Practical to reduce engine/ compression braking?

[petevb]

I'm wondering if there is a practical way to reduce engine braking?

The issue: I find that in autocross situations in 2nd gear engine braking is much more significant than at the track. In these situations the extra braking at the rear while turning in often causes the rear to begin to rotate more than ideal, slowing corner entry. Not unexpected with a high compression 3.6 spinning at 7500 rpm...

Of course normally this is dealt with using driving style, suspension, alignment and brake bias, but it occurred to me that reducing engine braking itself would be useful. I'm not sure there is a practical way to do that, however? I can imagine increasing the air past the throttle plate might have a small effect (increasing idle)? I'm not sure that would be noticeable, and I can't think of much else... I'm thinking specifically about 3.6 VRams and GT3 motors, but also in general.

Any ideas?

-Pete

[pete3799]

Pushing the clutch in would do it.
A compression release would be the only other way i can think of and that ain't happening easily.

[Flieger]

Bikes like Ducatis have used a technique where the rear cylinder had the throttle butterfly kept open a little while the rider was off throttle, with the front cylinder throttle closed to idle. Then the slipper clutches became more popular.

You could let your foot rest on the throttle if possible. You could stay in a higher gear but that would not be good shifting at the apex so you can accelerate.

[jpnovak]

That is an interesting problem. Can you program the ECU?

In my MS system I would probably try to write some code that wold open the air bypass (Idle air controller) if the rpm >5000 (or whatever threshold) AND the amount of snap throttle closing was severe. Use a change in manifold pressure for that time based threshold change. It might take a few tries to get the timing and amount of throttle opening correct but it should be doable.

You call this the AX program and load it before each run between the cones.

[petevb]

Originally Posted by jpnovak

That is an interesting problem. Can you program the ECU?

In my MS system I would probably try to write some code that wold open the air bypass (Idle air controller) if the rpm >5000 (or whatever threshold) AND the amount of snap throttle closing was severe.

That's an interesting thought. I can control the ECU, but I don't currently have an idle control. That would be another reason to add one, though.

Or an idle adjust switch that I can set on the start line? Flip to "idle" at 3k rpm, might let enough air in to significantly reduce engine braking? Similar effect to holding the throttle cracked... Right now the cup motor has very high tech idle control- a screw.

[J P Stein]

I've had the issue of rear end stepping out during AX corner braking.
You've got everything on that car of yours so I'll assume you have an adjustable proportional valve for the rear brakes. Keep backing off the rear pressure till the rear brakes work in concert with the engine braking, mark the spot, then return it to the normal position when the AX is over.

Back in the day when I was having an in-curable push, I liked the back end to step out under braking. It helped the car to "dirt track" around the tight stuff.
This was fine when I got it right, but doing that consistently is a bridge too far for me.
I do know a 911 AX driver that can pull it off and he is bloody fast. That makes one think the 911 needs to be AXed like that.

[petevb]

Originally Posted by J P Stein

You've got everything on that car of yours so I'll assume you have an adjustable proportional valve for the rear brakes.

Yes, dual master cylinders and adjustable prop valve.

Originally Posted by J P Stein

Keep backing off the rear pressure till the rear brakes work in concert with the engine braking, mark the spot, then return it to the normal position when the AX is over.

The issue is I don't think it's possible to adjust the brakes to fully work in concert.
I think about it this way- the first 10%, say, of braking comes only from the engine, and thus is only on the rear wheels. So if you're maxed out in a corner and then lift the rears will be braking, the fronts won't, so the rears are overloaded and the car will try to spin. As braking Gs build when you actually use the brakes you can eventually adjust the bias to work with the motor, but at low braking Gs the motor is always going to be braking the rear wheels with a much higher % than the fronts.

As you suggest some of this is a good thing- the 911 generally doesn't want to tuck the nose, and this helps. But lower the gear and the more compression braking the motor has the more "tucking the nose" becomes "totally sideways" when you lift. I suspect my car with the GT3 motor has twice the compression braking of most 911s, and geared to 66 in 2nd it's running a short gear.

Effectively I think I need to cut engine braking in half to have it behave like a normal 911 on lift-throttle. Not sure that's practical, but I suspect it would be helpful...
Edit: I see F1 cars used to have a knob on the wheel to control engine braking (before it was banned). And some motorcycle ECUs make a big deal out of adjustable engine braking. Perhaps it's a bigger issue at higher power to weight ratios...

[chris_seven]

Do you have an LSD?

A good Plate Type LSD with a shallow coast ramp (Say 20 degrees) may help to cure the trailing throttle oversteer.

It may introduce some undertsteer but you can probaly ovecome this with trail braking.

I would also be inclined to try a bias bar style pedal box to replace the proportioning valve which doesn't give ideal results in this situation.

[petevb]

Originally Posted by chris_seven

Do you have an LSD?

A good Plate Type LSD with a shallow coast ramp (Say 20 degrees) may help to cure the trailing throttle oversteer.

It may introduce some undertsteer but you can probaly ovecome this with trail braking.

I would also be inclined to try a bias bar style pedal box to replace the proportioning valve which doesn't give ideal results in this situation.

Yes I have a LSD. I also have an adjustable dual master cylinder. I believe you are correct that running with the prop valve set high enough that it effectively doesn't work helps this issue.

