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#1
03-19-2019, 01:08 PM
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Shift points & max rpm?
Realizing there are a multitude of contributing factors here, I am curious how people generally set their shift lights and max rpm for the track. (Foot brake, 10.5 sec bracket car)
In my case, max power at the dyno was right at 5900 or 6k. With that in mind, what is a "best practice " for setting the rev limiter in a race environment? Forged crank, rods, and ross pistons, should be able to handle a few r's. Most of my limited drag racing experience was used with a mechanical tach to determine shift points with a ratchet shifter. I installed a shift light last year, but only got out to the track one time with it. Although I like to think i have reasonably quick reflexes, i was definitely struggling when trying to set the shift light RPM while accounting for the time it took that signal to reach the transmission via my brain and arm! What is a good starting point for me to set as a base line? Thank you for the input! Sent from my SM-G965U using Tapatalk
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68 Firebird-- Street/Strip - 400/461 Eagle Forged Bottom End & Ross Flat top pistons. KRE 325 CFM D port, Ultradyne 263/271 @.050, .4267 lift. Crower Solid roller lifters and 1.65 stainless rockers. Quickfuel 1000 on Torker2 intake and 2" open spacer. Hedman 1.75" headers. TH400 w/brake. Ford 9" w/3.80 gears & 28x9 Hoosier pro bracket drag radial. Best ET: 1.35 60ft, 6.29 @ 107.20 mph, 9.99 @132.33 mph. 3,300 race weight |
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#2
03-19-2019, 02:50 PM
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For best ET, optimum shift points are where the torque (or hp) curve of one gear crosses over that of the next higher gear. Since you already have engine dyno data, this is easy to do. Just plot driveshaft hp (or torque) for all gears on one plot (to do this for hp, just plot engine hp vs driveshaft rpm - you will need to divide erpm by each gear ratio to get ds rpm). Where they cross is the best shift point. If they don't cross, optimum shift point will be the engine structural rpm limit
The pic is torque, and for a 6 speed but you get the idea 
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#3
03-19-2019, 04:32 PM
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Set it about 500rpm lower than you think it should be set, and make a couple of passes. Raise it 100 or 200 rpm, and repeat. If you go slower, then reset at an rpm lower than initial.
Too many variables, just test and find what works best for you. Always safer to start off low and work your way up.
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'73 T/A (clone). Low budget stock headed 8.3:1 455, 222/242 116lsa .443/.435 cam. FAST Sportsman EFI, 315rwhp/385rwtq on 87 octane. 13.12 @103.2, 1.91 60'. '67 Firebird [sold], ; 11.27 @ 119.61, 7.167 @ 96.07, with UD 280/280 (108LSA/ 109 ICL)solid cam. [1.537, 7.233 @93.61, 11.46 @ 115.4 w/ old UD 288/296 108 hydraulic cam] Feb '05 HPP, home-ported "16" D-ports, dished pistons (pump gas only), 3.42 gears, 275/60 DR's, 750DP, T2, full exhaust My webpage http://lnlpd.com/home |
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#4
03-19-2019, 07:12 PM
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These are a few graphs a made up some time ago. One shows the engine's HP and Torques curves and the other the rear wheel force along with rpm and mph.
Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
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#5
03-19-2019, 08:25 PM
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Figure out your max RPM, hit your shift points, look at you're worst RPM spread on shift points, and maximize the area under the power curve based on the prior. It's as good as you can get.
Last edited by Aaron Quinton; 03-19-2019 at 09:19 PM. |
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#6
03-19-2019, 08:44 PM
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Set the rev limiter to 6500 and shift light to 6000 rpm. Try to video the tach and increase the shift light rpm if necessary based on ET and hand reaction of shifting. I shift at 6500 rpm and have my rev limiter at 7100 rpm.
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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule. |
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#7
03-19-2019, 09:19 PM
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Do you have a power curve so you know how much power you're putting out vs. RPM?
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#8
03-20-2019, 06:06 AM
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I do not. I varied shift rpm for best ET and made sure my rev limiter was far enough away from my shift rpm to not interfere.
__________________
1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule. |
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#9
03-20-2019, 03:31 PM
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Optimal shift points will be different for each gear, unless the rpm drops are all identical
Sent from my SM-T817V using Tapatalk |
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#10
03-22-2019, 12:04 AM
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If you send me your engine set up, transmission and rear end I'll have a crack at where you shift the car. If you don't have a dyno curve I'll asking for a fair amount of detail to make sure the power curve is right. I'm happy to help. The key is maximizing area under the operating power curve.
You can hit me at asquinton@yahoo.com for a quicker response. Thanks, Aaron Quote:
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#11
03-22-2019, 06:15 AM
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Engine is a 498 ci (4.320" x 4.25") with 335 cfm heads and 266°/270° at 0.050" and 0.620"/0.637" cam and Victor/Dominator. Trans is stock geared TH400, 29"" tire and 3.90 gear. Car has a 3550# race weight and goes 10.0 at 134 mph.
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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule. |
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#12
03-22-2019, 11:41 PM
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Tony, I have a model started to get the ball rolling. Fill out what you can and we'll get you dialed in.
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#13
03-24-2019, 01:12 AM
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Check your email sir!
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#14
03-25-2019, 06:15 AM
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Im getting the info ready and will send it to you.
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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule. |
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#15
03-25-2019, 10:53 PM
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I'd be happy to have a second look. Thanks Tony.
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#16
03-26-2019, 10:43 AM
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I've used 8% above peak power as my starting point and haven't had to change much. Getting actual shift at that point not only involves your reaction time but reaction of linkage valve body and convertor(or clutch and/or tire slip). Due to all the reaction stack up you may be setting your shift light close to your power peak rpm !
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#17
03-28-2019, 08:12 PM
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This Is such a broad area!! It can depend on so many things, gears, How far after peak did the engine hang on? How is shift recovery? How quick does it drop after peak? The optimum shift will be the rpm range that runs its fastest, like say 6000, and drops to 5000 on shift maybe better then 6200 and dropping to 5200, lets see your dyno sheet?
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Darby 74 Grandville 2Dr 455 c.i 4550# 2011 1.60 60 ft,7.33@94.55-11.502@117.74  2017, 74 firebird -3600 lbs (all bests) 1.33 60 ft, 6.314@108.39 9.950@134.32 M/T 275/60 ET SS Drag Radial 2023,(Pontiac 505) 1.27 60 ft, 5.97@112.86, 9.48@139.31.... 275/60 Radial Pro's |
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#18
03-30-2019, 11:23 PM
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I haven't forgot about you Tony, been working day job too hard!
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