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Post by oldegreezer on Oct 17, 2014 21:36:07 GMT -6
Changing from 9" to QC for next season. ?1= When running 9" I started with J-bar level with yoke on pinion mount. Is that the way to start with QC. ?2= I've heard everything from a .10 to .30 gear ratio change needed when making this swap. ??3= This season I ran between a 83" to 85" right rear tire. Next season I will be running between a 87" to 90" right rear. Anybody have any tips on a way to calculate the difference in ratio I will need to put me in the ballpark. Lets say at a track that I ran a 6:31 at last year I'll need a 6:48 next year because of change to QC and a ?:?? because of tire cirf. difference. I know how to figure it out after I run my heat but I'd like to be in the ballpark when I showup at the track. I hope I asked these ?? correctly and I thank all for replys. Sorry it looks like one big sentence but I don't know how to do it differently. THANKS=#01H
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Post by confused on Oct 17, 2014 22:05:38 GMT -6
The pinion is about 1-1/2" to 2" lower on a QC than a 9". Your starting point will be above the pinion. I've heard the same thing about the gears. I currently run a 6.0 in a 9" Probably going to try something around 6.2-6.3 in a QC. Tire size I'm not sure about. I would think that you may need a little lower than what you were running. Maybe 0.1 or 0.2 lower. Just a WAG tho.
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Post by racer96m on Oct 17, 2014 23:42:33 GMT -6
I found this, maybe it will confuse you more than you already are, lol
The common interpretation of Drive Ratio for a quick change (QC) rearend is the combined ratio of the axle ratio and the QC gear ratio. We prefer to call this the Drive Ratio. We will use the word "final" later to include the tire size used in combination with the gear ratios. To find the Drive Ratio, we multiply the QC ratio by the axle ratio to find the Drive Ratio.
The tire size affects our true Drive Ratio (DR). So, if we include the tire size into our calculations, we will have the whole enchilada. To find this Final Drive Ratio (FDR) number we divide the Drive Ratio into the tire circumference. If we have a 4.11 axle ratio, a 30 tooth over a 22 tooth QC ratio being 1.364, then our DR is 5.6045. If we then divide the DR into the tire circumference of, say, 85 inches, our FDR is 15.1663. This is a number we need to record and remember.
The reason this number is significant is because we might find ourselves having to use a different size tire some weekend down the road. Suppose for whatever reason we arrive at the track late and all that is left in the tire truck are 87 inch or larger tires. Do we need to make a gear change? You bet. Here's why.
If we do the math in reverse and divide our FDR number into 87 (the new tire size) we get a new DR of 5.74. That is what our rear gear should be in order to keep our same FDR at 15.1663. That means we need to install the closest gear to that, a 28 tooth over 20 tooth gearset equals a 5.76 QC gear to maintain our previous performance. If we don't make this change, the car may be flat off the turns and our lap times will suffer.
Always keep a record of your FDR. This is the ratio that includes tire circumference information. You may have found the right gear ratio for your track and then lose the advantage when you are forced to run different sized tires. This is especially true when a track changes tire brands where we have seen the sizes of each brand being different by two inches or more.
To calculate the Drive Ratio for a quick change, divide the number of teeth in the top gear by the teeth in the bottom gear for the QC ratio, multiply that by the axle ratio (ring and pinion ratio of the QC rearend) and you have the Drive Ratio. For OEM-type rearends, you divide the number of teeth in the ring gear by the number of teeth in the pinion gear for the Drive Ratio.
Calculating the Drive Ratio
Axle Ratio QC Ratio Drive Ratio
4.11 x (30 / 22 ) = 5.6045
To calculate the Final Drive Ratio that includes the tire circumference, we divide the tire size in inches and decimals by the Drive Ratio to get the Final Drive Ratio. This number can be used to recalculate the correct gear that would go with a new sized tire.
Calculating the Final Drive Ratio:
Tire Circumference Drive Ratio Final Drive Ratio
85in / 5.6045 = 15.1664
In this example, we have a new tire size of 87 inches in circumference, up from the 85 inch tire we had in the previous examples. To find the correct QC gear ratio to maintain our Final Drive Ratio, we divide the new tire size by our Final Drive Ratio to arrive at a 5.74 QC gear. If we install this gear, or one very close to it, we will maintain our previous performance.
Calculating Correct QC Gear for a New Tire Size:
New Tire Circumference Final Drive Ratio New Gear Ratio
87in / 15.1663 = 5.74
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Post by confused on Oct 18, 2014 5:52:21 GMT -6
Thanks Dave. That helps me a lot. That is how I tend to think and why I get told "quit thinking" LOL
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Post by oldegreezer on Oct 18, 2014 22:15:58 GMT -6
Thank you both for taking the time to answer my ?s. Understandable and to the point. thanks again= #01H
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