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Learning how to build Lotus Seven replicas...together!
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PostPosted: August 2, 2019, 8:59 pm 
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Hey guys,

Kind of an odd one, and not something I ever considered while building the car. My Seven sees mostly autoX use for now. It has an overly large 3.8L engine with a twin-screw S/C on top. While racing I have to be veeeeery gentle when lifting off throttle as it will cause a lot of engine braking and lift-off-oversteer. I'm starting to get used to it, but was pondering if there was a viable way to reduce this. I can't think of anything that would allow the air to bypass the throttle w/o increasing engine RPM or going lean. Vehicle does not have DFCO or I'd play with that. I guess a one-way clutch of some sort would be the trick, but can't see it being viable.

To give an idea, here is a wet autoX run where I was focusing on feeling the balance of the car while engine braking. The engine did almost all the braking I needed on a wet day. I don't think I touched brake that entire run. https://www.youtube.com/watch?v=7wm1YklFYVI

Any input would be appreciated.

Cheers.

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PostPosted: August 3, 2019, 11:37 pm 
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Do you tune your own engine? Some earlier GM EFI had the stepper motor, idle air control motor, that also functioned as a throttle follower (opened in response to throttle input) and this could also be tuned for decel and downshift characteristics.
It's been a few years but that was part of the tinkering I was doing with a TBI in my truck. Could be an easy way to fix your problem without changing mechanical components.


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PostPosted: August 4, 2019, 6:51 am 
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Make sure the bypass valve and actuator are working properly. Check the diaphragm for vac leaks and check vacuum lines are to the right ports and not just capped.

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PostPosted: August 4, 2019, 9:17 am 
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Another factor is 100% fuel shutoff under throttle lift. It's used for emissions reasons, but in our light cars, it can produce exactly what you're running into. I'm pretty sure it contributed to my big off at Willow Springs raceway. I finally disabled it and that made a big difference for controllability.

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Last edited by KB58 on August 12, 2019, 1:21 pm, edited 1 time in total.

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PostPosted: August 4, 2019, 8:51 pm 
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I had basically the same issue with my car for autocross too, and my solution pretty ended up being learn to drive around the issue. It was so bad that when I first started racing the car is just push the clutch in when approaching a corner and coast through. Having a fairly long throttle pedal travel helped a bit too, easier to keep the throttle open just a little bit.
Kristian

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PostPosted: August 4, 2019, 10:14 pm 
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do you know if this is due to loss of fuel or air ?

if its fuel,

is this speed density or mass air ? if mass air then adjust the maf transfer at the low end, if its speed density then its in the fuel table as effected by the map sensor.

is there a "time to return to idle" in your particular calibration ?

can you see a vacuum when decelerating ? if so how much

what is your air/fuel ratio when decelerating ?

if its air,

disconnect the IAC and open the throttle plate just enough to make it idle. see if it improves

if it does, then the IAC is closed and your throttle plate is also closed, if you open the throttle plate a little to allow some air in, the IAC should still be closed until the revs drop down to the pre set idle RPM but will only open enough to correct to the pre set idle RPM as counted by the CPS so the idle should remain as set, you are looking for a balance between the two, you can take the IAC down until it only opens for cold start

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PostPosted: August 6, 2019, 7:00 am 
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A lighter flywheel will help, esp if you have an OEM one made for a car with 2x the weight or more.


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PostPosted: August 6, 2019, 12:23 pm 
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When I see a new (to me, anyway) concept talked about, I tend to ask those stupid questions nobody else wants to ask [LOL]. To me "engine braking" means using engine compression to slow down the car. But, I know some of the cars belonging to you guys are supercharged or turbocharged. Are we talking about something unique to those engines with the term "engine braking?"

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: August 6, 2019, 12:41 pm 
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The function you are looking for is "throttle cracker". The way it works is whenever the car is rolling (you need a VSS input) the throttle is cracked (IAC slightly open) until the vehicle speed drops very close to zero.

If you are using stock engine management, there should be some of this engineered in. Problem is it's calibrated for a car that's waaaay heavier than yours. You can increase the affect by following John's post tweaking the throttle plate to allow the IAC greater control over the idle.

If you run aftermarket EMS, look for IAC tweaks to enable throttle cracking.

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PostPosted: August 6, 2019, 5:51 pm 
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Lonnie-S wrote:
When I see a new (to me, anyway) concept talked about, I tend to ask those stupid questions nobody else wants to ask [LOL]. To me "engine braking" means using engine compression to slow down the car. But, I know some of the cars belonging to you guys are supercharged or turbocharged. Are we talking about something unique to those engines with the term "engine braking?"

Cheers,


A "Jake brake" is a compression release. They used to come on dirt bikes.

In this case, the supercharger is a significant load on the engine, especially if the bypass does not open when the throttle is closed. The bypass is similar in concept to a blow off valve (BOV) for a turbo application and serves to regulate pressure. It is like cracking the throttle.

The engine is dropping rpm too quickly when the throttle is closed (engine braking) to suit Cory, either by design or an issue. A lighter flywheel would allow it to drop rpm quicker (bad) as well as rev quicker.

Engine braking can damage automatics with locking converters, driving the internals against the sprags. It can also damage the supercharger if the intake pressure gets too high and no where to go (closed throttle when tb is after the supercharger) or in this case, a dead load on the engine of around 10hp that serves no purpose with a closed throttle. That's what the bypass is for.

He has a bypass in the pics.

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PostPosted: August 8, 2019, 10:27 am 
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@MV8

Thanks. I'm getting the idea. The increased pressure in a supercharger/turbocharger introduces a more complex dynamic for sudden throttle closure.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: August 9, 2019, 10:39 pm 
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Sorry for the late reply, and thanks for the help!

Bypass et al is working correctly, and I believe everything is working as designed. Pretty sure it's just a lot of displacement for the weight of the car. IAC has no real tuning options besides idle RPM. There is no Decell Fuel Cut Off on this car so fine there. I am learning to get used to driving the car like this (mostly just being super gentle on throttle release) but it's not confidence inspiring to have a car that will get loose when you come off throttle. The Throttle Cracker/Follower is a good idea (and you've also helped me realize why GM cars stall w/o a VSS signal). This car has a cable operated throttle, but the last years of this engine had electric throttle. I'll look into what it takes to swap, but likely needs the newer S/C and ECU as well so it will be a bit involved. Likely will be worth the effort though, and net me more as-yet-uneeded power as well.

The lighter flywheel: I mulled over the physics on that for a bit but am too tired to figure it out. Initially I want to say the more mass the flywheel has, the less impact the car's will have on it? But then I start thinking about RPMs and compression and.... ? The effects of the engine braking are troublesome when doing small mid-sweeper corrections, as well as sudden off-throttle events. There is a 7lb flywheel available to replace the almost 30lb OE flywheel so pretty significant.

Thanks again guys. Good help. Best "fix" for this car would likely be some 275 wide Hoosiers but it's not time for that yet.

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PostPosted: August 10, 2019, 5:02 pm 
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C10CoryM wrote:
There is no Decell Fuel Cut Off on this car.


Are you running the stock GM computer? And if so did you reprogram the computer to disable the DFCO. If you did then you can ignore this, If not then you probably have a DFCO problem. Basically all fuel injected gm vehicles have it to provide engine braking and better fuel economy. The conditions and how aggressive it is depends on how each vehicle was programmed based On speed, engine loading, and other variables.


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PostPosted: August 10, 2019, 8:03 pm 
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WelderLee wrote:
C10CoryM wrote:
There is no Decell Fuel Cut Off on this car.


Are you running the stock GM computer? And if so did you reprogram the computer to disable the DFCO. If you did then you can ignore this, If not then you probably have a DFCO problem. Basically all fuel injected gm vehicles have it to provide engine braking and better fuel economy. The conditions and how aggressive it is depends on how each vehicle was programmed based On speed, engine loading, and other variables.


I don't have access to any DFCO with HP Tuners so I assumed this was too old for it (1997 engine/ecu). I guess it's possible it's there and just not available to adjust. I'll check the injector pulse width during decel next time I am out in it.

Thanks.

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PostPosted: August 12, 2019, 10:40 am 
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C10CoryM wrote:
I don't have access to any DFCO with HP Tuners so I assumed this was too old for it (1997 engine/ecu). I guess it's possible it's there and just not available to adjust. I'll check the injector pulse width during decel next time I am out in it.


Yea I would take a look at injector pulse to see if it is happening, but taking a quick look online because I don't have a service manual and going thru the tables of trouble codes DFCO is mentioned a few times as a mode along with steady state throttle that the computer checks values and can issue a trouble code.
https://gsi.ext.gm.com/gmspo/mode6/pdf/1997/97c381_W__yE.pdf


I'm not familiar with HP tuners so I do not know how they handle modifications to DFCO values and/or enabling/disabling and if changing it is available on a 97 PCM. I do know that GM was using it in OBD1 as my 1991 Saturn OBD1 pcm has DFCO and is a standard GM PCM from the time. I did find this page on HP tuners that covers DFCO and how it works.
https://www.hptuners.com/help/vcm_editor_parameters_gm_eng_fuel_cutoff.htm


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