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Boost pressure and Gear changing...

4K views 21 replies 6 participants last post by  saabcog 
#1 · (Edited)
Just a question about normal behaviour for those of you with Manual gearboxes...

Say, for example, you are accellerating at full throttle through 3rd gear to about 5500rpm. If I flat shift into 4th (hard off throttle, hard on clutch, less than a second to shift and hard back the other way) the car pulls well, but not impressively.

If I shift more casually (take a second or two to shift and hold rpm in the new gear for a second before flooring it) it pulls much much harder that it would in the previous scenario.

I'm wondering if this is normal, so by shifting hard am I causing a boost spike or something similar that is causing a loss of boost when the ECU overrides?
 
#2 ·
I've thought alot about this scenario aswell, mostly from my time drag racing... but whats happing is that when you completly let off the throttle and quick shift, you lose boost and the turbo will spool down. On the other hand if you maintain some throttle but roll onto the clutch your keeping some boost and eliminating some turbo lag.

My time at the strip has lead me to beleive that modulating the throttle a little between shifts will give better times, basically maintaining boost. I could be wrong thou :confused:
 
#3 ·
Honestly, I thin it's mental because of what your expecting when shifting fast or shifting slow. The turbo won't lose momentum when you shift fast over slow, your just gonna loose time in the shift waiting for it to come down. In fact, the turbo will slow down more in a slow shift than in a fast one.

Think about it this way, when you shift fast, you stomp and expect it to just fly. When you shift slow, your more relaxed, and it feels much faster.
 
#4 ·
I've just been out with a friend and a stopwatch. When shifting into 4th at 140kph, it is definately faster from 140-180 after a slower gearshift. we're talking faster by over a second...
 
#5 ·
I notice some delay and feel the turbo not pulling consistently all the way through when I drop to third and floor it....especially when going up hills....this is at about 45mph. Maybe I should be in 2nd? If I don't floor it and give an even amount of pressure on the gas pedal the turbo seems to be able to spool up and give a more consistent feel. My car has 70k on it....
 
#6 ·
iDav said:
I've just been out with a friend and a stopwatch. When shifting into 4th at 140kph, it is definately faster from 140-180 after a slower gearshift. we're talking faster by over a second...
Try this:
After you shift quickly, watch your revs. If they drop to a point and kinda hang there while your accelerating, then your clutch is having a hard time holding the power from the abrupt shift.

This would cause the turbo to spool slower as well because the engine is not under full load while the clutch is slipping.

When your shifting slowly, your allowing the revs to drop and match the speed of the wheels. This would mean that the clutch would slip much less or not at all when engaged in the next gear.

I personally haven't noticed this phenomenon, but I'll have to go out and get a stopwatch to see if it applies to my car as well.
 
#7 · (Edited)
02BrnfrmjtsAero - This same thing happens to me - it almost seems to go faster at ~80% throttle rather than 100%

SectorNine50 - I certainly hope it's not my clutch, as I've just had a new Aero spec clutch installed less than 10k Km ago. It certainly doesn't slip like it used to. (I used to drop from 4th to 3rd and watch the tacho redline before any accelleration! - very very dead clutch)

It just seems that if you go straight to WOT, there's less boost. If you pause for a second at 50-75% throttle before flattening it, there's more boost.
 
#8 ·
Not saying the clutch is bad, but dropping the hammer while dropping the clutch is going to put some serious strain on that clutch. I'd be willing to bet it slips more than you think for even just a second.
 
#9 ·
Fair call...

I wonder how to prove it...
 
#11 ·
I'm not sure exactly what you mean saabcog... my foot wont reach the clutch and throttle at the same time...
 
#12 · (Edited)
saabcog, since he's talking about accelerating, heel-toe wouldn't really be used, but a double-clutch rev match would do the trick.


iDav, heel-toe downshifting is a pretty advanced maneuver, but it's a great skill to have; makes you a better & smoother driver, and it's way easier on your car.

I'm going to tell you to do an internet search to get full details and tips/instructions, but the gist of it is this:

It's a double-clutch rev match performed while braking... i.e., you're slowing down to approach a turn, and you want to shift into a lower gear so you can power through it without any coasting.

The basic steps are:

1. Right foot depresses the brake pedal to start slowing car.

2. Left foot depresses clutch pedal and shift lever is moved into neutral.

3. Left foot releases clutch.

4. While keeping toe pressure on brake pedal (maintaining deceleration), right heel rotates over to blip gas pedal to raise revs 1000-2000 rpms or so, depending on vehicle speed and number of gears you're skipping.

