A couple of questions???

1. Why run the engine to redline (6600 RPM) before shifting, if the torque curve maxs at 4800 RPM and HP maxs at 6200 RPM??? It would seem to make more sense to shift when you've maxed out torque, which is producing acceleration. I could see running to max HP if you want more revs before up shift in order to stay up on top of the torque curve. But at redline, torque has been steeply dropping for 1800 RPM. Its a big difference in velocity i.e. shift out of 4th at 90 (4800 RPM) or 120 (6600 RPM).

2. I'm still breaking my Z in and trying to keep under 4000 RPM. But, I'm only part way through and intersection with the gas only half depressed before I'm approaching 4000 RPM in 1st gear. Does it make sense to use 2nd gear and a little slower clutch release to take off???

3. I began by taking off in 1st at about 1000 RPM and letting the clutch out smoothly. The revs drop as the engine loads and so I'm not getting much of a launch. Guys in '56 Volvos are blowing my doors off. Pushed the launch up to about 2000 RPM, but still have to let the clutch out smoothly to avoid chug chug. Should I be pushing the pedal to WOT when I feel the clutch engage??? or just as I start to let it out??? If I pop the clutch, I seem to get chug chug unless I let it out slow. WOT at launch in 1st gear is not my smoothset move, yet. Any advice.

Sorry for the length

 

38 hits and no responses! Should I shift to another forum? I know there are people out there that know this better than I????

 

1. Not up for a TQ/HP discussion right now - I'll pass on this one.

2. Shift to second gear. You really don't hold on to first very long in daily driving.

3. Sounds like you are new to a MT, so I would just recommend practice 'normal' starts before 'launching' enters your vocabulary. After a while you won't need to over rev to get started and you will ditch that engine bog you are getting. THEN we can chat about launching!

 

Yah its been awhile. Last reg MT was '61? ford 3spd on the column followed by a '64 VW bug with all of 40+ HP (fyi both used - I'm not that old). The Z is way different, but seems forgiving. Normal starts are going ok, only the occasional engine bog in 1st, mostly when cornering from lights and stuff and when I get impatient. It just seems like I'm thru 1st before even exiting the intersection.

Thanks. POWERZ, I'm taking it smooth and slow, but looking ahead.

 

using gearing to your favor can create faster acceleration, also the torque may peak, but it doesn drop that rapidly, it stays higher then if you shifted at 4200 and then had the revs drop do 2800 or so. 6600 isnt nescesarrily the BEST place to shift, but the best place can be calculated from a dyno curve, a computer, and a good amount of calculus. however its much easier to say, shift close to 6600, but early enough that you run no risk of hitting the revlimiter. I could tell you 6512RPM is your best shift point(which will vary from gear to gear) but you cant shift right on that point, so dont worry about it, just get near to 6500.

you can start in 2nd, but its worse on the clutch, in snow, you should start in 2nd, but for regular use, 1st isnt long at all, I rarely take it to 4000. you just have to shift. if its a problem, you have the wrong model.

as for starting, I ussually start it up at about 1500-2000, as you engage you will have to learn how to give it more throttle to keep the RPMs constant through the engagment. itll just become familiar, you dont floor the gas, just give it a little more. starting at 1000rpm is tough, I rarely do it just cause its more work than its worth. Ill replace my clutch when I wear it out, no big deal, Im not gonna start at a snails pace and worry about stalling so I can get some extra miles on a clutch that will likely need to be replaced when I get a turbo anyway.

 

You will get there! I wasn't trying to be condescending, I'll be the first to admit I am no where near 100% driving the Z yet. I've always driven MTs, but this tranny is significantly different that what I'm used to. I would practice doing what ares suggested: holding the rpms at a consistant level while you release the clutch. This technique will help out a ton when you start slipping the clutch to control wheel spin!

 

Feeling no negativity- suggestions well taken. Definitely got the right tranny. Just trying to make sure I'm not missing something.

I did get a download of the stock Nissan Z dyno curve and have looked at dyno curves that others have posted with mods etc, most are really interesting, which leads to another question - how come you don't see the shift points and a drop in HP and TQ at those points on the curve. I've always assumed they are at WOT. Do they dyno the car in one gear and just smooth out the load changes when you shift?? (Believe it or not, as part of our MV inspection, the NJ Div of MV dynos your car every other year and measures emisssions - to bad they don't give you a print out of HP and TQ. Never had a MT car dynod by NJDMV - will be scared to givem my Z.)

