16 replies to this topic

[Mangel]

As far as i can understand, most sedans have the torsional rigidity to support a much more powerful engine that usually sits in most common sedans. 

 

But what actually would be the weakest link when increasing engine power in the chassis. Is it usually strength in engine mounting (chassis side), torsional rigidity or something like this? And what margin are we talking about here?

 

I'm thinking that increase in engine power for most cases won't be a problem because the chassis need to handle other forces which is stronger. Like the engine weight and the force it will put on the mountings when the car f.ex. hits a bump or the like.

 

Any thoughts on this topic?

 

[Greg Locock]

I agree, not an issue. You will tear the engine mounts (the rubber bits) apart if you use the original ones, but the ones designed for the engine in question should be ok. Same applies to the rear suspension bushes.

 

If you think about it the most mundane vehicle is capable of handling 1g loads from braking, and 7g from hitting potholes, so powertrain loads really aren't a big deal.

[Mangel]

Thank you! 

 

Do you know of any good reports, or the like, that talks about these forces?

 

The reason I'm asking is that I'm trying to write a application for engine power increase. And usually the response is that you have to document that the chassis can handle the power increase. But then i would need some sort of legislation to tell me how the bar is set for the chassis regards to engine power.. .well no wonder i can't find this if the engine power doesn't really matter...

 

 

 

But one question though.. cars like the bmw e46 m3, and other e46s, has a sad tendency to crack by the rear subframe mountings. I guess this has much todo with that the cars are driven hard. But i cant understand how this can come from normal acceleration (WOT) unless it is a design fault.

 

In most cases if this could happen over time with normal acceleration for such a car. I guess dumping the clutch, which probably happens to allot of these cars, would be totally catastrophic. 

[Greg Locock]

My brother managed to destroy the engine mounting system of a new car by sustained spirited use of its 1275 cc engine over the course of  a year. Mind you I once shredded a set of tires on a WRX in one tank of fuel, I don't know of any open source report, I do remember that a standing start maneuver will put something like 3 times the max engine torque through the clutch. That was back calculated from the rpm vs time trace. Torque converters typically give a factor of two torque multiplication.

 

However, the powertrain abuse can be much magnified by stick/slip at the rear axle, for instance the prototype Lotus Carlton had a nasty stick slip when you wellied it out of a corner, which destroyed the entire engine mount. I proposed to investigate it using ABS tone wheels down the driveline, that was regarded as too much like hard work and instead we spent a happy couple of days at the GM proving ground making random component changes. That worked.

[Kelpiecross]

I would think that that an "engine steady rod" as on a Morris Mini would protect the main engine mounts - especially if they are mounted high-up on the engine. Maybe you could add one or more to your engine?

Edited by Kelpiecross, 23 November 2016 - 03:52.

[Greg Locock]

Yeah, but when they failed (as they did) bang went your exhaust. There's an old, and somewhat discredited, engine mount theory known as the torque axis mount, developed by Plymouth in the 1930s, the idea there was you had two engine mounts on the axis of rotation of the engine, and then a torque arm (or two?) to react the engine torque. It didn't really work as advertised, but it did at least show some thought about separating support for the engine and the dynamics.

[Mangel]

Don't need to be a open or free report, the point is really the reference. 

 

Won't the "standing start maneuver" depend on how heavy the rotating mass in the engine be? How do you calculate it?

Tried to calculate it once.. think i got about 39k Joules from a flywheel with radius of 15 cm and 15 of weight at 6500rpm. seems a bit much maybe

 

Stick/slip situation don't really become a problem before you have slicks and enough power to lift the inner front wheel of the ground on the way out of a corner, snapping the axle when it lands...The car is Audi 90 quattro from 1988, for those who know these cars.

 

But these things aren't really the issue. The issues is proving that these modifications is safe regards to the power. I guess i can for the motor mounts show that the sheer strenght of the bolt (if it's bolted through all the way), that it's superior to force the engine will load on it.

 

And the biggest issue is really proving, or at least being convincing enough, that the chassis will handle the forces.

[gruntguru]

The standing-start-maneuver needs a rpm/time record to determine the peak deceleration of the rotating masses, The torque determined from that deceleration must be added to the torque being produced by the engine at that rpm.

[Mangel]

Yeah, figures. But what would these numbers be for an normal vs an aggressive clutch? And will flex/slack int the drivetrain play major role?

Edited by Mangel, 24 November 2016 - 08:04.

[gruntguru]

Well Greg says 3X engine torque for average road car. Maybe more for a race clutch, more for a rigid driveline (UJs vs rubber donuts), more for backlash in the driveline, more for heavy-road wheels.

[Kelpiecross]

Yeah, but when they failed (as they did) bang went your exhaust. There's an old, and somewhat discredited, engine mount theory known as the torque axis mount, developed by Plymouth in the 1930s, the idea there was you had two engine mounts on the axis of rotation of the engine, and then a torque arm (or two?) to react the engine torque. It didn't really work as advertised, but it did at least show some thought about separating support for the engine and the dynamics.

If I remember correctly (and I may not - it's been a while since I played with a Mini) the Mini had its engine mounts on the engine/gearbox centreline and must have relied on the steady rod to stop the unit twisting. Probably why I also broke the exhaust on my beloved 998cc Cooper.
But the very similar Morris 1100 unit had widely-spaced mounts on the front of the engine - again if I remember correctly.

