madsenj37

I have read that cars are drag limited and that cars are electronically limited. I have also read that the rx-8 is drag limited and that it is electronically limited. Is there a difference? (lets keep it on topic, this is not an rx-8 forum, it is merely used for example)

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stingc

I'm not exactly sure what you're asking, but there are three possible ways that a car can reach its top speed. It can be electronically, drag, or rev limited.

An electronically limited car means that the onboard computer will not allow the car to be driven above a certain speed (155 is a common limit in German cars for example). The ignition or fuel is cut if you try to accelerate beyond that point.

A rev limited car is geared such that you can reach redline in the highest gear. You could go faster than that, but it wouldn't be very smart to do so. If the car has a rev limiter also (most modern cars do), then you actually would be prevented from overreving the engine by an ignition or fuel cut.

A drag limited car has a top speed limited only by the engine and aerodynamics of the car (the gearing also plays a part). You can put the pedal to the floor, and eventually the car just won't go any faster.

madsenj37

That clears it up, thanks. Reading my original statement, i can see where its not clear. Basically, I have read in two separate places that my car is 1 drag limited while the other source said it was electronically limited. I wanted to know what the differences were, if any. I still do not know which one the car is, but you cleared up the difference for me between drag and electronic limiting.

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hahaha

If it is electronically limited I would expect you to hit a certain speed and then the car to stop accelerating all together, more adrupt than if the cars power was slowly being matched by drag. Just a guess though.

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JusticeForPhat

on the same subject, with electronic limiting what does it feel like when you hit the limiter? the reason i ask this is becuase on my ECU i have it set to a 11,000 rpm limit. (its a haltech E6K) When it hits the limit it cuts spark. Now this is great for limiting rpm but the car bucks like a bastard as would be expected. Gets a little scary on the dyno. Just curious if this process is a little smoother with fuel cut. Im just afraid to cut the fuel in fear of lean out.

potato

Drag means you can go as fast as the engine and wind force allows u to, electronically means that there is a limiter, say at 150, that u wont be able to go anyfaster,even if ur car can go faster.

MSD

On a turbocharged engine, an electronic limiter will force the ECU to open the wastegate as the speed approaches the limit. The reduced output of the turbo prevents the engine from getting the power it needs to go any faster.

Edit: I just realized that after multiple edits, my post was barely comprehensible.

JusticeForPhat

thats a good point about the wastegate, I hadn't thought of. Granted my turbo has no wastegate (doesnt need it, its a VATN) however im sure i can rig it to bleed off some boost at high rpms without worrying abot melting my engine.

Heres a good shot of the actuator that controls the turbo geometry:


thanks for that tidbit of knowledge MrSelfDestruct, i flat out did not think of that.

Elitegibson

The main reason they cut spark instead of fuel is that cutting fuel would cause a "lean" condition which, at that speed, could cause serious damage to your engine.

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amonkie

I've reached the top speed in my car (which was drag limited) and the top speed in a friends V6 Firebird (which was electronically limited). In my car, it just keeps going faster and faster, but slowly the rate at which you're going faster slows down and levels off until you can't go any faster. Drag limited cars will have a higher top speed if you have the windows up and the t-tops on (I had neither). Electronic limits feel like the driver snapped his foot off the gas...they aren't as violent as a rev limiter, and you don't 'bounce' off it...it just cuts spark until you're below the limit.

Coincidently, if you want to disable the limiter....if you have a manual car, you can sometimes disable the sensor that tells what gear you're in. The speedo will still work, but usually the limiter is tied to the gearbox.

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MSD

To be perfectly honest, I originally learned it while reading a Saab owner's manual and trying to figure out how to set the clock on my friend's car

WarWagon

Most modern cars are electronically limited. Most vehicles are capable of sustaining VERY high mph given the right drivetrain, which is why you can see musclecars and cars of the 80's moving well in excess of 150+ mph in the 1/4 mile alone. Many people confuse drag limit and rev limit, in that they assume that drag is the only thing holding them back when in fact their engine just can't move enough air to accelerate them further. When a car reaches its drag limit, it is very likely to become extremely unstable.

stingc

Warwagon, no remotely stock car runs 150 mph in a 1/4 mile.

