Diff Ratios?

[schroeder]

Does anyone have experience with different diff ratios?

I am having a 67 coupe built for vintage racing and have to choose a diff ratio. Choices are 3.55, 3.77 or 3.91.

3.55 sounds too high to get the performance I need out of the corners. My concern with the 3.91 is revving so high that I run out of top end. I know it all depends on the track etc.

Running my +2 road car at my local track in Michigan I hit 100 mph at the end of the front straight. I believe that this has a 3.77 diff but have never checked it.

I would appreciate anybody sharing thier experince using different set ups.

Another question that would help in my selection. How high can I rev the motor? It is an L block with steel crank and rods and o-ringed block and 11.5 compression. Does the valve timing go first?

Many Thanks

Stuart

[rgh0]

Stuart

A complex question and the answers even more complex but that whats this forum is all about.

What diff ratio suits depends on lots of factors as you touched on

eg.
Type of tracks
Engine development and power spread - essentially cams used and max rev limit target.
Car weight
Gear box ratios
Tyre diameter.
How you want to be competitive - best on one track or OK on all


Hard to comment on your particular siutuation for all these variable but this is how I approached the problem for my Elan.

I race on a wide variety of track from tight tracks where I never need get out of third gear to tracks with 1.5 km long straights and very high proportion of total lap time in top gear. I do not have time to switch diffs so I was looking for something that suited all this variation.

My engine development philosophy is to aim for good power but maximum torque and power band. Thus I use a high lift short duration cam that puts out good power from 4000 to 8000 rpm with peak power around 7000 to 7500 rpm and a 8300 rev limit. This is down maybe 10hp at peak on what would be possible with a longer duration cam and 9500 rpm rev limit but I cant afford the time for frequent rebuilds and in the end I dont believe lap times are much quicker on many of the tracks as the loss of torque and power band balances the greater peak power

The Elan FHC is relatively heavy when legal in my competition class at 700 kg compared to say a S4 seven. A plus 2 will be even heavier unless drastically lightened. Heavier cars need more power band and need to trade peak power for torque for optimum lap times.

How well your gear ratios match the corner speeds on any track also affect the right diff ratio. A higher or lower diff may help match corner speeds and gear change points. I run a close ratio quaiffe box which brings second gear into play on more corners where the standard box 2nd gear was almost never used on the tracks I compete on. I now get on average better corner speed matches to second or third gear.

Tyre diameters are one thing you can more easily play with to match tracks. The 185/60 Hoosier street TD is almost 10% smaller in diameter than the 175/60 Yokohama A-032R, this is equivalent to a 3.77 to 4.1 diff change and you can use it to match circuits better if you want depending on what tyres are legal in your racing class.


I aim to be reasonably competitive on all the tracks I compete on.

The outcome of all this is I settled on a 3.77 diff in my Elan. This enables me to get to a 200 plus km/hr at 8000 rpm on tracks with long straights while still given good second and third gears for corners and slower tracks. Smaller diameter tyres on the tight tracks are an option to further optimise this approach when I am feeling rich enough to have multiple sets of fresh tyres.


Been doing some development work with a friend who runs a S4 Seven in the last few months on the actual lap time changes from a lower peak power short duration cam versus a higher peak power but narrow power band long duration cam. The outcome has so far again reinforced the benefits of trading off power for torque and power band especially on tight tracks.

regards
Rohan

[schroeder]

Rohan

Thanks for the input. I guess I should have thought about different tyre sizes for different tracks. Much easier than diff changes. The elan in question is a strpped out S3. The +2 cpmment was simply for comparison.

I see you rev limit at 8300. Never thought that a t/c would rev that high let alone 9500 rpm. My cam set up doesn't sound as hot as yours. I am hoping for around 150 hp when I can get it to the dyne. It too will be connected to a quaife gbox.

Stuart

[msd1107]

Stuart,

Here is a more theoretical discussion on setting up your car for different circuit conditions. Theory assumes an infinite range of equipment and budget, so there will have to be compromises made.

I will discuss two general types of circuit, a road race type where you can get up to a high top speed and a handling type of course where the top speed is quite limited.

Estimate the top speed that can be achieved on the course, either from personal experience or other competitors. From this, your max RPM and the tire used will determine the differential ratio to be used. For the Elan, different R&Ps are rather coarse grained, being 5% to 10% different, so you may not get that close. If you can get tires with subtly different rev/mile figures that can be used to fine-tune the selection. This may be somewhat limited, since the tire compound is more important for lap time as opposed to an inferior compound in a slightly better size.

Determine the minimum speed on the circuit. The max speed divided by the min speed establishes the overall speed ratio spread that the engine/gearbox has to serve. If the course has very slow speed corners and long straights, you may need the wide ratio (2.97 1st) gearbox, even though 3rd gear is widely spaced from 4th. For many courses, the standard 2.51 1st gear box will be suitable, supporting a speed ratio of as large as 4.4 (30 to 130). For a course with a higher minimum speed or lower top speed, you could go to a Leeson or Quaife set with 2.2, 2.02 or closer 1st.

For Solo or Gymkhana courses, the top speed is much less than what can be achieved in top gear. It is probably best to set up the car so that you need only one shift, since shifting time negates the advantages of a closer ratio drop between gears. Again, determine minimum and maximum speeds for the course. From this you can determine whether to use 1st and 2nd, or 2nd and 3rd and whether to use the 2.97 1st or 2.51 1st and what R&P to use.

All of this takes much longer to write about than to use appropriate software. I use a spreadsheet that allows you to quickly model different parameters, and has help text for setting up a small formula car using a Hewland type gearbox. You could read it and adapt it to your situation. Email me at david_harralson at hotmail dot com to request a copy.

David
1968 36/7988

[davidholroyd]

4.4 if your engine revs to 8 + in top on 13" wheels and 205/60/13 yokos, ok for mallory park in the uk which is just short of an average lap speed of 100mph

[paros]

Non scientiific thought!
I believe the most important ofr diff ratio is acceleration and thus would always go for a 'shorter diff' even if this meant having to hold the car at 7700 in top for a few seconds. I once with a previous car did some calculations and found the differnce in a lap time for having perhaps 500 rpm extra in top for a second was not great compared with out dragging the competition out of corners. However if they all sweep past you into the breaking area doing 5 mph more than you then...............
As I said not very scientific but it is my opinion
Richard