Helical Gears And Forces Induction

Hi,
I have rebuilt an A+ Gearbox from a Mk2 1989 Metro.
It’s going in my, currently being built by me, 1310.

This will be a road engine for the odd track day maybe.
Hepolite P21253 +40 Pistons
4.5 thou Deck Height
AC RS Cam
MED ST1 Lightened Flywheel and Performance Road Clutch Kit
AC 1.3 Rockers (Not yet purchased)
Fitting a Cross pin Diff and uprated clutch.

Fitting a Supercharger with 8.5 CR running 10 PSI.
I’ve been advised that this will push out about 120BHP and 130ft/lb Torque, give or take depending on my head machining skills

Question:

Will Helical drop gears be ok or would the fail?

Edited by JonnyAlpha, 28 December 2021 - 03:58 PM.

People have run more & tried various tweaks to stop the idler eating in to the gearbox casing.

 

Mokespider has convinced me having its needle bearing well aligned is the key - search out his posts on the subject.

They are plenty strong enough. I would be more concerned with getting a gear set that suits the power profile.

 

the idlers with the big bearing modification would help.

They are plenty strong enough. I would be more concerned with getting a gear set that suits the power profile.

the idlers with the big bearing modification would help.

I saw a MeD video ages ago and they fitted one, but don’t seem to sell them now?

People have run more & tried various tweaks to stop the idler eating in to the gearbox casing.

Mokespider has convinced me having its needle bearing well aligned is the key - search out his posts on the subject.

This happened to a friend of mine, idler gear fell apart and destroyed the gearbox.

If you're going boosted, why didn't you go for AC FI cam?

If you're going boosted, why didn't you go for AC FI cam?

Because originally this was always going to be a naturally aspirated engine HIF44. Then I picked up a DCOE 45 with Mangalotsi Linkage for a real good price. This meant I’d have to put in a bulkhead box, which I have no problem with, but it hides the beauty of the Weber. Then I saw one of Vmaxcart’s Supercharger with a Weber hanging off of it and thought, WOW.

So a bit more info required.

Are there any strengthening benefits in fitting SC Drop Gears on an otherwise standard A+ Gearbox? Or is it a waste of time?

Would a standard Helical A+ Gearbox be fine with 130ft/lb Torque.

Can you get an Idler Gear modification to work with either Helical Gears or SC Drop Gears?

Found some info here but doesn’t answer all the questions:
https://www.theminif...cut-drop-gears/

I am fitting a Cross Pin Diff

You are confusing points. The drop gears (3) are a separate entity to the gear set.

 

the gears you need to run will be to suit your cam profile and the rough that you will select a suitable final drive. Straight cut gearsets don’t have the same ratios as a standard gear set as they tend to be designed to be used with a NA engine with a cam that comes on higher up the rev range. And then a final drive ratio is picked to get the moving in first.  The drop gears can have different ratios to standard which can be an easier option to swap out than swapping the CW/P.

 

 

you need to look at what your power curve is and what you want to use it for. 
 

then put the info into the gear calculator on Guessworks and see what you get. Don’t forget to ref it back to a standard set up so you can see the changes.

You are confusing points. The drop gears (3) are a separate entity to the gear set.

the gears you need to run will be to suit your cam profile and the rough that you will select a suitable final drive. Straight cut gearsets don’t have the same ratios as a standard gear set as they tend to be designed to be used with a NA engine with a cam that comes on higher up the rev range. And then a final drive ratio is picked to get the moving in first. The drop gears can have different ratios to standard which can be an easier option to swap out than swapping the CW/P.


you need to look at what your power curve is and what you want to use it for.

then put the info into the gear calculator on Guessworks and see what you get. Don’t forget to ref it back to a standard set up so you can see the changes.

Sorry of course, I now understand whet you are talking about.
I remember ages ago when I started the rebuild, communicating with John Guess and a few others about the Drop Gear ratio. I seem to remember know that Primary Gears come in various teeth numbers but didn't realise (or forgot) that the Idler and and 1st motion gear also differed?

Looking at the MiniSpares catalogues as a reference Primary Gears come (or used to) in two flavours - 29 tooth and 30 tooth (economy) (no listed as N/A).
The idler gear is only listed as 37 tooth
The first motion or drive gear also in two flavours 29 tooth and 28 tooth (economy)

I seem to recall that my Gearbox (which came from a 1989 1.3 Mk 2 Metro) had a 30 tooth Primary Gear and a 28 tooth - both economy (but I need to double check). These were what I was intending to use?
I did have a 2.95 Diff, but swapped this for a 3.44. I have now removed it and will be fitting a 3.1 Cross Pin Diff.

A set of Drop Gears has come up on FB with a 22 tooth primary gear but a 24 tooth first motion gear and a 30 tooth idler?

If Iv'e got this correct, using John Guess's Ratio Calculator, with my existing gear set and the 3.44 Diff, in 4th gear at 5000RPM reads 88MPH, however with the 3.1 Diff this increases to 98MPH. Swap out the Drop Gears for these Straight Cut 22/24 and it drops to 84MPH at 5000RPM.

All theoretical, but I could do with some help selecting the optimum gearing for this proposed engine / torque output?
If Helical Gears will be OK then that would limit the noise output, however if Straight Cut Drop gears are needed because of the increased Torque then I will have to look at those.

This will be a weekend car with maybe a Track Session once a year (Castle Combe) - I am not a speed freak

Edited by JonnyAlpha, 24 December 2021 - 04:30 PM.

Don’t just look at the speed in top as all 4 speed boxes are 1:1 helical or straight cut. It is all the other gears you need to look at. A SCCR box with a 3.1 CWP can make first gear fun and a right pain to use.

 

that standard set up is stronger than many people think. 
 

remember the Metro turbo only changed the bearing size for the input gear nose in the transfer housing.

 

work out it all out before spending

 

for example if you have no torque below 3k  a standard box means it would be fun until about 15mph  while the MiniSpares  clubman gear set raise it to about 20mph now if you live in the country this will not be an issue. But town/city traffic.

 

again I have no idea about how your engine will work but you need to consider it all. Is the 3.1 correct for the box you need?

Don’t just look at the speed in top as all 4 speed boxes are 1:1 helical or straight cut. It is all the other gears you need to look at. A SCCR box with a 3.1 CWP can make first gear fun and a right pain to use.

 

that standard set up is stronger than many people think. 
 

remember the Metro turbo only changed the bearing size for the input gear nose in the transfer housing.

 

work out it all out before spending

 

for example if you have no torque below 3k  a standard box means it would be fun until about 15mph  while the MiniSpares  clubman gear set raise it to about 20mph now if you live in the country this will not be an issue. But town/city traffic.

 

again I have no idea about how your engine will work but you need to consider it all. Is the 3.1 correct for the box you need?

 

The AC RS Cam delivers power from 1250 - 6000 RPM

 

From AC's documentation "In a typical example build (1293 to 1330cc) this cam will deliver 83 to 85bhp with a gas flowed head and 84 to 86lbft of torque with a single HIF44 SU and a  typical stage 1 kit fitted. Power would rise to a typical 92 to 95bhp with the addition of 1.5 rockers on the same engine."

 

Before adding the SC, I will be porting my head and using Metro Triple collet Valves (35.6mm Inlet and 29.2mm Exhaust). I will also hopefully be fitting a set of AC's zero tolerance 1.3 Rockers. Weber 45DCOE with the long inlet manifold, LCB and RC40 Exhaust.

This should produce around 85 lb/ft Torque.

 

I have been advised that, in preparation for going Forced Induction, I should swap the 3.44 Diff to a 3.1. 

 

So will the Eco Primary / Eco 1st Motion Gear on a standard A+ box DAM 5262, with the 3.1 Crownwheel be suitable? 

Put the figures into the Guessworks calculator to compare the 3.1 with 1:1 drops to the 3.1 with Eco drops and see what difference it makes (personally I think this will make it too long geared). Then compare the 3.1 with 1:1 drops to the 3.4 with Eco drops - these might come out quite similar.

 

If the second comparison does prove to be similar then you could run the 3.4 with 1:1 drops when NA and then change the drops when you go to SC - which would be easier than trying to change the FD on a built engine.

Don’t just look at the speed in top as all 4 speed boxes are 1:1 helical or straight cut. It is all the other gears you need to look at. A SCCR box with a 3.1 CWP can make first gear fun and a right pain to use.

that standard set up is stronger than many people think.

remember the Metro turbo only changed the bearing size for the input gear nose in the transfer housing.

work out it all out before spending

for example if you have no torque below 3k a standard box means it would be fun until about 15mph while the MiniSpares clubman gear set raise it to about 20mph now if you live in the country this will not be an issue. But town/city traffic.

again I have no idea about how your engine will work but you need to consider it all. Is the 3.1 correct for the box you need?

The AC RS Cam delivers power from 1250 - 6000 RPM

From AC's documentation "In a typical example build (1293 to 1330cc) this cam will deliver 83 to 85bhp with a gas flowed head and 84 to 86lbft of torque with a single HIF44 SU and a typical stage 1 kit fitted. Power would rise to a typical 92 to 95bhp with the addition of 1.5 rockers on the same engine."

Before adding the SC, I will be porting my head and using Metro Triple collet Valves (35.6mm Inlet and 29.2mm Exhaust). I will also hopefully be fitting a set of AC's zero tolerance 1.3 Rockers. Weber 45DCOE with the long inlet manifold, LCB and RC40 Exhaust.
This should produce around 85 lb/ft Torque.

I have been advised that, in preparation for going Forced Induction, I should swap the 3.44 Diff to a 3.1.

So will the Eco Primary / Eco 1st Motion Gear on a standard A+ box DAM 5262, with the 3.1 Crownwheel be suitable?

The power band is that, but doesn't suit boosted applications. You need to reduce the valve overlap, not many N/A cams provide that as they rely on scavenging to clear the cylinder and draw air in during the exhaust stroke.

Put the figures into the Guessworks calculator to compare the 3.1 with 1:1 drops to the 3.1 with Eco drops and see what difference it makes (personally I think this will make it too long geared). Then compare the 3.1 with 1:1 drops to the 3.4 with Eco drops - these might come out quite similar.

 

If the second comparison does prove to be similar then you could run the 3.4 with 1:1 drops when NA and then change the drops when you go to SC - which would be easier than trying to change the FD on a built engine.

 

Graeme - do you mean use the 1:1 selection located at the very top of the Drop Gear selector?

 

 

Any using this I have done the comparisons you suggested and the screen shots are below.

What do you by "to long geared" and which combination do you think would be best - I have been advised to fit the 3.1 Diff if going Forced Induction to make the car more driveable? 

 

Green is 3.1 with 1.1 Drops and White is 3.1 with Eco Drops:

 

 

Green is 3.1 with 1.1 Drops and White is 3.4 with Eco Drops:

 

 

So what combination do you reckon would be best? 3.1 Diff with the Eco Drop Gears?

Using this I realise that even though I changed the Worm and Pinion for the 3.44 to a 6/17 this tool suggest that I now need a 7/18 to ensure the Speedo is accurate