Why didn't the bearings fail?
Discussion
A month ago we had a Moggy Minor 1275 van in, it sounded a little like a diesel but ran sweetly. It produced the curve shown as red and around 65 bhp at the wheels, it responded well to the advance but the diesel noise increased too. We persuaded the owner to bring the engine in 'to have a look'. Low and behold, the big end shells were -30s when the crank was -40, way too much bearing clearance, amazingly after a hundred miles the bearing shells looked fine. We fitted new correct size bearings and disturbed nothing else and put the sump back on. First power run today was crap, yellow line 55 at wheels 
We noticed throttle wasn't full so we decided to sort that, even though it was as we dynoed it before, greenish line 58 bhp at wheels. We then started to retard the timing from where we found optimum when we first dynoed it. Power came storming back in and eventually we ended up with the blue line just under 70 at the wheels, we couldn't replicate the original power below 3500 but better above 3500 which we assume is full throttle, higher comp as the bearing clearances are correct and capacity increased also because of the stroke improvement? Timing backed off from 35 to 29.
Peter
,
We noticed throttle wasn't full so we decided to sort that, even though it was as we dynoed it before, greenish line 58 bhp at wheels. We then started to retard the timing from where we found optimum when we first dynoed it. Power came storming back in and eventually we ended up with the blue line just under 70 at the wheels, we couldn't replicate the original power below 3500 but better above 3500 which we assume is full throttle, higher comp as the bearing clearances are correct and capacity increased also because of the stroke improvement? Timing backed off from 35 to 29.Peter
,
We were wondering about friction as well, it must have been at a minimum judging by the rattle, I have never heard that noise before, it was almost like worn little ends for its sharpness,nothing like a 'run' big end sound which is always pretty terminal sounding to me. I don't feel the engine would have survived long in race spec!
I was surprised at how much differing ignition advances both builds required as the only difference was the big end clearances and whatever effects they would have under dynamic conditions.
I know the MGB race engine builders Stateside tend towards larger than max OE recommended bearing clearances but have always been wary of trying this route myself. We have become braver with oil pressures and run down at 60/65 hot nowadays on the race stuff, relying on the old adage of 10 psi per 1000 rpm and seeing what other race builders have done.
Peter
I was surprised at how much differing ignition advances both builds required as the only difference was the big end clearances and whatever effects they would have under dynamic conditions.
I know the MGB race engine builders Stateside tend towards larger than max OE recommended bearing clearances but have always been wary of trying this route myself. We have become braver with oil pressures and run down at 60/65 hot nowadays on the race stuff, relying on the old adage of 10 psi per 1000 rpm and seeing what other race builders have done.
Peter
You show up your lack of knowledge Dave, we have never had erratic readings in inertia mode. As long as nothing is 'broke' and we strap it down the same, have tyre pressures same and get everything to temp the same we get repeatable results.
it would be good if you put your mind to the timing effect rather than try and belittle my dyno as usual. The power curves before and after were very similar shapes just lower afterwards, yes I did check for change of rolling resistance and it was the same, the engine sounded as flat as a f*rt until we retarded the timing.
Peter
it would be good if you put your mind to the timing effect rather than try and belittle my dyno as usual. The power curves before and after were very similar shapes just lower afterwards, yes I did check for change of rolling resistance and it was the same, the engine sounded as flat as a f*rt until we retarded the timing.
Peter
Yes it was running same fuel. On the rollers, if the engine doesn't pink its nuts off and frighten you the power drops off and the engine goes flat with too much advance, same as with too retarded, flat.
We sometimes get the effect of losing some at say low revs when we advance it and gain at high revs which shows us advance curve wrong, conversely sometimes we gain at low revs and lose at high revs etc etc When the curve is good we either gain everywhere or lose everywhere when we change the static timing. In this case it gained when we pulled the timing back. Never had this effect before but I have never, to my knowledge, run an engine with massive big end clearances.
I have come across one B where I got erratic power plots but that was my influence. I plot data from low to high rpm and do a fair few runs until I get two or three same figure power runs when I take a plot. I sometimes use the brakes to drop the speed down to start the next run. On this particular B it turned out the remote servo was faulty and occasionally stuck on giving different power runs, once I worked out what it was we had reliable runs. I leave the brakes alone since I learned this one!
A couple of weeks ago I had another odd one, reliable figures but turned out the throttle pedal bar which runs across the clutch and brake pedal was just catching the brake pedal when foot flat. My stepson, Keith happened to walk round the back of the car and noticed the brake lights coming on and off every run
It took an hour to cut the throttle bar and reshape it so it missed the brake pedal, only a couple of bhp but I guess it didn't give such diametrically opposed signals to those following...brake lights come on and car disappears into distance
Peter
We sometimes get the effect of losing some at say low revs when we advance it and gain at high revs which shows us advance curve wrong, conversely sometimes we gain at low revs and lose at high revs etc etc When the curve is good we either gain everywhere or lose everywhere when we change the static timing. In this case it gained when we pulled the timing back. Never had this effect before but I have never, to my knowledge, run an engine with massive big end clearances.
