Dumb ? What is throttle response?

[wckrause]

I've heard this term used many times, and I'm not exactly clear what it means. Can somebody explain this.

Also, is throttle response measurable?

What effects throttle response?

[dtw]

Not a dumb question at all. To me it's a somewhat subjective term, but it does have some definite meaning. What's the reaction you get to pressing your right foot to the floor? Does the car take off, or falter? Does it feel sluggish? Technically, it relates to the ability of the intake system to quickly and efficiently get air and fuel to the engine in response to throttle input. This is where the K&N intakes, wide throttle bodies, modified mechanical injection space cams, modified chips, flowed heads, bigger ports, and all the rest come into play. They are all working to get the engine to respond quickly to the driver's input.

[jluetjen]

I think of throttle response this way:

If you think about moving the throttle and the car sounds like it is starting to accelerate and it feels like it is picking up. That is great throttle response. MFI's provide this experience.

If you move the throttle and the car responds by accelerating. Then this is normal throttle response and what you will find with CIS 911's and most cars on the road today. Once it starts to accelerate, it might pull like a train, but that is HP and Torque as opposed to throttle response.

If you have to plan ahead to ensure that the engine starts pulling when you want it to accelerate out of the apex of a corner, then this is "Turbo lag" or poor throttle response.

I'm not sure that I agree that big throttle bodies and space cams have a lot to do with throttle response. I think that it has a lot more to do with fuel delivery as soon as the throttle is moved. MFI's have it because they are pre-mapped and moving the throttle immediately moves it to the new point on the map. Instant Fuel!. Webers are close because their accelerator pumps give an extra squirt of fuel. CIS and other closed loop injections systems don't have instant throttle response because they need to wait for the air column in the intake system and plenum chamber to move the sensor plate which then meters out more fuel. Turbos have a lag because it takes time for the turbo to spool up.

[speeder]

Yes, good question actually. Very commonly used automotive term that cannot be measured, at least not w/ numbers.

Although I agree w/ Dave's explanation, another way to look at would be that throttle response can be affected w/o touching the engine; ie. lightened flywheel (or lightened car) would improve it. Literally means "what happens when you open throttle and how instantaneously it happens. A car w/ moderate HP could have excellent throttle response, under certain circumstances. Hope that adds something.

[wckrause]

This is where I get confused. Is throttle response the time delay between throttle movement, and change in torque? or throttle movement and change in RPM? A lightened flywheel wouldn't help the engine develop torque quicker, but it would help it change RPM quicker.

Is the difference in throttle response between a MFI and a CIS type system measurable, or can it at least be approximated? 1 sec , 1/2 sec? Can a dyno make these kinds of measurements?

[ChrisBennet]

Originally posted by dtw
...This is where the K&N intakes, wide throttle bodies, modified mechanical injection space cams, modified chips, flowed heads, bigger ports, and all the rest come into play. They are all working to get the engine to respond quickly to the driver's input.

To expand on what DTW said, in general, larger ports, wider throttle bodies and larger exhaust will hurt throttle reponse and low end. The larger cross section of gas = more mass and the resulting inertia of this greater mass slows changes in the velocity of the gas column.
-Chris

[RoninLB]

Bill...."can a dyno make these kinds of measurements"....IMHO, throttle response is, subjectively/emotionally, measured when my pants feel tooo tight.

[magilla]

My understanding is that the throttle response would be the change in RPM's. Torque would be a function of the engine (displacement, etc).
The test would be to sit in neutral, idling. When you press the throttle, does the engine immediately increase in RPM's? The faster it increases, the better the response. If you slam the throttle to the floor, and it takes a second or two to hit redline, that would be slow throttle response. When the car is at speed, there will be much more load on the engine, and all of this will change, but the principle will be the same. If the engine generated infinite horsepower and had infinite torque, the throttle response would be the same at idle as it would at speed.

Magilla

[BlueSkyJaunte]

Excellent question. I've wondered about this too, because so many people complain about CIS throttle response. I sometimes wonder if everyone isn't driving around in poorly-tuned CIS cars, because when I put my foot down, my engine certainly spins.

Granted, I don't have a lot of torque/HP below 3000 RPM, but I'm not driving an SBC.

When my engine is "on the cam" at 4500+ RPM, the response feels d@mn near instantaneous.

I would guess the only way to measure this would be to take a time-dependant measurement of throttle position vs. RPM. I guess the lag (difference between the two lines) would be your "throttle response" curve. It probably varies under load, accelerating vs. decelerating, etc....don't know how many dimensions you'd have to graph this in!

EDIT: D'OH! Magilla, ya beat me to it, man.

[Saffs]

Latency would be my description. The quicker acceleration appears, the better throttle response, and, therefore, less latency....
I've heard people say MFI has much better throttle response, but the examples I've driven - but not owned - I couldn't really tell that it was as far superior to CIS as people allege - bad examples, perhaps? However, say a newish BMW that has an electrical type accelerator, that I can really feel.
I think throttle linkages, cables, length of linkage/cable to the actually source, must play a big part too....