I understand that you should be able to evaluate shift points from the dyno curve, but it isn't making sense to me dimensionally. You have HP (ft-lbs/sec) and TQ (ft-lbs.) vs RPM (ft/min or sec) from a dyno curve. I understand the difference between the engine and rw, a relationship that is changing with the tranny gear ratio and also including the diff ratio, but that is constant. If the goal is to max Accel, you need to know the change in V (ft/sec) vs Time (sec) and be able to max that out - produce the steepest change in V vs T. RPM is dimensionally the same as V, but there's no T?? A dyno certainly should be able to give you that info, but I've not seen anyone discuss it or evaluate shift points and impact on V vs T. It also seems to me that there would be a lot of trial and error in evaluating the shift points and their impact on A.

Going to drop my takeoff speed to about 1300 and try to keep revs constant as the clutch slips out. Thanks.

 

Normal liesure driving, my car respond better when shifting between 3500 and 3800 RPM.

The clutch is a bit catchy (on or off) very short friction point compaired to the E36 M3.

 

Nice toy box zmadnez. My car seems to shift easily in that area and it keeps the wear down on the engine.

 

Thanks bfleming!! I am a new guy to the MT as well... and was wondering how to improve the take off speed too! Only had 460km behind my newly arrived Z..... and I have kinda got used to the clutch already... well... I would rather give it more gas for take off... around 1500 and release the clutch to the friction point and hold the left foot there for a sec or so to avoid the jerky take off.... well... I did a lot of practices on that and tried to get the best fast take off that I could get... so far so good!

Thanks god that no one will horn you if you stall the car in australia, because they do understand this kind of thing is really unavoidable sometimes on the roads, even to a guy who has 30 yrs behind the MT!!!

All the best over there in the States man!!

cheers,

richie

 

You only dyno in one gear (closest to 1:1 - 5th gear on the Z) which is why you don't see shift points. If you wanted to take shift points down to a science you could dyno in each gear and come up with some exact shift points for each gear.

 

This is probably more like the curve you were expecting to see - http://www.go-fast.org/z28/photos/engine/dyno021.gif - 1st thru 4th gear. Assuming you want the maximum speed, the goal is to shift at the point that will maximize the area under the HP curves. Typically that means riding past the top of the peak of one gear before shifting to the next. If you ran a dyno of each gear from low RPM to redline, then you could line all these up (like a mountain range) and see where the curves cross. Those would be theoretical shift points (assuming you could shift with zero delay).

Dave

 

Think I might be over "****" yzing this! (pun intended) Especially considering my skill level. To much down-time to think during TV viewing of Daytona or something. Caught the bug a year or so ago with my chevy, a little late, lot to learn.

It is starting to make sense. It didn't seem like it could be as simple as running to redline and then shift, as the Nissan brochure implies. And the HP/TQ curves didn't make sense, unless you know they were done in a single gear, as POWERZ explained.

The LS1 Langenfelter curves are really interesting (Talk about your major engine builder!). The area under the HP curve is ft-lbs/sec x ft/sec or ft-ft-lbs/ sec-sec, which dimensionally is Acel (ft/sec-sec) x TQ (ft-lbs). If TQ stays up and relativley constant, maximizing that area seems like it will max Acel, as Dave described. Keeping the TQ up would also be important and the Z curves I've seen show it dropping after 4800 RPM, albeit slowly. As I understand from one book describingf the relationshhip between HP and TQ, the reason HP keeps going up (to 6200 RPM in a Z) is because RPMs are increasing faster than TQ is dropping (HP=TQ x RPM/5252 . So, past 6200 RPM the torque must be dropping to fast for the slowed increase in RPM to continue to boost HP, and they both start to drop.

Wonder if anyone has actually run the individual curves for the stock Z on each gear and lined them up to see where the shift points should occur, as Dave described??? There are some V shift points listed in the owners manual, as I recall, for each gear, but I don't know their origin. Are they at Redline or actually points picked to max speed?? I'm sure Nissan developed such curves for the engine and tranny design - maybe they'll give them to us. Don't think I'll ask the dealer, though.

It still seems to me like "T"or time is missing in these relationships. Guess the starter gives that to you when the race is over!

Thanks all.

 

What you probably don't know is that the dyno used for emissions produces very little to no engine/drive train load. If you were to go by the dyno that it produces you would find that your z makes about 40-60 hp and about 100 ft/lb tourque! A dynojet loads the drivetrain and the engine since your rear wheels spin a huge heavy drum that is in addition mechanically loaded to get an accurate measurement. (Hope I got the details right on that one). Also, you are correct: most dynoing is done in one gear and for the most part it should be the 1:1 gear. For the Z that is the 5th since 6th is overdrive.

