ITA 240SX guys check out my vid

[DavidM]

Specifically the RPMs. This is from Road Atlanta last weekend.

[ame]http://www.youtube.com/watch?v=JyLeJQfSFLE[/ame]

This is the first time I've really looked at the RPMs I'm turning. I had been using the stock tach and it had been going up to close to 7000 before it died, but the data is only showing me going to just above 6000. I have a 4.36 in the diff and the computations seem to show that the RPM data in the video is correct.

I'm thinking I'm shifting too early and need to run the RPMs up a little more around 6200-6300 before I shift so that it drops to 5500 or so after the shift. I'm also thinking it may be possible to do the esses without shifting to 5th. I need to back calc from the top speed to see what the RPMs would be in 4th. What RPMs are you guys shifting at and what is a realistic top RPM that you can hold (not just hit for a brief second)? This is a built motor so should be able to handle whatever is at the top end of the range.

Thanks.

David

[erlrich]

David - I'm pretty sure the factory tach in my car is also off a little; I have a 6k pill in the shift light, and the tach reads just over 6200 when the light comes on. I usually try to shift 3-4 right at 6k, but will sometimes hold 4th a little longer before going into 5th. Even with the lower 4.36 gears 5th gear just doesn't have a lot of giddy up in our cars, so I will let it wind up a little more in 4th.

A lot of it depends on the track of course; my engine (also fully built) will turn 6500 all day without breaking anything, but it's not making much power up there - the power curve starts to dive pretty sharply at around 5700-5800. Conversely, power comes on pretty strong in the mid-4k range, so I don't think you're hurting yourself by shifting when you are. Of course, I have no test data to support anything - other than the dyno charts - so I might be totally off base.

[CRallo]

I'm with Earl... Have had our car on a Chassis Dyno, made more Torque than HP and our curve showed not much point in revving much over 6, gear ratio drop or not...

[DavidM]

Thanks guys. I pulled out a dyno sheet Bob gave me with the car. Torque peaks just above 4500, but the HP peak is in the 5700 range. The HP-Torque crossover point is right at 5500. HP trails off after 5700, but there isn't a dramatic drop off.

Interestingly, I have a dyno sheet from a previous motor and it shows a real drop right after about 5500 or so. I don't know if the motor I have was built differently or they tuned it differently, but it seems to have a much better power curve. Max HP is under 150 so maybe they gave up a little at the max to make the curve better. Or maybe it's a newer version of the Stretch header. Dunno.

If I run the ARRC I'll try running the revs a little higher before shifting and see what happens.

David

[Matt93SE]

If the HP and TQ graph crosses anywhere other than 5250rpm, then it's a bad graph... make sure your numbers line up on scale and make sure you're reading things right as well. most charts will have both HP and TQ on the same scale, but some will have separate scales for both and HP/TQ will appear to cross at a different point. the calculation for HP is HP=TQ*5252/RPM so the math shows it'll always cross at 5252).

the KA-E also isn't huge on top end. As has been mentioned, you're just making noise above about 6200, and you'll do better shifting around 6k.

What I did to calculate my ideal shift points was to plot wheel torque vs. speed in each gear and figure my shift points on where I have more torque @ the wheel in the next gear. I wound up with a different shift point in every gear, but the math doesn't lie.

1st gear I shifted at 7000rpm, 2nd gear at 6750, 3rd gear 6250, 4th @ 6000.
(These were for my old Maxima that I did HPDEs on....)

If you want to play with the file, it's on my server here: http://blehmco.com/stuff/Shift point calculator.xls
Simply grab your dyno numbers and dump the torque & rpm into the spreadsheet, then fill in the gearing & tire info (everything in green) the spreadsheet will calculate the rest (red), and plot the information.
You might also have to play with the scaling, but those scales should work for anything under 250hp or so.

[DavidM]

Yup. Crossover is at 5250. I had it wrong. HP is fairly consistent from 5000-6000 with the peak around 5500. HP falls off after 6000, but really only loses about 20hp at 6500. Torque is pretty good till the 5250 crossover, but falls fairly steeply after that.

I'll play with the spreadsheet and see what numbers I get. Should be interesting. Thanks.

David