Twin engined cars.

[Mr Dave]

OK I am thick.

Now we have that out of the way how do you work out how much power a car with two engines has.

Im meaning like the homebrew ones where say a corsa has a redtop in the front and another in the back. for example http://www.mig-welding.co.uk/forum/showthread.php?...

I just cant get my head around how powerful they are.

For example if each engine was 100bhp do you end up with a car that has the performance of a 200bhp car? How does each engine not do the same work at the same speed if that makes sense? Is the power somewhere in between?

[Mr Dave]

Surely with both engines being matched for throttle position and rpm and the gearing being the same then both engines can only do the same work as one?

Would the car not be only able to go through each gear at a certain speed? I dont see where this extra power is useful if you see what I mean?

But then 4wd cars? But like for like they are slower than a 2wd car of the same power and weight due to losses.

Like if two cars exactly the same were attached together one behind the other, (say two 1000kg 100bhp cars, with each car being stripped out to 500kgs to make weight a non issue), I cant see that being massively faster in a way that doubling the horsepower would.

I think Ive thought about it too much and really gotten myself lost. I think Ive properly confused myself and need a lie down and a cry.

[Mr Dave]

PhillipM said:

AS said before, you don't need to match the engines, all this crap about syncronising power and revs between them just came from a few bullst max power articles where the installers tried to make it look like they did more work than just bung a front subframe in the back.
The clutch and gearlinkage is the worst bit.

The engine is always, always, slowed down by the load - i.e. Accelerating the mass of the car - ergo, no matching is necessary as both axles on the car are connected by the road, and hence already synchronised.

Magic eh?

Anyhow, it seems a good idea down the pub until you realise that you've just added the weight of another crankcase, water pump, oil pump, crank, bearings, flywheel, clutch, transmission, shafts, hoses, radiator, etc, etc.
It's like that tandem bike someone mentioned earlier - great, you've now got twice the power for when you hit a hill!
But, you've also got two fat bds aboard to drag up there rather than one - the only thing it's helped with is it's made the bicycle feel a bit lighter..
Then you realise that a pair of 100bhp engines in a car are probably not going to be any faster than a nice little turbo bolted on your original 100bhp engine to give it 150-160bhp for about 20kg more weight....

Especially if it's the normal cut-and-shut and front subframe in the rear and hope-to-fk-it-doesn't-kill-you-in-the-wet approach to suspension geometery most of these heaps have...;)

Edited by PhillipM on Saturday 3rd July 00:33

Edited by PhillipM on Saturday 3rd July 00:34

But you could have two turbos bolted to your two engines and have 300-320 bhp for 40kg more weight....

Im still confused.

[Mr Dave]

CraigyMc said:

Mr Dave said:

But you could have two turbos bolted to your two engines and have 300-320 bhp for 40kg more weight....
Im still confused.

You'd have 2x the power, and you'd need 2x the radiators, transmission,exhaist, etc, etc.

You'd have a heavier car because of all the extra kit required to get the second engine to work, so 2x the power wouldn't equate to 2x the speed.
The extra engine adds weight so you will end up with a power to weight ratio of somewhere well under 2x the original, one-engined car.

You asked about performance. If all you want to do is go in a straight line, then 2 engines isn't so bad. If you want to corner, though, you should be aware it will always be worse than a single engine.

As a matter of fact, with most cars, 2 engines means one in the front, and one in the back.

There's a thing called a polar moment of inertia: It's hard to describe but an example would be to imagine the car as a barbell (like in weightlifting).
The weights at each end of the barbell are like the two engines in one of these monster cars.
Try turning the barbells as if they were a car going round a corner - rotating in the same way as a helicopter rotor.
The inertia of having these two big weights at each end means that it's

1. hard to start turning
2. hard to stop turning, once you've managed to get it going

Now, imagine the exact same weight, but the bar is shorter - like a dumbbell. The weights being closer to the middle means that they are:

1. easier to start turning
2. easier to stop turning

That's polar moment of inertia, and it's why instead of having the weight at each end of the car, it's much better to have the weight near the middle of the car (as in all proper racing cars, and exotic road cars).

If you want to go round corners, you'd be better off with the original, single-engined car than the two-motor thing.

Round a track, 2x the power with more weight than the original car, and an inability to corner fast would mean the original car stands a fighting chance of being quicker.

To put it another way: what do you think, when you say "performance" ?

Craig.

So you are saying that the chavs that do this to their corsa/golf/other chav hatchback are chav and havent a good grasp on vehicle dynamics.

But it does give you double the horsepowers, although its best not asking about actual real world performance.


So a car with 2x 100bhp engines that all up weighs 1000kg isnt as fast as a car with 1x200bhp that also weighs 1000kg?

[Mr Dave]

PhillipM said:

The point is the 2 x 100bhp car isn't going to weight the same as the 1 x 200bhp car in the first place.

But yes, the handling would be worse.
Want more power? Tune the original engine.
A bit extra again? Turbo time
Want even more power? Fit a bigger engine.

Edited by PhillipM on Saturday 3rd July 18:53

I could make it weigh the same.


And in extension to your post
Want more power? Tune the original engine.
A bit extra again? Turbo time
Want even more power? Fit a bigger engine.

Want even more power? Fit 2 bigger engines with turbos.

[Mr Dave]

PhillipM said:

Mr Dave said:

Want even more power? Then you didn't fit a big enough engine or turbo

Fixed.

Or the right amount of engines or turbos.

[Mr Dave]

CraigyMc said:

Mr Dave said:

Or the right amount of engines or turbos.

How many engines do the car manufacturers put in their cars? Even their fastest ones?

Exactly.

Moving on...

Penny pinching gone too far I say.