ITA 240SX guys check out my vid

D

DavidM

New member
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
 
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. :shrug:
 
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...
 
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
 
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.
 
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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
 
Thanks for the video David. :) Gives me a chance to watch my car and my driving.


I will say this, regardless of the shift points, there are some corners here that you could gain some time. I think that you are not using enough of the track at turn one, which would gain you some. Not that I am an expert but we should chat one of these days.
 
Yeah. I'm still working on T1. I definitely need to use more track on exit. I was trying to follow Willie and you through there to see your lines. Hoppe says I need 15 mph through 1. :o I'm still pretty much butchering T3 through T5 as well. One reason I want to see if I can run 4th through the esses is so that I don't have to downshift before T5. I loose a lot of time there because I'm not confident coming out of the esses and doing the downshift before turning in. Always something. :)

We should get some beer next time you're home.

David
 
Sorry I did not get to say hello at Road Atlanta. It turned out to be a short weekend as I blew the clutch in the Saturday race on lap 3. Left out early to try to get it fixed, but no clutch available. Hope to see you at Barber. Thanks to you and the others for the info regarding HP and Torque curves. I build my own engines and don't have a dyno yet. I also use a 6200 pill for my shift lite. I have Tristan's 4.6 diff. This was the first time at RA with it and I liked it alot. I have the poor man's Data Acquisition system, i.e. a wider focus on the camera. For reference on my tach 5k is exactly at 9 o'clock.

Here is the vid from my short but fun race.

[ame]http://www.youtube.com/watch?v=cK7JFMhLTE8[/ame]

Cesar Pascual
ITA Nissan 240sx #75
Atlanta Region
 
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Cesar,

You should have bought that "ZOOM" clutch I was selling. That thing was great.

I wish now that I had kept that 4.6 R&P. I am thinking of putting a shorter gear in the ZX. I was hitting 6600-7000 rpm at the end of the back straight at RD ATL in 4th gear. Never got in fifth on either straight.

I think I did figure out what the motor cutting out was caused by....looks like there was a rub on the PTU harness, and grounding out on the engine block.

None the less, I have decided to pull the motor and get the full race build going instead of doing the Barber race. Somthing tells me that the "great white whale" would not have been too competitive there anyway. Shame, 'cause I like that track.
 
Wish I had bought that clutch, so many parts so little funds. But the 4.6 is great! Glad you have an handle on your engine cut out problem. Sorry to hear you will not be at Barber. My wife Gerry is coming, her and Kathy could have entertained each other talking that FA stuff. Hope the engine build goes well, when do think you will be out again?
 
The car wont be out until next year where I hope to get in a full season. First I have to do a full build on the motor to prove the car will NOT make the HP numbers that it was was classed with, to get the weight down a bit. :dead_horse::D
Kathy will be bummed that she will be missing Gerry. It's usually just us guys for her to hang out with (what...that's a bad thing?).
 
Tristan Smith said:
First I have to do a full build on the motor to prove the car will NOT make the HP numbers that it was was classed with, to get the weight down a bit.
:dead_horse:
Click to expand...

Given the current ITAC's (i.e. the CRB's) state of mind, I wouldn't hold my breath on that one...but you already knew that, didn't you?
 
Sorry to have missed you Ceasar. Saw you out there, but then you were gone. Now I know why. Pretty sure my car has the stock clutch FWIW.

I bet that short was a bitch to track down Tristan. Good you found it. I'm curious to see what sort of numbers the built motor puts out.

Hopefully the cage on the new tub will be mostly done by the end of the week. The exhaust, drive shaft, and rear shocks are off the current car in preparation to drop the rear subframe. The plan is to just move the entire subframe, diff and all, to the new car. Gonna get a couple dollys to drop it onto to roll it around.

Anybody ever replace the rubber piece on the drive shaft where it connects to the transmission? Looks like it's meant to isolate driveshaft vibrations from the transmission. Mine looks like it's getting old and cracked. There's also a seal on the little piece of driveshaft that connects to the transmission that is toast. Looks like there's a little bearing that it's supposed to keep greased and free of dirt. Don't know if the seal is replaceable or not.

David

PS: If anybody is around labor day weekend and would be willing to help swap motors, I pay in good beer and food.
 
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DavidM said:
Anybody ever replace the rubber piece on the drive shaft where it connects to the transmission? Looks like it's meant to isolate driveshaft vibrations from the transmission. Mine looks like it's getting old and cracked. There's also a seal on the little piece of driveshaft that connects to the transmission that is toast. Looks like there's a little bearing that it's supposed to keep greased and free of dirt. Don't know if the seal is replaceable or not.

David
Click to expand...

I checked CourtesyParts.com, Nissanpart.cc and a couple of parts houses and the rubber doughnut (guibo) is not sold separately. I don't think I've ever heard of one failing.

There is (should be) a seal on the tail of the transmission where the stub fits in. It keeps the transmission fluid in and the dirt out of the output shaft bearing. It's available at any parts house. The center support bearing is available and Neapco even makes universal joints for rebuilding the non-rebuildable driveshaft, so you can get everything but the guibo! If it looks that bad, head for the junkyard.

Thanks,
DJ
 
David, I would go to U-Pull-it and see what is available on the "used market". There are places on the web you can buy a refurbished driveshaft. Every thing is new on them but you'll spend 300 - 400 dollars.
 
I don't think the rubber piece is "gonna fall apart" bad, but I figured I'd replace it while I had the driveshaft out. Guess I won't worry about it if you can't get the part. Seems like the people rebuilding the driveshafts would have to get them from somewhere. Maybe there's aftermarket pieces somewhere.

I am concerned about the seal though. This is actually on the little part of the driveshaft that inserts into the transmission. It goes through the rubber piece and connects to the main part of the driveshaft. I might be able to customize something from the driveshaft off the new tub. It's an auto, though, so may not have the pieces I need. We'll see.

David
 
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