Power or Gear ratios?

[rdodger]

I am putting together a budget to build a lightweight (550ish kg) Thruxton



It will be used mainly for track days with a 10 day Alps Tour thrown in each year.

I am considering engine options as most things would fit. I have pretty much discounted Bike Engines as though when I recently drove one on track and thought it was amazing I feel a car engine would give me more flexibility and always gives the option when touring to put it in a higher gear and chill for a few minutes.

So I have been thinking about a 2 litre Duratec on throttle bodies (approx 200bhp) using the MK3 MX5 6spd gearbox.

The reason for the Duratec is it's pretty cheap and reliable and the tuning parts to get to 200bhp are all external so if the engine breaks £500 buys you a new one. It's the same argument the BEC boys have been using for years!

The thing that concerns me is the gear ratios. Looking at S2000, MX5, Sierra both standard and modded, the ratios don't seem ideal.

MX5 Honda S2000 Caterham R500
1st: 3.709 3.13:1 2.68:1
2nd: 2.190 2.05:1 2.01:1
3rd: 1.536 1.48:1 1.59:1
4th: 1.177 1.16:1 1.32:1
5th: 1.000 0.97:1 1.13:1
6th: 0.832 0.81:1 1.00:1
FD : 3.73:1 4.1:1 3.92:1



The S2000 is a bit odd in that it has a 1.1:1 gear output on the gearbox so the actual ratios are a bit different.

So at last the questions!

Would my money be better spent on a Quaife or similar box with close ratios like the Caterham to make the most of the 200bhp?

Spend more money on the engine to get 240bhp and make do with the ratios?

Or will a standard mx5 gearbox do the job ok with 200bhp?

Thoughts? Experience?

Sorry about the gear ratio's the forum keeps removing the spaces!

Edited by rdodger on Thursday 8th September 12:12

Edited by rdodger on Thursday 8th September 12:15

Edited by rdodger on Thursday 8th September 12:17

Edited by rdodger on Thursday 8th September 12:20

[The Black Flash]

Can't help directly, but a program like Gearcalc is a great help: http://www.locost7.info/gearcalc.php

You can plug in the different ratios, final drives, wheel sizes etc and see how it relates to engine speed and road speed in each gear. Most of the common gearboxes are in there already.

[Sam_68]

rdodger said:

Spend more money on the engine to get 240bhp and make do with the ratios?

Or will a standard mx5 gearbox do the job ok with 200bhp?

There's no simple answer to these questions, because the peak power is meaningless - it's the shape of the power curve (ie. the area under the graph) that will tell you what you want to know, but only if you feed it into a really quite complicated computer program, including not only the power data and gearing, but also potentially the graph of speeds around a particular circuit... the answer might well even be different depending on what circuit you're driving around!

[LooneyTunes]

Sam's right - peak power is pretty meaningless.

You'll be able to get a reasonable feel though by having a chat with other folk running the same powerplant and gear ratios.

For me the main factor in making a decision would be the cam profile - the main advantage to a CR box is being able to keep "on cam" instead of falling out of the powerband when you change gear. Likely to be less of a problem with something with more linear delivery but if it's a lumpy cam then quite difficult to determine without seeing the charts.

Also, if you're looking at cruising, you may want to consider a long top gear just to keep the revs down on the motorway?

If it were me, I'd probably want to keep costs low and initially stick with as much standard stuff as possible - get to know it and then look at where to spend the leftover cash on upgrades.

Look at layout though and what the upgrade path could look like - not sure if going MX5 'box initally will make it harder to swap cogs/boxes later? (not suggesting it will, I just know nothing about anything Japanese)

PS - you'll probably end up wanting to do both anyway in the end!

[Paul Drawmer]

Sutters didn't complain about that drivetrain here:
http://www.autocar.co.uk/car-video/ginetta-g40-vid...

[Sam_68]

LooneyTunes said:

...PS - you'll probably end up wanting to do both anyway in the end!

I guess, looking at the OP's anticpated use (mainly track days, with a few Alpine passes thrown in), the standard production car gearboxes from the MX5/S2000 are going to be way too tall and widely spaced even with a standard engine, and any later modifications are just going to make that worse: I've never known an engine get less peaky, when you modify it for extra power?

It might be more logical to get the gearing right first, therefore, and add the power afterwards?

[Mr Sparkle]

Havent tried an MX5 gearbox but the S2000 feels nice out of the box.

If it's reliant the max speed in gears @9000rpm are: -

Gear	Mph
1	43
2	65
3	90
4	115
5	138
6	Approx. 165

Its power only really comes on at 6000rpm in the car so you have to be pushing into the revs if you want performance.

I take it you would be using the cars original diff's? Is there much weight difference between your options?

[jason61c]

Sam_68 said:

I guess, looking at the OP's anticpated use (mainly track days, with a few Alpine passes thrown in), the standard production car gearboxes from the MX5/S2000 are going to be way too tall and widely spaced even with a standard engine, and any later modifications are just going to make that worse: I've never known an engine get less peaky, when you modify it for extra power?

It might be more logical to get the gearing right first, therefore, and add the power afterwards?

True. You could have it mapped for more midrange however if you're getting 200+bhp out of a duratec you're pushing for power at higher rpm.

As the Americans say "there's no substitute for cubes". What about a 2.3 duratec?

