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Hitting redline during AX?

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Portland, OR, USA
#1
Friday I was able to attend a practice day. The course was an abandoned airstrip, so long and narrow. There was 3 separate points on the layout that got me bouncing off redline. 2 of them for about 1 second, but the 3rd spot was for what seemed like 3-4 seconds. Since it was practice, I tried going into 3rd a few different times. First try went for the downshift while trying to turn and it messed up my line. 2nd time I got the shift in the braking zone, but the run wasn't any better then without shifting. The last time I really went for it, and ended up carrying more speed through a lane change section, and took out a row of cones...

So... how often do you hit redline during an autocross? I was reading a article about running a fatter A052, but slightly taller. Their conclusion was the 0.8% higher gearing cost more then the 20mm extra width of tire.
https://www.turnology.com/tech-stor...V9nlh41IH2-hnRQek68oWAmlqn8lNlNjfF2je6ZkAB_lc

After this event, I wish I had the taller tire, I already am on the RE71 205/50R16, so taller then the taller tire in this article by 0.4%.

Im guessing this layout was an anomaly. What have you experienced?
 


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#4
If you can do it smoothly and not during a high G turn ... Short shift HS into third rather than shift at redline, the meat of the power and is 3K-5K.. it may feel slower but it’s generally not, and 3rd gear has slightly more available power in the stock ECU so perhaps that’s helpful.
 


OP
Rexracer
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Thread Starter #5
If you can do it smoothly and not during a high G turn ... Short shift HS into third rather than shift at redline, the meat of the power and is 3K-5K.. it may feel slower but it’s generally not, and 3rd gear has slightly more available power in the stock ECU so perhaps that’s helpful.
Yeah going up to 3rd wasn't bad, had a good spot to do it in. It was the point that I needed to go back to 2nd that caused the problem.

My question is more, are these cars geared short where you are hitting redline regularly. If you are, are you shifting? Also, if redline is a regular occurrence, has anyone gone to a larger tire for a little more gear, and to use the meat of the power band, that 3-5k rpm zone.
 


OP
Rexracer
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Thread Starter #7
Raising CG on already a high CG car is detrimental and even dangerous in my opinion.
I can understand that concern, but the tallest tire I am seeing (in 16" that could/should fit), the A052 in 225/50R16, would only raise the car 0.4". This isnt insignificant, but if these cars are that close to rolling over, then they shouldn't be competing.

But then again that difference in size only equals 1.7mph at 50mph.


1591197055296.png
 


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#8
I have some real reservations about how they did the test in the article you linked to- I'd think it would be beneficial to optimize temps and pressures for each tire/wheel combo rather than to simply duplicate pressures across both. The 0.4% difference in height (calculated from values based on tire designation, rather than actual measurements) is hard to blame for a 1.11 second difference (measured by GPS, which introduces it's own set of problems). They pointed out that there were differences in wheel size, and there were comments to that effect. In the end, they tried to say this was a scientific exercise into the effects of tire size for track times, but all they can really say is that their driver (drivers?) were faster on that one particular set of wheels and tires on that particular day, at those temps and pressures than they were on another. Which is cool, I guess, but pretty damn problematic if you are hoping to derive some real info that would help you choose tires and wheels.

Given our cars wide torque curve, 0.4% in tire height is probably the least important difference between those setups.
 


PunkST

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#9
Theres always one spot where i do. Still at stock redline even on my tune. Haven't felt a need to change it.
 


OP
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Thread Starter #11
I have some real reservations about how they did the test in the article you linked to- I'd think it would be beneficial to optimize temps and pressures for each tire/wheel combo rather than to simply duplicate pressures across both. The 0.4% difference in height (calculated from values based on tire designation, rather than actual measurements) is hard to blame for a 1.11 second difference (measured by GPS, which introduces it's own set of problems). They pointed out that there were differences in wheel size, and there were comments to that effect. In the end, they tried to say this was a scientific exercise into the effects of tire size for track times, but all they can really say is that their driver (drivers?) were faster on that one particular set of wheels and tires on that particular day, at those temps and pressures than they were on another. Which is cool, I guess, but pretty damn problematic if you are hoping to derive some real info that would help you choose tires and wheels.

Given our cars wide torque curve, 0.4% in tire height is probably the least important difference between those setups.
After posting this up, I was reading the article closer. I had only jumped to the end for the conclusion before. I think more then anything its showing the importance of the right wheel for the tire being used. I mean going down to a 6" rim for a 225....

But Mark Scroggs had great success with the Turbo 4 Camaro running a 275 on an 8" rim.
 


OP
Rexracer
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Thread Starter #12
I avoid it. It's harsh over time on the engine, and we've got a really responsive torquey engine. Just go up a gear and let the torque move the car.
You avoid hitting redline? I was talking with a friend about this. What aspect of this is harsh on the car? I assume at redline it just cuts spark/fuel.

It seems the shift to 3rd then back down to 2nd in a 1-2 second span would be harder on the transmission and the engine jumping RPM suddenly would be harder on the car, then having the engine cut fuel for a second or two.

I am honestly asking, I am trying to understand.
 


the duke

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#13
You avoid hitting redline? I was talking with a friend about this. What aspect of this is harsh on the car? I assume at redline it just cuts spark/fuel.

It seems the shift to 3rd then back down to 2nd in a 1-2 second span would be harder on the transmission and the engine jumping RPM suddenly would be harder on the car, then having the engine cut fuel for a second or two.

