Re: I bore TBs ...

I'm using two different tools for reading the throttle position sensor, and they are giving me two completely different answers. What?!

For a couple of races earlier this year I was disappointed in my performance. On a whim at the next event we decided to check the throttle cable, and found two problems. First there was something wrong at the linkage point under the car where the pedal cable meets the engine cable. Then I found further that the height adjustment on the Rennline pedal was preventing the pedal from going all the way to the floor! Using the Durametric, and checking both with the pedal on the floor and by manually pulling the lever on the throttle body (to achieve 100% throttle plate opening), we found the following using the Durametric cable and software:

Manual test at throttle body: 76 (presumably that is 100% but ... who knows?)
Starting point: pedal 50 -- NO WONDER I WAS SLOW!
After linkage fix: pedal 65
After pedal height adjustment: pedal 76
Back on track for the next session, the car was noticeably more responsive to mashing the gas pedal and I went faster immediately.

I knew I might need to take a closer look at the pedal and make some adjustments. Because the Durametric is kind of a pain especially when trying to read realtime values on a computer across with a tiny font and horrible charts several feet away, I researched and bought a GoPoint BT-1 which plugs into the OBD port and via Bluetooth displays values on your phone. Here's what it reads:
Manual test at throttle body: 100
Pedal: 98-100

Plug the Durametric back in and check it back-to-back:
Manual test at throttle body: 76
Pedal: 76

Why do the two devices display the data differently? Is it just a number coming through the OBD, or could the BT-1 app be saying "oh, an OBD value of 76 on the throttle means 100% so I will display 100". ? Are there two sensors and I'm looking at a different one on each device?



Edited 1 time(s). Last edit at 11/06/2016 10:59PM by Trygve (Oakland, CA). (view changes)

But not pushing my memory too hard the throttle plate never fully closes and under full throttle will not fully open.

There is a voltage range that is less than the nominal 5V that is acceptable, that represents the throttle not open (at least not open by some signal from the gas pedal) to fully open.

I believe what you are seeing is normal but after I get to work let me refresh my memory...

Mark, this is an original model (1998). Throttle cable all the way from pedal to throttle body.

Trying to understand why one device seems to say "76" at WOT and the other says "100". I would think they are reading the same actual value from the OBD.... unless... ?

If its not e-gas, its a cable. So is there even a sensor to know where the metal butterfly is? I doubt it - I would guess that it is reading airflow and working backwards (but maybe you know better). Given that, i'm trying to think what it can read except something off the MAF, and somehow adjusting for temperature, altitude (pressure) and anything else it needs to compensate for.

I guess one could be an absolute scale and another one which, for some set of circumstances, is scaled to 0-100?

Just a thought. Else i would imagine it might rarely actually hit 100, or Zero.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

This is all happening without starting the engine. Just with the key on and pressing the gas pedal or pulling the throttle cable spring on the throttle body housing.

The sensor outputs a voltage, not a percentage. One or both of the two pieces of software is misinterpreting the meaning of that voltage.

You can measure the voltage output yourself and compare it to spec to determine who's right. Or maybe you can adjust the settings on the software to understand the correct voltage range.

... is mounted right on the throttle body.
It's a simple rheostat so voltage varies according to the butterfly's position.
Depress the accelerator pedal using a piece of lumber to hold it completely down.
Remove the air intake tube from the throttle body and look at the position of the butterfly.
At WOT it should be horizontal, meaning completely open.
If it's not completely horizontal with the pedal completely depressed, adjust the throttle cable until it gets horizontal.
Then read your Durametric or other tool and that's your 100% value.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

two side draft Webers on my Datsun 510 the butterfly valves -- all 4 of them -- didn't open to a fully horizontal position. I was told they were not supposed to but I can't recall the reason now. Just like they didn't fully close. I remember the reason for that: So the butterfly valve couldn't get jammped stuck shut.

Pedro, thank for the good ideas. It seems to me that the Durametric might be misinterpreting the voltage as Boxsterra mentioned. They're both giving wildly different "100%" numbers (assuming it is wide open, which I can verify by looking at the butterfly as you suggested). My guess is that the BT device is correct and the Durametric simply misinterprets the voltage range such that the max value is displayed as 76.

I'd be curious if anyone else can hook up a Durametric or other reader and check what their reading is when the pedal is to the floor. You just have to turn the key on, not start the engine.

PST-II I get 100% with WOT in my car.
I have replaced the rheostat (throttle position sensor) twice in 19 years.
They will get dirty and not give the correct sliding resistance as they get old.

