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Thread: Rear gear selection
05-21-2020,†02:38 AM #1
Rear gear selection
While sitting here idle due to the virus watching the racing season go down the toilet, I’ve been looking a lot through dyno records, et slips, and 60’ times from the past couple seasons. I’m going to try to think through some issues on my e.t.
Currently run a full spool, bigger shafts, baseline suspension outlaw kit, etc, but still stock 3.27 gear. Also running a stock gear 4R trans with 28 10.5 m/t slick out back. My best 60’ to date (before aluminum block rebuild this year and some other weight reduction) has been 1.40, which I believe is pretty sad. I run mostly 1/8th mile which I shift into 3rd right at the traps at 6.668 at right around 100mph. I have the notion to swap in a 3.73 and see if it would be better. By raising the rear gear, I would not have the tendency to over power the tire at the shift point which, by looking at the datalog info, I see a spike before the shift which would indicate such. As I shift, it’s two-fold, rpm drops several hundred rpm and wheel speed decreases which would indicate it gaining traction again during the shift.
By looking at dyno curves, I’m hitting 615ftlbs at 4k and flatlines to redline, and 650hp at 5500rpm, also launching around 35-3800rpm, shifting around 6500, by the datalogging I can see the 1-2 shift drops quite a bit of rpm and I believe that’s where I’m losing the most power and time. I am able to build 10psi on the turbos off the transbrake at launch and it seems to hook, but as my suspension guru says, it doesn’t seem to launch very hard.
I have a lot of people telling me I should be going faster than I am and I’m trying to encompass the big picture. By raising the rear gear, I could possibly change my 60’ but would it change the overall picture? My thinking is the wheel speed would be slower throughout the 1/8th without the loss of traction, but because of such the rpm drop between shifts would not change.
I guess what I’m trying to see here is if a gear change is the right route, or if the rpm drop on shifts that is essentially dropping me out of the power and is the underlying issue. Obviously the only fix seems to me to be raise the shift point higher, therefore the 1-2 shift would still be in the peak power rpm band, but like I said, I’m suspecting loss of traction as a factor at the shift as a factor to the bogging of second gear.
Let me know all your opinions, I’m welcome to suggestions!
05-21-2020,†03:00 AM #2
Before track change (crappy prep) a tire change (Hoosier QTP to M/T slicks,) and baseline suspension, I was at a 1.98 60’ rolled through the traps at 98.7.
After all those changes early in the year, and some great tuning help from Kevin, In one weekend I went from a 1.98 to immediate 1.65 then finally whittled down to 1.40 A few weekends later at the end of the year. I believe the engine was at the end of its rope due to compression loss and ring gap erosion therefore warranting its rebuild this year. If it were making the power it did recorded during the time of the dyno sessions, it’s safe to say I’d break the 1.3 barrier.
I believe there is more in it I haven’t figured out yet and I should be deep in the 1.3’s maybe even 1.2 range, and should also have 110-115 traps. At least by some calculators, I’m WAY slower than I should be.
But who knows, it could be all pipe dreams and I need to turn up the boost. But I believe I’ve maxed out the suspension and it’s as good as it’s gonna get, so I’m looking elsewhere.
05-21-2020,†04:28 AM #3
If your wheel speed decreases after you shift, your car would be nosing over and slowing down, indicating a different problem.
A simple example;
Assume it takes 100 revolutions of the tire to cover a finite distance, your race track. One revolution of the tire is 72 inches or six feet. That means your race track is 600 ft long, the tis 100 revolutions x 6 feet. Lets examine two different passes in the same car. Pass #1 is done in 6 seconds and pass #2 is done in 7 seconds. Both passes are 600 feet long. Pass #1 the wheel turns 100 times in.6 seconds (0,100 minutes) meaning it was spinning 1000 rpm, average. The wheel in pass # 2 covered the same distance but it took 7 seconds (0.1167 minutes) meaning it was spinning 857 rpm average.
Higher wheel speed => higher performance.
I would give some thought to a 5000 rpm stall converter with a lockup capability for quarter mile tracks.
05-21-2020,†11:21 AM #4
Thanks Ed, I’m running circle d’s multi disk 3800 stall. I don’t want the headache of tubbing for a taller tire, I've finally found one that seems to hook. That being said we were on the same page, raise redline/shift points above 6500.
Only reason I shift to third before the traps is because I’m on the rev limiter in 2nd right before I cross.
By raising shift rpm I’m likely to be in second through the traps at redline.
If I do get the opportunity for 1/4 mile, I still have another gear and lockup to.
05-21-2020,†01:43 PM #5
Fact of the matter is, when shifting at 6500rpm, I drop to 3500rpm in second gear. Because I cannot change that EASILY, even by raising to 7k, would drop to 3800, 7500 would drop to a good goal at 4100rpm.
