while racing, not lifting foot off trhottle may cuse compressor surge?

J

javieringo1

Member +
Purpose

Blowoff valves are used to prevent compressor surge. Compressor surge is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The driver will notice a fluttering air sound.
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this is some info I found. the situation is this, while drag racing, the best way to save time while shifting, is to shift around 6500rpm, right? (this is on a starlet gt). Well, I was told that you save time by NOT LIFTING YOUR FOOT OFF THE TRHOTLLE. Does that work with turbo charged vehicles? because I tried it already, and it seemed to help me on my ETs, but the BOV was't going off during shifts, so, doing that, may cause wear to the engine or turbo? is it better If I lift my foot rapidly to let the BOV release preassure?

Thanks for the advice.
 
H

-Harry-

Member +
Look at your quote then look at your post. Youve answered your own question.

when the throttle plate closes you will get a spike in pressure in your piping which can stall the turbo unless vented. If you changing with your foot flat to the floor (known as flat shifting) then the throttle plate isnt closed is it.

Flatshifting can be veryhard on your syncros though. Also the rpm at which you change is dependant on many factors including when your car makes peak power and where your powerband sits in the rev range. For a bog stock GT Id be changing at 6000rpm.

I really wouldnt bother though. My gt runs mid 11's and I dont flat shift. I have enough trouble trying to get the gears to engage.

Hope that helps
 
Last edited:
goldenvtr

goldenvtr

Member +
i dont fully lift off the throttle when drag racing and it added roughly 2 mph to my trap speed. tried it many times and from my experiance my times are slightly faster when i keep the throttle depresses whilst gear changing
 
H

-Harry-

Member +
If you have I bog stock, (I mean air box exhaust, the works) then there is no point going over 6000rpm. Ive got one and Ive tryed it many times.

As for flat shifting, Id say it would depend on your power levels. Anything over 240hp at the wheels tends to put a lot of load on them. 300 at the wheels makes them hate me.:(
 
J

javieringo1

Member +
well thanks a lot for clearing things up. I guess no problem cause by not lifting foot of gas right?

and mine's stock, so ill give it a try shifting @ 6000, 6500 and 7000, and see which one gives me better ET.

also, I should be taking off @ 3000-3500 rpm right? and slowly lifting foot of the clutch and not slamming down gas pedal right, doing it not so fast???
 
J

javieringo1

Member +
gt's make 133bhp@6400 rpm, so shouldn't I be shifting at that range? please correct me if I'm wrong.

and btw, 300whp is crazy man, your car must fly hitting 11's :rockon:
 
H

-Harry-

Member +
I meant by stock airbox and stock exhaust.

Depending on your mods will depend where you shift. You really gotta just see what works for you and your car.

As for launching. Its dependant greatly on tires (size, width,type,pressures), diff, suspension, castor, camber.......the list goes on.

In my stock gt I launch it around 4000rpm and the slide the clutch a lot with about 3/4 throttle till the clutch is out then flat to the floor. Once again though you gotta work out what works best for you and your setup.

That power figure youve got there for a stock gt is at the engine too not at the wheels + factor in that its a 17 year old engine.
 
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alfradio

alfradio

Member +
(Wow! Another new term! "flatshifting".... nice!)

I shift TERCEL1 at around 6800 RPM... for what it's worth and at the track, it's
always "flatshifted" or, as some say, over here in the Colonies, "WOT"
(which stands for Wide Open Throttle) Take a look HERE!
 
GTti

GTti

Member +
So people are suggesting that by keeping the turbo spooling and the intake presurized they experience better response.

If you don't run a BOV and change gear quickly, the intake will remain charged, the compressor won't have a noticeable reduction in speed and you will barely hear the 'flutter' = better response.

Flat shifting is fine but you'll just wear components quicker as suggested.
 
J

javieringo1

Member +
alfradio said:
(Wow! Another new term! "flatshifting".... nice!)

I shift TERCEL1 at around 6800 RPM... for what it's worth and at the track, it's
always "flatshifted" or, as some say, over here in the Colonies, "WOT"
(which stands for Wide Open Throttle) Take a look HERE!
Click to expand...

Awesome video! how do you launch? do you press down the gas pedal all the way when you release the clutch? or do you do it gradually?
 
C

Chris

Member +
buy a bee r rev limiter, good for launching ( and flames ha :p) and you can set your engine rev limiter, althou theres not ,much point if its bog standard.
 
J

javieringo1

Member +
GTti said:
So people are suggesting that by keeping the turbo spooling and the intake presurized they experience better response.

If you don't run a BOV and change gear quickly, the intake will remain charged, the compressor won't have a noticeable reduction in speed and you will barely hear the 'flutter' = better response.

Flat shifting is fine but you'll just wear components quicker as suggested.
Click to expand...

yeah, too hard on the clutck I guess, and the gearbox :-(

Chris said:
buy a bee r rev limiter, good for launching ( and flames ha :p) and you can set your engine rev limiter, althou theres not ,much point if its bog standard.
Click to expand...

right, I'm not planning on modifying it, so no point there, I just want to make the best ET the car can make, when I take it to the track (about once a moth, tops) :) It's my DD :)
 
alfradio

alfradio

Member +
javieringo1 said:
Awesome video! how do you launch? do you press down the gas pedal all the
way when you release the clutch? or do you do it gradually?
Click to expand...

Pretty much all the way... I think (according to the video) I'm holding the RPM
at about 2000-2500 and then as soon as I launch (when the last amber light
is going out) I floor the accelerator (increasing the RPM) while letting up on
the clutch quickly (but, NOT, popping it!)

Once under way (I've lowered my RPM shift point from 7K (in the video) to
about 6800), it's peddle to the metal through 2nd, 3rd and 4th gear... If you
play close attention to the boost gauge, you'll see that once the boost
reaches its peak (around 10 psi from me) it stays pretty well at THAT point
througout the 1/4 mile run with WOT shifting!
 
M

Mikey B

Lifer
lol flat shifting (as you put it) as your doing that the throtle is wide open so theres no need for the blow off valve.
 
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