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Kirkham Motorsports

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  • 1 Post By Tommy
  • 1 Post By Cobra #3170

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  #1 (permalink)  
Old 08-04-2015, 02:28 PM
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Default Gearing for autocross

I went to an autocross at Thompson Motorspeedway this weekend and had a blast. It was a tight course that you had to go through twice. It was confusing at first but by the end of the day I felt comfortable and always improved my time. I started in 1st,went through a slalom and hit 2nd for the remainder of the course, probably no faster than 35 mph I think. After thinking about my day and what I learned and how I can improve I thought about gearing. It seemed to me that I may have been better off just leaving it in 1st and use the engine for braking and the rpms for accelerating faster. The problem is most competitors used 2nd and probable had better range. So here are my thoughts. By the way I drove the car to the autocross at a comfortable 74mph at 2600rpms
I have a tko600 .82 5th, 3.27 rear, 26.5 tire a 351w about 488fwhp and a very nice broad torque curve from3000 rpms to 5000rpms 435 to 455. And 28lb. flywheel
Currently with 3.27 1st and 2nd mph looks like this at 2500 to 5000 rpms 21.0-42.0 mph-------31.89-63.78 mph
With 3.55 its 19.34-38.69 mph---------29.38-58.75 mph
My question is am I better off with the 3.55 rear and getting into 2nd and staying there with a better mph range?
I realize my highway cruising will be compromised but there is always a trade off.
By the way street tires and 15 inch wheels is not the way to race, and I realize seat time is 1st on the list.

Your thoughts please


I posted on ffcars also because the two guys that helped me throughout the day use that site. and I wanted to thank them.
Lou
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Old 08-04-2015, 04:05 PM
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I'll bet that if you sent Cobra #3170 a PM, he'd tell you more than the rest of us combined.
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Old 08-04-2015, 06:08 PM
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I'd be happy to be corrected by the experts, but I'd suggest that suspension set up (including: corner weighting, spring rates, dampening, ride height, tires & tire pressures, etc), and and seat time could see you improve seconds, where as swapping your diff ratio probably only tenths of seconds.

Now while you've identified seat time, have you considered what suspension tuning is available to you?

I'd have thought like most racing, it would be about keeping as much rubber on the black stuff as possible at all times. "Staying planted" and thus having the grip available to push for quicker times.

That's my thoery
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Old 08-04-2015, 06:28 PM
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I checked out some of the topics cobra #3170 posted, with 8000 rpms!!! Yikes! and one gear. I should have tried staying in first gear. I would have never reached my chip.
I also realized and mentioned in my post that seat time wheels and tires would be some of the first moves. It's just that gearing could have been handled Sunday at the track and I did not try staying in 1st gear.
The first few runs I spent getting through the correct gates, forget going fast. Until I learned the course analyzing my car was secondary
It wasn't like a goodguys autocross where the cones lined a chalked out track, which seems easy to follow. Gates and an intersection on a double layed out course at 30 mph was confusing at first. Walking it and running it were not the same
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Old 08-04-2015, 09:12 PM
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RET_COP,
I consider myself an inexperienced novice whose autocross experience is approaching ten years old, which is why I recommended Bruce Cambern for advice. But my recollection is that the hardest part of autocross the first half dozen times I did it was not getting lost. I too struggled with the gear shift question and settled for shifting into second one time and then staying with it. It was hard enough wrestling that beast around those tight corners without worrying about shifting gears too. ... My only other thought is that courses change from event to event. So the ideal rear end ratio for one course might not be right for the next. .... Good luck at the track. And when you get tired of long days with little track time to show for it, look into track days. I found them more to my liking.
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Old 08-04-2015, 10:47 PM
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Default Gear ratio

