Landracing Forum
Tech Information => Technical Discussion => Topic started by: Ron Gibson on March 23, 2017, 12:40:36 AM
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Looking for some input as to what ratios to run in a 4 sd Jerico in a 4500 lb A/STR. At the present I have two available. The ratios I have now are as listed. Can change with main drive sets.
#1 2.829 #2 2.681
1.776 1.841
1.286 1.326
Ron
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A lot is going to depend of course on tire size, rearend ratio, and what rpm range the motor is going to operate in.
Download the "Four Speed Car --RPM in gears for 4 Speed Car Transmission" spreadsheet here...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#Four%20Speed%20Car
and insert the different ratios and ...
(http://purplesagetradingpost.com/sumner/bvillecar/Spreadsheet%20Car%20Image-1.jpg)
... you can quickly see the speed/rpm relationships and the rpm drop on each shift. If it won't let you input data, save it under a different name and reopen that one. Also if you can change the input/drive cluster gears also, download the second one as it will let you input those.
We have found that with a 800 HP+ car that even with a 1.90 first and 2.47 rear it is very hard to run over 50% throttle in first with a 6000 lb. car without spinning the tires. With that in mind you might want to get a high first and cut the gaps between there and the 1 to 1 high.
Some of the other spreadsheets here...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html
... might also help,
Sumner
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Thanks Sum, I've had your spreadsheet downloaded for awhile. I guess I'm looking for a little practical experience.
YEP, ACCORDING TO THAT THAR SHEET, THAT SUCCUR WILL RUN 357 MPH" FAST HAIN'T IT?
Kidding aside, guessing, about 900 hp 2.75 gear 32" tires. Comp Cams says cam is good 5800 to 7800 RPM.
Ron
Yes, I know I should have dyno sheet on it.
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Spreadsheet, smeadsheet.
You can control the traction in 1st and 2nd (and even 3rd) with the throttle. You can control the RPM loss going into 4th with getting 3rd as close to 4th as you can. And if you're running thru the 5, sit there for over 4 miles wishing you didn't drop so much when you went into 4th.
Stan
(5x 5-mile record holder.)
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I purchased a used Jerico from a local drag racer and put a Gearvendors unit behind it. I don't know anything about gear ratios, I just stuck it in the old Stude and went 248, and drove it off the line.
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Yes, I know I should have dyno sheet on it.
If the motor is in the car take it to a chassis dyno, you really need to know this info. Unless you just want to make some passes and not try for the record then it is less critical.
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Jerico's are like Tex's there are literally hundreds of combinations. When I had mine built I choose what I wanted even the main set to keep parasitic load at a minimum. I can run as close as 1.06 third if I remove it as an overdrive which it is now. If I did it the gears would be 1.61, 1.23, 1.06, 1; but I'll keep it as an overdrive if needed.
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(http://1fatgmc.com/car/misc-pics-1/ron-1.jpg)
If you are looking to run B'ville I see on SCTA's site that the A/STR record in '15 was 233 (not sure if that is still the case). If you are running on that record and have the HP I think you need smaller tires or a lower rearend ratio. With the data you posted 235 in 3rd would be 8734, over your rpm range, and 4th would be 6792, a fair ways below your max HP.
I picked the set with the lower first gear since the 3-2 and 2-1 ratios were closer and I think first in either one of those two sets of gears is going to be pretty useless as low as they are.
With a 28 inch tire 235 would be 7762. If you made your 3-4 shift at 7858 at 185 your rpm drop would be 1748 to 6111. If you kept the 32 inch tires and went with a rear gear around 3.15 you would have about the same numbers,
Sumner
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Ron, I agree with Stan, go with the closest 3rd gear and let her pull in 4th as long as you need. The real challenge for you will be the salt conditions, if the salt is "good" that is it has some traction in , say a coefficient of friction (traction) of .5 you can probably apply up to around 625 hp to the salt at the class record speed of 233 mph (this is assuming that the weight on your drive tires is 1000 lbs) but if the salt is wet and slick and the coefficient is say around .3 then you need to start bolting weight on the car to be able to hook up with the salt and apply that 900 hp. So don't forget your lead plates! The 625 hp number has nothing to do with aero drag it is simply a number based on the salt coefficient of friction and the weight on the drive wheels and the desired speed.
