First, the cost to remove and re-install a tranny shouldn't even approach $1500. It's a tough job for a DIY garage mechanic, but a pro shop should be able to do this without trouble. Yeah, major ripoff.


Second, the rules for transmission are pretty clear for SCCA classing (this is for Evos):

AS, STU -- transmission must be the stock transmission that came with your car, or repaired to a "stock" configuration. You can use any fluids you want, but the mechanicals must be identical to stock.

BSP -- transmission must be a stock transmission that came with any BSP-legal Evo. You can swap between the 5-speed and 6-speed Evo transmissions as you want. Otherwise, the same as AS & STU -- no internal modifications.

SM, SM2, FP, XP, EM, AM -- transmission can be anything you desire. If you can dream it, you can run it.




Back to the Thread Topic...

In every case where I've had a chance to try both 1st gear and 2nd gear on an autocross course, 2nd gear has been faster. That includes courses which were small and tight enough to use 1st gear all the way without hitting the rev limiter. (I should point out that my XEDEFlash raises my rev limit...) It's better to have 200 WHP which is usable at any steering angle and any car attitude than to have 300 WHP which can break traction. IMHO.

 

exactly.

 

I've never had the need to downshift to 1st. I'm on the stock 9.8 2003 turbo, so I don't need the RPM THAT bad. Maybe if you're running a GT30... another story. Also is heavily dependent on track situations... I suppose my tracks may be more open than yours. I get to a max of 60-80 most times.

 

Wtf, you cannot compare or replace downshifting with left foot braking, this is stupid advice. You forgot the fact that when you left foot brake your NOT DOWNSHIFTING and not accellerating anywhere like you would be if you downshifted. If he is shifting it is hopefully becuase if he didnt his car would be out of the power band and slow as ****, at which point braking makes it worse.

Scorke

 

I just feel bad you guys have to run autox courses that require first gear.....

 

WRONG! the point of left foot braking is to create artificial load on the ECU, which in turn causes the turbo to stay spooled at lower rpms where the turbo would not normally spool because you are stepping on the gas and the brake at the same time. A lot of experienced drivers do it as it is faster in some cases than down-shifting. This is especially useful on a stock turboed EVO ...
I agree a lot with what Racerjon has to say through personal experience ...

 

You're okay with the description here until you bring the ECU into it. The ECU isn't involved in left-foot braking any more than a carburetor is (on those dinosaur cars that still use carburetion). The load is placed on the engine.

And the load isn't "artificial" at all. Maintaining speed against a braking force is a very real load. If you have a boost gauge, try maintaining a constant 65 mph on the freeway in 5th gear while progressively pushing harder on the brake and throttle. You can probably get 10-15 psi of actual boost or more, even though you're under 3000 rpm. That's a real engine load, and the engine is putting out real horsepower, which is rapidly heating your brake pads and rotors...

But yes, LFB can allow you to maintain boost far higher than you would maintain without LFB.




But like racerjon1 said, turbo lag in an autocross corner is something of a non-issue if you're leading the throttle like you should. Get into the throttle before you want full-on boost, because if you wait until you want it, it's too late.

From my experience, turbo lag is more of a problem and more unmanageable at the high end of 1st gear than in the midrange of 2nd gear. When you're driving an autocross course at 5500-7000 rpm in 1st gear, and getting in and out of the throttle, that's a bad way to go. It's damn near impossible to effectively modulate the throttle. In my car, you go from 300 WHP to negative HP (engine compression braking) and back to 300 WHP and you just end up being slow.

 

I agree ... that the load is on the engine but there are complex algorithms in the ECU that use various factors that determine the load being placed and how much boost to set, to keep the engine generating the power it requires to maintain the necessary speed/acceleration ...
The reason I refer to it as artifical is because it is a load you are creating (braking) as opposed to something that would happen naturally (going up hill or being in a higher gearer trying to maintain speed at a lower rpm) ... it's just my terminology ...

 

You seem to be attributing the ECU with more capability than it really has.

The ECU doesn't set boost at all. It's not capable of doing so. It can limit boost, but it's unlikely that will happen during left-foot-braking.

Boost is determined primarily by two factors:

1. How fast the turbo is spinning.

2. How fast the engine is "emptying" the intake manifold. This is basically a function of the RPM of the engine and the throttle position.

The ECU can open the wastegate in order to limit (1), and it has no control over (2). (And before you tell me that the ECU can limit RPM -- go downshift into 2nd gear at 90 mph and tell me how the ECU is going to save you...)

In particular, the ECU cannot increase boost above the "natural" boost achievable at a particular turbine speed/RPM/throttle position.

And if you're triggering boost control (opening the wastegate) during left-foot-braking, you're riding the brakes (and throttle) harder than you need to.


There's no ECU "magic" (or complex algorithms) required to explain why left foot braking works -- it works because you're expending more horsepower, which requires more air and fuel, which results in more exhaust gas, which results in a faster turbine speed, which results in higher boost. With or without an ECU.

 

I see, thanks for explaining that ... I do know from experience that left foot braking does work and it builds up boost at lower rpms and keeps the turbo spooled. I've always thought the TPS reports back to the ECU which then determines how much fuel to dump into the combustion chamber from the rpm vs fuel table and some other factors ... whether it is based on a simple spreadsheet style table or the values in the cells dynamically change due to alogrithms built into the ECU, I don't know ... but it all seems to go through the ECU to me.

How come when driving on the highway, if you keep your throttle position the same and go up hill the your boost starts to climb? It's as if the car is compensating for the extra load with boost .... I don't know, if you could explain that it would be great.

From some of the threads I have read in the past, it has been mentioned that the ECU maintains a fuel vs rpm table, boost vs rpm table (limiting) and load factor, etc ... personally don't know because I haven't messed with the internals of the ECU yet. Someone mentioned the load playing a factor in the boost see at certain rpms, are they wrong?

 

Assume a perfectly efficient turbo engine, where the engine will perfectly burn all of the air which is forced into the combustion chambers, and will extract the maximum thermodynamic energy possible. Also assume that turbine speed is dependent only on exhaust gas volume per unit time.

Assume a car with such an engine, cruising at constant speed on flat ground.

Now the car hits an uphill stretch, and throttle position remains the same. What will happen is that the turbine speed and the power output will remain constant. Why? Because the only way for the exhaust gas volume to drop would be for the intake gas volume to drop, and that won't happen as long as the turbine speed stays the same.

So when this ideal turbo car hits an uphill stretch, power output remains constant. Which means that speed drops, because maintaining the original speed uphill would require an increase in power. Since engine speed falls, intake manifold pressure ("boost") increases.

On this ideal car, boost pressure increases with increased load, assuming constant throttle position. All explainable with simple mechanical arguments.

A real car deviates from this ideal car obviously, and there are circumstances on real turbo cars where power and boost will both fall when a hill is encountered. But for a car close enough to the ideal, boost will increase under increased load.

See above.

 

Sorry for the long bump, but at least I'm using the Search button.

I did my first autocross last week and, out of 15 total runs, I downshifted from 2nd to 1st for the last slalom of my final 2 runs. I know I'm probably being paranoid, as my car is still shifting fine, but what sort of damage could downshifting to 1st actually do? Is it a wear/tear thing where each time you do it, something gets weaker and weaker, or is it more like everything is fine until something just breaks when you do it? Do I have any reason to be concerned?