Optimum shift rpm

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ziggy34

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Hi guys I am after some advice on what is the best rpm for me to be shifting on the 1/4 mile. I have a 1972 Australian Valiant Charger with a 10.2 to 1 cr mopar 528 cam 750 dp carb with eddy heads and rpm air gap manifold, 727 auto 2500 stall with 26 inch tyres and a 3.45 ratio diff. I currently shift at 6500 rpm and the limiter is set at 7000 rpm which it seems to easily rev to.
My question is should I be shifting sooner or later. The cam specs say 2800 to 6900 rpm range and the manifold says 6500. P.s I don’t have access to a dyno and only get to race a few times a year so I can’t just go to the track for testing.
 
Try a few runs a 6600 if it slows down go the other way a 100 rpm at time until you find the best results. Racing is all about trial and error. Thats why top motorsports are so expensive lots of R&D.
 
The theoretical shift points are where the curves for the various gears cross on a dyno graph. You don't HAVE to have a dyno, you can also get this info from onboard data equipment, there are a few, one is the old G-tech, and accelerometer info works as well. If you have a current dyno curve, you can rework the curves based on gearing and plot them that way

Here is an example: NOTICE that shift points are DIFFERENT depending on the gear you are changing to/ from

You can plot this from acceleration, torque, hp at the very least

12bolt-ShiftPoints.jpg
 
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DavidLee said:
can you give us your times that show on the slip?
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The best time I have done was on a street quarter mile sprint. No burn out allowed and ran a 13.00 with lots of wheel spin. No 60 foot times or mph recorded just the e.t. When I ran at a proper drag strip I had fuel issues so the time slip doesn’t mean a lot but The best e.t was 13.6 with 4.10 gears.
 
ziggy34 said:
The best time I have done was on a street quarter mile sprint. No burn out allowed and ran a 13.00 with lots of wheel spin. No 60 foot times or mph recorded just the e.t. When I ran at a proper drag strip I had fuel issues so the time slip doesn’t mean a lot but The best e.t was 13.6 with 4.10 gears.
Click to expand...
to really get an honest reply, it would be best for you to get your suspension issues fixed.
 
We did what 67Dart said and used dyno results with calculator.

But you can still get within decent range by just running it safely on abandoned street from a roll (so you have traction) and feeling where it slightly lugs/ loads the motor in a shift ( and you essentially stop accelerating momentarily) and where it more smoothly continues to accelerate

Of course, your “*** calculator” won’t be as precise as proper science calculation. Lol. But it will get you in the ballpark. Then as 273 said. Get timeslip runs. (But also remember to compare same day slips .... atmospheric conditions changing day to day will affect your final ETs of course )

At least this is what I would do without dyno and tools. But hey ... in the end ... it’s for fun and enjoyment. Go make hits, be safe and have fun !
 
273 said:
Try a few runs a 6600 if it slows down go the other way a 100 rpm at time until you find the best results. Racing is all about trial and error. Thats why top motorsports are so expensive lots of R&D.
Click to expand...
This^^^^

Track time
Make your run shifting at 6600
Then try 6500
Compare slips
Keep trying different shift points until no more improvements are made.

Be consistent!!!! In your testing.
 
Ok thanks for every body’s advice. I just wanted to know if 6500rpms was around the right time to shift with that cam. When I get a chance I will try different shift rpms and see what happens.
 
Download a g-meter based on the accelerometer chip, not GPS. Secure the phone in view of the tach, experiment shift points just before the g-meter drops. Each engine/vehicle combo may be different.
 
IMO 6400-6600 is going to be the sweet spot.
As others have mentioned try varying shift points. I would try 6200, 6500, 6800 this will let you know. 100 RPM shift point changes will not be tangible.
 
Sometimes reving past optimum (not beyond engine's safe capability) produces better results as it sets the next gear up better. Experimentation will tell the truth.
 
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