[rdodger]

The Duratec will do 200 bhp standard with just throttle bodies and ecu. That's a big reason for me thinking it's a good bet. That and the torque being pretty good and available from quite low down.



The kits standard fit diff is Sierra so there are a few options on ratio. I guess 3.9 is the most popular?

I have a mk2 mx5 at the moment that is supercharged and has pretty much the same power output with 214bhp and 174lbft. It has the 5 speed gearbox and to be honest is pretty good. 1st is a little short but 2,3,4th are good with 5th being decent for cruising. I recon that the 6 speed would give me a pretty decent 2,3,4,5th with a 6th for cruising.

[LooneyTunes]

Sam_68 said:

It might be more logical to get the gearing right first, therefore, and add the power afterwards?

Possibly, depends on whether power upgrades could significantly change the curves - last thing he'd want to do was change the ratios again if it turned out the upgrades meant coming off cam.

Might even decide he likes the standard ratios... in which case ££££ saved.

[Sam_68]

LooneyTunes said:

...thing he'd want to do was change the ratios again if it turned out the upgrades meant coming off cam.

As a general rule, though, higher tune = narrower power band, so if you've fitted a close ratio box, you're always going to be headed in the right direction... worst case is that he might have to change the final drive?

Realistically, unless you've got something like a Hewland transaxle, where you can swap the ratios in the paddock, you're going to be sticking to one set of ratios in the gearbox, in which case close ratio=good, wide ratio=bad?

[LooneyTunes]

Sam_68 said:

As a general rule, though, higher tune = narrower power band, so if you've fitted a close ratio box, you're always going to be headed in the right direction... worst case is that he might have to change the final drive?

If it is a close ratio box then yes, the risk of falling off cam can be reduced. How close you need to go will be determined by the engine's useful power band (in the highest envisaged state of tune) but you then need to take into account the following:

The power band could well move higher as well as narrower with a tuned engine; and

If you're going to end up upping the revs at which the engine works, this will have in-gear speed implications (unless final drive is changed obviously) which could impact the driving experience.

So IMO at least, it's not necessarily clear that a CR box "future proofs" things if you're looking at going down the tuning route.

[Furyblade_Lee]

Roger, why dont you just put a full MX5 Supercharged drivetrain in???

[rdodger]

Furyblade_Lee said:

Roger, why dont you just put a full MX5 Supercharged drivetrain in???

Cost really Lee. Well value anyway.

The Duratec on bodies would be about the same price and a good deal lighter with very similar power.

I can get 250bhp and 200lbft from an MX5 but it is starting to get pretty marginal at that.

[antnicuk]

not sure what the ratios are but the 5 speed (182) and 6 speed (231) gearboxes from the rx8 will bolt straight to an MX5 belhousing, might be worth looking into as the rx8 doesnt have much torque but has loads of revs. It runs a 4.44 diff ratio

[Russ Bost]

Just to throw a spanner in the works! I don't think that weight is realistic, many 7's don't get down to 550kg with a car engine (my bike engined Furore weighs in at a genuine 560kgs & it would be difficult to take a lot off that without spending a large amount of money)- the AGM WLR weighs in at around 700kgs with less bodywork & fibreglass seats so I'll be amazed if you can get a Thruxton down to that level.

My personal opinion would be more cubes (or turbo/supercharge), more power over a wider rev range & forget about gear ratios - if everything is so marginal that gear ratios become really important (bearing in mind the 2 completely different types of usage mentioned) then I think the basic concept is wrong. All IMHO of course

[Paul Drawmer]

Well, the AGM thingy must be rather solidly built as this

Libra VVC by pdrawmer, on Flickr
was 800Kg with doors, heater and wind up windows.

The Caterham R400 is 525Kg

[Sam_68]

Yep, I'd agree with Russ' post. In terms of gearbox, I'd suggest that the basic decision will be between a 'road' gearbox (with wide ratios to accomodate the ludicrously tall 'economy'/cruising top gears required by conventional road cars these days, or close ratio/track gearbox (eg. the Caterham 6-speed or something with Quaife internals), giving direct drive in top. Once you've made that basic decision, the actual ratios (provided they're reasonably well spaced) won't be that critical.

It's not as though you're going to be fitting something like an old race-tuned and carburettored Imp engine that won't pull the skin of a rice pudding below 6K rpm and has a 2K rpm power band; modern, EFI engines like the Duratec and Honda VTEC are comparatively tractable even in high states of tune.

[Russ Bost]

Paul Drawmer said:

Well, the AGM thingy must be rather solidly built as this

Libra VVC by pdrawmer, on Flickr
was 800Kg with doors, heater and wind up windows.

The Caterham R400 is 525Kg

Ok so having established that doors, heater & wind up windows weigh around 100kg (sounds about right!) & a monocoque SHOULD (theoretically)be lighter than a spaceframe & bodywork, & that an established, very, very basic car costing a great deal of money & from the oldest & most well developed kitcar manufacturer of the lot weighs less than a bigger car with full bodywork would your point is ........................................

I said "many 7's don't get down to that weight with a car engine" & believe me a lot don't, I didn't say it's impossible, merely that it would be difficult to get a car engined version of the Thruxton down to that sort of weight

[Paul Drawmer]

My point is: The AGR is heavy.

I think Roger's target weight for the Thruxton sounds ambitious, but I don't expect it to be a lot more than that.