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Hi all,

I am a late in life(comparatively) car guy, I've been into cars for the last 5 years; though only gas ones. Right now I mostly drive a 2001 Porsche 911 C4 Cabriolet that I received with a blown engine and swapped in a slightly smaller engine from a Boxster S. I like driving a small sports car, I like the feel and noises, and I like shifting my own gears. But I'm also somewhat environmentally conscious amd don't like the gas mileage; even if I only drive it twice a week and ride my bike and take the bus the other 3 days a week.

I've always thought it would be cool to have a through the road hybrid system. From a packaging perspective I could connect an electric motor to the front diff, and load the driveshaft recess(or frunk) with batteries, and my car has an electric throttle body for sending a signal to the controller. The front diff on my car is a 1:1 open diff. The car has no signals or sensors between the rear engine/trans and front diff so lots of people convert from AWD to RWD. I think something like this motor would fit behind the front diff and augment my car's driving pretty well with its low end torque (I can imagine how I would attach the diff and motor but it would obviously be one of the challenges) https://www.evwest.com/catalog/prod...ucts_id=283&osCsid=8d935rm17vt5k8ioh5b0kbn9o5

I get that this is a board about purely ev conversions but I'm not there yet. I want to explore the idea of a hybrid first. I'm going to ask a variety of questions that I've tried finding answers to without much luck in the scope of a through the road hybrid conversion...

Is there a good resources I'm missing to help answer these questions?
Would an electric motor in this arrangement get any regenerative braking(for some reason I have a hard time visualizing the regenerative effects with an open front diff)?
Would a 1:1 diff be okay for the speed of the motor?
Could I have the electric motor shut off at a certain speeds/RPMs through the controller? Is there any downside to doing so?
Could I have the electric motor shut off if the battery gets to a certain power level? Is there any downside to doing so(driving around in just gas mode)?
Would a standard ev motor controller be adjustable to get the driving characterstics dialed in? (I'm imagining i may want to tune both the gas and electric aspects of the driving experience but not sure if that's all that realistic)

Again, feel free to poke holes in the idea. But please be kind. Thanks!
 

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It's interesting that while the most cost-effective and efficient vehicle configuration for many applications is a hybrid, it is also the most difficult to design and build... and yet many DIY enthusiasts want to build them. Among hybrids, a thorough-the-road configuration is the least effective and least efficient, and again that's the most popular type of hybrid proposal in this forum.

It would be an interesting project, but it's very unlikely to be worthwhile.
 

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To add to the above.
Weight!, a lot at the front and a lot at the rear. You just ruined an adequate sports car. There’s a reason most hybrids have a tiny gas engine and a poor EV system. It’s like the merging of the worst parts of each system. I’m surprised EV west didn’t mention their bolt in SDU to replace the already wrong sized engine that lets face it, will fail soon enough. I spent many years rebuilding and relining these engines and always wondered why didn’t they make a modular frame to swap different drivelines. I didn’t know much about Tesla until years later but it’s a known conversion for the 911 series and if you are thinking a hybrid because you have a good engine you want to use, pushing the extra weight around will soon see the bores scored as it’s the low end that kills these motors, most engines living in the higher rpm didn’t suffer anywhere near as much. Take a look at how much weight will be added and then decide if you want to take the hybrid route. It’s going to increase your poor gas milage and be a poor EV performance vehicle. You have a vehicle that’s ripe for an EV conversion but a hybrids like the bmw i8 use a very small engine, not a 2.7l or whatever you have in there now. A better choice would be going to the bullet proof 993 turbo engine or full EV. Just my opinion. Do a search and you will see a few 911’s done already.
 

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The differential just allows the two wheels on the same axle to turn at different speeds, so the car can turn corners; the propeller shaft (driveshaft) speed is simply the average of the two axle speeds, divided by any reduction ratio in the final drive gears (1:1 in this case, so it's just the average). As long as neither tire is slipping, the differential makes no difference to the rest of the drivetrain: driving and regenerative braking work the same as if there were no differential.
 

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The 1:1 final drive ratio is unusual, but is sometimes found at the secondary axle of AWD systems that incorporate the required reduction gearing I'm another part of the system (the 'transfer case' or equivalent). The wheels of a car only turn at a few hundred rpm at typical speeds; that's undesirably slow.

Mechanical power is just torque multiplied by speed. A useful motor running that slowly must produce a lot of torque; if it could run (for instance) ten times as fast, it would only need to produce one-tenth of the torque. Motor size and weight is more closely correlated to torque than to power, so a low-speed/high-torque motor is a heavy motor for its power.

The usual fix for this is an extra gearbox between the motor and the final drive (differential).
 

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Although they are not through-the-road systems, a hybrid with electric-only drive to one axle is now routine (pioneered by Toyota but now used by most manufacturers of AWD hybrids). For efficiency, in these systems the motor is never used to drive unless there is insufficient traction at the other axle, or maximum total drive power is needed. Of course it is used for as much braking as possible.
 

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Discussion Starter #7
Thanks for the feedback.

The 1:1 diff is indeed the front diff only; as the final drive and other gearing are all handled by the transmission attached to the engine at the rear.

Im not sure why low end bore scoring would be an issue. The electric motor would spin low rpm but my 3.2 would keep its full range. And a 993 turbo engine would cost 2-3 times what I’m discussing.

I Actually had an alternate idea but you’d need a prettty good idea of 996 drivetrains to visualize. I’m thinking of attaching an electric motor to the driveshaft up front (rwd only) which runs back to the transmission and through what is essentially a torque converter attached to the output shaft.

I’m not too worried about weight assuming I maintain a similar speed acne improve gas mileage.
 

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The 1:1 diff is indeed the front diff only; as the final drive and other gearing are all handled by the transmission attached to the engine at the rear.

...

I Actually had an alternate idea but you’d need a prettty good idea of 996 drivetrains to visualize. I’m thinking of attaching an electric motor to the driveshaft up front (rwd only) which runs back to the transmission and through what is essentially a torque converter attached to the output shaft.
There are some variations within the 996 series, but my understanding is that there is no central differential splitting power; the rears are always driven and the front suspension are driven through a viscous coupling at the front (which is a bit like a fluid coupling, and less like a torque converter).

As I understand this alternative proposal, the entire front drive unit (including viscous coupling) would be replaced by the motor. This would result in a straightforward parallel hybrid, leaving three issues:
1. AWD has been lost (for some reason many owners of this model seem okay with that)
2. To have any benefit, suitable control logic is needed (just powering in proportion to throttle and regenerative braking probably won't be enough)
3. If the front final drive ratio is really 1:1, then the motor will be turning at axle speed, it would need to be large to be effective, or a gearbox would be needed.

A 1:1 ratio doesn't make sense to me for this system. One source which looks pretty sound says 3.444:1, which makes more sense and reduces the gearing issue.
https://books.google.com/books/about/Porsche_996_The_Essential_Companion.html?id=RaanuEGHWuwC
 
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