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MrJoshua
MrJoshua UltimaDork
4/11/22 8:55 a.m.

The brake question: Autocross guys tend to use the lightest components with the least rotating mass. Heat soak is not a problem like it is on track cars. You want components that get up to good operating temperature immediately, and have just enough cooling/mass to make it to the end of the run before overheating. I've read stories of brake downgrades on FWD cars to drums on the rear if they were ever an option on that car just to save that small amount of rotating mass.

 

The brake rule Stampie keeps mentioning is you get to replace the brakes on your car with new (or rebuilt) replacements for the exact part that came on your car with no budget hit. You can't use that rule to switch to better brakes from another year. Fresh factory brakes should be good enough for an autocross. If you want more you can buy more autocross specific pads and count them in your budget. Then you can switch to a more road race specific pad compound and add cooling and you might be OK for track work.

 

The autocross: The challenge autocross tries to like both high horsepower cars AND small great handling cars. With a lot of horsepower and grip you can count on being over 60mph at least once with some cars topping 70mph. The low speed corners will probably not drop below 25mph.

Road race vs Autocross: From what I've read one of the differences between road race setup and autocross is the stiffness of the suspension and looseness of the car. An autocross car needs to react NOW to steering inputs, not take a moment to settle into the corner. You can also set up the car to be easier to rotate based on suspension. The low speed and short corners can sometimes benefit from a slightly tail happy car. 

Gearing:  If your car can stay in one gear for most of the course it would be beneficial. In a perfect world you just tap redline at the fastest point in the whole course, and drop to the bottom edge of your best power range at the slowest point. You have a version of the TH400 right? You have plenty of options with that gearing. Assuming a 25" tall tire and a 3000-7000rpm range you could use a 2.88 final drive and have first gear from 31-72mph. Using the same tire size and RPM range, and switching to the 3.54 rear gear and you get 25-59mph. Using second gear and the 3.54 rear gearing puts you working in the 2000-5000 range, but that may be preferable because it prevents throttle inputs from being so hair trigger and upsetting the chassis. No matter what it looks like you will be spending some time in the low RPM range so I wouldn't go crazy with cam selection pursuing high rpm power.

I think your car will be fine for autocross. Your power will be good, fresh brakes should be fine, you have room for a LOT of tire. I think you should practice a bunch. Find an autocross that gets the cars over 60mph and has the fastest cars finishing in the 40 second range and you will be close enough to the challenge course setup to predict how your car will behave. It is an adjustment, but your road racing experience should help.

 

Tires tires tires tires tires! Get sticky tires. Regular road race tires won't cut it. Autocross specific slicks are the best bet. If you are trying to save money using take offs, get the softest qualifying compound you can find in a tire designed for a car 1000lbs less than yours.  Autocrosses are short and the tires need to heat up immediately. Formula Atlantic uses some really wide rear rubber. Maybe qualifying compounds from them?

Stampie
Stampie GRM+ Memberand MegaDork
4/11/22 10:06 a.m.

In reply to MrJoshua :

We ran SM7s on the Q45 and they worked great. But that's your use a tire from a smaller car thought. I'm sure Frenchy knew all this because he's been a professional race car builder/driver for 40 years.

SV reX
SV reX MegaDork
4/11/22 1:47 p.m.

For autocross, and especially for the Challenge, I'm gonna suggest that brakes are extremely important, perhaps even more important than horsepower. 
 

I let professional drivers drive my cars, and every one of them comments on my brakes. I'm pretty good at setting up brakes. When a pro driver gets in my car and realizes how good the brakes are, it gives them a senses of confidence in the car in the first turn. From then on, they are able to dive deeper into every corner before hitting the brakes, because they know they will hold. 
 

You'd be surprised how much difference that can make on an autocross course. The driver can stay on the gas longer heading into every single corner. 
 

Brakes matter. Well adjusted OEM brakes are usually sufficient for the Challenge autocross. 

frenchyd
frenchyd MegaDork
4/11/22 4:39 p.m.
MrJoshua said:

The brake question: Autocross guys tend to use the lightest components with the least rotating mass. Heat soak is not a problem like it is on track cars. You want components that get up to good operating temperature immediately, and have just enough cooling/mass to make it to the end of the run before overheating. I've read stories of brake downgrades on FWD cars to drums on the rear if they were ever an option on that car just to save that small amount of rotating mass.

