C3 Corvette Speeds

[Roscobbc]

One of the things that has always fascinated me about the earlier L88 powered C3 Corvettes is actually how fast they could be under race conditions back in the late 60's and early 70's often with only modest power outputs. When I originally have my engine built-up the intention was to 'replicate' the HP figure of a period L88 but in a far more driveable, relatively economical package, using a larger capacity engine to achieve it.
The amazing thing to me is that in the early 70's privateers were racing L88 Corvetttes for several years after all of the major teams had moved on to other cars. For me it is the near 200 mph speeds at LeMans the privateer teams were seemingly still achieving with engines supposedly 'de-tuned' to low/mid 500hp for reliability and longevity over the 24 hour race.
It's truely fantastic that in '68/'69 an L88 with an extra low 2.53 rear end was timed at 213 mph on the full length Mulsanne Straight reputedly with 600+ hp.
All this got me in to a conversation with Andy Maskery (Essex Rep) about transmission ratios of Muncie and Borg Warner 4 speed gearboxes vs rear end ratios and the effects they might have on theoretical maximum speeds in individual gear ratio on our respective cars.
Andy's C3 has a 383 stroker, BW 4 speed and 3.08 rear end. Mine with its stroker 489, Muncie M21 and the same rear end ratio as Andy's - (3.08). Obviously both cars having the same rear end ratios maximum speeds rpm for rpm would be similar provided that each car could 'pull' a specific rpm figure.
Andy drew-up a spreadsheet for his car showing ratios and rpm's. I did the same for mine plus an additional sheet showing it with a 2.73 'economy' gear available from GM as an option on certain years. There are some interesting conclusions with Andys car and mine sharing the same rear end ratio. Whilst the M21's ratios are quite close and allow for potential 70 mph + speeds in first gear...... in fourth (direct) gear Andy's car (theoretically again) is actually faster at the same rpm's owing to slightly taller tyres. Even more fascinating is the difference a 2.73 rear end would make to my car at maximum achieveable speed (presuming that aerodynamics would allow and the engine would hold together at 6500 rpm +)

 

[Oneball]

Greenwood’s car that did the 213 was reputed to have close to 700bhp. The Mulsanne was properly long back then and as prototypes were limited to 3litres then it’s no surprise the Corvettes were fastest.

Mine has a 3.36:1 diff so should top out at 180mph.

70s and 80s race cars had significantly higher too speeds than they do today. To put it into perspective based on a modern GT3 race car. They both weigh the same but Greenwood had another 200bhp

 

[teamzr1]

I took your C3 as to engine size, flywheel HP/Tq using M21 tranny and benchmark them through my simulator
with the only difference being a 2.73 and 3.08 rear gear ratios

In short, the 2.73 was able to get 7 MPH more on top end than 3.08

	2.73 Gear	3.08 Gear
Time to Speed sec
0- 30 mph	2	1.87
0- 45 mph	3.04	2.8
0- 60 mph	4.08	3.73
0- 80 mph	5.61	5.91
0-125 mph	12.77	12.29
0-150 mph	18.6	18.98
0-170 mph	28.47	28.05
Time to Distance sec
0- 60 ft	2.24	2.18
@ mph	33.45	34.92
0- 120 ft	3.25	3.14
@ mph	48	50.54
0- 600 ft	7.62	7.46
@ mph	93.31	95.59
0- 1320 ft	12.1	11.96
@ mph	121.14	123.09
0- 5000 ft	28.71	28.53
@ mph	170.37	170.63
0- 50 ft(1/4 mile)	2.03	1.98
@ mph	22.15	22.77
Top Speed mph	187	180
70- 90 mph sec
2nd Gear	1.97	1.81
3rd Gear	2.61	2.27
4th Gear	3.51	3.05
90-120 mph sec
2nd Gear	3.95
3rd Gear	4.38	4.01
4th Gear	5.91	5.03

 

[teamzr1]

At a drag strip, compare 2.73 V 3.08 Gears

 

[Roscobbc]

Interesting figures JR - When I had the original L36 in the car my laddo bought one of those GTech performance meters. It measured 1/4 miles and 0-60's etc.
After 4 or 5 attempts I managed to record a 0-60 sec's of 4.5 seconds.........I always doubted the accuracy of that figure (and especially so when reading magazine road tests of the cars) - after seeing your figures perhaps the GTech meter was correct. We do have the benefit of far better traction with our modern tyres. I was surprised also how little a difference the change of rear end ratio from 3.08 to 2.73 actually made to the performance figures. I'm guessing its the torque spread that minimises that difference. I have noticed that on all performance calculators the M21, M22 and Ford Top Loader gear sets always show the best acceleration figures over all other transmissions.

