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RE: Noob Question - 2/3/2006 12:45:48 PM
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LORCLAS
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Because you can hold the shift points longer with a manual and also keep the engine in its power band by keeping the RPM's high. With an automatic your RPM's drop significantly with every shift and the engine has to "start over" and bring the RPM's back up with every shift. With a manual every shift can be at 6,000 rpm.
This is obviously cliff Notes explination.
< Message edited by LORCLAS -- 2/3/2006 12:51:11 PM >
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RE: Noob Question - 2/3/2006 2:21:20 PM
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swartlkk
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Nope, wrong entirely... Well, not entirely, but still not the main reason why a manual can be faster than an auto. I say can, because a manual transmission requires MUCH more driver skill to make it work to it's potential. The reason why a manual can be faster than an auto is the parasitic loss through an automatic transmission is ALWAYS higher than through a manual. Some manuals get down to a very small 5% loss through the tranny. The best autos can barely manage 20% loss. So take your 120hp and wack off, even say 10% loss through a manual, 12hp. With an auto, that same vehicle would loose 24+hp (and a 20% loss on an auto is a very low parasitic loss for todays trannies).
Autos actually do very well at keeping a transmission in its powerband. That is designed into the gear ratios, the design of the engine, and the rear end gear ratio inorder to keep the vehicle in it's best operating range (with several sacrifices along the way). If your engine is only designed to rev to 5400rpm, putting your shift points at 6000rpm could actually hurt your acceleration up through the gears. Yeah, it may pull very nice after a shift, but because the hp peak was back at like 5000rpm, the fall off of the hp and torque makes it dog up in the rpm (which many people do not realize when messing around with shift points). You really need to look at the hp curve and tailor your shift points in such a way that you maximize the area under the curve.
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RE: Noob Question - 2/3/2006 3:18:15 PM
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LORCLAS
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Yea. Go with swartlkk. Looks like he forgot more about transmissions than I will ever know about them.
Great explaination swartlkk.
< Message edited by LORCLAS -- 2/3/2006 3:36:38 PM >
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RE: Noob Question - 2/3/2006 3:30:28 PM
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swartlkk
Posts: 15304
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Another thing is that the more gears you have to play with, the better acceleration you should have. Again, I say should, because the more gears at your disposal gives you, the driver, more times to mess up.
So, having the same car/engine, a 5 speed should beat a 4 speed. This is because the engine can stay more closely inside it's power band much better, plus the gearing is more closely matched. Some of the older vehicles used to come with a CR or close ratio transmission which was great for getting off the line and up to speed, but these transmissions were limited in their ability to leg-it-out, so to speak, because the final gear would be lower due to the closer ratios throughout the transmission. The rear axle ratio will also have a bearing on the acceleration of the vehicle due to the torque multiplication, but a lower gear ratio will increase cruise rpms, thus affecting fuel mileage.
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RE: Noob Question - 2/3/2006 3:59:26 PM
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strongboy2005
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quote:
ORIGINAL: swartlkk
Another thing is that the more gears you have to play with, the better acceleration you should have. Again, I say should, because the more gears at your disposal gives you, the driver, more times to mess up.
So, having the same car/engine, a 5 speed should beat a 4 speed. This is because the engine can stay more closely inside it's power band much better, plus the gearing is more closely matched. Some of the older vehicles used to come with a CR or close ratio transmission which was great for getting off the line and up to speed, but these transmissions were limited in their ability to leg-it-out, so to speak, because the final gear would be lower due to the closer ratios throughout the transmission. The rear axle ratio will also have a bearing on the acceleration of the vehicle due to the torque multiplication, but a lower gear ratio will increase cruise rpms, thus affecting fuel mileage.
At 80 mph my car isn't even close to shifting into 4th, and 80 is about my 1/4 mile trap speed, so would the 5-speed still make a difference over the 4-speed?
Is there a way to determine how much loss I'm getting through the transmission? I thought most cars only lose about 15% from the crank to the wheels, if this is so how can the transmission be taking 20%?
Kind of an unrelated question, but my Programmer is set to arrive today along with my cold air intake (pure coincidence how that worked out btw, i bought from two different people in different parts of the country), and I was wondering if it would be bad to set the shift points at WOT to 5500-5600 RPMs... I know the redline is 5600, but would it offer too much stress in the long run and not enough of a gain to make the wear on the engine worthwhile? Will this cause additional wear on the engine as opposed to, say, shifting at the stock 5200 RPMs? (I am running Prolong lube in my oil, which is supposed to reduce wear significantly)
I made an excel document to determine my relative shift points, i'll try to post it on here to see if what I determined was accurate...
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RE: Noob Question - 2/3/2006 4:30:15 PM
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swartlkk
Posts: 15304
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When I say 20% loss through an auto, that is from the crank to the wheels. Many manual trannies have an overall loss from 5-15%, but the best of autos average a loss of 20%, crank to wheels, most being around 23-25% (chrysler 42LE is 35% - OUCH!). Taking the hp/torque dyno from Hypertech for the stock configuration, the max torque is 155hp@4500rpm and max torque of 201ftlbs@3500rpm. So with GMs published hp/tq, thats a loss of 22.5% ((190-155)/155) in hp and ~24% loss in tq ((250-201)/201). The published parasitic loss for the 700r4/4L60/4L60E is 22%. In fact, most GM auto-4s have a parasitic loss of ~22%. These losses are mostly in the torque convertor and transmission oil pump. The generation of heat is the result of these losses (all energy has to go somewhere).
That site is a great resource and if you used that to find your shift points, then they should be correct. You should use a readout of hp based on an actual dyno done either on your truck or a similar one. Be a little conservative if using a dyno not from your truck especially if using the Hypertech dyno. Shifting at 5600rpm shouldn't be a problem, but much faster and you may have trouble with valve float due to the relatively weak valve springs used on these heads when compared with the weight of the valve.
*EDIT* - corrected a conclusion
< Message edited by swartlkk -- 2/3/2006 4:37:36 PM >
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RE: Noob Question - 2/3/2006 4:45:22 PM
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swartlkk
Posts: 15304
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I'm not sure what the maximum PCM allowable rpm is for the 4.3L... I'd have to do some searching to find that out. At worst, it would cut out the spark to limit the rpm and you'd feel it right before it would shift.
The max pcm allowable rpm is not the rpm at which the engine will blow. These bottom ends can handle probably upwards of 6000rpm given the proper valvetrain components, but I wouldn't do it on a daily basis or sustained. One problem that would also arise is that since you are falling back to a lower rpm (but still higher hp than the stock shift point), you may run into trouble with the internal components inside the transmission (much like Hanr3 is going through now although his blazer is stock).
I would increase the line pressure in the transmission before messing around with shift points, but I don't know if your hypertech can do this. This will make the shifts quicker and less traumatic on the transmission, but more harsh to you. The clutches will slip less during a shift, reducing the wear on them as well as the amount of heat generated.
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RE: Noob Question - 2/3/2006 6:14:57 PM
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swartlkk
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Will it work, yeah. Can I guarantee that nothing will break, no. You have to realize that modding and breaking go hand in hand. Now, nothing that you are going to be doing should stress the transmission or engine too much, and it's not like you are going to be pushing WOT all the time. You have to show restraint in using WOT, this is true for stock applications, but even more so modified. You should always try to roll into and out of the throttle, never stab it or let off suddenly. By doing so, you will not lose any time, but you will save the life of your vehicle tremendously.
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