G.C.

 

I guess the bulk of my experience with this is skewed, as all of my racing experience is in an RX-7. With my little rotary, the HP & TQ "curves" are more linear -- keep going up with RPM until you blow. We shift at 8000 in race conditions; factory redline is 7000.

In the Z, I set the shift light for 5500. That gives me a bit of margin since it winds up so fast under full throttle. You want to straddle the peak of the curve. Yes, you shift after the peak, but you don't fall too far back below the peak. When on it hard, I shift a bit after the light goes solid, so a bit over 6k.

Stare at the tach less and the road more

For a normal street start, my left and right feet pass each other going opposite ways. I bleed on throttle as I let out clutch. Nice and smoooooth.

THe best thing about practicing is that you're driving your Z!!

 

Didn't realize the dynos at NJDMV weren't the same as those used in performance shops. Some time I'll have to get to a shop and check it out. Now that you mention it, the dynos at DMV do not seem to load the engine. All four wheels just sit on them and they spin as the inspector pushes the gas down. Saw a thread a few days ago arguing about the comparability and validity of two different types of dynos in evaluating engine mods??

 

For maximum acceleration you want to optimize force at the rear wheels (Acceleration=Force/Mass). This force is engine torque times gear box ratio times rear end ratio divided by tire radius (minus losses of course). That is why you go past torque peak before shifting because you want to take maximum advantage of the torque multiplication of the lower gears (higher numerically). There is also the factor of the power consumed in accellerating the rotational inertia of the power train but lets not worry about that for the time being. Since the rear end ratio is constant and the tire diameter is nearly constant you just have to plot the engine torque times gear ratio for each gear vs vehicle speed to see the optimum shift point. If you had constant torque, the optimum shift point would always be at redline. Since torque is dropping off at higher revs the optimum is usually at or just past peak power rpm.

 

The force developed at the rear wheels is, therefore, directly related to TQ and the gear ratio. So, as dr_gallup indicates, lower gears with higher numbers should produce more Force and Acceleration for any given torque.

But, because you are dividing by the outside tire radius - r, the amount of Force and Acceleration is inversely related to that r. Decrease r and you'll get more Force and Acceleration and increase r and you'll get less Force and Acceleration.

Why buy bigger radius wheels and tires?? Looks?? I can see wider tread to provide more frictional surface (i.e. racing slicks), thus dropping the needed fritional forces to avoid tire slippage and get traction.

Assuming a 4" high tire on a 20" rim, r = 28/2/12 = 1.17'. If you use a 4" high tire on a 17" rim r = 25/2/12 = 1.04'. For every 100 ft-lbs of torque with the 20" rims, you produce 85.47 ft-lbs, while the 17" rims produce 96.15 ft-lbs. That's an additional 10.68 ft-lbs for every 100 ft-lbs the engine and drive train deliver to the rear wheels by going from 20" wheels to 17" wheels. That's 12.5% more Force and Acceleration from the 17" over the 20"!

Am I missing something?? I realize this is getting off the subject of shift points and there are other considerations in wheel tire selection. But????

 

Most people going to a bigger wheel are doing what's known as a +1 or +2 conversion where you incease the wheel diameter 1 or 2 inches but decrease the tire sidewall height to compensate, keeping the rolling diameter the same. Since we already have a 45 series tire with the 18" wheels there are limits on how far you can go with this, particularly if you go to significantly wider tires. If you don't keep the rolling diameter the same the odometer and and speedometer calibration is affected.

The cheapest way to reduce your overall gearing for faster acceleration is to go to a smaller wheel and tire. Not too many people do this cuz it looks goofy. Most wheel and tire packages are sold based on looks alone. Go to tirerack.com for a more indepth explanation of +1 and +2 conversions and how to calculate the rolling diameter.

 

Yah, I've been checking some of the wheel/tire stuff and actually found a FAQ section in another forum that goes over the whole wheel/tire thing for the Z and what you need for wheels and tires to minimize change to the speedo, fit in the wheel well, have space for the brakes and etc. Just never realized the negative impact on F and A until you posted above. I've seen lots of posts by people going to 19" and 20" rims and I'm sure tires with smaller height. My Enthsiast came with 17" wheels and I was feeling inadequate. But, if I put the same height tires on that they do, I've got 12.5% more TQ (assuming a 4" high tire) - A little less of an advantage if the tires aren't quite as high.

There's also lots of posts by people spending $$$ for mods on their car to get even a little more TQ and HP. CAI's for example. If they're also putting bigger wheels on, its self-defeating.