Thank you for your constructive input Fran.
If you have a 1/3 scale model of all the options, I have access to an appropriate wind tunnel, and a group of people who normally work with Gordon Murray.
The meat on the bones here is "does one really need a ZF or ZFQ" to build a car with a comfortable 170mph top speed? As we discussed on the phone, I am at that 'decide before you buy' phase. Transaxle is a critical point here, and I plan to try to identify just how limited the perfomance of a car with an Audi or OEM Porsche G50 transaxle would be, compared to a ZF.
Given the relationship between engine power and aerodynamic drag is a constant , and assuming level surface and sensible transmission losses, its possible to work out your VMAX.
What is also important, is not to set your ratio's any taller than they need be, and also to find the compromise between shift duration and ratio shortness in order to maximise torque multiplication.
Armed with all this information, I can, as Jac Mac to some extent rightly points out, go build an engine. I would however want to build an engine which places maximum power at the most appropriate RPM for the given gear ratios, to deliver the required level of performance.
If you recall Chris Melia stated he was doubtful many 40 replicas could acelerate beyond 165mph. The relevence here is that if a powerful replica with say a 500bhp motor, which needs a ZF or ZFQ box can only hit 165mph, what does this say about a car with a 350bhp crate engine and an Audi transmission?
I prefer to establish the results in a scientific fashion rather than taking a suck it & see approach. Shoving a powerful engine and a tall CWP set in, is not science.
So with all the options Fran cites, perhaps a best and worst CdA are the figures I am looking for.
Gulf flares, snorkels, Gurney bubble, double nostril and MK2 rad won't be making it onto my car I don't think.
If we consider the Lamborghini Muria, a car from the same era as the GT40, which "borrowed" some of the styling clues, the CdA for a Muria is .577m²
Pumping that into the equation which has been simplified remove the unit correction factoring:
vmax ^3 = cda / power
we get the Muria with 500 net bhp having a theroetical Vmax of 193mph
This is assumptive that the engine delivers the quoted power figure at the same engine rpm required to deliver the engie speed, hence calculating gear ratio's is also important as previously stated.
If the Audi box is only good for 350bhp on the first motion shaft, lets allow for 15% losses, and suggest net bhp is 297.5bhp. Then the Muria has a Vmax reduced to 163mph.
As a side note I would argue (and based on the fact that I just broke a Quaife sequential box in another car for this very reason) torque is the evil in terms of damaging gearboxes, power only being an expression of torque at a given rotational speed. Lets agree to discuss engines producing max power at 6000 -6500rpm, well within the 1st motion design criteria for all potential transmissions to be fitted to a GT40.
Now based on this theory and wildly guessed CdA, any of the following could apply:
The Muria is one hell of a lot more aerodynamic than a GT40 replica
You don't need that much power to get 165mph from a GT40 replica
Most GT40 replicas are capable to well over 165mph
If you only want to go 163mph, an Audi transaxle may well do the job nicely. If the CdA is lower, you'll go even faster.
In Europe, somewhere like Spa or Monza or the Ring are a few of the numberous places you'll easily be pushing significantly more speed than this. Great circuits though and places I shall be visiting in mine Dalton, so not so academic after all.