 

The brake rule Stampie keeps mentioning is you get to replace the brakes on your car with new (or rebuilt) replacements for the exact part that came on your car with no budget hit. You can't use that rule to switch to better brakes from another year. Fresh factory brakes should be good enough for an autocross. If you want more you can buy more autocross specific pads and count them in your budget. Then you can switch to a more road race specific pad compound and add cooling and you might be OK for track work.

 

The autocross: The challenge autocross tries to like both high horsepower cars AND small great handling cars. With a lot of horsepower and grip you can count on being over 60mph at least once with some cars topping 70mph. The low speed corners will probably not drop below 25mph.

Road race vs Autocross: From what I've read one of the differences between road race setup and autocross is the stiffness of the suspension and looseness of the car. An autocross car needs to react NOW to steering inputs, not take a moment to settle into the corner. You can also set up the car to be easier to rotate based on suspension. The low speed and short corners can sometimes benefit from a slightly tail happy car. 

Gearing:  If your car can stay in one gear for most of the course it would be beneficial. In a perfect world you just tap redline at the fastest point in the whole course, and drop to the bottom edge of your best power range at the slowest point. You have a version of the TH400 right? You have plenty of options with that gearing. Assuming a 25" tall tire and a 3000-7000rpm range you could use a 2.88 final drive and have first gear from 31-72mph. Using the same tire size and RPM range, and switching to the 3.54 rear gear and you get 25-59mph. Using second gear and the 3.54 rear gearing puts you working in the 2000-5000 range, but that may be preferable because it prevents throttle inputs from being so hair trigger and upsetting the chassis. No matter what it looks like you will be spending some time in the low RPM range so I wouldn't go crazy with cam selection pursuing high rpm power.

I think your car will be fine for autocross. Your power will be good, fresh brakes should be fine, you have room for a LOT of tire. I think you should practice a bunch. Find an autocross that gets the cars over 60mph and has the fastest cars finishing in the 40 second range and you will be close enough to the challenge course setup to predict how your car will behave. It is an adjustment, but your road racing experience should help.

 

 

Tires tires tires tires tires! Get sticky tires. Regular road race tires won't cut it. Autocross specific slicks are the best bet. If you are trying to save money using take offs, get the softest qualifying compound you can find in a tire designed for a car 1000lbs less than yours.  Autocrosses are short and the tires need to heat up immediately. Formula Atlantic uses some really wide rear rubber. Maybe qualifying compounds from them?

That's really great information.  For some reason I was blocked from responding when I first read it.  
     I like that you knew my rear end ratio's to choose from are 3:54& 2:88.   The most common take offs would be former NASCAR tires and they tend to be close to 27 inches. 
         I'm not afraid to run the engine over the 6500 Red line.  Especially for less than a minute. The factory kept it at 8300 for 3 hours.  
    The stock cam really starts to fall off above 5000 rpm and even a XM3 Isky cam  is all done about 7000. ( I'm not sure I can change the cams inside the budget yet.).  I do have a T5  etc I can use but I question if it will survive.   So to play it safe I'll send it with  the Turbo 400 

  The stock brakes are 4 piston vented 12" rotors . Not what I'll vintage race with but  I'll check them completely prior to sending them. 
      Using those 1/4 mile calculators they claim that if I get the engine where the computer is telling me I should be.  Keep the weight down to under 2700 pounds  they should be in the 9 second time range.  But I actually doubt that they will do that well.    10's should be plenty respectable.   Test and tune should tell me how close I am.  
 Both Jeff and  Erik  are active autocrossers. Both have newer Corvettes so they should do reasonably well.  
            

Stampie
Stampie GRM+ Memberand MegaDork
4/11/22 5:04 p.m.

In reply to frenchyd :

Just to point out the obvious that I'm sure you are aware of, the Challenge uses NHRA rules.  You built your cage to NHRA specs for the speeds you think you can run right?

SV reX
SV reX MegaDork
4/11/22 6:07 p.m.

In reply to frenchyd :

I'm not trying to be a wet blanket, but if you've never been drag racing before, your car will not be running 9's. I know that sounds harsh, but it's just true. There is MUCH more to drag racing than the online calculators can ever tell you. If it did run 9's, you'd need an NHRA competition license. 

Go to a track and watch a 9 second launch  It will take you less than 2 tenths of a second for you to see how hard it is.  You will see in the very first launch.

And honestly, it's unlikely it will run 10's. Unless you put in 25 or more passes before you get to Gainesville. 
 

These are all some of the reasons it's called "The Challenge". 

MrJoshua
MrJoshua UltimaDork
4/11/22 6:44 p.m.

