Track times after 2.87 to 3.54 change (info)

[dfw81shark]

I finally got to get to the track tonight to see how much better the numbers are on the vette. Since the last time I raced, I've changed the rear gear from 2.87:1 to 3.54:1 and I've added Hooker Headers and sidepipes with Spiral Turbo mufflers instead of the stock manifolds and Flowmaster 40s. I didn't record the temperature last time, but it was probably pretty warm and today it was about 70 when I ran. Don't know the barometric pressure of either day. Altitude is 930 ft. if I remember right.

Here's the best old time (of 2 runs)
R/T .476
60' 2.330
330 6.653
MPH 53.32
1/8 10.110
MPH 70.78


Here's the best new time (of 5 where 4 where within about 1/10sec)
R/T .425
60' 2.175
330 6.173
MPH 58.16
1/8 9.392
MPH 75.22


My know my reaction times are horrible even though those are the best runs. I really had the R/Ts nailed on my motorcycle but couldn't get them in the vette. Maybe with a bit more practice.... Another thing I noticed is that I barely got out of first gear with the 2.87s and I ran through all 3 gears easily with the 3.54s. I'm pretty happy so far, except that it runs high rpm on the highway. And I'm having a hard time keeping the Goodyear Eagle GTIIs planted (esp on the track).

 

[81 Corvette]

I don't like the high rpm's on the highway either, but you get used to it, and after a while like it. I remember when mine was stock and the car's reaction to stomping the gas when going 70. zzzzzzzzz Now its weeeeeeeeeee!!!!!!

Looks like the mods are paying dividends. Have you done anything else to the engine?

Did you change the speedo gear? If so, what gear did you put in? I can't seem to find the right answer, so the tach is my speedo now. 2600 rpm = 60mph

Any idea on your 0-60 time

 

[dfw81shark]

Looks like the mods are paying dividends. Have you done anything else to the engine?

Did you change the speedo gear? If so, what gear did you put in? I can't seem to find the right answer, so the tach is my speedo now. 2600 rpm = 60mph

Any idea on your 0-60 time

Haven't done anything to the motor yet. It annoyingly smokes when and only when you start it up in the morning (my 79 Camaro smokes too at startup, but not THAT BAD) so we're going to change out the valve seals first and then if that doesn't fix it then the rings. I think the previous owner put a mild cam in it but other than that no plans and no other modification has been done to it that I can see. I'm pretty happy with the power now. It still has the original-style air cleaner and carb, although I may go to an open air filter because I like the intake sound my Camaro makes with its filter.

Haven't changed the speedo gears yet. I just know to keep it around 85mph(which is ~70mph) for the highway. I'll look into that soon, since I'm counting mileage 17% faster. I'll start by going by bowtie overdrive's calculator, which says for a 255/60/15 and 3.54 rearend to use a 15-tooth drive and 39-tooth driven gear. If you've used that combo and it didn't work, lemme know so I don't make the mistake.

My guess on the 0-60 time is about 6.2-6.3, since the 330ft was 6.1@58mph.

 

[KANE]

I don't like the high rpm's on the highway either, but you get used to it, and after a while like it. I remember when mine was stock and the car's reaction to stomping the gas when going 70. zzzzzzzzz Now its weeeeeeeeeee!!!!!!

A 700R4 will really wake your car up!!! A 3.07 to 1 versus a 2.87 to 1 first gear makes a bid difference. It is like stuffing a 3.73 in the tail. Oh, and best of all- your cruising speed will be lower on the highway. At 70mph, I could still drop to second an take off.

I think the saving grace to the 82 Vettes was the 700R4. With the acceleration and gearing donated by the tranny, it didn't matter that the intake runners were only half height. The overall performance was stronger than the previous smog sharks.

Here is my experience with overdrives-
My old 1978 with a 700R4/TPI/3.01 rear end pulled 3K at 100mph!!! Weight isn't a big deal either. I weighed in at 3300lbs with a 1/2 of gas, 80 style seats, carbon fiber rear spring, Goodwrench block, and AC.

Your 81 should produce similar rpm numbers. It may weigh even less, giving you that much more acceleration.

 

[76 Sting]

Hey guys...while your on the topic what are the actual rpm's you both have using you gear combos. I have a 76 with I think 3.08 (stock), but my rpm's are saying something higher closer to 3.50's. I am at: 55mph 2500 65 mph I am 3000, 75mph 3500, 100mph 4500. So my concern is highway travel and high rpm's cruising. My off the line performance is horrible...no tire squeal, but I can catch 2nd gear with an automatic.

 

[john1977]

My 77 was the same as 76 stings, 3:08s same RPMs at same speeds except had my motor rebuilt to 350hp. Just had a differential rebuild and gear change to 3:55s and a 2400 stall convertor,new 9 leaf. Not sure what expect now,weather has been to bad to try it out yet. So its up on stands getting body mounts changed.

