Digital v Analogue Right Foot...

Zippy · Post by **Zippy** » Sat Jun 17, 2006 10:57 am

Right, I'm in danger of making an ar$e of myself here but this is something I'd like explained properly. I've noticed an interesting phenomenon relating to my use of the throttle pedal. I have my own thoughts but I'm not sure that I'm right...

DIGITAL...
When looking to accelerate hard if I plant the throttle the car picks up pace pretty quickly but it doesn't 'feel' quick.

ANALOGUE...
If I slowly squeeze the throttle as the revs build I can induce the magical 'push-in-the-back surge of power'

feeling from when I start accelerating all the way through until I have to change up.

I played with this quite a bit over the weekend and it was quite repeatable, the squeezy, throttley bit though requires a bit of practice to ensure there's not too much throttle application, e.g. if I push too much to start I don't get the push-in-the-back, then if I stop momentarily & let the revs build a bit then start squeezing again I can induce the 'surge'.

I'm not sure which is quickest however I know which feels quickest. Is my brain playing tricks on me here or is there a straightforward explanation relating to the fuelling that needs explained to me?

Over to you, guru's...

Thanks,
Iain.

Uldis · Post by **Uldis** » Sat Jun 17, 2006 11:36 am

The mapping is not perfect.
The quickest is probably what you feel as quickest.
I've had this before on a big Honda V8 after changing carburators. Basically you're having a more correct AFR with partial throttle than with full. There is also the full throttle "squirt" emulation that maybe is putting in too much fuel.

Is your air filter clean? this could cause those symptoms...

BTW, which is your car with which ECU?

GregR · Post by **GregR** » Sat Jun 17, 2006 11:49 am

Zippy, noticed exactly the same thing driving back from Motherwell yesterday! I had been applying throttle progressively and getting a nice big grin feeling, so I thought flooring the loud pedal would create a greater surge and it was actually worse!

Looking forward to the responses here, and my filter is only 2 weeks old, so I doubt that's my problem.

mac · Post by **mac** » Sat Jun 17, 2006 2:35 pm

Old cars, as Uldis has said, used to have a fuel enrichment mechanical system which literally pumped fuel into the carb. The ecu tried to mimick this but since there is no physical link (the link is physical to the throttle flutterby and then from there it's electrical impulse to the ecu and then back to the injectors)

There's also the possibility that when you stab open the throttle the extra air kills the vacum (sp) so all the air isn't drawn into the cylinders in that cycle.

For what's it's worth I find progressive application of the loud pedal better than jumping on it.

Mac

tenkfeet · Post by **tenkfeet** » Sat Jun 17, 2006 3:23 pm

Top tip .Just been out testing the theory and there is a definite difference . Hitting the throttle fast there is a very slight delay like the engine is thinking about it . Using the fast progressive method it appears to rev immediately which continues up the rev range. Using timing equipment calibrated in elephants it was about half an elephant faster 30 to 60 mph in third.

I also checked the other day if throttle was opening fully. It wasn't so after quite a large adjustment its even better . I think that was also posted on here .

Post by **ironside** » Sat Jun 17, 2006 7:30 pm

mac wrote:Old cars, as Uldis has said, used to have a fuel enrichment mechanical system which literally pumped fuel into the carb. The ecu tried to mimick this but since there is no physical link (the link is physical to the throttle flutterby and then from there it's electrical impulse to the ecu and then back to the injectors)

When you floor the throttle suddenly the induction mixture will weaken a little for a short time because air is lighter than fuel and is drawn in more rapidly. There can also be a quick stutter/hesitation depending on the timing of the flooring because the fuel may already be injected and the inlet valves open.

An ECU with acceleration fueling capability will keep track of sudden changes of the throttle (using the throttle pot) and enrich the mixture a bit when this happens and decay it over time.

The Emerald does this - you can find it under Additional Settings/Accel Fuelling in the software. You can set enrichment % and clamp/decay time there.