With carburetors you want to have a bit of turbulence at full throttle because the mix is being created at the carb itself.
With fuel injectors the mix happens just before the intake valve so you want fast flowing air at the TB.
With carburetors you also need for some air to pass through at idle speeds so the butterfly needs to stay a bit open.
In the non-egas 986/996 engines there's an idle control valve that opens or closes a hole in the throttle body allowing enough air for idle while the butterfly is completely closed.

Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

dial in the desired idle speed. While there is a gap at the throttle body it is not sufficient to provide all the air needed. There also needs to be an adjustment to deal with differences in engines, elevation, etc.

Furthermore, if the butterfly valve closes completely there is the issue of it sticking closed from the pressure difference between the cylinders creating very low pressure on one side and the high pressure -- atmospheric pressure -- side of the butterfly valve. There is also the issue of wear both of the throttle body bore and the butterfly valve.

If the butterfly valve opens all the way -- becomes completely horizontal -- there is a concern that the valve gets "stuck" in the completely horizontal position due to the air flow allowed by a fully open butterfly valve. In this situation the spring or butterfly control hardware would be unable to close the valve against the air flow and then one would have an essentially run away engine operating at (a stuck at) WOT.

With the E-Gas setup there is a mechanical stop to prevent the butterfly valve from closing too far and and a mechanical stop to prevent the butterfly valve from opening too far. A reference is made that in the "fully" closed position at the lower electrical limit stop -- which is determined during adaptation -- that this prevents the throttle from making contact with the throttle housing bore where it could cause wear and eventually get jammed.

The upper electrical limit stop is determined by the DME 7.2 control module and does not require adaptation.

... and include a new butterfly valve to fill in the new bore.
On cable-operated TBs, I assure you that the butterfly closes all the way.
It has a very powerful spring that keeps it completely closed.
The only air that gets in when the pedal is not depressed is through the idle control valve.

Here is a side-by-side picture of a standard non-egas TB on the left and a bored out non e-gas on the right.
Note how the butterfly perfectly seals the bore on both TBs.

[pedrosgarage.com]

Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna




Edited 1 time(s). Last edit at 11/08/2016 10:57PM by Pedro (Weston, FL). (view changes)

Quote
Pedro (Weston, FL)
... and include a new butterfly valve to fill in the new bore.
On cable-operated TBs, I assure you that the butterfly closes all the way.
It has a very powerful spring that keeps it completely closed.
The only air that gets in when the pedal is not depressed is through the idle control valve.

Here is a side-by-side picture of a standard non-egas TB on the left and a bored out non e-gas on the right.
Note how the butterfly perfectly seals the bore on both TBs.

[pedrosgarage.com]

Happy Boxstering,
Pedro

Sorry to nitpit but "perfectly seals"? There has to be some clearance between the butterfly valve and the bore or the valve would/could jam.

The heavy spring keeps the valve closed. But what if the spring breaks? What closes the valve in this case if it is wide open? Besides a heavy spring is not ideal as it increases wear on the butterfly valve shaft and the throttle body bore the shaft resides in. As these wear the butterfly valve then moves from its centered position and contact with the bore is made. An important step in the carb rebuilding process involved replacing the worn butterfly valve throttle shaft and addressing the worn throttle shaft bores in the body of carb.

Quote
MarcW
Sorry to nitpit but "perfectly seals"? There has to be some clearance between the butterfly valve and the bore or the valve would/could jam.
The heavy spring keeps the valve closed. But what if the spring breaks? What closes the valve in this case if it is wide open? Besides a heavy spring is not ideal as it increases wear on the butterfly valve shaft and the throttle body bore the shaft resides in. As these wear the butterfly valve then moves from its centered position and contact with the bore is made. An important step in the carb rebuilding process involved replacing the worn butterfly valve throttle shaft and addressing the worn throttle shaft bores in the body of carb.

Yes, perfect seals.
When I started boring these TB, the slightest gap, and I mean if you could see a spec of bright light shining from the back side, it would not let the car idle properly.
The only way to properly bore them is with precision milling.
As I've stated three times before, the amount of air needed for idle is governed by the idle control valve which requires that all of the air gets metered through it, hence a perfectly sealed butterfly.
If the butterfly spring breaks (which I've never seen happen) the gas pedal return spring would probably shut the butterfly., if not I guess that what the ignition is for.
I feel MUCH safer in my cable-operated, spring-returning butterfly than in a fly-by-wire, e-gas car. Do you remember Toyota?
Up until the 991 every GT3 racecar still had a cable-operated TB, even the 997s.
TBs are very different and much simpler than carbs. The only moving part is the idle control valve and the butterfly's shaft. Even with extreme high mileage (over 250K miles) I've never seen on that needs rebuilding.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna