Everyone,
Call me crazy as a lot of people do, but during my last few times on the track as I was breathing off the throttle after a long straight run(approx 124mph in 4th) I SWEAR my butt felt the car acclerate more strongly. Of course this lasted only a split second. At approx 9/10 full throttle. After that it was normal. Thoughts? Anyone else ever notice this? This only occurs when the throttle is lifted slowly and after a full throttle run. I havent noticed it in 1st,2nd or 3nd gear at all.
I want my cable and holly carb back!
olddragger

 

O.D., this issue was discussed a while back and a conversation on the concept of "leading throttle" ensued. You may find it still around on the forum.

CRH

 

Ok thanks man.
OD

 

Guys,
This may be an issue when tuning the cz's or greddy's set up. (And Charles you were right this was discussed. Youre memory amazes me. This car IS very responsive to a leading throttle).
Just using logic consider this. If this car/ecu is "reading" the throttle position in such a sensitive way then the ecu management not only reads and responds to air flow, load, etc,etc ad nausem BUT it also reads how FAST the throttle is "Mashed" (thats opened to you non southerners!). So in essence if a person is tuning custom maps then differant readings will show if there are no other variables except has fast you push the throttle to the floor! ? ANd on the cz thread some are wondering why they are getting differant reads on differant runs with the same map,run and air temps etc. I wonder if this has something to do with that. Just another wall to jump in dealing with this ecu.
Guess I'll have to change my driving style a little!
How can I be "Olddragger" if i can't put the petal to the metal! Damn!
olddragger

 

Only problem is that the ECU doesn't "read" the throttle position, it commands it.
You can stomp on the gas pedal as fast as you want, the ECU will open it when it damn well pleases.
That doesn't mean you aren't experiencing a lift-throttle surge. It just means your idea on the cause isn't right.

 

Thanks MM you know more than I do, I readily admit. But please explain (in simple terms) what is causing this? Does this car respond better to a easier breathing on and off of the throttle? Maybe respond is not the word I should use. Does it matter how the throttle is used in referance to power? Faster raw engine acceleration with a fast linear push of the throttle OR faster with a pedal to the medal? OR both would be the same?
thanks
OD

 

jeff, than why does leading the throttle seem to give a better response than flooring the pedal? also why does it seem to accelerate faster at about 95% wot pedal position than 100%?

 

Well, my first inclination would be to think that the tuning is intentionally deficient in this regard.
Probably an attempt to lessen shock loading on the drivetrain and such.

Also, the butt is not a dyno. The perception of a loss of torque at a certain throttle opening may be something different entirely.
Some really specific CANScan logs might help here.

However, I have had several cars that weren't fly by wire exhibit this behaviour, though my RX-8 does not, so I am stumped at the moment.

 

As I was reading the latest issue of Hot Rod I got to an article about the new piggyback/stand-alone ECU's out there for domestics and they discussed the open loop/closed loop situation. I was also remembering an explanation of the same by MM, and another thought came about. Since the RX-8's ECU goes into open loop at WOT, does it make sense that the "surge" we feel when returning to less than WOT is the ECU switching back to closed loop? I am thinking that the surge may be the ECU setting the ignition timing to a bit more advance, slightly leaner A/F mixtures, and etc. This may also explain MM's quandry on the cable-throttle cars, too.

The only other possibility that I can come up with has to do with the location of the highest vacuum signal in the intake tract at various throttle positions/loads, although that phenomenon occurs moreso in carbureted engines than FI engines.

CRH

 

Now that's definitely an interesting theory and one worth thinking about. I'm very familiar with the problem, although, not on my 8.

Back in the day AKA early 1980s. I had a VW Sirocco 4 speed with Fuel injection, a Cat (ahem) “Test” pipe, Anza exhaust, etc. While it always had a surge on lift from full throttle "mash," I really saw an increase when I threw a Webber big diameter throttle body on it.

The Webber definitely gave it a bigger punch at higher RPMs but you traded some torque down low for it. It replaced two smaller throttle plates with one huge one. So at lower RPM the air wasn't flowing as fast in the intake manifold because it didn't have as much velocity from rushing through two smaller holes in the throttle body. So torque was down a bit.

Anyway, I really only noticed the problem coming off WOT when I was a few thousand below redline. Backing off WOT at redline never caused the problem. So my theory was that when I backed off the throttle and the throttle plate in the throttle body partially closed, that caused the air to accelerate as it squeezed through the narrower opening and gave more velocity to the air running down the intake manifold into the engine, which resulted in a surge. Then as the throttle continued to close, air was restricted enough that power fell off and the surge went away. Note: since I was at full throttle, but not at redline the engine had access to more air than it needed for that RPM, so I believe it's possible to have more HP by increasing air velocity through pinching off the airflow with partial closure of the Throttle body. Anyway, that was my theory.

 

^^ Good possibility.

My Miata *had* that issue before I got the E-Manage on it.

My MX-3 had this problem right from the factory - holding the throttle open just slightly less than WOT produced more power in all RPM ranges.
I tried adjusting the TPS to no avail. Probably this odd combination of open-loop and higher velocity air.

 

I guess the real solution is to avoid the open loop/closed loop conflict. That's what FAST has done with their new XFI system. As stated in Hot Rod, "Thanks to the added accuracy the wideband O2 sensor affords, the system can run in closed loop under WOT.". They then go on to explain, "The results are greater adapatability to changing atmospheric conditions and more leeway for the end-user in dialing in the tune.". What strikes me as odd is that it seems what the domestic community is now realizing, the imports have known for a while with regard to hardware. On the other hand, why have I not seen any advertising for import-based ECU's mention the open/closed loop dilemma? What we 8'ers really need is to either better plot WOT and partial-throttle maps or eliminate the switching, altogether. Why do we need to have two loops, anyway? It seems to me that the modern ECU's are trying to do the same job that my old cars did using power valves in the carbs and vacuum advance diaphragms in the distributors. In this case it seems as if the ECU's can sometimes overcomplicate a rather simple situation. I realize the processing power of the modern PCM's better allows the cars to meet regulation requirements but things get a lot easier when the end user can ignore certain federal regs like economy and emissions. I am sure all of this has been discussed before but I am now exploring thoughts about the electronic end of things. Please bear with Rainman as he tries to catch up with the crowd.

CRH

 

To further complicate matters, the RX-8 PCM goes into closed-loop at WOT at a few different RPM points for some reason.
This is what the dongle on the E-Manage harness in the Greddy kit attempts to aleviate.

 

isnt wot timing at high rpm at 30 degrees already? I could be wrong but I thought that was a decent number. I wonder if we will notice this with the n flash. if my failing memory is correct I didnt notice this with the k or l flash.
My thought is that power is there and the ecu is holding it back. (I need a tuned cz unit!!) That surge is very noticeable to me in 4th gear. To the point if I dont lift quick enough It puts me past my braking point and screws the chassis balance up.
this is interesting.
olddragger

 

MM has previously noted that the ignition curve on the factory map(s) is set pretty good, as is. If I remember it was also noted that the fuel curve really needs some help and that's where the hidden h.p. resides. O.D., as a general concept with engine tuning we try to best tune an engine for proper, reliable, power production at WOT throughout the widest and most likely rpm range and then set up various systems such as vacuum advance and power valves to help us achieve better fuel economy and emissions levels at throttle positions less than WOT. That's why we have less igniton advance at WOT than we do at lesser throttle positions. Of course, electronic controls have eliminated the mechanical aspects of power valves and vacuum advance diaphragms but the functions are still used.

CRH