I am honestly asking, I am trying to understand.
Your engine, specifically your entire rotating assembly. Under power you're cramming all kinds of air and fuel in there. You're containing explosions and thousands of revolutions a minute. This causes flex, distortion, and compression and tension in parts, etc. Suddenly you hit your revlimiter and all that force from the previous few revolutions has to go somewhere. The pistons was flying upwards, ready to be counter-acted by compression and the resulting explosion to slow it down, but that doesn't happen, the force has to go somewhere. Now your wrrist pins, and fasteners have to take ALL the force, as opposed to the explosion providing a "buffer" if you will. Things relax, move back into position, spring back etc. But not all of this happens in unison, and you can get contact. Too much and boom, excess wear or even failure. There is a lot of energy in a rotating engine. Removing the explosions removed a check feature that slows things down if you will.

There are different ways to limit rpm. Some just cut spark, some cut fuel. Some hit harder while others are softer on the engine for this reason. It's less the rpm limiter and more the drastic change between load and unload. I've seen motors blow when shifting at high rpms for the same reason. Stability under consistent/ramping stress, but when the clutch went in to shift the drastic loss of load caused the previous flex to spring back and grenade the motor. Like most in life steady-state isn't the problem. Shock loading is.

Depending on the course design you shouldn't have to shift much if at all. Use the high torque high response engine and leave it in gear. Most autocross course designs shouldn't really get into 3rd anyways. Or shift under braking, no time lost.
 


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kivnul

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#14
For me it depends on the course, but I rarely shift into 3rd. I do hover at the rev limiter sometimes. I also have much taller tires @ 215 45 r17
 


OP
Rexracer
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Thread Starter #15
Your engine, specifically your entire rotating assembly. Under power you're cramming all kinds of air and fuel in there. You're containing explosions and thousands of revolutions a minute. This causes flex, distortion, and compression and tension in parts, etc. Suddenly you hit your revlimiter and all that force from the previous few revolutions has to go somewhere. The pistons was flying upwards, ready to be counter-acted by compression and the resulting explosion to slow it down, but that doesn't happen, the force has to go somewhere. Now your wrrist pins, and fasteners have to take ALL the force, as opposed to the explosion providing a "buffer" if you will. Things relax, move back into position, spring back etc. But not all of this happens in unison, and you can get contact. Too much and boom, excess wear or even failure. There is a lot of energy in a rotating engine. Removing the explosions removed a check feature that slows things down if you will.

There are different ways to limit rpm. Some just cut spark, some cut fuel. Some hit harder while others are softer on the engine for this reason. It's less the rpm limiter and more the drastic change between load and unload. I've seen motors blow when shifting at high rpms for the same reason. Stability under consistent/ramping stress, but when the clutch went in to shift the drastic loss of load caused the previous flex to spring back and grenade the motor. Like most in life steady-state isn't the problem. Shock loading is.

Depending on the course design you shouldn't have to shift much if at all. Use the high torque high response engine and leave it in gear. Most autocross course designs shouldn't really get into 3rd anyways. Or shift under braking, no time lost.
All good information...but... I am still struggling to see much of a difference between hitting the rev limiter and say going from full throttle to hard braking for a turn, we don't push in the clutch relieve that pressure.

I will agree to the point that some cars are harsher then others. Our cars its almost not noticeable when it hits the rev limiter, but my friends NB Miata its BANG BANG BANG BANG BANG.

And yes, we shouldnt be going to 3rd, in my Vette, I never once felt the need to go to 3rd (2nd was good to 66mph), but I am trying to understand other peoples experience with the Fiesta on different types of AX layouts.
 


OP
Rexracer
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Thread Starter #16
For me it depends on the course, but I rarely shift into 3rd. I do hover at the rev limiter sometimes. I also have much taller tires @ 215 45 r17
Thats another 1/2" taller of a tire then I am running (205/50R16), so 2.1". Its giving you another 1.2mph at 60mph.

Compared to stock its 4.7% taller, so 2.8mph at 60mph.

Any issue with feeling more high centered with the taller tire?

PS Wife has family in Spokane, if you guys are running an event up there when we come to visit, I will try and convince the family to do the trip in the Fiesta, its a good long distance family road trip car...right? lol
 


kivnul

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#18
Thats another 1/2" taller of a tire then I am running (205/50R16), so 2.1". Its giving you another 1.2mph at 60mph.

Compared to stock its 4.7% taller, so 2.8mph at 60mph.

Any issue with feeling more high centered with the taller tire?

PS Wife has family in Spokane, if you guys are running an event up there when we come to visit, I will try and convince the family to do the trip in the Fiesta, its a good long distance family road trip car...right? lol
We have lots of events, https://asnw.org/ , have a autocross this weekend even. I cannot really tell that the car is taller (and I keep the suspension @ stock height). I do have more roll than I would like though, so someday I may switch to the new Advans that have some nice 215 40's that would work well and let me drop the car some.
 


OP
Rexracer
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Thread Starter #20
We have lots of events, https://asnw.org/ , have a autocross this weekend even. I cannot really tell that the car is taller (and I keep the suspension @ stock height). I do have more roll than I would like though, so someday I may switch to the new Advans that have some nice 215 40's that would work well and let me drop the car some.
Great, will look it up when we are coming next!
 




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