Best Performing Overdrive Ratio,gt6 J Or Dtype With 327:1 Diff

[TIMSGT6 X 5]

I'm getting down to finishing off a couple of trans and OD rebuilds for my GT6 and am exploring my options for installation.

If I am reading my manual correctly a Dtype OD serves up a final drive ratio 0f 8026: 1 or roughly 25% faster than 1:1 Correct?

This is predicated on a GT6 running a 389:1 diff ratio.

I am running a 327:1 currently and plan on keeping it, is this going to be less than optimal for a GT6's power band?

I also have a Jtype that show a 27% overdrive value, considering the same 327 diff ratio is this going to be any better or worse, particularly for fuel economy.

I would intend sustained speeds of 75MPH.

My thoughts are now leaning to reserving the JType combo for my Bonneville offering to the Salt Flat Gods, thinking a very high lift cam and mechanical fuel injection to give up the most HP at higher revs (this keeps the car normally aspirated and without EFI which keeps it production stock) 

Hoping to get maybe 125MPH, not setting a record for 2liter but possibly for a production GT6.

Cheers Tim

[zetecspit]

I doubt you would notice a lot of difference between the 2 overdrives in practice.

The 3.27 is a little long legged for my liking, and better suited to the 2.5 (but that is me, but then again a 2.5 can eat that ratio, mine did!)

 

As to fuel economy, again very doubtful you would notice the difference. I would even say a 3.63 may be a better choice, more revs don't always mean less economy. But that would be confusing the issue.

 

Have you tried the numbers out on the minty lamb site, I usedit t hoose a diff ratio so the engine didn't drop out of my perception of its power band when changing gear, and would give you hard numbers to consider

http://www.mintylamb.co.uk/gearspeed/

[spitfire6]

Hi,

 My Gt6 2.5pi convertible had a 3.63. No good on autobahn, 3.27 much better to keep revs down at autobahn speeds. No problem at low rpm. No wish to numerically increase final drive, would decrease numerical ratio if possible.

 

Cheers,

Iain.

[Nick Jones]

Looking at Mintylamb,

 

With a 3.27 diff and 175/70 13" tyres direct top gives 123 mph at 6k so 125 is possible with only a slight bending of the factory redline.  25% OD then gives enough gear for 155 mph though you'll be needing a WELL tweaked 2.0 to pull that!

 

With revvy 2L you might even do better with a 3.63 diff which give 111mph and 139mph respectively at 6k or 130 mph and 162 mph at 7k.  You don't need the taller OD.

 

Nick

[oldtuckunder]

  25% OD then gives enough gear for 155 mph though you'll be needing a WELL  

 

 

 

My brain leapt ahead there and inserted "sealed underpants" 

[TIMSGT6 X 5]

It is definitely a chess match with power bands and about 4 other major factors when the goal is to top out. My current street car will respond well enough with its configuration, about 2800 RPM @ 75MPH.

Going flat out brings up the limitations, particularly on salt, high altitude, high desert heat.

Just from driving my current engine it hits a wall around 5 grand , and takes a long haul to get there. The course for my intended division, production GT 2 liter , is limited to 3 miles, the last mile is averaged for the speed. so you need to be near top end by 2 miles.

The near stock engine I have now, run on pavement, is topping out around 105 -110. The only solution is going to be more HP, delivered , at an RPM close to what realistic top end is expected. There is no use of a cam peaking at 6000RPM when the wind resistance and other factors won't allow that speed to be obtained with or without an overdrive.

It may be a case of picking a goal speed and matching things up to suit that. Just getting to Bonneville is expensive enough  and not getting near a goal. There is no opportunity to tune under the conditions there, or see if these engines are capable of pushing near 130 mph with about 150HP and added weight.  

Getting to the HP again shifts the power band around, has anyone played in that arena before?

Cheers Tim

[Nick Jones]

Yes, well sealed underpants too probably.......

 

With the old (near-stock apart from FI), engine and about 120 bhp the Vitesse would manage 108 mph on the satnav - with a longish run-up.  Managed that in two places on the Nordschlief on the same day and on a random bit of smooth German autobahn a couple of years later.  Both times in 5th with the top up.  It has a 0.78:1 top gear and a 3.63 diff - I forget the rpm it topped out at - somewhere around 4,500.  I'd expect the same powertrain in a GT6 to manage a bit more as the Vitesse is an aerodynamic disaster!

 

Haven't tried it with the new engine, which should be about 25 - 30 bhp more (not measured so far) and higher up the rev-range.  Frankly it's scary enough and uncivilised enough at 108!

 

Nick

[TIMSGT6 X 5]

Imagine doing that on a gravel road and you will be close to what the Salt Flats offers.

There is a point where HP cannot overcome the resistance and it is quite low. I don't know if the final drive ratio dictates most efficiency or if it doesn't really matter where the ratio comes from, a tall gear from the trans/ OD and say a 4:11rear or straight 1:1 from trans and a 3:27 diff.

Presumably a combo that allows the engine to rev high at its peak output and creeping up  to a higher speed with overdrive, maybe staying in 3rd gear with OD

Considering someone last year managed an average top speed of 124MPH in a MG Midget with 1200cc means it can be done.

Sounds like a new topic to me, probably a few hooligans here have flouted the law and have some experience in speed attainment. 

[oldtuckunder]

Bearing in mind I know nothing, there were a couple of things bouncing around in my head reading this.

 

One was that I'm sure in the past reading a few car performance specifications of cars with overdrive that max speed was always achieved in 4th i.e. engine gearbox at 1:1 now I suspect that this may be an artefact of the final drive ratio as well where possibly without overdrive max speed might be achieved from an engine/car combination using a 3.89 diff as opposed to a 3.27.

 

The other thing that occurs that if using overdrive on such a set up requires that when say you have reached max rpm in 4th 1:1 and you kick in the overdrive (and assuming it doesn't go bang) you are getting a further 25% jump in gearing, you'd better make sure that the drop in rev's still keeps you in a very sweet spot in the torque/power band so that the engine want's to still pull from that point.

 

Wondering (and I mean that) if actually as you have 2 miles wind up if junking overdrive, going for standard Vitesse/GT6 gearbox ratios, a 3.27 Diff and 185/85/13 tyres (I assume that in a production class you are limited to original wheel size?)  might be the way to go sacrificing acceleration for attainable top speed. The above combination on MintyLamb shows that the 150 would be (theroretically) achievable within a 6.5K rev band.

 

Alan