which is a great advantage.

Kevin.



Kevin Jackson

Kevin, I only bought the Cyclone because it is advertised as being specifically "Designed for 3.9 Hot-Wire injection engines" (Quote from Real Steel's brochure). Why would they say that if it wasn't true? I was advised to use Real Steel by Peter Burgess, so assumed that they are reputable. I must say that he didn't recommend the Cyclone camshaft, but thought the standard 3.5 camshaft would be better, but that was after I had already built the engine.

You are not the first person to sound a warning about RPI, but that is the kit I have. The Optimax chip is a Mark Adams design - I have tried to contact him directly, but he will not answer emails and I have no other way of finding him.

I can see that a programmable ECU could be an advantage, but only if the user understands how it works and how to set it up, and I haven't a clue. I'm afraid I am absolutely in other people's hands with this kit. I plugged it all together, and it worked. To me that was nothing short of a miracle. So, as someone for whom this is a black art, what would you recommend? As I am going to strip the engine anyway, should I ditch the Cyclone?
Mike Howlett

Oh and to answer your question, the idle is stable at around 800rpm.
Mike Howlett

Mike,

Before you do anything else try running with the vacuum disconnected, if you are getting excess advance on a light throttle this could be whats causing the problem.

As to real steel I have had very good service from them and am curious about the cyclone cam being that it has a LCA of 108 degrees which is normally considered too hot for standard hotwire, but a Mark Adams chip should get round the problem if it's the right spec. Just buying a chip from RPI although from Mark Adams doesn't mean its right for your tune.

Did you tell RPI what cam you are using?

I'll try and find a number for Mark Adams.

You should have some decent heads from Peter Burgess.

What RPM does the snatch become apparent 1500 RPM or less?

Kevin.
Kevin Jackson

Mike,

Marks company is Tornado Systems Ltd. in Shropshire.

Tel. 01694 720144.

I found him very helpful whem I was considering using the Hotwire ECU but decided to go Megasquirt insted as it does away with the restrictive airflow meter, and give you fully programmable ignition for £300 for the ECU.

I will need some help installing it and loading the maps as i haven't got a clue although thankfully i do know people who have done it.

Kevin.

Kevin Jackson

Hi
Interesting this
I would have thought that idle smoothness would be more dependant on lobe spread than lobe centre angle.
Be carefull not to get mixed up with these specs.
Lobe spread as you are probably aware is ground into the cam grind selected but lobe centre angle can be moved around a bit by adjusting cam timing by you.
Basic statement would be the wider the lobe centre spread the smoother the idle. ish.
Hope this is of some help Willy
WilliamRevit

Willy,

I think you are referring to lobe seperation angle which is the same thing as lobe centre angle, these are fixed at the time of the cam being ground and cannot be changed, you can of course advance or retard a camshaft to fine tune the the power curve, normally up to a max of 4 degrees in either direction, so if the cam is normally timed at 112 degrees you could time it at 108 to 116 degrees.

Kevin,
Kevin Jackson

Have you extended your throttle lever?
The MGB pedal has a very long travel that you can take advantage of. I almost doubled the length of the rover 3.9 EFI throttle lever so it would match the full travel of the MGB pedal. Allot more control, and easy to do.
P.N. Sherman

Thanks Peter, that's a good thought. I did extend it a little but not by as much as double.

Kevin, I disconnected the vacuum advance and plugged the line to the inlet, and it seemed to make absolutely no difference to the engine at all. I know the vac unit works - if I suck really hard on the line I can see the base plate move. Do you think I should leave it disconnected?

Really the snatching is not a huge problem - I suppose I've got used to it. I think Peter may be on the right track in that a tiny movement of the throttle disc makes a big difference in the revs.
Mike Howlett

Vacuum advance usually only makes a difference to light-throttle cruising, giving better economy for the same speed and distance. Can also make part-throttle acceleration a bit sharper, rather than pushing the pedal a bit more. For various reasons both seem harder to detect on the V8 in use than the 4-cylinder, where it can be quite noticeable. The vacuum capsule was punctured in mine, i.e. giving a weak mixture on one carb, but I only found out when I tested it on a routine service.
Paul Hunt

Wasn't double, just almost.
Measured the max and the min positions of the cable for the max and min of the pedal movement. Then bolted on a small extention lever using the existing hole. It hooks over the top at the bottom.
I was finding it difficult in wet weather, a tendency to spin the rear wheels a little at traffic lights.
With the extension I was able to control that.
I can't measure it right now I'm sorry. Car is still at the painters/resto' shop. FIVE going on six weeks overdue!
This blurry photo might help.


P.N. Sherman

actually the extension lever isn't perfectly aligned with the original lever underneath. It's a little more vertical than the original section. That also would give more control because with the lever at a greater forward angle to begin with a small movement of the cable will produce a larger movement in the lever as it rotates back through its radius of movement.
The original cable attachment point is where that small bolt is.
P.N. Sherman

Mike,

I picked up on another thread somewhere that you had a engine balance problem.

Some of the larger stock Rover V8's have an element of external balancing and this is provided by the crank pulley, if you had a mismatch this could be the source of the problem.

When I built my 4.35 engine I had the whole rotating system balanced at Real Steel and they wanted the crank, rods, pistons, flywheel, clutch and the crank pulley, it is now very smooth.

How you getting on getting it sorted?

Kevin.
Kevin Jackson

Kevin,
The components you listed are now with Real Steel. I drove all those bits and the block 400 miles to get them there (I did fit in a visit to my son, daughter-in-law and granddaughter in London at the same time, and visited the Classic Car Show at the NEC on the way back). They are also doing a rebore and regrind as there was light scoring on the big end journals and signs of oil burning in the combustion chambers. So while I had it all in bits, I thought I would go the whole hog.

As for external balancing of the engine, I believe the 3.9 is the only one where this is the case, and I deliberately retained the original crank pulley for that reason, even though it meant some difficulty with the anti-roll bar. I'm convinced that the unbalance is due to the SD1 flywheel on an engine that used to be attached to an auto transmission. Hopefully the balancing will sort it all out. Apparently the special machine needed to balance a V8 is not at all common in the UK, Real Steel being one of the only people who have one.

Mark at Real Steel was most helpful and I should get the bits back on a pallet before the Christmas holidays.
Mike Howlett

This thread was discussed between 08/10/2009 and 29/11/2009

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