A&Q about 350Z
Q:
Can someone tell me what will happen to the redline of an engine if the size of the cylinder bore is increased. I know increasing the stroke lowers the redline, but nothing about what increasing the bore does. Any help will be highly appreciated. Thank you for your time.
A:
A:
It may depend on how much of an increase. Lots of oversized piston weights are matched by the manufacturer for balance and the redline would not change. This is for stock type oversizes of 1/16 " or less.
A:
Well there are two "redlines" to consider; there's the one where your engine makes peak power and the one where your engine may mechanically fail.
Lets say for instance your engine makes peak power around 6000. That would be the redline that the manufacturer would put on the tach. But (I'm using your VTEC as an example) that engine would physically hold together till more like 7000.
Increasing bore size may reduce the 7000 by a tiny bit if the pistons were heavier AND if their weight was a limiting factor in its mechanical ability. But careful shopping could yield pistons that were lighter and the mechanical redline wouldn't change. Either way, there's no reason to rev an enging up to 7000 if your power peaks at 6000.
Increasing bore will increase displacement and shift the power peak down a little, but not much. Even in the case of large V8s the additional displacement isn't enough to change where you'd shift for max acceleration, so the published redline on the tach will stay true if all you're doing is the pistons.
A:
So how does increasing the bore affect the powerband?
A:
Increasing the bore SHOULD increase the power throughout the complete range.
But really, How much of an increase in bore are you talking about? A compression change would be more significant than a redline change with a bigger bore.
A:
Not on the street of course, but for performance, you ALWAYS go past the peak HP RPM. That is some of the valuable info you get from a dyno run..
Lets assume that your HP is the following:
4700 RPM = 270
4800 RPM = 280
4900 RPM = 290
5000 RPM = 300
5100 RPM = 310
5200 RPM = 320
5300 RPM = 330
5400 RPM = 340
5500 RPM = 350
5600 RPM = 360
5700 RPM = 350
5800 RPM = 345
5900 RPM = 340
Now assume that when you shift from 4th to 5th the engine RPM is 1000 RPM less.
If you shifted at 5600 RPM, you would be wasting the bulk of the "HP under the curve". In the example above you would shift at 5800 or even 5900 RPM.
After a dyno run, here, we always calculate the gear ranges and then determine maximum utilization of HP under the curve for the shift point. It is ALWAYS beyond the peak HP RPM point. And, unless significant mods have been done to the engine, it is almost always below the manufactures "red line" on the tach.
The "red line" the manufacturer has on the tach is independent of peak HP. It is just a recommended "do not exceed" RPM.
The balance on most modern performance engines will allow exceeding the tach "red line" RPM in most any case. However, unless significant breathing mods have been made it isn't required and shouldn't be done.
Jim
A:
The transmission also needs to be taken into consideration since the accel gears spin way faster than the engine.
A:
What do you mean by accel gears?
A:
what he's saying is the tranny causes the gearing ratios to change. It seems like an obtuse indirect message, but it turns out, thats what trannies are designed to do, change the final gear ratio.
Sorry. I thought that was funny.
A:
good, except that most city driving happens at partial throttle, where the curve is even more different (for lack of a better word)
if you are tuning for the street, look for mods that add torque at low RPMs and shift the powerband down in RPM.
if tuning for the track, it is OK to sacrifice low end power because the only time you will use it is at the starting line.
A:
that is not what he's suggesting grey. What he's saying is, to put it simply, the highest average HP per RPM range for a gear, will cause the highest acceleration for a given vehicle.
If city driving = highest HP at lowest RPM possible, then everyone would be driving diesels.
There is a wide variety of vehicles and engines to chose from on the market. I don't like low-reving engines personally. There is disadvantages with any configuration. Low reving = low HP/L. Can't have your cake and eat it too.
A:
"The curve is different"? There is no real curve for a part throttle HP run. It's a non-sensical point.
And since the thread was regarding "red-line" or performance shift points, city driving is not at issue. And, I clearly pointed out that this was for performance.
If you are tuning for the street, strip or road course, or oval. You tune for HP at the RPM you will be operating at. This "tuning" is building the engine, gear ratios and tire sizes to keep the engine in the max HP RPM as long as you can. If you are tuning for economy, none of this thread makes much sense.
A:
All makes sense except for the partial throttle curves. If you never saw one, it does not mean they do not exist. I'd been drawing quite a few of those when worked for the industry...
A:
sorry, thats what i thought you were talking about...
i like diesels for the simple fact that they are more efficient.
i like high revving engines better otherwise.
does hp/liter really matter?
torque is a lot better in real world driving.
thats just my opinion tho. ;-]