[Robert Coats]

Originally posted by magilla
My understanding is that the throttle response would be the change in RPM's.
Magilla

The key word is "response." Thinking about this, it's how quickly (time) does the engine respond to throttle position change. A test for "throttle response" could be like this:

1. Position a block under the throttle pedal, such that when fully depressed, the engine is running at maximum horsepower, say 6,200 rpm.

2. Connect a switch to the block and link it to the START button on a stopwatch another sensor on the engine that will press the STOP button when the engine hits 6,200 rpm.

3. Start the engine, and let it fully warm up. From idle, press the pedal all the way down. The will be a time delay from the time the pedal "requests" the engine to achive the speed and when the engine actually delivers it. This could be a measured form of "throttle response," in my humble opinion.

[Randy W]

My thought is that low "flywheel effect" is a contributing factor to good "throttle response". If you define "throttle response" as the ability of the engine to deliver an immediate response to throttle input, then all those things that have been mentioned contribute. IMO, it is the ability of the induction/injection system to provide optimized fuel, air and spark in an immediate manner consistent with best performance (not noise, pollution and fuel mileage) that best defines throttle response. This is why modern day cars bound for the US cannot provide the same level of throttle response as pre '74 models. IMO, MFI provides the best throttle response - it is immediate, linear and ultra-smooth. Electronic fuel management systems like MoTec are a close second. Carbs, even tricked out PMOs, although great, will not consistently provide the same level of throttle response as MFI or EFI because of their inherent design. Bosch DME systems can be tweaked to provide good throttle response. CIS is unfortunately a design that provides only medium response to throttle input. Now if you were to put someone that had been driving an early 80's Celica into an SC, they would probably think the Porsche had great throttle response - so everything is relative, as usual.

[rob911]

In addition to what saffs said
"However, say a newish BMW that has an electrical type accelerator, that I can really feel. "
Guess we cannot easily switch to fly-by-wire throttle.

However people do switch to individual throttle bodies with one butterfly per cylinder and well engineered solid throttle links instead of cable. This might help throttle response as well as improving the breathing.

Could adjusting all the slack out of the mechanism help?
Does my '84 3.2 have a cable or rod mechanism?

[Clark Griswald]

I believe short intake runners improve throttle response. Stands to reason that a change in throttle opening will be felt at the cylinder faster with a short runner.

MFI and carbs are very short compared to the jumbo runners of CIS or DME.

Further it makes sense that closed loop systems like DME will take longer to reach an optimal mix due to feedback lag.

MFI and carbs are pure open loop.

[RoninLB]

Picking up on where Clark left off is that sudden throttle response is related to intake volume size, IMO. The smallest volume possible that doesn't incurr power loss is best for throttle response. A larger than necessary intake will slow gas/mix. speed. A optimized intake volume may allow smaller jet/injector size. A "right" combo. will probably lead to more consistant spark plug readings. A "not-right" combo. will probably lead to difficulty getting good tuneable readings, IMO.

[Early_S_Man]

Also ... the closer the throttle bodies are to the intake valves, the quicker the response! This is somewhat independent of the intake runner volume, and closely inter-dependent on cam timing/overlap specs, since MFI 'T' engines obviously have smaller ports than S/RS engines ... yet provide better response under load than the upper-echelon 'brothers' ... at lower-to-mid rpms!

One thing that had not been mentioned thus far regarding any measurements is 'real-world' response under driving conditions, i.e., checks in the driveway from idle vs on the road at a specific speed in a certain gear! You won't find an engine that revs quicker from idle than an MFI 'S' engine, except maybe a '69 911R with a 916 DOHC engine, or an RSR ... but compare the throttle response of any of those to an MFI 'T' car in a 30-50 mph test in 2nd gear ... if you want some shocking differences from their 'driveway' test!

Probably measurable but certainly a subjective thing. The output of the motor combined with the weight of the car are other contributors to the experience. Randy's early car (72S) with it's light weight and RS spec MFI motor would provide great throttle response as street cars go....damn thing probably literally leaps forward when he steps on the gas!

When I asked my buddy about the "throttle response" on his 1200 lb. single seat Formula-C racecar (has a Pinto motor!) he said he wouldn't know because his foot was always pressed to the floorboard!

[wckrause]

It seems like the definition of "Throttle Response" includes every aspect of performance between stepping on the gas and reaching the desired speed.

I had a much narrower view. I thought of it as the time lag between when you step on the throttle to when the intake is moving the correct amount of air with optimal fuel air mixture. This would include turbo lag (on turbo systems), reaction time of any air metering device (CIS sensor plate or MAF, etc), and any time required to close loop the system.

[J P Stein]

I concur, Bill.
Dumping a bit of raw fuel into the intake stream the moment the throttle is opened is the reason that Webers will "out reponse" MFI every time.

I'm open to a "driveway challange" with any pretenders

[Early_S_Man]

Bill,

I agree with you, and if a person had a data acquisition system with custom controls and triggers that could start readings and signal events such as throttle pedal opening, throttle plate opening, and acceleration in three planes ... that is just the sort of data I would be looking for in the real-world test I suggeasted! From the instant that the throttle plates opened ... until the ramp-up of acceleratiopn occured, there would be the time interval you described! Of course, there would be variance among different cars of the same model, but I would expect similar trends, especially with larger sample sizes!