Simon
(not a guru - but have read the manual)

robin · Post by **robin** » Sun Jun 18, 2006 10:13 am

Actually I don't believe it has as much to do with fuelling as it does with the mass of air you're trying to acclerate. It's basically the same explanation as to why variable valve lift makes sense - at low power you want small valves which don't open much - the volume of air that the engine needs is tiny and it's best drawn through a small hole - as the engine's power output increases, it requires more air and this is best drawn through a progressively larger hole.

I believe that the effect will more-or-less vanish above ~4,000rpm so is mostly a road-phenomenon ...

BTW - all ECUs with cable throttle systems have fuelling enrichment on throttle transient - it's a way of making sure the engine doesn't run lean when it suddenly gets more air and the ECU's sensors and control loops haven't caught up with that fact yet.

And all of this neatly explains why drive by wire is better than cable throttle - the pedal position indicates driver's required engine speed, computer can open throttle to ideal opening position to maximise power output at current engine speed (and thus accelerate to desired engine speed ASAP). By controlling the throttle opening the ECU can reduce the need for throttle transient and so on the road the car will be more economical.

Cheers,
Robin

(P.S. I don't know if they actually implement the drive-by-wire the way I explain it).

Zippy · Post by **Zippy** » Sun Jun 18, 2006 10:23 am

Typical, Robin beat me to it. Here's what I'd originally written below. Thanks for the feedback Robin, Uldis, Mac, Simon.

Tenkfeet, where did you come across the 'Elephant' calibration system? I assume this is the one used in Gregory's Girl and the UK equivalent of the American 'Mississippi' system.

Cheers,
Iain.

Uldis, I've still got a fully standard base S2 setup - 120hbp K, std K4 ECU, std induction, exhaust etc.

My thoughts were along the lines described by yourself, Mac and Simon. Given that the throttle - Mac, I say butterfly, you say flutterby (que for a song) - position is manually controlled by the right foot, the air flow is fully under control of the driver, and that the ECU then has control of most of the other stuff, then the ECU's desperately trying to make the best of the air flow it's fed but might not be able to do enough to cope with the sudden change.

So since the driver's in control of one of the biggest variables if the use of this isn't optimal for the engine speed it'll have a detrimental effect on accelleration.

This gets me wondering about these new fly-by-wire systems which I'm led to beleive control the throttle butterfly position as well, use the throttle pedal position as a guide for how much additional power is being requested by the driver. Does this mean that in these cars 'flooring it' could give a better response than progressive application of the peddle if the ECU's programmed to respond to the pedal application in the right way since it has control of the air flow as well?

Anyway, time for me to read the Engine Management manual I suppose.

Thanks,
The phantom dish-washer.

tenkfeet · Post by **tenkfeet** » Sun Jun 18, 2006 12:59 pm

Zippy wrote: Tenkfeet, where did you come across the 'Elephant' calibration system? I assume this is the one used in Gregory's Girl and the UK equivalent of the American 'Mississippi' system.

.

I had forgotten it was in Gregorys Girl . One Weegie Elephant is the equivalent to 0.8 US Mississippi's . .

Post by **Shug** » Mon Jun 19, 2006 12:04 pm

The problem is also minimised with DTH TB's. No big volume of air inside a plenum to accellerate - butterflies about 60 or 70 mm from the inlet valve. That's definately one of the first things I notice about driving a standard car - that almost elastic throttle feel (proportionally - still miles better than 99% of boxes on the road).

It has it's disadvantages - I'm having to be really carful with throttle application in the rain on these maxies with -2.5 deg camber on the back - it gets very twitchy, very quickly and I feel that little cushion in the standard throttle pickup would be quite nice ATM!

Rich H · Post by **Rich H** » Mon Jun 19, 2006 12:53 pm

I'll swop you my pleum for your DTHTBs if you want Shug Hurricane and all

Post by **Shug** » Mon Jun 19, 2006 12:57 pm

Hmmmmmmmm...... <ponders>

No. GTF

Rich H · Post by **Rich H** » Mon Jun 19, 2006 12:59 pm

No pleasing some people