FD Dynamics meet the Mid Engine Corvette: a cautionary tale
 

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The September Road and Track interview of Tadge Juechter / Chief Engineer 2020 Chevrolet Mid Engine Corvette was highly relevant to the FD.

the first question:

WHY Mid Engine?

"Biggest was the limit of performance. We knew we were in trouble bringing out the 638 hp (542rwhp) C6 ZR71 and we had a hell of a time beating the 505 hp (429 rwhp) Z06 0 to 60. It was only because of the Michelin tires that we were able to... We couldn't hook it up. So that was kind of the start of it. Fifty-fifty weight distribution is great when your not power limited. That's why race cars evolved with the engine in the back. You really want more like 40/60 so you can get the power down."

With 215 rwhp our 50/50 FDs, as well as the Miata, have no traction issues. Unfortunately it is all too easy to add power and a 350 rwhp single turbo setup is considered conservative, 400 still not crazy, 500 is sporty and 600 is within reach on pump gas and meth AI with pretty good durability.

yet Tadge is saying that 429 rwhp is about the limit for the 3130 pound C6. at 50/50. The 638 hp version ended up with 335/25 tires on a 20 by 12 inch rear wheel. weight was 3353 with a 51/49 distribution so the all important rear weight was 1643 pounds.

the mid engine Corvette weighs 3366 dry (as per November Hot Rod Magazine) so call it 3400 curb. no distribution numbers but mid engine cars generally range from 56.5 to 59.3 (as per August Car and Driver) so let's use the middle:

3400 X .579 = 1969 rear weight... so a primary reason for the mid engine Corvette was to gain 350 pounds of rear weight.

FD 2862 X .5 = 1431

recap:

2013 Z06 505 flywheel/429 rwhp, 3199 curb weight, 1600 rear weight, 335/25/20. 3.73 pounds rear weight per one rwhp

2013 Z06 ZR1 638 flywheel/542 rwhp, 3353 curb weight, 1643 rear weight, 335/25/20....3.03 pounds rear weight per one rwhp

2020 mid engine Corvette 495 flywheel/421 rwhp, 3400 curb weight, 1969 rear weight, 305/30/20... 4.68 pounds rear weight per one rwhp

FD

255 flywheel/217 rwhp, 2862 curb weight, 1431 rear weight, 225/50/16... 6.59 pounds rear weight per one rwhp

let's solve for rwhp per rear weight of the 505 Z06:

1431/3.73 = 384 rwhp

Chevrolet had to upsize the rear tires on the 638 hp C6 to barely "hell of a time" beat the 505 hp Z06 which is the equivalent of a 384 rwhp FD!

obviously there are many qualifiers here but there is some truth also. ask most really experienced track (we are talking road courses) guys and they will tell you that they really don't need much above 400 rwhp to get the most out of the FD chassis. an awful lot of additional hp sort of goes up in tire smoke and also becomes extremely difficult to manage. there is nothing wrong w additional hp and it certainly results in faster times in the quarter mile etc.

as you get up above 500 rwhp, given our rear weight it does become easier to go sideways or lose traction. of course you can engineer around almost everything but for many of us in this web based world where everyone wants 600 rw (bigger is better?) perhaps, given our 1430 rear weight a bit of judiciousness might be in order as we contemplate our FD mods.

my background is road racing and i have always considered front weight to be evil, which is another way of saying rear weight is good. i have around 52% rear bias. i do recall talking w a 630 rwhp 20B FD owner at the Rotary Revolution. he had his car running well w a smallish turbo to limit the power but said he had to be very careful as even at 140 mph he could make his car go sideways if he wasn't careful.

having power is one thing, being able to efficiently use it may be another.

I think gross vehicle mass and front/rear bias is the important factor here. Not rear wheel weight alone.

"obviously there are many qualifiers here"

i could have gone on and on re other factors but the primary fact remains that our cars are driven by the rear wheels and weight on them is the primary factor that determines traction. more weight, more stick. just look at the efforts to increase front to rear weight transfer in drag racing to gain traction. it is no different in road racing. two equal cars, one w more rear weight... the car with more rear weight runs faster laptimes because the driver can put his right foot down earlier on corner exit.

according to the Chief Project Engineer for the mid engine Corvette, "the biggest limit on performance" of the front engine higher power recent Corvettes was the 50/50 weight distribution. they actually added a bit of overall weight, which of course is a negative to performance, to gain just over 300 pounds of rear weight.

my point is we are around 50% rear weight and it is very easy to make lots of additional power w the FD so we are ultimately limited as to being able to hook it up. this factor is worth consideration as we plan our mods.

the talk is the mid engine Corvette will offer a huge amount of options. engines, brakes etc.

now that they have 1969 pounds of rear weight, if we look at the two ratios in post one...

505 flywheel hp was 3.73... 1969 / 3.73 = 528 rwhp / 621 flywheel hp a nice efficient conservative yet close to the edge of traction level

638 flywheel hp was 3.03... 1969 / 3.03 = 650 rwhp / 765 flywheel hp probably max

Reminds me of reading the Road N Track story about the "King" on the 'Tail of the Dragon'. His RX7 was "About 300 hp at the wheels, and it weighs close to 2600 pounds. You can't use more power."

Yes Sir!
 

Sgt,
That's pretty cool that you mentioned that... I have it bookmarked.
It's a good read, not bad for a 20+ year old car..... sometimes less really is more!

https://www.roadandtrack.com/car-cul...eciale-dragon/

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I understand the overall point, but I think it's important to remember that the FD was engineered 30 years ago so there are limits to any comparisons with modern sports cars.