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4 & 5 Speed Transmissions With an Over Drive Option:
Our goal for 2007 is to get Hooley his AA license, so we have to run over 250 mph. On this page I'll try and figure what we need to do to achieve that based on the data we have at this point. Let's try for 255 mph so we have a little cushion.
First we need to run about 7500 rpm with our present gearing and the HP spread sheet tells us that will will probably need about 830 HP at the crank (700-715 rear wheel HP). Using the 830 HP/255 mph figure in the "Force, etc." spread sheet we can see we need probably about 1750 lbs. on the rear wheels if the salt is really good (.6 traction coefficient) 2100 lbs. for good salt (.5 traction coefficient) and 2625 lbs. for crappie salt (.4 traction coefficient).
Let's get the traction problem out of the way first. For really good salt the car's present weight of about 3900 lbs. would probably get the job done on really good salt and might just make it on average salt. I feel we should add about another 300-400 lbs to be safe and probably forget getting the job done if the salt is crappie.
Now for the 830 crank HP needed. Since we were only running about 8 lbs. of boost this year going to 12-14 lbs. of boost should get us another 100 hp over the HP the motor makes at 7400 rpm now. Since our cam is making max HP now at about 7400 rpm and that would get us about 252 mph we might be good there with just a change of pulleys on the blower.
Sounds good, but me being the worrier that I am I'm afraid this isn't a good route to take, meaning just turning the boost up might not be the best plan. There are a couple problems here, not so much the one of turning the boost up, which despite what I just said is what we will have to do. The problem lies in getting the motor and transmission to live and can we reach the new top speed with the gears we now have with the addition of another 300-400 lbs. of weight.
First the motor. We are using a great after-market block with all the good parts, forged crank, good rods, pistons, etc., with one exception, the crank has a short coming. It has the regular sbc (small block chevy) snout vs. a sbc crank with a larger big block snout. Not a huge problem if we were say running at the drags, but with the extended high rpm periods on the salt and driving the blower off the front of the crank they have been known to break at the snout. For this reason I'd like to see us keep the rpm down around the 7000 rpm level and 7000 won't get us 250 to 255 mph. I think with the added boost we can still make the 130 additional HP we need at 7000-7100 rpm.
Another problem area is the transmission. It is a 4 speed close ratio Muncie and we are probably pushing the safe HP through it now without it blowing up. One answer to both of these would be going to a Jerico or G-Force transmission that would take the added HP and also let us customize the gear sets. One interesting feature of both of these transmissions is that their 4 and 5 speed transmissions allow you to setup one gear as an over drive. You are still going to have one gear as 1:1 though.
With the 4 speed transmission 3rd can be setup as an overdrive with ten ratios between 1:1 and .0849 overdrive being available. For our purpose a .958 overdrive would be about perfect as using the "4 speed" spread sheet 7055 is 250 mph and 7196 is 255 mph. A .922 overdrive would give us 250 at 6790, 255 at 6926 and 260 at 7061. I like the .958 better as it is working closer to the right HP and rpm range we want to be in.
One thing to keep in mind is that with the "tall" gears we run that 100 rpm is almost 5 mph at 250 mph. Having a quick change or a transmission such as this where you can really fine tune the gearing and that might make the difference in a record or not. If you try and run a 4 or 5 speed street transmission or something like a gear vendors the jump from the 1:1 gear to overdrive is going to be 18% or more. That is a big jump. Look above and you will see what going from .958 to .922 does to the rpm at these speeds. A mere 3 percent difference between the two gears and just 6% to 7% past the 1:1 and you have made a big difference. Now imagine the impact of going 15% to 20% with an overdrive gear.
With the 4 speed we could run to 7000 in 3rd (now the 1:1 gear) and then only loose about 300 rpm in the shift to the overdrive 4th and keep motoring hopefully up over 250 at a safe rpm for the motor. The down side to the 4 speed is before we had 3 gears before the 1:1 4th. Now we only have 2 gears before the 1:1 3rd. Remembering that we are still in a drag race up to the beginning of the 5th mile that could hurt us. With another 300-400 lbs. in the car we will have more traction in the lower gears also, so we could get in it more in those gears and try to accelerate faster than before, but with just two gears before 3rd we might not get that done. You can put any gear ratio imaginable in these transmission for 1st and 2nd also. So looking at the spread sheet lets make 1st a little taller than the Muncie since we push the car off and it has been spinning the wheels in 1st. We will change the first from a 2.2 to say a 2.0 that will give us 120 mph at 7000 in 1st gear. 2nd was a 1.64 and 3rd was a 1.28. Lets change 2nd to a 1.38 and see what the spread sheet tells us.
We can run 1st out to 7000 and be running 120. We will loose 2192 rpms on the shift to 2nd with the rpms dropping to 4878 rpm (probably in the power band good enough for this speed). Run 2nd out to 7032 rpm when we will be running 173 mph. Shift into 3rd (our 1:1) and drop 1936 rpm's down to 5096 rpm. Run 3rd up to about 6900-7000 at a little over 235 mph and drop about 300 rpm on the shift into 4th and hope we are entering the 5th mile and can accelerate quickly enough to average over 250 in the last mile.
