Well its rebuild / change season during the winter so here's what I'm wondering. When running a race car that leaves the starting line under its own power no push that uses a manually shifted three speed auto trans the rear gear being a 270 ratio I'm wondering what transmission gear ratios would be needed if the transmission was a manual 5 speed with a clutch. The idea is to still drive off the line no push using the 5 speed and clutch. My thinking is first and second gear ratios in the transmission need to be low enough to leave the line with the 270 ratio. Then the remaining three upshifts a closer ratio with fifth gear 1 to 1.
Anyone have ideas of ratio percentage's for such a change?
Ronniroadster
Ronnie,
Since you drove the car off the line, I am guessing you were using a torque converter or? Was it a stock stall converter, or did it have a higher stall converter? Are you going to a clutch that will allow some slippage or? What is your driving technique for the stick shift, high RPM and drop the clutch and spin the tires, or try to slip the clutch to get moving? There are a lot of variables involved.IMO
Tom G.
Quote from: desotoman on January 24, 2021, 08:18:48 AM
Ronnie,
Since you drove the car off the line, I am guessing you were using a torque converter or? Was it a stock stall converter, or did it have a higher stall converter? Are you going to a clutch that will allow some slippage or? What is your driving technique for the stick shift, high RPM and drop the clutch and spin the tires, or try to slip the clutch to get moving? There are a lot of variables involved.IMO
Tom G.
Hi Tom here's some more info. Yes we use a torque converter that's tight not a high stall to drive off the line. The change would require a clutch leaving the line on the salt so it will require some slippage by the driver on the clutch pedal.
PM sent.
Tom G.
As observed by manufacturers there are many that used rear gears in the 2.70 range. Most had 1st gears in the 3.00 to 3.25 range. I take it that your going to use a 5 speed with a 1 to 1 5th gear like a Jerico. They offer 1st gears in that range. If so it would be good to know more about the engine and it?s torque. I have a trans that only needs a clutch for starting out and not shifting and it works fine. Since you had a fairly large split with automatic I would bring the select ratio for second a little closer to your tight selection of 3-4-5 if you need to not loose rpm because of torque.
I used the site GearCalc to figure out what to buy by trying different ratios available and checking RPM drop between shifts. Took a while but worth the time. Good luck.
Quote from: jimmy six on January 26, 2021, 09:55:38 AM
As observed by manufacturers there are many that used rear gears in the 2.70 range. Most had 1st gears in the 3.00 to 3.25 range. I take it that your going to use a 5 speed with a 1 to 1 5th gear like a Jerico. They offer 1st gears in that range. If so it would be good to know more about the engine and it?s torque. I have a trans that only needs a clutch for starting out and not shifting and it works fine. Since you had a fairly large split with automatic I would bring the select ratio for second a little closer to your tight selection of 3-4-5 if you need to not loose rpm because of torque.
I used the site GearCalc to figure out what to buy by trying different ratios available and checking RPM drop between shifts. Took a while but worth the time. Good luck.
Thanks for the guidance comparing what manufactures used with a similar rear ratio is very helpful. I'm going to look at the GearCalc site. This change is keeping me awake at night so much to think about.
Ronnieroadster
I agree with Jimmy that you really only need first gear to be a relatively low gear, this gets you moving the last 4 gears need to be close ratio. You need to make the rpm drop between the last for gear get smaller and smaller to provide the best acceleration and pull against the aero forces that are increasing at the square of the speed increase (aero resistance measured in lbs is a force and increases as a square function, the power to over come this resistance , horses power, increases as a cube function) and the drive train is only seeing the force. Having a good dyno plot of your engines horsepower curve is a great place to start to select the right ratios. Top gear should always put the engine at its maximum horse power at your desired speed. If you have an engine with a flat torque curve then you can stand a bigger drop between 4th and 5th. The narrow the power ban the closer the ratios should be and the more tranny gears ratios , 6,7,10 speeds?, the better.
I have always found that a first cut on ratio selection is to make the rpm drop between each of the close gear around 1/2 of the drop before it. i.e. if you have a 1000 rpm drop between 2nd and 3rd then you should have a 500 rpm drop between 3rd and 4th and then a 250 rpm drop between 4th and 5th. But you really need to look at your power curve and if you have any aero info on your car that can assist you in finding the desired drop between the top gears and do the others accordingly.
Have fun! Also Jimmy said that top gear needs to be 1:1 he is right again if your tranny's top gear does not require the counter shaft to carry any power. i.e.The main shaft is locked and no power goes through the counter shaft. Most efficient arrangement.