[Greg Locock]

The Austin Maestro team came up with two completely different engine mounting location strategies, and went right through several prototype phases in both versions. Back then (1980) figuring out how to mount FWD engines was a black art. Mind you the XJS went for quite a long time with no gearbox mount, we used to run them on chassis dynos with the gearbox propped up. If you ever see one in a scrap yard it is worth having a look at the trans mount, it is pretty special.

[mariner]

I do'nt if it was published but Chrysler had a test of , I think, 400 standing start launches for its 1960's/70's muscle  cars.The tire grip was obviously poor by modern standards but the tramp etc was probably severe.

 

Maybe a report on these tests lies deep inside the SAE archives , Chrysler did publish a lot of stuff in those days.

 

BTW I've sometmes wondered there is any benefit in directly linkng the torque reaction at the engine mounts with the rear, or front, roll stifness via hydraulics? The idea would be to have diferent roll couple in and out of corners.

[Greg Locock]

Lotus Esprits were rated for about 50 full throttle journalist style launches.  Talking of which they also had a policy whereby if the purchaser felt that their car was not performing to spec they could bring it to a circuit and one of the ride and handling or powertrain guys would wring it out for them and give them the printouts. And on a closely related subject some American rag (CR?) got a bunch of consumers to whine that their cars didn't meet the claimed performance figures. So some lucky dude got the job of demonstrating to each of these owners how to get the factory type numbers from their pride and joy. One new clutch and a set of tires later ...  I wish i could find that article again it was quite funny.

Edited by Greg Locock, 10 December 2016 - 00:42.

[mariner]

I'm not sure about the US rag but I do recall that when a UK mag couldn't repeat the GM claim on a Corvette they apparently sent a company driver who could hit the time - he simply didn't lift on the full power shifts .

 

Muncie and Dana built serious transmissions then!

[Greg Locock]

One time I was asked to do a durability test on an auto trans. I was not best pleased, so asked for the procedure, and grumpily went out to do it. (Durability tests are a pain in the arse). This particular one consisted of driving gently forward, put it into reverse, drive gently backwards, put it in to drive, drive gently forward. 50 times. It's an abuse test. How gently is gently when you don't want to do the test? Not very. Broke it.

 

Another fun fact is that if you are driving at 80 kph on a wet road or gravel, in a 4 speed Falcon if you floor the throttle and shift into reverse the wheels break traction and spin backwards,gently slowing the car. Then keep the throttle on and put it back in drive and they break traction the other way. I can't remember who showed me that one, but it was in heavy 3 lane traffic, they were driving down the road with rear wheels spinning continuously, throwing out steam and spray all over. Doesn't actually bang the car up. I should point out we used to have $1 cars that were no longer assets, basically we used to use them for transport and any old experiments we felt like (the rules have changed). One car was left instrumented for a year and every day the engineer went and repeated the exact same noise test. Absolutely useless information... until he demonstrated that our practice of adjusting the calibration of the sound level meters to a calibrated  noise source every day was actually decreasing the accuracy of the measurements. On another car he built a duplicate of the ideas in a Lexus LS400 air intake, and ended up with a significantly quieter car for the cost of a few bits of plastic tube. Hardly any of our stuff got into production, but messing about with zero value assets at least made you think about techniques. I cut a big hole in the front of the the car and measured the velocity of the crankshaft pulley with a laser, that was pretty interesting (the pulley is a big, known, noise source, but you usually infer the vibration from the noise generated, this way we measured it directly). Sadly the new program schedules don't give development engineers any time to just play with the cars, not that we have $1 cars, or are allowed to drive experimental cars on trade plates, or any of that fun stuff. Somewhere on a program scheduling website there is a DAY by DAY diary of the decisions or activities that are supposed to happen on that day. Needless to say I sing a little song every time something comes out of left field and blows our schedules into tiny pieces. Last year i was working on one safety related job that would routinely took 2 weeks, but for strange reasons took 4 months. It was an absolute grind, but I learned a lot, and got put onto the special matlab license server for priority 24/7 access-usually there is a slight chance that a license will not be available as we share a limited pool. Still got that access, I suppose I should turn it back in. Nah.

[Lee Nicolle]

Old front drive cars usually feel dreadfull, remember the whole engine and transaxle are riding on those rubber mounts which get soft and doey when they are old. While the engine officially cannot fall out some have in larger crashes.

Never use uerathane engine mounts, they transmit that much noise back into the car you think everything is worn out. Replacing all those mounts on a 10 y/0 160k car makes it feel nearly new again. Though not inexpensive.

The majority of common rear drivers usually have cheap engine mounts, there is exceptions. On the odd occasion they get soft and transmit horrid noise as well but the car is not scrunching over speed humps and normal driveways. And a 10y/o 160k car that has not been driven hard still drives very well with all the original components.

I have never been a fan of solid mount engines in race cars [you would NEVER do it in a roadcar] The engines usually are bolted to the original tags, lugs on the sides of the block and cast iron or alloy does not flex very well!

Proper Racing engines usually take all this into account.

Proper engine plates front and rear are a good deal better but then stuff up transmission mountings etc. Though solid mounts do require lots of loctite to counteract the viabration loosening fasteners.

And yes I have experienced all of the above!