Because the car was set to redline at 6500rpm, what are any other limiting factors on raising shift rpm? Valve float, running out of fuel, timing are some things that come to mind initially.
Also, as I said, launching at 4k....should I start creeping up until I find the fine line between over powering the tire and the highest launch rpm possible?
05-21-2020,†02:15 PM #6
To size your stuff go to the TToC and download a copy of the Fuel System Calculator and the instructions PDF document. The calculator will provide all the obscure, hard to get answers.
05-21-2020,†02:55 PM #7
Ed, I’ll check out the fuel system on the ttoc, I doubt I’ll have other issues.
Here’s another thought, by going with a 4.56 gear and launching in second, even with a 6500 redline I'm in the same ballpark as a 7500 redline.
I believe the only reasons the redline was set at 6500 was because it wasn’t making any more power, and it’s still stock computer, other than that I have no reason to believe it couldn’t turn 7500.
05-21-2020,†03:40 PM #8
I guess a second gear launch wouldn’t work, I just thought about that, transbrake is on first, duh.
05-21-2020,†05:04 PM #9
Last edited by eschaider; 05-21-2020 at 05:08 PM. Reason: Spelling and Grammar
05-21-2020,†06:30 PM #10
I contacted the tuner to see what he could dig up, I doubted it was fuel, and I know the stock ecu “can handle” over 7200, with the right components. You are correct, it may have been snuffing the spark and that’s why it leveled off.
Regardless you are correct, about the only good way is to raise redline. Every other way is a bandaid.
05-21-2020,†06:33 PM #11
05-22-2020,†03:51 AM #12
05-22-2020,†09:12 AM #13
What's the cam spec's? 7500 RPM may be stretching it for the cam in use.
My cam is spec'd to work with a 6500 max red line and by 6500 mine is comfortably done so I doubt it would be beneficial to go beyond 7000. Dyno pulls were stopped at 6200 or until I made the 900+ HP goal but a few lower HP pulls went to 6500 to check fueling.
Watching Mat's vids' the car doesn't explode off the line to get a hard launching 60'. It needs a lot more power in the form of a lot of boost or RPM or a combination of both. Now that he has good tires, a higher stall (4500 or more) would be fantastic because then some boost could be built up easily enough to try and over power the tires on the hit instead of 3-8ft out. Once that kinda hit is achieved then we can get the rest of the chassis dialed in to handle it. The first 6"-3ft is where it needs help.
Last edited by KEVINS; 05-22-2020 at 09:23 AM.
05-22-2020,†10:53 AM #14
It is possible to install the cams to 'lay down' somewhere north of 7000 rpm, but you have to intentionally take that step, Kevin. My bet is it is a tuning and potentially an ECU issue. Most people would not and/or do not know how to install cams to do that. Usually when you do that, it produces a huge increase in low speed power because of how you have shifted the power curve downward in the engine's operating range. In Matt's case he is not dealing with huge low speed power issues, quite the contrary. The cams that JM uses are 12mm lift (0.470-iish) relatively smooth profiles installed to favor upper engine speed to allow his 10K+ rpm engine speeds. Low speed power is hard to evaluate on that car because it launches several thousand rpm higher than most modmotors shift at. I would be surprised to find that style of cam or cam phasing in Matt's engine.
The higher stall converter would be a significant benefit on two different fronts, one certainly is launch and the other would be cushioning the rpm drop between gears to provide a higher sustained power level to the tire. On any gear change that is below the converter stall threshold the engine speed would rise to the converter's stall speed. Higher stall means higher sustained engine operating speed (and power) throughout the rpm range. If the car is to be a dual purpose car high stall converters can make daily driving less enjoyable but race track operation will be quite nice. A lock up converter can go a long way to mitigating some of the bad manners a high stall converter has in daily driving.
The real need (like you suggest) is for more torque boost at launch and that is going to require mods to the tune and more sophistication in the ECU to allow access to and modification of the tune to produce and manage that. The stock ECU can get you well down the road but when the low speed power does start to rise the stock ECU lacks the sophistication to manage it, so you can actually get the car to launch hard w/o blowing the tires off ó i.e. traction control among other things.
All that said Matt should still be able to see low 1.3 second, possibly better, 60 ft times. Like you suggested it is going to take more boost to get the steam in the boiler up for launch.
Last edited by eschaider; 05-22-2020 at 10:57 AM.
05-22-2020,†01:36 PM #15
What a blessing having two of the most respected minds on my thread!
I guess Kevin can agree, the thread should be renamed to “why my 60’ sucks!”
Let me read everything y’all posted and gather my thoughts. Also, my tuner agreed to raise redline to 7500, he said it shouldn’t hurt a thing.