 

[teamzr1]

Oh, yea, I still own a G-tech sitting in a drawer since 1980s,
used it on my C4s but never believed the results and even glued a level to the side to try and make it work right



I looked in my 1968 Corvette database and see multi different tranny types and difference was what each gear ratios are

 

[CaptainK]

Interesting numbers and fascinating to see the difference between the cars and so forth.

Anyone fancy running the numbers of my 1968 327/300 4 speed manual to see how slow it is in comparison to the more powerful beasties above? (last rolling roaded at 250bhp fly / 300lbs/ft torque). It should have a 3.70 diff in it I think, but from recent looking at it, it might have a later 3.08 in it.

I used to have a G-Tech back in the mid noughties. Not had it long, lent it to a mate and then never saw it again. So didn't really get to compare many cars at the time, and then never bothered getting another.

 

[Roscobbc]

The 'real' issue with our earlier cars is that they lack some of the body changes that help improve aerodynamics at higher speeds (and cooling air thru' the front end) - the early C3 is notorious for its 'front end lift' at three figure speeds. I can recall back in the early days with the car on stockish sized tyres becoming very noticeable 'unconnected' up front at about 115/120mph......scarily so. Once I'd swapped over to the 18" wheels and tyres this front end lightness was less noticeable (only to be replaced by excess tramlining lol!)
Seemingly the 'real' trick (other than adding some aero) is to drop the front end an inch or so.......having the nose down stance seems to help to push the front 'down' rather than allowing air to push it 'up'. Its certainly a mod the Bonneville speedster guys seem to use with great effect. I'm sure TeamZR1 will have some useful imput here!

 

[Forrest Gump]

……. I was surprised also how little a difference the change of rear end ratio from 3.08 to 2.73 actually made to the performance figures. I'm guessing its the torque spread that minimises that difference.

I originally had a 3.55 in my car, but when a 3.08 was offered to me I grabbed it, even though I knew would theoretically take away a bit of acceleration. The resulting lower revs at cruising speed (even though it’s only a few hundred rpm) does make it some much more bearable, and the extra speed in gears just adds to the flexibility when driving. At these times of expensive fuel, an even lower ratio would be nice for cruising but then I might start to feel the loss on acceleration.
In a racing car you can see how having the right diff ratio for a given track is very important though - that 0.3 seconds or whatever difference getting up to top speed on every straight and multiplied for all the laps (or 24 hours) adds up to quite a distance on the track.

 

[teamzr1]

Interesting numbers and fascinating to see the difference between the cars and so forth.

Anyone fancy running the numbers of my 1968 327/300 4 speed manual to see how slow it is in comparison to the more powerful beasties above? (last rolling roaded at 250bhp fly / 300lbs/ft torque). It should have a 3.70 diff in it I think, but from recent looking at it, it might have a later 3.08 in it.

I used to have a G-Tech back in the mid noughties. Not had it long, lent it to a mate and then never saw it again. So didn't really get to compare many cars at the time, and then never bothered getting another.

CaptionK, all I can say is never ever get caught at a stoplight with Roscobbc next to you
He will stomp you bad by 2 seconds in a drag race !

