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TBH, with the Sprint's brick-like aerodynamics, I'm not convinced it's physically possible!
Steve
The TR7 Sprint was supposed to top out at 120, with 0-60 in 8.5. They are the same as the specs for the 135bhp US TR8. But I don't know if the TR8 was 135 SAE or 135 DIN. Where I know the Dolomite Sprint was 135 SAE, but only 127 DIN. This does imply that German horses are stronger than American ones, but I'm not sure that's true. Perhaps they are just encouraged more efficiently.
But the TR7's plaid cloth seats may be an issue in driving it to 120 mph, as the stains will never come out.
As to how many horses to go how fast - if it's limited by aerodynamics, so rolling resistances can be ignored, and ignoring ground effect (cos the maths is too hard), it should take 8 times as many horses to make the same shape car go twice as fast: Power = Force x Velocity (Power = Work / Time; Work = Force x Distance; Velocity = Distance / Time). So 4 times the force (drag being proportional to V squared) moving at twice the V.
Graham
Well the 0-60 of 8.5 is close or the same as that widely advertised for the Dolomite Sprint in the UK. Non emasculated UK Tr8s (not that there were many) running on carbs probably shoved out a bit more, that V8 engine in the P6B was rated at 155bhp (DIN) slightly less in the worse breathing but much lighter MGBGTV8. But the TR7 Sprint (as a hardtop as they all were) is clearly a much cleaner shape aerodynamically so, along with a tad less weight, a V-max of 120 is a fairly reasonable assumption. The same engine in a convertible would probably struggle to make over 100mph top down or maybe 105 top up. That was my experience with my GT6 convertible in the 80s, from experiments conducted in the same place in the same car on the same day. With the (nothing special aerodynamically) Spitfire hardtop on, it showed me a true V-max of 117mph, (a "proper" GT6 MkII with the sloping tail was supposed to top out at 120) with the hardtop removed and the soft top errected it could only make 104mph where you'd think the weight loss would make it faster and top down it REALLY didn't even want to make the ton, I reckon 98 was about it's lot!
Which matches all i've learned about aero. Aerodynamic drag doesn't seem to increase linearly (is that a word?) but in a curve which is like the reverse of an engine power curve, where an engine starts off on a steep curve which tapers off towards the top, a drag curve starts off shallow and doesn't seem to really climb to significance until around 80 mph. Till, on a stock Sprint, by 110 or thereabouts, it's near vertical. Sure, with enough extra power you can overcome this resistance, you just pay the price in fuel consumption. As you so correctly pointed out, the engine needs to work harder the faster you go, it can only work so hard at max power output which gives you a V-max for any given engine in any given car shape, where power output exactly equals aero drag. Other parasitic losses are tiny by comparison , so I'm ignoring them! Add more power and the V-max will rise but the engine needs more fuel to make that extra power so fuel consumption rises, also by that 4x figure unless i've done my sums wrong. Certainly it's much more than a linear progression anyway. It's a big part of why the average car is so much more economical at 60mph than at 90, at 60, the drag curve is still fairly flat whereas at 90 it's much steeper.
Lousy as the Sprint's aero is, it could be worse! When first built, at standard Toledo ride height and sans the standard chin spoiler, the Carledo was "nervous" at 80mph or more. Dropping it an inch and a half and adding that chin spoiler made a surprisingly big difference to high speed stability.
So, in one way, by picking a Dolomite to make quicker, i'm starting in the wrong place, I should choose a body shape that is much cleaner aerodynamically. But I LIKE the shape so i'll live with it! In any case, plod will take your licence if they catch you at over a ton so, whether my V-max is 120 or 150, is really irrelevant!
As far as tyres go, even when on a trackday at Castle Coombe, there's only one spot where I can touch 100mph before braking hard for the next corner, so from a purely tyre safety point of view, I could manage comfortably on T or even the the now nearly obsolete S rated tyres and the V raters i'm using are a bit overkill!
Steve