147 TS throttle behaviour?

[johnl]

My TDC resetting topic has me thinking about throttle behaviour, or rather engine response to how far the pedal is pushed. My engine seems to stop responding once the pedal is about 2/3rds depressed. That is, the last third or so doesn't seem to make any more power.

Judging by feedback from the arseometer, it feels as if the butterfly is already fully open by the time the pedal is 2/3rds to the floor? Or, as if perhaps throttle body might be bigger than it actually needs to be? If so (?), then perhaps opening the butterfly past X° doesn't really do anything much because the constriction isn't the throttle body ID, but is instead say intake valve or port size, or something, maybe?

I don't know, but I do subjectively feel that the last 1/3rd or so of throttle pedal travel is rather pointless, at least with my engine. Anyone else?

Regards,
John.

[bazzbazz]

Have you carried out a "Throttle Reset" ?

[johnl]

Have you carried out a "Throttle Reset" ?

Yes, but not for a long time, maybe about a year or so ago. Afterward there didn't seem to be any noticeable change in the way the engine felt or worked. I'll have another calibration session, but I don't expect any improvement.

I'm finding that this characteristic affects how the engine feels to drive. It contributes to a subjective feeling of the engine being underpowered in some circumstances, whereby you make a big input with the right foot and so 'expect' a big output. Instead, the engine only responds up to X pedal movement, then from there on is unresponsive to further opening, so feels 'wrong' and 'soft'.

I don't think there is an actual performance problem here, the engine is running sweetly and objectively accelerates well, feeling responsive up to about 2/3rds pedal movement. After this (with further movement) the performance feels as if it goes 'flat' because further pedal input does not generate an increase in power output. It is only a psychological issue to do with linearity of control and 'feel', not a real problem re power output.

I'd much rather a short pedal travel where all the travel feels to have some affect, than a long travel where much of the travel feels to do nothing. I do prefer a short pedal travel rather than a long one (I always have a very short travel adjusted into my kart pedal), but I'd be OK with a long pedal if it was reasonably linear in its' affect from closed to fully open. My throttle pedal lacks linearity, i.e. response to pedal movement seems so 'digressive' that it (response) becomes zero well before the pedal hits the floor.

It works the other way round too. As it is, when the pedal is already open a long way (floored or near to it) and I want to modulate acceleration or actually reduce speed somewhat, lifting off a bit doesn't do anything. I have to back off quite a lot to get a power reduction, because it takes a big lift to get any reduction in acceleration rate or actual speed, which is actually quite annoying.

This tends to suggest that the throttle body ID is not a constriction to airflow until the butterfly has been closed quite substantially, which tends to imply that the TB is too large in ID. If this is actually what is going on, then one 'fix' might be to place a block under the throttle pedal to eliminate the 'useless' pedal travel...

I'm fairly sure this isn't all just in my head, just wondering if anyone else has felt the same thing...

Regards,
John.

[bazzbazz]

If you have diagnostics you can actually graph the pedal movement, & throttle body movement together & compare.

[Cool Jesus]

Alfa's electronic accelerator is tuned down with respect to pedal position so as not to allow the end user to be too abusive on the go.

See Novitec GP1.

http://shop.alfisti.net/Tuning-Styling/Alfa-147/Motor-Sports-Airfilters/Novitec-GP1::6627.html

[johnl]

"If you have diagnostics you can actually graph the pedal movement, & throttle body movement together & compare."

I haven't desperately needed "diagnostics" as yet, so don't have it. I should get something...

"Alfa's electronic accelerator is tuned down with respect to pedal position so as not to allow the end user to be too abusive on the go.

See Novitec GP1."

I do think car manufacturers may well play games with what the throttle actually does, made possible by the electronic nature of FBW. Personally I wish it was just a cable connecting the pedal to the butterfly, without a computer making decisions on my behalf.