Quote:
Originally Posted by RET_COP View Post
I went to an autocross at Thompson Motorspeedway this weekend and had a blast. It was a tight course that you had to go through twice. It was confusing at first but by the end of the day I felt comfortable and always improved my time. I started in 1st,went through a slalom and hit 2nd for the remainder of the course, probably no faster than 35 mph I think. After thinking about my day and what I learned and how I can improve I thought about gearing. It seemed to me that I may have been better off just leaving it in 1st and use the engine for braking and the rpms for accelerating faster. The problem is most competitors used 2nd and probable had better range. So here are my thoughts. By the way I drove the car to the autocross at a comfortable 74mph at 2600rpms
I have a tko600 .82 5th, 3.27 rear, 26.5 tire a 351w about 488fwhp and a very nice broad torque curve from3000 rpms to 5000rpms 435 to 455. And 28lb. flywheel
Currently with 3.27 1st and 2nd mph looks like this at 2500 to 5000 rpms 21.0-42.0 mph-------31.89-63.78 mph
With 3.55 its 19.34-38.69 mph---------29.38-58.75 mph
My question is am I better off with the 3.55 rear and getting into 2nd and staying there with a better mph range?
I realize my highway cruising will be compromised but there is always a trade off.
By the way street tires and 15 inch wheels is not the way to race, and I realize seat time is 1st on the list.

Your thoughts please


I posted on ffcars also because the two guys that helped me throughout the day use that site. and I wanted to thank them.
Lou
Generally speaking when you start autoxing you are better off with a lower numerical ratio because it will avoid wheel spin and make your run smoother.
You are on the money about seat time run every event you can and sign up for fun runs too if they are available. Check out the Evolution autox schools they have them all over the country and they are a very good place to start.
Most of the time will be gained through driving experience if you decide that you like it then you can begin to change the car to improve performance but I would not do anything until you have some seat time and decide you want to go on and upgrade the car. Tires are the most important change you can make to gain time but the car has to be balanced to take full advantage of autox tires. Starting in first and shifting quickly to second is a good strategy and lots of very fast Corvettes do just that so I would leave the car as is for now. An experienced autox driver could drive your car and give you an idea of what adjustments might help to make it easier to drive fast, might just be air pressure settings but could be spring or bar change too.
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Old 08-05-2015, 03:07 AM
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Every car engine and gear combination is different. That said I run in 2nd the entire course, just bumping 1st to start. For my car, 3.90 gears and a truetrac work best. Top speed on our local courses is typically 60 mph
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Old 08-05-2015, 05:20 AM
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One more thing - A technique that helped me learn each course better was to sketch the course on a pad as I walked it, noting the areas that were easier and trickier to navigate. I also noted which side of the first slalom cone to use. Then I attempted to memorize the key points on the course using single words for each. I'd end up with 8-12 words that I'd repeat aloud as I looked at the sketch and drove the course in my head. Finally, as I sat in the car awaiting the start, I'd repeat the mantra to make visualizing the course easier.
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Old 10-18-2016, 02:14 PM
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Just an update after this autocross season. I found that staying in 1st gear even if hitting the chip at the end of some long straights was faster than 2nd gear. I am able to drive my car harder with Avons compared to last year. I would like to have the broader range of the 2nd gear but that would mean 4.10s which would render 1st useless on the street and 5th on the highway driving to the events a slow trip. I'm stuck because I have a dual purpose car. 3.55 and 3.73 would do nothing but ruin 1st for the autocross and not give me enough for 2nd. I'm hitting the chip at 55mph and 6700rpms so I'm good on tight courses. I think I'll be good in 2nd for the long airfields. I'll let you know. I need more seat time anyway. It's fun for sure.
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Old 09-06-2018, 12:38 PM
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Sounds fun!

I've been autocrossing about 11 years now. You are getting good advice above...nothing to add.

Where is every body getting Avons today?
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Old 09-06-2018, 03:29 PM
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SASCO Sports for me on the east coast. I also did change my gears to 3.73 and autocrossed with them last year. I actually liked them. I finished 3rd in my class. The guys with really good times were using A6s on 18" wheels and trailering their cars. I am somewhat caught between a tight course and an airport type course. I've had to shift sometimes and it takes a bit getting use to but all in all always fun. My only problem in my old age is getting the course down when it's on a tight parking lot and overlaps itself in certain areas. I've embarrassed myself a few times but the young guys always helped me along. Anyway it was a bucket list thing to do and I accomplished that. I've been doing more just cruising this year.
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Old 09-06-2018, 06:47 PM
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Quote:
Originally Posted by Tommy View Post
I'll bet that if you sent Cobra #3170 a PM, he'd tell you more than the rest of us combined.
I agree.....with Tommy........Bruce has more experience at Autocross and a very refined Cobra to match.....but you better be ready to make some good changes to your car....it is addicting......
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