 

[dfw81shark]

Hey guys...while your on the topic what are the actual rpm's you both have using you gear combos. I have a 76 with I think 3.08 (stock), but my rpm's are saying something higher closer to 3.50's. I am at: 55mph 2500 65 mph I am 3000, 75mph 3500, 100mph 4500. So my concern is highway travel and high rpm's cruising. My off the line performance is horrible...no tire squeal, but I can catch 2nd gear with an automatic.

Are you sure you have 3.08s and are running in 3rd on the highway? Your numbers don't sound right. You should be doing around 2600rpm at 70mph with 255/60/15 tires. Your numbers are more in the 3.92 range. With that gearing you should have a heck of an off-the-line performance.

I haven't gotten my speedo gears changed out yet, but I am going by the rpm guide here:
http://www.2004r.com/speedoCalc/rpmcalc.shtmlhttp://www.2004r.com/speedoCalc/speedocalc.shtml

Input your tire size and it spits out the rpms in the 1:1 column. Right now I pretty much go with the flow of traffic on the slow streets and keep in mind I'm doing 3000rpm at 70mph on the highway. The sound is a little annoying, and I'd like to run lower rpm on the highway with the overdrive, but I can't see the immediate cost savings with what I can see of about 3-4mpg. My TH350 is built, so I'm going to deal with the droning, I mean awesome sidepipe sound, for now.

 

[john1977]

Im sure my speed/rpms are correct have clocked it with another car and know i had 3.08 gears because i counted the gears on my old ring and pinion before the change to 3.55s. Dont know if my stall conv. might still be slipping at that speed. Also had to change speedo gears to compensate for the knew gears. I'm running 225/70-15 tires. Old speedo gears were 21t driven and 7t drive.

 

[Glensgages]

Here's the best old time
R/T .476
60' 2.330
330 6.653
MPH 53.32
1/8 10.110
MPH 70.78


Here's the best new time
R/T .425
60' 2.175
330 6.173
MPH 58.16
1/8 9.392
MPH 75.22

Here are my times before (2.87s) and after (3.73s):
0060': 02.349 02.253
0330': 06.645 06.337
0660': 10.189 09.751
MPH: 70.880 71.370

All above ETs reflect my penchant for 'deep-staging', which drastically increases ET, especially in the 'short' times:
for comparison, here is a run leaving with both bulbs lit, same day as the earlier 9.751/71.37 'blast':
60': 2.118
330': 6.157
660': 9.562
MPH: 71.58

As can be seen, my 60' times decreased .135 alone!!!!!

 

[MoeJr]

dfw for an approx 1/4 mile. Your 10.11 1/8 mile = approx 15.8 1/4 mile.
your 9.39 1/8 mile = approx 14.6 1/4 mile.

www.ondoperformance.com has a great calculators section which is where the conversion chart I used to get those numbers is at.

 

[Glensgages]

dfw for an approx 1/4 mile. Your 10.11 1/8 mile = approx 15.8 1/4 mile.
your 9.39 1/8 mile = approx 14.6 1/4 mile.

www.ondoperformance.com has a great calculators section which is where the conversion chart I used to get those numbers is at.

...and if you don't have a 'puter handy, NHRA factors 1/8-mile racing at .64 ET compared-to a full 1/4-mile.....
in-other-words:
10.11 divided-by .64 = 15.796
9.39 divided-by .64 = 14.671

Outrageous gearing will effect this calculation, by it might come in handy to our 1/8-mile friends!

 

[dfw81shark]

...and if you don't have a 'puter handy, NHRA factors 1/8-mile racing at .64 ET compared-to a full 1/4-mile.....
in-other-words:
10.11 divided-by .64 = 15.796
9.39 divided-by .64 = 14.671

Outrageous gearing will effect this calculation, by it might come in handy to our 1/8-mile friends!

14.6 is definitely nice. This is a good acceleration for me. Not too scary, good for everyday and the occasional squirt into traffic. One of these days I'll get to a big-boy track and see if that 14.6 holds up.

:L

 

[Glensgages]

14.6 is definitely nice. This is a good acceleration for me. Not too scary, good for everyday and the occasional squirt into traffic. One of these days I'll get to a big-boy track and see if that 14.6 holds up.

:L

Didn't mean any dis-respect to the 1/8-mile racers:
done both 1/4-mile and 1/8-mile racing, and truly prefer 660' racing better.

I believe your 'guesstimated' 14.6 1/4-mile ET will be fairly accurate:
old-time drag-racers used a 'hook-factor', where you take the ET x MPH and try to get as close to 1320 as possible.
If above 1320, you could ET quicker at that given MPH, or need more gear-ratio:
if lower, you should be running more MPH, or you may be running slightly too-much gear ratio.

I recall the ultimate 1/8-mile 'hook-factor' to be around 680-690, and your 1/8 9.392 @ MPH 75.22 equates to 706, which isn't too-far from excellent!
My Z28 goes 695 (8.171 @ 85.08), my rear-engine dragster went 672 (6-flat at 112), and my '82 goes 684 (9.562 @ 71.58), and I'm not sure why the pathetic Cross-Fire hook-factors that well.....

3.73 rear-gears and 3.06:1 First Gear ratio???

?????

 

[dfw81shark]

Didn't mean any dis-respect to the 1/8-mile racers:
done both 1/4-mile and 1/8-mile racing, and truly prefer 660' racing better.