It occurs to me that there could be a reason a car manufacturer might be tempted to fit a TB that is somewhat larger than actually needed. A 'too big' TB will flow more air at smaller throttle openings, so will tend to make the engine feel more powerful / responsive at lower rpm / lighter throttle opening (i.e. feel more 'sporty' for a given max power). This would make a car feel 'zippier' in say an urban environment, where it is constantly accelerating from rest / low speed to moderate speed.

The downside is that this would adversely affect linearity of engine response as the throttle is opened further and further. The overly bored TB may well be capable of flowing significantly more air than the engine is capable of ingesting, because the 'bottleneck' is not the TB bore but something like valve or port size etc etc. This may make the last part of the throttle opening more or less redundant.

Of course, this being an electronically controlled FBW throttle, it should be possible to create any pedal to butterfly motion relationship the engineers may want. Of course for this to be useful the TB bore would need to be at least X size, or more. Keeping in mind that a TB that is too large won't restrict flow and thus won't reduce power, there might be some incentive to specify a TB that is just a bit too large just to ensure it doesn't ever choke the engine?

A clue might be found in how modified an engine needs to be (i.e. how much of an increase in air flow due to cams, porting etc.) before the stock TB starts to become the limiting factor. If the stock TB is fine with modifications that modestly increase airflow and thus power output, then this may imply that it could be a bit oversize for an unmodified engine?

Anyway, you can try this at home; accelerate hard in say third gear (or whatever works for you and whatever speed and safety restrictions apply on your test road) until rpm climb reasonably high, then back off just a bit. Does the rate of acceleration diminish? Or, do you have to lift off quite significantly before it does? If your engine is like mine, then the latter is the case.

I sometimes find that when I lightly back off from WOT there is a sensation that the rate of acceleration increases slightly, when it should be decreasing. I don't think the acceleration rate actually does increase, but I think that it doesn't decrease until the pedal has been backed off to a significant degree.

Subconsciously you (I) expect an instant drop in acceleration / speed the moment the pedal is lifted, but because this doesn't happen there is a feeling that acceleration increased. A bit like when you lock up the brakes on a slippery surface, the car feels like it starts to go faster, even though physics dictates that it can't.

Regards,
John.

[Citroënbender]

I run my diagnostic software on an old IBM ThinkPad with dock. They're cheap and tough. Mine uses XP Pro but you could probably also install an OS like Mint. There are many times it's handy, you can use it to test presumptions, identify certain referenced components in generic fault codes, check for swapped modules and confirm firmware version.

There's stuff MultiECUscan doesn't do, like show individual coil or injection data, but it's still a worthwhile product in the paid format.

[johnl]

If you have diagnostics you can actually graph the pedal movement, & throttle body movement together & compare.

So if a substantial mismatch were seen then this could be a cause of the characteristic I'm seeing, maybe.

'Normal' would be to see pedal and butterfly movement 'graphs' match each other from idle to WOT? Lets assume that the 'throttle system' does this, but a significantly non linear relationship between pedal position / butterfly angle and power output still exists.

This would at least hint that the throttle body is capable of flowing significantly more air at WOT throttle than the engine can actually ingest (full flow into the engine being achieved before the butterfly is fully open), which would cause the last X° of throttle opening to be ineffective with respect to increasing power (wasted pedal motion, adding to the FBW sensation that the pedal isn't really quite connected to the engine (though of course the same issue could exist with a cable TB)).

The effect of an overly bored TB would be to increase initial throttle response, but diminish response the closer to WOT the butterfly moves. The full flow that the engine can actually use would occur at less than fully open butterfly. So, the driver sensation is that the engine seems to become less responsive to pedal motion the closer to the floor the pedal is pushed (this sensation may exist to some degree anyway, but I think the 'dead' pedal motion makes it feel significantly worse). Also, backing off from WOT won't result in the engine backing off from the power it's making, unless the pedal back off is significantly great enough (which adversely affects throttle control and sensitivity).

If this anaylsis (i.e. series of guesses and speculations) is correct, then ideally a slightly smaller TB could be installed. Failing that, a reprogramming of the ECU throttle maps might work, limiting butterfly opening to only that needed to provide the maximum air the engine itself can actually use (if one knew what the hell they were doing). Failing either of these things, a block of wood under the accelerator pedal...