I'm not real happy with the 4 speed overdrive in this situation, but still feel it is stronger that what we have now and gives us much more flexibility in gear choices for all the gears except the 1:1 you are stuck with. If we felt the motor was good to 7500 and would not break the snout on the crank we could run similar gear ratios in it as we have now. One note is the motor without the blower could run all day at over 8000 since it has really good parts in it. If this motor/transmission combination went into a car that was more aero than Hooley's Stude and could run over 260 then the 4 speed would be a better deal as the overdrive would grow "taller" and there would be more of a difference between the 1:1 3rd and the overdrive 4th.
Now what I would really like to see us get is one of the Jerico or G-force 5 speeds, but they are pretty pricey at over $5500 for the tranny and shift mechanism. If you look at these transmissions, either the 4 or 5 speed be aware they come in NASCAR/road race configurations and different drag strip configurations. You do not want one of the "drag" ones that are designed to pop out of gear as soon as you let off the power so you can slam it into the next gear on the drag strip. Since you can get in and out of the gas on the salt if you spin the tires having to manually hold the transmission in gear is not a good idea.
What I would really like is the 5 speed. The advantage of the 5 speed is we could keep 1st, 2nd and 3rd closer together before going into the 1:1 4th and then into the overdrive 5th. This could maximize acceleration by lowering the rpm drop between gears, which could be a crucial part of the equation.
If we can't come up with the money for the transmission we will go back with what we have and turn the boost up and see if everything lives. The ultimate combination in my mind would be a 5 speed with a quick change. If you have the money go for it.
(Note: Today (10 -19-06) I got a really good e-mail on this subject from Tom Burkland that will probably change my mind on using the 5 speed. From what he is saying it looks like we might actually be able to accelerate faster with the 4 speed. I'll give you a teaser from it and after I get it all figured out for our situation and maybe even have a new spread sheet to help in the decision I'll share all of it, but for now here is the main point he was making:
"You expressed some concerns about the ratio spacing required to accelerate these cars (comments aimed specifically at the Studebaker and five speeds versus four speeds). The same traction/thrust/weight required calculations apply across the entire speed range of the car so you can determine a required engine torque to slip the tires for each gear ratio with the given weight applied to the drive tires. This torque figure can be spread across the engine rpm range as determined by the ground speeds, shift points, and ratio splits of the transmission. If the actual available torque curve is at or above these levels across the range to deliver maximum thrust on the ground then you really do not need any more ratios in the transmission. The additional gears result in more rolling friction, which has the same impact on car performance as increased aerodynamic drag.
The Datsun comp coupe was a good example of this approach. It ran 4 times in the low 270’s with direct drive (no transmission) and a three disc slider clutch. Clutch slippage was used to get the car from push truck speed up to about 110 mph where the engine speed was about 2500 rpm at clutch lock up. From this speed and engine rpm on up the engine had enough torque to spin the tires so there was no need for transmission gear reduction. Below this speed the engine was run up to about 3000 rpm and the clutch slipped enough to apply tire slippage torque to accelerate the car at the traction limit. The driver technique required to do this takes a little practice since you drive the first half mile with the clutch pedal and then switch feet to run the throttle to the other end of the track. It probably is not a good approach with your blown small block gas engine as the “lugging” created by this procedure may flush the bottom end right out of it."
[( Update 12/03/06) We are ready to order a G-Force transmission this next week. It will be their newest model GF4A, but will be a refurbished used one with the gears of our choice. We are taking Tom's advice and it will be a 4 speed with pretty close to the ratios I described above. 1st = 1.923 (the tallest 1st gear they have). 2nd will be 1.331. 3rd will be the 1 to 1 and 4th will be a .931 overdrive gear. I worried about going to this tall of a first but checking the spread sheet it isn't that much worst than the 2.20 we are running now. Hooley usually pulls away from the push vehicle at about 3100 rpm which is about 47 mph. The 1.923 first gear will mean 3100 is about 54 mph, so we will only have to push a little harder. That will help the rpm drop into the relatively tall 2nd. Shifting at about 7000 (124 mph) we will just fall to about 4900 rpm going into 2nd. Then the shift from second at 7000 (180 mph) into 3rd will result in the rpm dropping to about 5300. The final shift from 3rd to 4th could also happen at 7000 (239 mph) and the rpm would drop just to about 6700 rpm.
Our goal is to try and move our average in the 5th mile from about 237 to 250+ so Hooley could get his AA lic.. With the above gearing we have to possibilities of doing this. First if the motor is pulling good past 7000 and can pull to a little under 7500 that would be 250. If can go to fourth we can get to 250 at about the 7000 we ran this past year.
Another thing I saw from last years time slips is on the back up record run we were about 7 miles an hour faster at the 2 1/4 than on the qualifying run. At the end of the 5 we were about 6 miles an hour faster than the qualifying run. So it is important for us to accelerate to the 2 1/4 as fast as possible.]
Time will tell if all of this figuring will help, but I'm getting to become a firm believer in data acquisition at the salt to use there and into the future and using these spread sheets to get you into the "ball park" before you get to the salt.