I used a 3.08 rear end gear and all else stock for your engine and HP/Tq



Here is a compare straight up

Battle	CAPTAINK	Roscobbc
Time to Speed sec
0- 30 mph	2.48	2
0- 45 mph	3.86	3.04
0- 60 mph	5.32	4.08
0- 80 mph	8.81	5.61
0-125 mph	21.19	12.77
0-150 mph		18.6
0-170 mph		28.47
Time to Distance sec
0- 60 ft	2.43	2.24
@ mph	29.43	33.45
0- 120 ft	3.58	3.25
@ mph	41.96	48
0- 600 ft	8.79	7.62
@ mph	79.92	93.31
0- 1320 ft	14.1	12.1
@ mph	103.87	121.14
0- 5000 ft		28.71
@ mph		170.37
0- 50 ft(1/4 mile)	2.19	2.03
@ mph	20.55	22.15
Top Speed mph	145	187
Gear Start sec
70- 90 mph sec
2nd Gear	3.13	1.97
3rd Gear	3.9	2.61
4th Gear	4.94	3.51
90-120 mph sec
2nd Gear		3.95
3rd Gear		4.38
4th Gear	8.86	5.91

 

[Roscobbc]

5.32 seconds on the 0-60 and 14.2 on the quarter are very respectable and should burn off most others at the traffic lights. And lets be honest here - JR's figures are theoretical and most likely 'best case' scenario.

 

[teamzr1]

The 'real' issue with our earlier cars is that they lack some of the body changes that help improve aerodynamics at higher speeds (and cooling air thru' the front end) - the early C3 is notorious for its 'front end lift' at three figure speeds. I can recall back in the early days with the car on stockish sized tyres becoming very noticeable 'unconnected' up front at about 115/120mph......scarily so. Once I'd swapped over to the 18" wheels and tyres this front end lightness was less noticeable (only to be replaced by excess tramlining lol!)
Seemingly the 'real' trick (other than adding some aero) is to drop the front end an inch or so.......having the nose down stance seems to help to push the front 'down' rather than allowing air to push it 'up'. Its certainly a mod the Bonneville speedster guys seem to use with great effect. I'm sure TeamZR1 will have some useful imput here!

I took your '68 as it is and then made some front end areo changes

Testing for 0 to 180 MPH

Yours as is taking 37.77 seconds
Whereas with my changes was 34.43 seconds so 3 seconds less to get to that 180 MPH

Changes I made

Shifted weight by 3 percent more weight (52 to 55%) on front end to simulate downforce only on front
Lowered front end 1 inch less in height
Changed to your 18-inch wheels
Reduced front coefficient of drag from 0.45 to 0.41

Using Silverstone track in UK (newer track layout) yours as of today was taking 2 seconds a lap longer

 

[teamzr1]

5.32 seconds on the 0-60 and 14.2 on the quarter are very respectable and should burn off most others at the traffic lights. And lets be honest here - JR's figures are theoretical and most likely 'best case' scenario.

Yes you are correct but both '68s were treated to the exact same conditions
I even made it good weather, no elevation, no headwind, no tire expansion, shifting times, losses,
etc so differences of head-to-head results are of car makeups only

Conditions specs :

 

[Roscobbc]

As an 'aside' this whole thing about gear ratios and the effect they have on performance has highlighted how good the Muncie gear ratios have been............except for reduced rpm cruising. In 'the day" there was the option of using an aftermarket overdrive unit (like Hone) or much, much later Gear Vendors. The thought of a aftermarket 5 or 6 speed overdrive gearbox with single or double overdrive ratios is interesting untill you look at the ratios available on the market.......one might think that they would replicate the same first three ratios used on the M21and simply chuck-on a .60 or .50 for the 5th speed......but no - most ratios available are typical CAFE ratios and less than ideal for acceleration 'out of the hole'..........interestingly the OEM selected transmission for the late ZL1 Camaro was the closest thing ever to an M21 on the lower ratios.....wonder why that wasn't available on the 'aftermarket'. Perhaps JR knows?.....

 

[teamzr1]

As an 'aside' this whole thing about gear ratios and the effect they have on performance has highlighted how good the Muncie gear ratios have been............except for reduced rpm cruising. In 'the day" there was the option of using an aftermarket overdrive unit (like Hone) or much, much later Gear Vendors. The thought of a aftermarket 5 or 6 speed overdrive gearbox with single or double overdrive ratios is interesting untill you look at the ratios available on the market.......one might think that they would replicate the same first three ratios used on the M21and simply chuck-on a .60 or .50 for the 5th speed......but no - most ratios available are typical CAFE ratios and less than ideal for acceleration 'out of the hole'..........interestingly the OEM selected transmission for the late ZL1 Camaro was the closest thing ever to an M21 on the lower ratios.....wonder why that wasn't available on the 'aftermarket'. Perhaps JR knows?.....