5. With right toe on brake and right heel keeping revs up on gas, left foot depresses clutch pedal and shift lever selects lower gear.

6. Left foot releases clutch, right heel comes off gas pedal pretty much simultaneously.

7. You are now engaged in your lower gear, and can transition off the brake and onto the throttle as usual, making your way through the turn with much more speed and control than you would've done otherwise.

8. Your friends in the car praise you for your serious driving skills. Hot chicks are waiting anxiously for you at your destination.


EDIT: reading through the thread again, I should clarify -

iDav, the double-clutch and heel-toe stuff I described above, those are both downshifting techniques... like if you were already in 5th and wanted to punch it in a lower gear to accelerate faster, or the heel-toe deceleration scenario I described.

In your original question, I understand it as you're accelerating hard from a lower speed... so you're only upshifting. In that case, neither of the above techniques would come into play.

My thoughts are pretty much that when you shift more slowly, you're essentially holding the car at steady throttle until the turbo is completely spooled up and ready to dump full boost immediately, whereas on your hard acceleration right after the shift, the turbo has had time to slow down, so you're asking for full throttle right off the bat... and the first couple seconds of that will be spent spinning the turbo up.

As far as letting the driveline slow down to match the wheel speed, I don't believe it'd be a factor once the clutch is let back out (except in terms of driveline strain). There is a remedy though, that would improve the smoothness of those fast shifts - you could install a lightweight flywheel. The car will be a bit more touchy, but those revs will fall faster, so you can perform a rev-matched upshift in less time than stock.
 
#13 ·
Sounds good, but my wife may not be pleased with Step 8.

I also agree about not solving my issue with upshifts.

Something to practise for my next track visit though...
 
#14 ·
Really hard to do with a size 13 foot... :cry:

iDav, to fix the slipping issue, try to time the throttle after you let out the clutch. This will give the clutch a second to seat onto the pressure plate and not be so much of a jolting motion. In reality, before you shift you should try to get off the throttle fully before depressing the clutch as well. Good habit to get into, gets the revs down quicker between shifts especially on these cars that rev so slowly.
 
#15 ·
Dangers of Heel-toe?

 
#16 ·
SectorNine50 said:
Really hard to do with a size 13 foot... :cry:

iDav, to fix the slipping issue, try to time the throttle after you let out the clutch. This will give the clutch a second to seat onto the pressure plate and not be so much of a jolting motion. In reality, before you shift you should try to get off the throttle fully before depressing the clutch as well. Good habit to get into, gets the revs down quicker between shifts especially on these cars that rev so slowly.

I thinks that's basically whats happening when I shift more slowly. It just gives it the second it needs to grab properly.

It disappoints me a little that an Aero clutch can't take Sub-Aero power without slipping...

Would a lighter flywheel make much difference?
 
#17 ·
iDav said:
I thinks that's basically whats happening when I shift more slowly. It just gives it the second it needs to grab properly.

It disappoints me a little that an Aero clutch can't take Sub-Aero power without slipping...

Would a lighter flywheel make much difference?
Well if I'm reading your stats correctly, your 20hp over stock Aero power (at least sub 2002 9-5's.) However I don't really notice this issue with my Aero, you got the pressure plate replaced with the Aero one at the same time, right? (I assume that the Aero one is a tad heavier, I'll have to check part numbers to be sure of that though. I'm installing the EPC as we speak.)

A lighter flywheel may allow the rpms to drop faster, but it still wouldn't fix the fact that if you throttle before the clutch is all the way out it's gonna slip a bit.
 
#18 ·
iDav, you replied before I finished editing my original post - take a look and see if what I added makes any sense.

Also, another danger of heel-toe is looking like a complete moron when you're practicing and jerking the car all the place. It does take a while to let it all sink in. If you do want to practice, I recommend getting good at the double-clutch rev-matched downshift first, then try incorporating the heel-toe bit. The instructions are exactly the same, except that you're never on the brake pedal... so it's left foot clutch, right foot exclusively gas pedal. Also, the hot chicks can be omitted.
 
#19 ·
SectorNine50 said:
A lighter flywheel may allow the rpms to drop faster, but it still wouldn't fix the fact that if you throttle before the clutch is all the way out it's gonna slip a bit.
True true, good sir... I concede and agree.

Also, apologies for post-whoring.
 
#21 ·
Yes, the pressure plate was done at the same time... Everything in there was new and the flywheel was resurfaced.

So it's a combination of poor throttle application on my part, the 9-5's heavy flywheel and Turbo Lag.

All good fellas. Thank's for the advice. I was just hoping to find out if this was normal behavior. It seems, for my car at least, it is.

Cheers! :D
 
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