Question on top speed Options

[Judd]

I know this is a autoX/road course site but I though some of you might have some insight on this question so I decided to throw this out. I'm thinking about running my car at Maxton N.C. at the E.C.T.A. 1 mile event. I'm putting down around 540RWHP and right now I'm running a 4.10 gear which should max out somewhere around 165 or so. I should get to that which is cool by me since my tech speed should be 175mph. To go over that, I will need a real cage which I'm not willing to do right now.

Later, I would like to replace the gear so I can get some more mph but I'm a bit confused. I've heard that you really should do your best to run 4th gear {1:1} for top speed events because that is the most efficient but my my set-up would really be doggy with something like a 2.73 in the rear. I'm really thinking of running a good dose of nitrous in an effort to get to 200mph {one of my goals} and I think I should have the power to do it but just how much will running 5th gear hurt me? Sure would like to run a 3.42 or 3.73 so I wouldn't have to swap out axles all the time.

Also, I will be carrying a trailer with extra wheels, tools and possibly a bike so I really would like to keep a low gear.

[Blainefab]

I know this is a autoX/road course site but I though some of you might have some insight on this question so I decided to throw this out. I'm thinking about running my car at Maxton N.C. at the E.C.T.A. 1 mile event. I'm putting down around 540RWHP and right now I'm running a 4.10 gear which should max out somewhere around 165 or so. I should get to that which is cool by me since my tech speed should be 175mph. To go over that, I will need a real cage which I'm not willing to do right now.

Later, I would like to replace the gear so I can get some more mph but I'm a bit confused. I've heard that you really should do your best to run 4th gear {1:1} for top speed events because that is the most efficient but my my set-up would really be doggy with something like a 2.73 in the rear. I'm really thinking of running a good dose of nitrous in an effort to get to 200mph {one of my goals} and I think I should have the power to do it but just how much will running 5th gear hurt me? Sure would like to run a 3.42 or 3.73 so I wouldn't have to swap out axles all the time.

Also, I will be carrying a trailer with extra wheels, tools and possibly a bike so I really would like to keep a low gear.

I took a swag at your 4.10 baseline: 6600rpm 25.5" dia tire .74 5th gear = 165mph ???

Just switching to a 26.8" tire (275/40-18 ala C5) gives you 173.5mph, close enough to your tech speed for now. Your driveshaft rpm will be crazy high, so definitely get a carbon shaft - there is GP pricing available on the ACPT shaft.

With the tall tire and 3.73 you'll get 191mph in 5th. 3.42 goes to 208mph, given enough room. Getting there in a standing mile involves more than a gear chart, but it's someplace to start.

I wouldn't worry about using 5th - you'll need the higher acceleration rate that the 4.10 gears will give you in the lower gears until you start spraying it. 5th is only a little less efficient than 4th, the only difference is that in 4th gear the input shaft to main shaft bearing is not rotating. Both gears load path is via the countershaft. I ran a couple thousand miles of ORR in 5th > 150mph.

[AndyJ]

Judd,

Jeff Creech AKA Milkman has run a fairly serious car at Maxton several times. Maybe he will chime in.

[roadracetransam]

I ran around 170 (based on RPM) with 3.23 gears and A4 in 4th gear. 4th gear is 0.70 gearing. It was with the stock motor so around 310 RWHP. I am sure with your HP you won't have trouble pulling in your top gears.
I don't think you will need nitrous.

This post has been edited by roadracetransam: Dec 25 2007, 06:33 PM

[AndyJ]

I ran around 170 (based on RPM) with 3.23 gears and A4 in 4th gear. 4th gear is 0.70 gearing. It was with the stock motor so around 310 RWHP. I am sure with your HP you won't have trouble pulling in your top gears.
I don't think you will need nitrous.

170mph on a standing mile course?? At the Texas Mile, I ran 158mph with my modded LT1/ A4/ 3.42 rear. The A4 really hurts you in the standing mile.