None taken! We just don't have 1/4 mile tracks around here. I'd love to try one.

 

[81 Corvette]

Hey guys...while your on the topic what are the actual rpm's you both have using you gear combos. I have a 76 with I think 3.08 (stock), but my rpm's are saying something higher closer to 3.50's. I am at: 55mph 2500 65 mph I am 3000, 75mph 3500, 100mph 4500. So my concern is highway travel and high rpm's cruising. My off the line performance is horrible...no tire squeal, but I can catch 2nd gear with an automatic.

sounds like 3.5x's to me. i've got 3.54 and i figure 2600 rpm = 60 mph and 3500 is around 75-80 (i forget the exact number)

your best bet is to jack up the rear, put it in nutreal spin teh tires until the drive shaft makes one full revolution. the number of tire spins equals your final ratio

 

[76 Sting]

DFW--- I am moving down to your area in approx 6-8 months building a home in Lantana. Hopefully I will see you at a few car shows :}

I am not sure about the 3.08 that's the reason I was asking for some RPM #'s something to compare to. Before I bought the car the rear end was rebuilt and because of the RPM's I am now displaying they did not look right. I mentioned this to someone before and they thought i might have someting closer to 3.5's, but I am not to sure. If I did have 3.50's or very close I woudl hope to at least spin them from 1st, but nothing until 2nd. Something does not make sense :{

 

[76 Sting]

sounds like 3.5x's to me. i've got 3.54 and i figure 2600 rpm = 60 mph and 3500 is around 75-80 (i forget the exact number)

your best bet is to jack up the rear, put it in nutreal spin teh tires until the drive shaft makes one full revolution. the number of tire spins equals your final ratio

Hey "81" thanks for the reply.

This is exactlly what I was told by someone else given the RPM's and MPH. We seam are very close in that respect. Tires are 255/70/15 ( I do not know if that makes a big difference though). How accurate is that spin test?? So if I am 3.54 lets say I should get 3.5 spins out of the tire. What about the size of the tire does this impact it at all?

 

[MoeJr]

76 Sting those tires if they are 255/70/15 are 29" tall. On a Vette those are mud boggers. Stock tire height is 27.4". At 29" it would be hard to spin those tires. One thing we don't know is whether or not the rear gears are the same ratio as the original and if not then did the speedo get calibrated to compensate. If it is correct then based on your numbers you have 3.73's. if that 76 is a stock or close to stock motor it will have a hard time spinning those tires off the line. Also if you have a big cam that doesn't turn on the power till it is up in rpm would also cause a lack of spinnage.Keep us posted.

 

[Glensgages]

What about the size of the tire, does this impact it at all?

Of-course it does..... just for giggles, blow the question out of proportion:
if you run tires twice as big in diameter, you'll travel twice as far (distance) with each axle revolution, and therefore, twice as fast via MPH.

Here are a few mathematical-formulas I've used over the years as a drag-racer (and friend of several circle-track racers) to calculate gear-ratios, tire diameters, MPH, and RPMs.....

Since (I'm assuming) that most of us C3ers are running automatic transmissions that came in our Corvettes (THM350 or THM400), the formulas are based on 10% slippage of the torque converter (hence you'll see either " x .9 " or " divided/by .9 " in these equations):
if you run the THM350C (lock-up torque converter) or typical 4-speed of the era, disregard the ".9 " portion of the formula.....

Much like figuring triangles, where you need to know two sides and an angle, or 2 angles and a side, you'll need to know 3 of the 4 possibile entries:
RPM, MPH, tire diameter, or gear ratio.

To calculate MPH:
RPM x .9 (slippage) x .006 ( a simple 'factor', including pi ), x 1/2 tire diameter (1/2 TD) div/by gear ratio (GR) = MPH

To calculate gear ratio:
RPM x .9 x .006 x 1/2 TD div/by MPH = GR

To calculate RPM:
MPH x GR div/by .9 div/by .006 div/by 1/2 TD = RPM

To calculate tire diameter:
MPH x GR div/by .9 div/by .006 div/by RPM = 1/2 tire diameter


dfw81shark:
Other-than tuning for top-end power, and general-conversation (very few people even know what their car run in the 1/8-mile), 1/4-mile racing offers no real advantage:
the real fun is in the launch!

 

[dfw81shark]

DFW--- I am moving down to your area in approx 6-8 months building a home in Lantana. Hopefully I will see you at a few car shows :}

:{

I joined Cowtown Vettes a couple months ago. Things are wound down for the holiday, so I haven't really done anything with them yet. I plan to hit at least a couple of track days and I know they host a carshow in the fall every year. Most of the guys there run C5s. There are a handful of C3s but I'm pretty sure I'm the only guy who owns only a C3. All the guys I've talked to seem to have a C5 or C4 driver and a C3 in the garage that they never mess with. I could definitely dig another C3 guy to talk to who's keeping theirs on the road.

On a side note- the club is about 200 members strong, so I haven't gotten to meet everyone yet- please forgive me if I offend a cowtown vette member.