Regards,
John.

[johnl]

See Novitec GP1.

According to some graphs I've seen, I think that this and similar devices have a similar affect as the 'sport' button in our Saab 9-5.

My understanding of such devices (FWIW) is that they act as a digressive multiplier of pedal input. The signal sent to the ECU (and hence to the TB) with pedal motion is modified by the device, i.e. initial pedal motion is multiplied by X at the TB, but as the pedal motion increases the degree of multiplication increasingly decreases (can that be said this way?), creating a (more?) digressive relationship between the pedal position and the butterfly angle.

With the device, the butterfly initially moves more per unit of pedal motion, but it still takes the same amount of pedal motion for the butterfly to reach WOT. At first the butterfly moves more, then it moves in a manner increasingly closer to the original stock motion, reaching WOT at the same pedal position as would occur without the added device.

The device will make the engine feel more responsive / powerful with smaller pedal movements, but will be less effective at larger throttle openings since the modified and standard pedal / butterfly response curves will converge as pedal position moves closer to the floor. The tendency will be to make the engine feel more responsive with small pedal movement, but less responsive with very large pedal movement.

Don't get me wrong, I quite like our Saabs' 'sport' button, it's my preferred mode. However I tend to doubt that fitting a device such as the GP1 will have much affect on the issue I'm discussing, i.e. possible oversize of the TB and the driveability issues this may cause.

Regards,
John.

[Cool Jesus]

Can't comment much on the technical side of the Novitec GP1 module other than their literature makes comment on the ECU making changes to variables it can control (such as fuel delivery) in order to haven't he vehicle meet emission standards. So it may not be just a direct input correlation between pedal and throttle plate.

What I can say is, I installed the module on my 159 yesterday and acceleration response to pedal input is now day compared to the OEM night. Huge difference to now be able to take advantage of the engine's power output. Highly recommend the gadget. There are other brands also, not just Novitec. I think I went with Alfisti.net because of price and an offer of returning it within a month if you're not happy with results.

[Colin Edwards]

Hi John,
The missus had a 147, however I don't recall what type of throttle it used.  If its just the plain old butterfly type then that type of response is typical of the design.  All things being equal, the larger the throttle body / throttle plate the more likely the non-linear response. 

Using a karting analogy, we went to great lengths to ensure the butterfly on the Walbro carby was exactly in-line with the air flow through the venturi.  Not so much to do with air flow but more to ensure correct mixture creation at high airflow / high rpm.  Could be worth 400rpm if not correct.  On a flow bench it was proven to me the airflow through the Walbro and other similar pumper type carbies like the Ibea or Tillotson, the butterfly could be only 70% open and potential airflow would be at around 97%.  The disturbance of the fluid through the carby caused by a not fully open butterfly had a significant impact on emulsion creation and flow, not so much on total air flow.   Not so much of a problem on the slide type throttle of the Dellorto fitted to a Rotax.

Maybe car manufacturers compensate for the logarithmic type response of the simple butterfly valve with a bit of electronic wizardry in the servo system between pedal and valve.  It would make sense.  The "throttle" on a fuel injected car is only responsible for fluid control rather than mixture creation so would make sense to let the inlet valve events control ultimate airflow.

Its a bit like a steam locomotive.  The most efficient mode of operation is to run with a wide open throttle and modulate power / speed via the infinitely variable valve events / travel provided by this very elegant design.   Funnily enough Alfa Romeo copied the steam locomotive with the multiair engine!!!

[johnl]

Hi Colin,
I thought your name was familiar, and it has occurred to me that you could possibly be the kart racing Colin Edwards. We may have previously talked on Kartbook...?

I can't see the precise butterfly plate position being an issue with this (but I've been wrong before, maybe a couple of times...). And of course a fuel injection TB has no direct bearing on emulsification / dropletisation (I made that word up) since the fuel is entering the airstream nowhere near the TB.

