mrklaw

Just had a bit of a fright, and was wondering if anyone could shed some light on it.

I was tailing a little fiat 500 or similar on a National Speed Limit, doing a steady 40. A clear section came up, so I overtook. I was in 3rd at the time, and dropped to 2nd. I floored it and pulled out. As I got alongside, there was a sudden and quite dramatic loss of acceleration. I put it in 3rd and completed the overtaking, but it was quite a shock.

I don't think I was revving into the red, but it was quite high. Is there a built in limiter if you rev into the red that cuts the acceleration?

I was low-ish on petrol (I put in 45 litres about 10 minutes later), but it was a straight, flat road, so I would hope it wasn't the fuel cutting out or anything.

Anything to clear this up or help me avoid it in future would be appreciated.

johnfelstead

you either hit the rev limiter, hit boost cut or had fuel surge. Try it again when nothing is about and see what happens.

SiPie

There is a fabled ECU bug in MY99's (only I think) where if you hold 3000rpm in 2nd for too long the ECU believes it is being emission tested and therefore effectively runs into a sort of safe mode

The only reason I mentioned this is that I had this happen to me some time ago when trailing a long queue of traffic and when I finally floored it there was hardly any power

However mine was not as high up the rev range as you were and yes there is a limiter so you may of hit that

Hope that helps to completely confuse you

johnfelstead

It seems all Subaru ECU's have this emisions cheat function. What it does is limit the boost to 0.6BAR until you release the throttle and reaply it. It's not this particular problem though.

mrklaw

its a MY03, and i wasn't at any revs for very long

Might have been the limiter.

I tried it a little later on when the road was clear again, but couldn't recreate it. Maybe I was being more concious of the revs so didn't let it get into the red - I changed up at around 6k anyway.

I guess in 2nd it goes through the rev range pretty quickly if you put your foot down...

So does the limiter actually back off on power? It didn't feel like it was leveling off, more like a sudden drop.

bluenosewrx

iTrader: (2)

i've got a MY02 WRX and this happened to me,it was a bit of a fright at the time but it had turbo cut out and i was in the high revs,dont worry its a safety measure built in,saying that i did sh*t myself at the time.bluenosewrx

GTMR

Almost certainly the rev limiter. Used to happen to me until I learned to keep half an eye on the rev counter.

rossyboy

iTrader: (8)

I need to keep both my eyes on the rev counter when booting it. The damn thing whips round so fast! Then I look over at the speedo

-=Buzz=-

no, look at the ROAD! Use your ears to tell you when to change gear...

johnfelstead

And if you cant feel the shift point, fit a shift light. Rev limiters are there to save the engine, ideally you should never hit that.

Martyn Tonks

As every one is saying the rev limiter is there to save the engine, once you hit the red line the limiter cuts in. I believe the limiter system is one where the each cylinder will only fire on alternate firing strokes, therefore a dramatic loss of power and a feeling of fuel starvation is very prominent.

Once you are used to the engine note through the gears you can really get the car to perform by upshifting just before the rev limiter cuts in, takes practice but is good fun.

Meanwhile keep your eyes on the road not on the rev counter to check your gear changing.

big500

Have you got a boost guage in the car? It sounds like the rev limiter but if it was a really dramatic loss of power it could well be fuel cut due to overboost. I reckon find a nice Striaght bit of open road and floor it in second and third.
If its the rev limiter it will hold off at the same rpm , if its fuel cut you should hit max boost in 3rd gear at a lower rev figure.In my experience fuel cut is very dramatic like someone has turned the ignition off then back on for a split second


Steve

bartmanuk

failing this get ya motor on a rolling road and find out wot rpm gives you the max power i.e 250bhp @ 6365rpm,then aim to change gear at that point...

BM

johnfelstead

No, thats not the way to get the best out of an engine. What you are looking for is to maximise the largest area under the power curve, if you change gear at max power you will drop below the level of power you would have had available if you let the engine rev past that point.

What you need to do is plot your engine power curve against your gear range, that way you know how much the engine rpm drops in each gear and that will allow you to pick your shift point to maximise the power available.

The ideal shift point changes from gear to gear, it is always above the max power point.

johnfelstead

Can you tell i have nothing to do tonight?

Here is what i mean, a plot of gear ratios against engine power used to calculate shift point. If you were to change at peak power the area under the power curve would be less so you would have slower acceleration. In practice i would set the shift light at 7050rpm as that is the best average shift point, even though peak power is at 6200rpm.



Hopefully this makes sense.

[Edited by johnfelstead - 1/10/2004 10:15:22 PM]

chrome

nice one john



*god you were bored weren't you

johnfelstead

you have no idea. LOL

Here is a plot with the shift point set at the maximum power point (in blue) and the best shift point (in black) so you can see the effect.

johnfelstead

These are for my engine/gearing combination, every car is diferent but you should be able to see the theory. Some engine/gear combinations have the ideal shift point in the lower gears past the rev limit so with those you would shift just before the limiter in the lower gears.

You can also produce a diferent data sheet that you can use for drivability if you arnt after max acceleration using the torque curve instead of the power curve.

bartmanuk

*** me john you must have been bored m8 lol
cheers for that tho m8,thats taught me lots.so where can i get one of these done?as i would like to know more things about my car like this.
i did go to power engineering in november and found out my max power and torque,but wot you have there is very interesting.

BM

Scrappydoo2003

Or maybe the Fiat 500 had a tractor beam fitted to it? Beam my up Scotty

johnfelstead

You can produce a gear ratio sheet quite easily if you know the diff ratio, gear ratio, max engine rpm and tyre size. You then need a dyno plot of the engine to superimpose. The easiest way to get this is either by putting the car on some rollers and getting a printout or by using delta dash to give you a power at the wheels plot.

Do you have a printout of the power curve or did you just get a peak figure?

RON

Fiat 500's are Ace, I passed my test in one, am I'm now thinking of buying another, just for the grin!!