Rex
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Given Rex’s scenario above, the 625 hp number would be for 2000 lb on the drive wheels, or 1000 lb of thrust at a 0.5 friction coefficient. Hp = (lb)*(mph)/375.
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Here we go again. We're talking Street Roadster. 625 or so won't get you to 230. Maybe for a Modified Roadster, not a brick with fenders as most Street Roadster are (were).
(33s and 34's don't count.)
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I'm trying to find the spreadsheet or slide rule that factors in the aero.
And then someone who knows the aero value of differing body shapes and styles and classes.
If I wanted to go 235, I'd gear for 235 at peak HP. If I didn't go 235, I'd add HP to get there. And maybe make a few aero tweaks in a class that severely limits them.
We did this successfully for a number of years -- 199, 201, 206 and 210 with LOTS of class competition. Then they went 229. More HP. More aero. Better starting point.
Start over? Not me.
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Here we go again. We're talking Street Roadster. 625 or so won't get you to 230. Maybe for a Modified Roadster, not a brick with fenders as most Street Roadster are (were).
(33s and 34's don't count.)
Ban those pesky 33/34 street roadsters :-D
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I'm trying to find the spreadsheet or slide rule that factors in the aero....
I have one here ( Horse Power Needed For A Lakester or a Car to Run a Certain MPH )...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#HP needed for A Lakester (http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#HP needed for A Lakester)
.... that can help, but you need some of the data to make it work and the results will only be as good as the data.
(http://1fatgmc.com/car/misc-pics-1/ron-2.jpg)
For example, and I'm not saying to use these but it is a place to start. I put in 5000 lbs for the car weight to get the required weight on the drive wheels to factor in a .4 to .6 friction coefficient for the salt (see bottom of picture). I put in a frontal area of 21 square feet and a Cd of .45 and a speed of 235.
The answer in 'this case' is 873 RWHP and you can see the weight needed on the drive wheels for different salt conditions. If anyone wants to share what HP it took them to run a certain speed with a street roadster we could extrapolate to some degree what HP it would take to run 235 with this spreadsheet (Horse Power Needed)...
http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#Horse%20Power%20Needed
(http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html#Horse%20Power%20Needed)
Sumner
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Sum
I'm amost totally stupid when it comes to computers, Sometimes I can find the keyboard :-D
How do I create another file for your spreadsheet to compare ratios without changing all info on existing file? Or do I just print and change info?
For all, I have 3 more set of drive gears. If I go with 27/22 M/D in the 2.68 low it comes up with 1st 2.382 2nd 1.636 3rd 1.178. To me third seems pretty close, but thats why I'm asking those that have been there.
Ron
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Ron;
Sum has made provisions for three different sets of conditions in his spreadsheet. Just scroll down to #2 & #3.
Regards, Neil Tucson, AZ
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Sum
I'm amost totally stupid when it comes to computers, Sometimes I can find the keyboard :-D
How do I create another file for your spreadsheet to compare ratios without changing all info on existing file? Or do I just print and change info?
For all, I have 3 more set of drive gears. If I go with 27/22 M/D in the 2.68 low it comes up with 1st 2.382 2nd 1.636 3rd 1.178. To me third seems pretty close, but thats why I'm asking those that have been there.
Ron
Ron what I would do would be to input the basic data (rear, tires, gear sets) into one spreadsheet and then save it under 3 or 4 different names. Then open all three or four and change the drive gears in the transmission in each one.
I personally like the ratios you posted way better than with the other drive gears. I think they get the splits down where they would be a lot better and the 17% going into 4th would be good. We ran with a .93 (7%) overdrive the year Hooley got the second record with the car and liked that smaller drop to run the last part of the 5 miles with, so I don't think the 17% is too small at all,
Sumner
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I say pay attention to Jimmy Six--he gets it ----in my opinion--
-I am trying to run 100 MPH more so my splits have to be wider and I have 2 more gears to try to help but my splits are very similar to his that last split needs to be fairly close mine are.
22 21 17 11 10
His with a 4 speed are: 25 14 6