Again I do believe this can be mitigated with setup and style, but I suspect that's more of a compromise vs addressing the root cause...

[Tyson Schmidt]

Can you eliminate the decel fuel shutoff? That will significantly reduce engine braking.

[gearheadgreg]

You could probably rob from a car in a salvage yard or make a slow-decel throttle stop, so that the engine drops quickly (normally) to around 2500rpms or whatever you want, and then slowly drops to idle. Reducing the engine braking when you are jumping around on the pedals, concentrating on cones.

Makes it work for auto-x and still streetable, etc. totally mechanical solution, and very simple.

[Tippy]

Put a sprag in the clutch disk. Just kidding.

[petevb]

Originally Posted by Tyson Schmidt

Can you eliminate the decel fuel shutoff? That will significantly reduce engine braking.

That's a thought- I'll check on that. Ideally I could perhaps make that one of the three map switch positions, just flip it on and off...

Originally Posted by gearheadgreg

You could probably rob from a car in a salvage yard or make a slow-decel throttle stop, so that the engine drops quickly (normally) to around 2500rpms or whatever you want, and then slowly drops to idle.

Unfortunately I need to reduce engine braking most at the top of the rev range, where it's most of the problem. So I'm not sure going quickly back to 2500 would help, but I'm not familiar with how those throttle stops work. What cars did they come on?

Originally Posted by Tippy

Put a sprag in the clutch disk. Just kidding.

Yea, that'd be interesting to spec. Zero engine braking doesn't sound like fun either...

[Flieger]

You need some sort of visco-elastic material. Like silly putty or that non-Newtonian cornstarch+water mix. It behaves sort of like an inerter or damper. It gets stiff when compressed quickly but will flow continuously under low shear stresses.

Some sort of steering damper, maybe? Lots of high speed compression damping but little low-speed damping?

[J P Stein]

It has been my experience that one needs instant throttle response or as close as possible ( both on & off) mid corner.

My 911 AX experience is limited but the overriding feature seems to be on throttle push...... drop throttle oversteer is another. Guys that use throttle steering seem to have an edge.

[LJ851]

if you can adjust closed throttle ignition timing, advancing the timing at the rpms you want to affect will help. you can run quite a bit of timing since there is no load on the motor.

[914efi]

Nobody has (directly) suggested left foot braking.

[petevb]

Originally Posted by J P Stein

It has been my experience that one needs instant throttle response or as close as possible ( both on & off) mid corner.

My 911 AX experience is limited but the overriding feature seems to be on throttle push...... drop throttle oversteer is another. Guys that use throttle steering seem to have an edge.

You absolutely must throttle steer a 911- no other way to do it.

The issue is this car in 2nd has nearly as much lift-throttle oversteer as my 3.6 VRam did in 1st. It's requiring a lot of adaptation; car setup, style, etc.

Yes, left foot braking is another usual answer- haven't gotten that dialed in yet.

I'll see if I can play with idle to see if it's a step in the right direction- sounds like that's the best quick and easy test before I go ripping into stuff...

[914efi]

If you can, get into a racing kart, preferably one with only a rear brake. You will adapt to the (forced) left foot braking really easily. They also force you to brake in a straight line, if you turn in under braking you will spin fast!

[Grady Clay]

Pete,

Good question. Lots of good ideas.
I agree with the above that programming the ECU is the solution.
You could even have a ‘thumb control’ to change the characteristics.

However, I’m of the ‘old school’ where I like to use the engine braking to maximum effect in many situations.
I’ll even ‘dump the clutch’ to initiate rotation in some situations.

Part of this is toning your ability to left-foot brake, heal-toe operation of the brake and accelerator and being willing to use (‘abuse’) the clutch.

In the situation you describe, I might enter the corner with the clutch disengaged, maximum braking and holding the rpm at some high speed. When I re-engage the clutch, I can choose to match rpm, have trailing throttle or applying acceleration power.

I agree with some shifter kart ‘testing’.
Careful, they are adictive.


What gears do you use?
You may want to build a transmission with several gear ratios close to your current 2nd.
This will allow you to pick a gear that will minimize shifting.

Another issue is your choice of clutch.
The 215 mm push-type is far more tolerant of engine-braking loads than the 225 mm pull-type.
A common failure of the pull-type is buckling of the three flat tension springs.
They are designed for high power loads and can fail on engine braking loads.
Of course the 215 mm clutch won’t long tolerate a 3.6 in a autoX intentional slipping situation. The 215 discs are cheep.
The other extreme is a huge, multi-disc clutch that will tolerate slipping heat (drag race style?).

What hand brake do you have?
One more ‘tool’ in autoX performance.

What LSD? What settings?
One difficulty with LSDs is the transition from decal to acceleration where the differential releases during this transition.
I prefer the predictability of a ‘spool’ on race tracks.
I suspect the low-speed (autoX) ‘push’ would be intolerable.


A couple of years ago my son (then 22) was in a Formula Mazda Pro Star car with all the ‘driver aids’.
In a slow turn, the ‘traction control’ wouldn’t let him use the engine braking to assist the ‘turn-in’.
He was eventually fastest with it turned completely off.
Same with the ABS.

Some times ‘kiss’ is best.

Best,
Grady