I have come across one B where I got erratic power plots but that was my influence. I plot data from low to high rpm and do a fair few runs until I get two or three same figure power runs when I take a plot. I sometimes use the brakes to drop the speed down to start the next run. On this particular B it turned out the remote servo was faulty and occasionally stuck on giving different power runs, once I worked out what it was we had reliable runs. I leave the brakes alone since I learned this one!
A couple of weeks ago I had another odd one, reliable figures but turned out the throttle pedal bar which runs across the clutch and brake pedal was just catching the brake pedal when foot flat. My stepson, Keith happened to walk round the back of the car and noticed the brake lights coming on and off every run
It took an hour to cut the throttle bar and reshape it so it missed the brake pedal, only a couple of bhp but I guess it didn't give such diametrically opposed signals to those following...brake lights come on and car disappears into distance Peter
What oil was it running? some uber thick 20/50 from millers or the like? that would possably cover the not knocking, i've wangled many a s
tter to extra life with some nice thick 20/50, even to the point of iliminating small end knock on a few pintos (at least for long enough to sell them) 
the power band shift i'm not really qualified to even guess at 
tter to extra life with some nice thick 20/50, even to the point of iliminating small end knock on a few pintos (at least for long enough to sell them) the power band shift i'm not really qualified to even guess at PeterBurgess said:
You show up your lack of knowledge Dave
Peter
Lol, it's clearly just dyno errors. Power doesn't change by 50% at 2000 rpm. You were chasing advanced ignition timing because your dyno kept reading higher every run that day, and then next time it was different again. You're just chasing ghosts. Try not to blow up too many engines by detonating them to death.Peter
The Moggy only had a telltale light for oil pressure, it would have been interesting to see what the pressure would have been ( may be frightening too!). That gives me a thought, thanks for your input...if we drop the pressure from say 100 to 65psi on a race B ( the 100 was before I shortened the relief valve spring to produce the desired pressure) we had about 3-4 bhp more at the wheels. There would probably be an element of this with the Moggy.
So,........ Differences which probably had a power effect
1) Oil pump taking less power to run because of (probably) significabtly less pressure.
Other suspects......
a)effect of less friction?
b) effect of piston movement caused by bearing clearance?
c)?
The igintion timing for each version of the engine was the sweet spot, either side and bhp suffered at all rpms.
We think the Cr was around 9.75:1 with the correct bearings. Karl Norris next door reminded me lower CR produces more torque at lower rpms when the cylinder filling is more efficient as there is more room to cram in fuel and air, but this is usually with much lower CR!, red herring?
I am trying to get a feel of what was happening in the engine but I am struggling, all supposition and not 'testable'.
Peter
So,........ Differences which probably had a power effect
1) Oil pump taking less power to run because of (probably) significabtly less pressure.
Other suspects......
a)effect of less friction?
b) effect of piston movement caused by bearing clearance?
c)?
The igintion timing for each version of the engine was the sweet spot, either side and bhp suffered at all rpms.
We think the Cr was around 9.75:1 with the correct bearings. Karl Norris next door reminded me lower CR produces more torque at lower rpms when the cylinder filling is more efficient as there is more room to cram in fuel and air, but this is usually with much lower CR!, red herring?
I am trying to get a feel of what was happening in the engine but I am struggling, all supposition and not 'testable'.
Peter
Just noticed you are back on Dave, you have always been brilliant at this sort of maths, does it make much difference to the dwell? Enough to upset the timing so much?
I can remember you doing calculus in your heard to work out a formula based on valve rock would not give correct guide clearance! You woke up next morning and ...BOOM! you had worked it out!
Peter
I can remember you doing calculus in your heard to work out a formula based on valve rock would not give correct guide clearance! You woke up next morning and ...BOOM! you had worked it out!
Peter
PeterBurgess said:
Whilst sitting on the loo just now and 'getting rid of my good ideas'.....I had a thought, what about the dwell time of the piston at TDC?
Maybe Stan could play with this thought? 70.62 mm stroke 146.05 mm rod length between centres.Big end journal 41.275mm.
Peter
It's pretty unlikely the piston would dwell at TDC given cylinder pressure from the burning mixture should keep it tight against the big end pin but I guess it depends on the magnitude of the forces acting. I suppose I could work those out if I put my mind to it.Maybe Stan could play with this thought? 70.62 mm stroke 146.05 mm rod length between centres.Big end journal 41.275mm.
Peter
Mignon said:
It's pretty unlikely the piston would dwell at TDC given cylinder pressure from the burning mixture should keep it tight against the big end pin but I guess it depends on the magnitude of the forces acting. I suppose I could work those out if I put my mind to it.
Yup, easy enough calculation. Combustion forces would exceed conrod little end acceleration forces at those rpms and keep the piston pressed down against the rod.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff