Differential gear ratios

[2cams70]

Using light weight pistons and rods and then carefully balancing everything should get a 1700 CC long stroke twin smoother than a standard one at 5000 rpm. it will not be a smooth as applying the same treatment to a 1600 CC standard stroke engine at 5000 rpm but it will still be better than a typical standard stroke, standard rod and piston weight engine, without precision balancing of all components as is normal for a road car.

There's no escaping the fact that for engines of equivalent bore the engine with the longer stroke has a higher piston speed at any given engine RPM regardless of the weight of the components which is not conducive to the engine being smooth and free revving. Personal choice but I'd rather keep the stroke as standard and work on other ways to increase performance.

[rgh0]

I dont think we are disagreeing I am just taking the question and answer a bit further into its engineering principles








Stroke and thus piston speed and thus piston acceleration is one side of the equation the other side is the weight of the components being accelerated i.e. the rods and pistons. Reducing the weight can compensate for increasing the stroke and thus acceleration forces which cause the vibrations through the imbalance in the internal loads on the crank. While a short stroke helps so does light weight components which is why modern engines and F1 to an extreme have both !


Designing and balancing engines and other reciprocating components of various configurations is a complex topic and there are various books you can study for further knowledge. I stay away from the mathematics as i don't think this would be of interest to most people and I try to stick to the concepts behind the maths. which helps people understand the options rather than stick to a mandated one directional approach.

cheers
Rohan

[TBG]

I have jumped off the diving board and ordered a set of 3:1 CWP.

Watch this space in about 12 weeks time and I will let you know if I have re-invented the wheel!!

[Foxie]

RE: Diff ratios.

the standard 3.55 is a 9/32
The other 3.55 is a 11/39

Mixing the two gives 11/32 or 2.909, a little too high.

The 3.77 is 34/9. Mixing it with an 11 tooth gives 11/34 or 3.091, still a little high
4.11 is 37/9. Mixed with an 11 tooth gives 37/11 or 3.364. This might work.
<Snip>
David

Both the ratios quoted are approximately 3.55, but for the purpose of distinguishing them, one is more accurately 3.55 recurring, and the other is 3.5454 recurring.

However my main point is that I understand that whatever the ratio, the teeth cut match only for that ratio; a 9 tooth pinion from a 9/32 set will not mate with a 39 tooth CWP from an 11/39 set, or any other ratio. You cannot mix and match to give a numerically different ratio.

Is this not correct ?

[HCA]

+1.
But, what do I know

[Foxie]

I dont think we are disagreeing I am just taking the question and answer a bit further into its engineering principles

cheers
Rohan

I have to say here I lately built an 1861cc 85 x 82 using the new QED 681F block (non-tall block ! )

It runs like a sewing machine.

[Davidb]

TBG: I think you are doing us all a service!

Isn't the reduction in rpm with your proposed change about 17%? That is not huge - generally 20%+ is what people want with a fifth gear but since this is throughout the gear ratios it will certainly effect performance.

Let us know!

[Craven]

For what it’s worth, 3.55 ratio 9/32 was the period diff used by Lotus but apparently the 9/32 combination had inherent problems of vibration at a certain speed. Later when a 3.5 diff was required a 11/39 giving 3.54 was introduced by Ford. However this diff has metric prop flange that needs changing for it to be used in an Elan.

[TBG]

Davidb says

Isn't the reduction in rpm with your proposed change about 17%? That is not huge - generally 20%+ is what people want with a fifth gear but since this is throughout the gear ratios it will certainly effect performance.

Yes, but if I get a 17% reduction in rpm in top I am pretty close to a 5th gear and as I have more than a 17% increase in power - weee hee!! Should be fun and 3rd gear is I think going to be a blast...............

[Quart Meg Miles]

Davidb says

Yes, but if I get a 17% reduction in rpm in top I am pretty close to a 5th gear and as I have more than a 17% increase in power - weee hee!! Should be fun and 3rd gear is I think going to be a blast...............

From what youi've written already, 3rd gear will be like top is at the moment so you won't be gaining much in that department and you may be disappointed by the 2nd-3rd gap.

What Elan version do you have and what tyre sizes (I'm too lazy to work it out from your gear speeds)?

[1owner69Elan]

My initial thought about the 3.0 diff is that it's not anything I would want to try. But, a quick calculation shows that the 3.0 diff compared to a 3.55/3.54 transforms the semi-close box into something similar to the close ratio box but with an overdrive 4th gear.

3.0 diff "effective" semi-close ratios (compared to 3.55/3.54) : 2.52 / 1.70 / 1.18 / .847

Lotus close ratio: 2.5 / 1.60 / 1.23 / 1.0

So, if one is perhaps satisfied with the performance/tractability of the close ratio box (paired with a 3.55 diff) for gears 1,2, and 3 coupled with a long 4th then maybe this 3.0 diff approach isn't as unsuitable as it first sounds.

There are members on this forum (e.g. msd1107, Mazzini,..) that, in fact, have (or had) the close ratio box paired with a 3.55 diff and seem to like it. Reviewing their experience may give insights into how this 3.0 diff "experiment" might perform. There is always the concern about standing or hill starts being more problematic, I would think. But, perhaps less of an issue with an uprated motor. I do think that it is more common to find the close ratio box paired with lower diffs (3.77, 3.9, ..)

[Davidb]

1owner: I also have a close ratio box and 3.54 rear end with a 1720 engine so this does have some appeal

For the type of event I drive my Elan in where first gear is seldom used and third and fourth gear are most used with long periods in fourth this just might be the answer.

But I am skeptical about how it will feel in first and second.

I look forward to TBGs report on its effectiveness.

[TBG]

Meg

S3 on 165/70/13 - hope that this helps your deliberations.

1owner69 - your remarks are most encouraging but I am going to have to wait. 10 week production time and then fitting so perhaps mid July all will be revealed..................

[prezoom]

I have a close ratio gearbox, coupled with a 3.54 and 165-13 tires in my Plus2. Though, powered by a 2L Zetec, and would not change it for the world.

[TBG]

Rob - do you think I have not got it all wrong then? Encouragement is a wonderful drug!!