I don't know what your talking about with engines. If the engine can't "move enough air," that means it can't generate enough force to overcome drag - hence "drag limited."

There's no reason to say a drag limited car is unstable. That has to do with suspension design, tires, and most importantly the body's aerodynamic lift. Many cars get light at high speed because typical car shapes tend to generate positive lift. Its not always true though, and it also depends on how powerful the engine is. A car with a weak engine will only get unstable if the suspension is sloppily designed.

pocon1

The European limit for Mercedes and such is a gentleman's agreement because getting stock tires that can handle greater speeds safely is not that easy. I believe There are cars that cut the fuel pump to slow the car down.

rider6061

[QUOTE]Originally posted by JusticeForPhat
[B] Granted my turbo has no wastegate (doesnt need it, its a VATN) however im sure i can rig it to bleed off some boost at high rpms without worrying abot melting my engine.


What exactly is VATN?

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JusticeForPhat

A VATN turbo is a variable geometry turbocharger. VATN means Variable Area Turbine Nozzle. The basic principle of a turbo is that it converts waste heat (exhaust) into energy (boost). All this you probally know. However most people have the wrong assumption that the turbo performance is based on how fast the exhaust is flowing or how much pressure is built up in the exhaust. This isn't really true. The exhuast (turbine) side runs off of heat. As the hot gasses pass through the turbine blades which are arranged so that each blade next to each other forms a nozzle. As the hot gas passes through this nozzle, there is a change in volume do to cooling and the little bugger spins. Normally the angles are set depending on what "trim" turbo you have and the only way to alter it is to buy other "trims" to swap in and out.

A VATN takes an interesting approach to this phenomena. It changes the angles in-between the turbin blades. By doing so it changes the profile of the nozzle, and in the end at which speed the turbine spins. Basically it gives you an infinite number of trims in a certain range.

In theory you can do a whole lot for the system especially by computer controlling it. For one you can almost totally eliminate turbo lag. The two leading causes of lag is 1.) A huge volume to boost before the head, ie the 17 feet of 3 inch tubing and monster front mounted truck intercooler my friend has on his Supra. With a whopping 6psi of boost that thing sucks. and 2.) Rotating inertia inside the turbo. What the VATN allows you to do is optimize the shape of the turbine wheel for lower rpms and to keep the turbine spinning. Once you stomp the gas and the engine makes some heat, you can vary the shape to get it up to operating RPM as fast as possible. Without that at idle the rpms of the turbo fall down pretty fast so it takes longer to get them back up.

Also with the VATN under computer control, it allows me to tune the turbo's amount of boost according to how I want the car to run. With a turn of the know you can trim the turbo for minimum lag times, for an autocross event. Or you can trim it to maximum boost for flat out speed. Much better than a standard turbo with a boost controller in my opinion. Sure you can control how much boost you make with it, but you cant control how the turbo responds.

RaDiAn

I now know more about a turbo and limiters than I ever did. Thanks all!

Can the trims be set during (on the fly) driving/racing? Does the VATN computer remember the many different trims positions?

JusticeForPhat

thats a good question. Currently the VATN controller is setup through our haltech in a closed loop system. Basically the haltech measures pressure post turbo. It then interprets the map we have programmed in. At this point it then sends a signal to a solenoid valve that controls the VATN. This happens every few milliseconds.

Now currently im working on two projects with this. One is to implement one of the Haltech's spare inputs as a boost controller. Basically a little potentiometer that will change the trim a bit. The other project is a seperate computer altogether to control it. Based off a basic stamp it would perform the same function as the haltech but it would monitor for different settings. Essentially it would max out boost at low rpms for straight line drag racing so I can have a monster launch (fuck wheel spin, if we integrate it with our custom traction control system we are working on . . . . . ) and then limit boost to 12 - 15 psi. For most other conditions it would be wired to a new knock sensor we are developing.

In short, if you can dream up a situation that you would want to change the amount of boost / spool time, we can figure out some way to control it. I think this isnt too bad for an entire team of Mechanical Engineers. Not one EE in the group of 7.

Variable geometry turbos are becoming standard items in diesels. The controllability is mainly the reason.