In reply to frenchyd :

Yeah, I echo what SV reX said. Those calculators are optimistic for non drag racers. If you hit 11's you will likely be in the top 5 in the drags. Even with the power and weight to run 9's, 11's will probably still take some work. 

frenchyd
frenchyd MegaDork
4/11/22 11:58 p.m.
SV reX said:

In reply to frenchyd :

I'm not trying to be a wet blanket, but if you've never been drag racing before, your car will not be running 9's. I know that sounds harsh, but it's just true. There is MUCH more to drag racing than the online calculators can ever tell you. If it did run 9's, you'd need an NHRA competition license. 

Go to a track and watch a 9 second launch  It will take you less than 2 tenths of a second for you to see how hard it is.  You will see in the very first launch.

And honestly, it's unlikely it will run 10's. Unless you put in 25 or more passes before you get to Gainesville. 
 

These are all some of the reasons it's called "The Challenge". 

I don't doubt a word of what you are saying.  I've watched Calvin Nelson on his UTube channel. He makes it look easy.  I know it's anything but easy.  I will be driving kids to and from school.  But Jeff and Erik both have autocross experience with their Newer Corvettes  so they should do decently and drag racing?   The car will do whatever.  
   I just hope you guys enjoy listening to a cheap V12 at full song. 

   As I've repeatedly said.  1 pass, a little plastic trophy  is the sum total of my drag racing experience . 
     Same with autocross. Sir Stirling Moss went just before me,  I just tried not to knock over any cones and when they awarded me 1st prize for that  pass they could have knocked me over with a feather.  While I'm happy to repeat that story the simple truth is in both events  the car did the work. I just didn't screw up. 

        

MrJoshua
MrJoshua UltimaDork
4/13/22 2:02 p.m.
frenchyd said:
I don't doubt a word of what you are saying.  I've watched Calvin Nelson on his UTube channel. He makes it look easy.  I know it's anything but easy. 

And I'm pretty sure I watched his dad run a measurably quicker 1/4 mile in the same car at the same track on the same night. I'm not sure if the gap has been eclipsed since, but at the time Calvin already had more 1/4 mile passes under his belt than I am likely to get in my entire life.

cruisermatt
cruisermatt Reader
4/13/22 2:33 p.m.

It took us two years to go 10's in a 2500lb car with a turbo V8. and it didn't even happen until after the event. 

frenchyd
frenchyd MegaDork
4/15/22 5:00 a.m.
cruisermatt said:

It took us two years to go 10's in a 2500lb car with a turbo V8. and it didn't even happen until after the event. 

I don't doubt it for a second. Drag racing looks easy. And the one pass I made that I won a trophy was easy. Go on green,  stop after the finish line. Collect little plastic cup. 
      Yet I'm sure extracting everything from a given car is many multiples harder. That's why I hope they can talk the Nelson's into driving for us. 

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
4/15/22 8:58 a.m.
SV reX said:

In reply to frenchyd :

I'm not trying to be a wet blanket, but if you've never been drag racing before, your car will not be running 9's. I know that sounds harsh, but it's just true. There is MUCH more to drag racing than the online calculators can ever tell you. If it did run 9's, you'd need an NHRA competition license. 

Go to a track and watch a 9 second launch  It will take you less than 2 tenths of a second for you to see how hard it is.  You will see in the very first launch.

And honestly, it's unlikely it will run 10's. Unless you put in 25 or more passes before you get to Gainesville. 
 

These are all some of the reasons it's called "The Challenge". 

I've built and driven plenty of 9-second cars... but I laid down 11s in them because I'm not usually the driver.  Even though I have hundreds of 1/4 mile runs under my belt ranging from 16 seconds to low 11s, I am far from a 9 second driver.  It takes a special kind of crazy to go 9s.  Watching the Nelsons almost break 8s last year made my butt pucker in ways that made me concerned for the future of my ability to poop.

Captdownshift (Forum Supporter)
Captdownshift (Forum Supporter) GRM+ Memberand MegaDork
4/15/22 10:11 a.m.

In reply to Curtis73 (Forum Supporter) :

16.9s, but that's still technically 16s :lol: 

frenchyd
frenchyd MegaDork
4/15/22 11:30 a.m.

In reply to Captdownshift (Forum Supporter) :

Nothing wrong with that sorta pass.  I can't remember what that Honda Civic turned. Or what light, 60ft times etc. but one pass and they handed me a trophy.  Maybe they put me in the wrong class or nobody had run a car that small before.  
 Who knows I tried to give it back but they said I trophyed out and couldn't run anymore. 

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