It wouldn't surprise me if the Walbro et al were at max airflow before WOT, especially if fitted to something like a J engine. It also wouldn't surprise me if at least slightly 'oversize' TBs are quite common in road cars...

Of course running a constant WOT is how diesels operate, and is known to be more efficient than choking the throttle to control power output. It's my vague understanding that a significant amount of research has been / is being done into trying to run petrol engines with a constant WOT, controlling engine speed / power output with extremely variable valve timing and / or lift. But it tends to be very complicated to achieve, and complication = cost and reliability issues...

Regards,
John.

[Colin Edwards]

The Fiat / Alfa Romeo MultiAir engines fitted to the Giulietta, the Mito, the 124 and I believe Giulia Veloce do not feature a "traditional" throttle.  The throttle-like device associated with the Multiair appears to be only required for low < medium rpm. Valve timing, lift, travel and duration etc provide the control of airflow and ultimate power output.  Innovative use of high speed servos and hydraulics.  Very clever!

[johnl]

I know of the MultiAir engine, but don't know much about it. I've been meaning to check out how it works, but for some reason just haven't gotten around to it yet. It does sound very clever as you say.

Regards,
John.

[johnl]

So I used a lump of wood attached to the underside of the pedal, blocking approximately the last 25mm or so of pedal travel. The result is interesting.

The size of the wood block (a bit of dowel) is fairly arbitrary. I just attached the block and tried it out (engine not running), then cut some off, then cut a bit more off until the pedal travel felt (underfoot) about an inch or so less than stock, roughly equating to how much of the last % of pedal travel felt (to me) to be more or less superfluous (i.e. no significant engine response to using the last 25mm or so of travel, as very roughly estimated by my ankleometer).

The stock pedal travel was about 70mm or so. It's now about 45mm, so throttle pedal travel has been quite significantly reduced. Does this cause the engine to make less power at pedal WOT? Does this mean that full butterfly WOT is not actually reached? Does butterfly WOT actually make more power than some lesser butterfly opening? I don't have any hard data, can't say.

I can say; quite subjectively, that the engine feels somewhat more responsive and powerful than with the stock pedal travel (which is a bit weird). The butterfly obviously cannot be opening further, so airflow cannot have been increased, at most the flow is relatively unchanged, or at worst is less, than before.

I think it's partly psychological, i.e. a placebo of sorts, related to the perceived reduction in pedal travel. The mind is tricked into feeling a more enthusiastic engine response simply because a smaller action (foot movement) was needed to achieve it. However, I really don't think there has been any significant actual loss of power (the sensation that power is increased is fairly compelling, so I tend to think there is no actual reduction).

With the stock pedal travel there came a point where further movement really didn't feel like it was doing anything more, just wasted woolly pedal motion. Even if the mind is not conscious of this, I suspect it is still registered subconsciously and contributes to 'feel'. With the shortened travel the pedal now does not reach a point where it ceases to feel that it is having some effect on power output. The engine response feels less 'doughy' (especially) near 'full' throttle, and in general feels more responsive.

Lifting off the throttle, the rpms feel to drop more immediately. Full throttle acceleration in first and second gear (using higher rpm) now feels at least as 'energetic' as before, but with better controlled and faster shifts. I think this is because the pedal now no longer needs to be 'lifted' that 25mm before any actual power reduction starts to occur. In stock form this 25mm of 'lift' takes some amount of time, not a lot but enough to be felt as a lessening of responsiveness to the lift, and contributing to less precise timing of all the actions the driver does when changing gear (all I really know here is that the shift feels better...).

Whatever, some may scoff but I really like the affects of this unlikely pedal modification, in whatever %s they might be real and / or imagined...

It does seem to me at least not unlikely, given this subjective 'evidence', that the TS TB is at least a bit oversized relative to the maximum airflow that the engine can actually achieve, and / or, the the butterfly reaches WOT significantly before the pedal contacts the pedal stop on the floor...

Regards,
John.