Ever consider Hewland who is in the UK and design trannies

I used them for my C4 ZR-1 racer back in early 1990s, not cheap but design allows high outputs, and can change gears quickly

Hewland Tranny

 

[antijam]

Ever consider Hewland who is in the UK and design trannies

I used them for my C4 ZR-1 racer back in early 1990s, not cheap but design allows high outputs, and can change gears quickly

Hewland Tranny

Years ago I was pit crew for a friend racing Classic F3.


Here we're discussing whether the circuit demanded a ratio change in the Hewland. Access is easy on a rear engined open wheeler. We could do a swap trackside in less than an hour.

 

[teamzr1]

Years ago I was pit crew for a friend racing Classic F3.

View attachment 18107
Here we're discussing whether the circuit demanded a ratio change in the Hewland. Access is easy on a rear engined open wheeler. We could do a swap trackside in less than an hour.

Hewland makes solid trannies that takes abuse and gears changes were easy,
It was the cost that was the killer, like $400 plus per gear

 

[teamzr1]

There are other ways other than tranny types and gear ratios to induce more performance and top end speeds

One way is to reduce the heavy mass of the flywheel, like stock at 40 pounds to I've gone with a flywheel that really is nothing more than
a starter ring, another type I show below, only weighed 7 pounds
Those alone allow spinning up RPMs really fast, the negative would be if on street driving as without the mass of flywheel
then the harder it is to get the mass of the car moving from a dead stop

Then is clutch types, 1 below I had been from Tilton who makes race setups,
This used 3 5-inch clutch discs with floaters, that also reduced weight
Or another I used was from McLeod who does street and track clutches, dual 8-inch discs




Then were my sneaky ways of gaining top-end, most did not catch on to this and the A-pillar we kicked from the top raked rearward 5 degrees
this allowed even better flow of air above the roof and higher MPH, you notice how laid back the windshield is

Another was the design of the side rockers around the side exhaust, both of these mods were tested in a wind-tunnel and saw nice MPH gains



Flipping the car over :

The real sneaky was notice all the holes cut in the rails to reduce weight
and the front under tray to help reduce wacky airflow patterns causing lift

Shot of the Hewland

 

[CaptainK]

CaptionK, all I can say is never ever get caught at a stoplight with Roscobbc next to you
He will stomp you bad by 2 seconds in a drag race !

I used a 3.08 rear end gear and all else stock for your engine and HP/Tq

Thank you so much for running those numbers through, much appreciated. Its very interesting to see the comparison between a base model Corvette versus one far superior. Granted its all theoretical though, as I know my car will actually be a lot slower in reality. I'd really love it if my car was 0-60 in 5 and a half seconds, but realistically its around 8 or 9 seconds at a guess. Purely by comparing it to my other cars - my Corvette is slower to accelerate than my FTO, which is about 7 seconds 0-60.

I am planning on getting my Vette's engine tuned up a bit more in the long run, but the usual thing of time and money gets in the way. Still, I'm enjoying it for what it is at the moment, and it makes for a great driving experience.

 

[Roscobbc]

Thank you so much for running those numbers through, much appreciated. Its very interesting to see the comparison between a base model Corvette versus one far superior. Granted its all theoretical though, as I know my car will actually be a lot slower in reality. I'd really love it if my car was 0-60 in 5 and a half seconds, but realistically its around 8 or 9 seconds at a guess. Purely by comparing it to my other cars - my Corvette is slower to accelerate than my FTO, which is about 7 seconds 0-60.

I am planning on getting my Vette's engine tuned up a bit more in the long run, but the usual thing of time and money gets in the way. Still, I'm enjoying it for what it is at the moment, and it makes for a great driving experience.

Capt - it would be interesting to get your car on a rolling road to see actually how much power its potentially lost over the last 50 or so years......