Rex
I would like to echo what Rex has said. My engine has a relatively narrow HP range (about 900 rpm) that falls off steeply either side. I have looked a lot for a multi speed trans that has or can be configured with very narrow spread particularly in upper gears. Mostly non existent if you want more than 5 speeds as they tend to go to overdrives and too wide splits. Some sequential transmissions get close but the price goes way way up. Liberty and Jerico do decently and if you have the grunt may work. My small engine has trouble. I have a GForce GF5R that can be configured with many many ratios and has been working for me. Can be had clutch or clutchless. The website has a ratio chart. However, there is a wider choice of gears available and can be seen in the ratio chart for the GF4A road race section which is what they used to build mine. The trans is not cheap at 6500 or so but is better than the 12-18K I have seen for multispeed sequentials (like used in the Hoonigan "Hoonicorn" Mustang Ken Block drives. I looked at every set of trans ratios I could find including the new 10 speed autos and such (likely impossible to adapt as are all computer controlled) and ended with the G Force. Good luck.
I appreciate the guidance everyone has given. Its been over 10 years now running the same basic configuration gaining a bit more speed each time. Every winter we begin the rebuild/changes to be ready for August. The transmission change is the next step to this never ending experience which we truly enjoy. Our combination is rear engine so that adds to the decision on which way to go the new 5 speed will be air shifted now to find room for the C02 bottle. Still lots more work ahead.
Ronnieroadster
Ronnie in our 5 speed we chose 1.91 1.52 1.22 1.11 1.0 but we have a GV on the back of ours so that we have a clutch on either side of the trans
We push off to 15-20 mph and engage the trans with out touching the clutch we ran a auto with out a torque converter for years
Quote from: SPARKY on January 27, 2021, 12:15:27 PM
Ronnie in our 5 speed we chose 1.91 1.52 1.22 1.11 1.0 but we have a GV on the back of ours so that we have a clutch on either side of the trans
We push off to 15-20 mph and engage the trans with out touching the clutch we ran a auto with out a torque converter for years
Sparky were pretty close on what were looking at from second on up 160, 126, 110 ,1.0 still sorting out first gear. Not sure about a push but its looking more like I might being doing that also.
We had success (up to 210 MPH) with a 4-speed Super T10. I selected the closest 3rd to 4th and always considered the first three only used to get to 4th. We could spin the tires (some) in the first three, especially at El Mirage. Ron, I guess you haven't been pushing off and run faster that we ever did. Our rule was that the driver "pulled away as soon as he thought he could outrun the push truck."
Ronnie, I feel your pain is choosing ratios. I spent months looking at configurations before ordering my GF4A. I ended up using the GF4A because I could do a shorty conversion to fit in my space and the ratio selection available. We always use the push truck on the salt and have done some not very pretty non push truck launches at Loring and Ohio.
If you are interested in the GF4A somewhere I have an excel file with every ratio available and you can plug in your ratios and shift points to create a chart of RPM drops.
Andy
Quote from: Stan Back on January 28, 2021, 02:23:30 PM
We had success (up to 210 MPH) with a 4-speed Super T10. I selected the closest 3rd to 4th and always considered the first three only used to get to 4th. We could spin the tires (some) in the first three, especially at El Mirage. Ron, I guess you haven't been pushing off and run faster that we ever did. Our rule was that the driver "pulled away as soon as he thought he could outrun the push truck."
Jim your roadster is moving lots more air than our roadster configuration that's why with the small cube vintage blocks we run our speed is a bit higher. Aero we have found helps a ton also something the street roadster rules do not allow.
Ron
Quote from: awelker on January 29, 2021, 08:12:50 AM
Ronnie, I feel your pain is choosing ratios. I spent months looking at configurations before ordering my GF4A. I ended up using the GF4A because I could do a shorty conversion to fit in my space and the ratio selection available. We always use the push truck on the salt and have done some not very pretty non push truck launches at Loring and Ohio.
If you are interested in the GF4A somewhere I have an excel file with every ratio available and you can plug in your ratios and shift points to create a chart of RPM drops.
Andy
Andy Thanks for the input. I recall talking with you years ago at Ohio about that transmission I was interested in how you devised the linkage system which was well thought out. My decision has been made about the trans its going to be a Liberty 5 speed. Having reliable rear wheel HP and torque numbers for our combination has given us a good direction to go. First and fifth gear will made extra strong to deal with the first to second hit. We hope to test the first gear ratio in July at Loring on the test runway this will help us decide if can we drive off or should we push. Then the final test/decision will be after a few 1-1/2 mile runs.
Ron
Hey Ron, I know you run a C4 & the late C4 was available with a lockup converter. Going to an O/D trans then to the o/d spur gears in the QC that's spinning a 3.something set of gears is going to suck up hp you can't afford to lose in the flattie!
Sid.
. . . but won't Ron's proposed 5-speed manual have its 5th "gear" in straight lock-up?
Quote from: Stan Back on January 29, 2021, 01:49:35 PM
. . . but won't Ron's proposed 5-speed manual have its 5th "gear" in straight lock-up?
Yeah we talked on the phone after I posted that, his 5th is 1 to 1 not an O/D.
Sid.