This post has been edited by AndyJ: Dec 25 2007, 07:13 PM

[Judd]

I ran around 170 (based on RPM) with 3.23 gears and A4 in 4th gear. 4th gear is 0.70 gearing. It was with the stock motor so around 310 RWHP. I am sure with your HP you won't have trouble pulling in your top gears.
I don't think you will need nitrous.

If I were on a nice 5 mile stretch you would be right but from what I'm told,,, hitting 200mph in a mile is pretty hard.

Thanks for the replies. Especially the part where I could still use 5th gear and only give up a smidge of efficiently. Man, I was dreading running a 3.08 or 2.73 gear and now it looks like I will be fine with a 3.73 or a 3.42 and that if cool by me.

Thanks for the help.

This post has been edited by Judd: Dec 25 2007, 07:29 PM

[roadracetransam]

I ran around 170 (based on RPM) with 3.23 gears and A4 in 4th gear. 4th gear is 0.70 gearing. It was with the stock motor so around 310 RWHP. I am sure with your HP you won't have trouble pulling in your top gears.
I don't think you will need nitrous.

If I were on a nice 5 mile stretch you would be right but from what I'm told,,, hitting 200mph in a mile is pretty hard.

Thanks for the replies. Especially the part where I could still use 5th gear and only give up a smidge of efficiently. Man, I was dreading running a 3.08 or 2.73 gear and now it looks like I will be fine with a 3.73 or a 3.42 and that if cool by me.

Thanks for the help.

Sorry, I had to re-read the question. Mine was not a standing mile just top speed.

[GlennCMC70]

i know plenty of guys w/ 100rwhp less than you hitting 160+ in the Texas mile. if i was you, 3.42's and an RPM limit of 7K should be plenty.
if you don't like that, 3.23's would be a good option. i know one guy who has done that w/ his 6 speed car, and he likes it for the standing mile stuff.

[Judd]

i know plenty of guys w/ 100rwhp less than you hitting 160+ in the Texas mile. if i was you, 3.42's and an RPM limit of 7K should be plenty.
if you don't like that, 3.23's would be a good option. i know one guy who has done that w/ his 6 speed car, and he likes it for the standing mile stuff.

Yea,,, I'm pretty sure I will get over 160mph in a mile but I'm going to have plenty of time to work up to faster outings. Shoot, I run 127-128mph in the 1/4 mile as is with a "safe" tune. I think I'm going to try to make 3.73s work. Should be perfect as long as I stay N/A. If and when I decide to try some nitrous I could run a 27-28 inch rear tire and should be able to crack 200mph. Now I gotta find a 27-28 inch tall tire and figure out how to make them work?????

[Teutonic Speedra...]

Not sure what what the rules are, but I would work on any aero advantages that you can (narrow tires, no mirrors, covered up fog light openings, etc.).

[v7guy]

Are there any driveshafts other than the carbon units you guys would recommend. I know the LT1 shaft isn't good for those speeds but isn't there a cheaper option while still being safe?

[StanIROCZ]

Are there any driveshafts other than the carbon units you guys would recommend. I know the LT1 shaft isn't good for those speeds but isn't there a cheaper option while still being safe?

Has anyone really had critical speed problems with a stock 3" Al driveshaft? Theoretical critical speed 11,600 rpm. From my experience you should feel comfortable with a 30% margin, so that brings it down to 8,118. With a .7 overdrive engine speed would be 5,683. That's pretty damn fast no matter what rear gear you have. If you had problems I would suspect they are for other reasons like a bad balance or loose bushings in the tailhousing.

Do the same math on a 3.5x.125" Al shaft. Theoretical critical speed 13,423 rpm. 9,396 rpm with 30% margin, 6,577 rpm engine speed. You should be able to get a custom 3.5" shaft for about $4-500 or so from a DS shop.

I have a 4" Al shaft because that's what was "available" wink wink. It wasn't easy to fit in the tunnel but can be made to fit.

[AndyJ]

Are there any driveshafts other than the carbon units you guys would recommend. I know the LT1 shaft isn't good for those speeds but isn't there a cheaper option while still being safe?

Jason,

I ran the aluminum drive shaft for many miles at speeds > 150mph with no problems. I changed to a CF driveshaft because of just a *barely* perceptible vibration at 145 and greater.

[dailydriver]

Not sure what what the rules are, but I would work on any aero advantages that you can (narrow tires, no mirrors, covered up fog light openings, etc.).

AGREED!! I would even think about a splitter, maybe even one of those aero noses. You do NOT want that front end 'dancing' around or "getting light" at those (200 mph (IMG:http://www.frrax.com/rrforum/style_emoticons/default/gr_eek2.gif) ) speeds!!!

[Blainefab]

Are there any driveshafts other than the carbon units you guys would recommend. I know the LT1 shaft isn't good for those speeds but isn't there a cheaper option while still being safe?

Jason,

I ran the aluminum drive shaft for many miles at speeds > 150mph with no problems. I changed to a CF driveshaft because of just a *barely* perceptible vibration at 145 and greater.

My 1LE aluminum shaft started howling around 150, and was a deafening shriek by 170. Changed to the ACPT CF shaft and it was smooth past 190.

There are others that sell CF shafts, some don't have the protective layer of fiberglass that ACPT uses.

[Blainefab]

Are there any driveshafts other than the carbon units you guys would recommend. I know the LT1 shaft isn't good for those speeds but isn't there a cheaper option while still being safe?

Has anyone really had critical speed problems with a stock 3" Al driveshaft? Theoretical critical speed 11,600 rpm. From my experience you should feel comfortable with a 30% margin, so that brings it down to 8,118. With a .7 overdrive engine speed would be 5,683. That's pretty damn fast no matter what rear gear you have.

The late T56 5th gear is .74, at 6600 engine rpm the ds rpm is 8919, well into the 30% margin. My 1LE shaft started making noise about 6500 ds rpm.

[Blainefab]

Not sure what what the rules are, but I would work on any aero advantages that you can (narrow tires, no mirrors, covered up fog light openings, etc.).

AGREED!! I would even think about a splitter, maybe even one of those aero noses. You do NOT want that front end 'dancing' around or "getting light" at those (200 mph (IMG:http://www.frrax.com/rrforum/style_emoticons/default/gr_eek2.gif) ) speeds!!!

Actually, for top speed you want neutral downforce, low drag aero - positive downforce just increases rolling resistance. As Dennis said - smooth out the openings on the front, minimal tire and spoiler.

This would cruise at 185 on a calm, level road with less than 350rwhp:

(IMG:http://www.skyhighway.com/~bfpix4/dscn0072.jpg)

(IMG:http://www.skyhighway.com/~bfpix4/ablaine1.jpg)

[Crazy Canuck]

with your present power, you can reach ~130mph on the 1/4 mile alone.
You don't need nitrous to reach your goal... with a 200 shot, that'll put you 140+ mph or so in the 1/4 mile alone... provided you can cope w/ traction.
You'll probably go through a 10lbs bottle on every 2 runs if that.

[02midevil]

Don't worry about 5th gear at Maxton, use it. Your rate of excelleration may fall off most noticable by not being pressed back in the seat.

A pair of TN brothers run a sbc first in a volvo and now in an older RX7 with enough nitrous to push it to 200mph everytime. My guess is a 450-550 hp motor and 200-250 plate, if you ask em they may tell you. A bbc 4th generation has enough juice to send it to the moon and runs 200 all day too.

One of the problems is getting traction at Maxton on the line, the 200 mph cycle guys need a perfect launch to pull 200+ mph every time or else they fall short. So in short Maxton is a 1 mile drag strip.

One car that speed inc did showed up at Maxton and went 190+ with a turbo LT1 (he posted on here once or twice). The video may still be on speed inc's website. That car did not even whisper when it idled.

One other thing people do is run to the 3/4 mile and slow down, listen and you will hear revs and then fall off somewhere down the track. An LS2 GTO goes 150mph at the 3/4 mile and a Cobalt SS was doing the same 150mph with a belly pan.

Also like stated if you can get more aero.