How is engine redline determined?

[KrisZ]

I read somewhere that some car companies are starting to think about building engines with longer stroke again (yay for low end torque), in order to meet the tuff fuel economy standards.

I think it's about time this happened, the horse power race is just getting silly, and most of today’s I4's and V'6's are gutless off idle, which just kills everyday drivability.

I say leave the high revving engines for sporty applications, and give us some low end torque in small and mid size cars.

 

[rslifkin]

I agree with Kris. My Jeep has that low, torquey powerband. Redline is definitely higher than usable power though. It redlines at 5300, but it's done making power around 4500 - 4600 (WOT shift at 4500). It pulls it's best from 2800 - 3800, with gobs of torque from 2000 - 3500 or so. It'll pull comfortably in OD with the TC locked down to 1100, and still accelerate up hills.

I only wish most modern cars had a powerband like that, as they would get better gas mileage, and be much more fun to drive around town.

 

[CBR.worm]

I have blown engines multiple times by over revving and having the valves float and hit pistons.

This was usually within 1,000 rpm of REDLINE. Sometimes it was to win a race, other times it was due to a missed shift or shifting the wrong way.

My experience has been that the valve train is the limiting factor.

piston speed is also a huge concern, think about the G-forces. It's interesting that lighter engine components can frequently raise RPM ceiling, the opposite of what you would sometimes think - bigger rods, etc.

Diesels burn slow, they typically don't make power beyond a fairly low RPM, so the engines are designed and built to run at that low RPM.

 

[Nick R]

hmm.. WOT shift in my old cavalier was 5500RPM, but redline was about 6,200RPM iirc, and I'd shift imanually there, and it did make a difference, but it didn't sound happy.

 

[Scott_Tucker]

Originally Posted By: Spazdog

Yeah....not so much.

Maybe in my old '85 LX 5.0. Redline in that car was 1000 rpm higher than useable power.
3.00 x 6000 x .166 = 2988
Okay, that works there.

But lets take a look at Honda's D16Z6:
3.50 x 7200 x .166 = 4,183.2
Holy cow! that engine must have titanium connecting rods and a forged crank!!!
It doesn't have anything special internally. in fact, the parts are rather slight even considering the displacement of the engine. thin connecting rods, small journals... In fact if you compare the D16's rods to the B16A2 (with only 800 rpm higher redline) they are toylike.
It's so undersquare that it actually has a longer stroke than the Ford 302.
So it has a sigificantly higher redline and a longer stroke than the venerable 4.9L Ford 302. Why?
an extra set of intake lobes. At 4800 it switches over to a higher lift longer duration intake lobe. The engine can breathe.


Maybe I should have elaborated. Most passenger cars are about 3000 fpm. That's not exact. As material science has improved, piston speed has increased so piston speed in modern engines is increasing. The D16Z6 is in a performance car so its not unusual that it would be that high, although 4183 fpm is a little in the high side for a production car. The internals are strong enough whether they look it or not. Most high performance cars will be in the 3000 - 4000 fpm range. Well built engines can rev to 5000 fpm but durability is reduced but that doesn't matter in a race engine that is going to be rebuilt often. A top fuel dragster engine may see 8000 fpm but they are rebuilt after every run. Formula 1 engines do not exceed about 5000 fpm.

O.E.M's and professional engine designers use piston speed and the corresponding G forces to assign a redline. It is the most important factor and that is why we use it.

Valvetrain could have been part of the formula in the past but it is a relative non-issue these days as the technology can support high rpm.

Redline is not something that is just guessed at nor do manufacturers just rev it up until it explodes and back it off a percentage.

 

[CBR.worm]

Yeah, my valve float related failures were at over 14,000 rpm

Not something seen on too many car engines.

 

[Scott_Tucker]

Originally Posted By: CBR.worm
Yeah, my valve float related failures were at over 14,000 rpm

Not something seen on too many car engines.


That's about the limit for coil springs.

 

[Scdevon]

Redline is determined by the maximum usable RPM in an engine's power band, not necessarily the speed that an engine would blow to pieces.

The stock rods / crank / pistons in most gasoline passenger cars would survive 7000 RPMs easily. The risk of valvetrain damage at this speed would be high though and most cam profiles are designed to make peak HP / torque below 6000 rpms,

 

[Spazdog]

That's what I'm saying.

If you check the dyno charts for most cars, redline is just a little past peak horsepower production and just before the curve takes a serious nosedive.

There are exceptions. The few DZ302 Camaro owners that actually drive their cars will tell you that there is ample power to be made above redline - some say as high as 8500 rpm.
Maybe reliability was a concern, maybe GM just didn't have any 8500 rpm tachs. But that was 41 years ago.


The Acura Integra LS is the other way around. It's redline is like 6600 but the power falls off pretty bad long before then. At redline it was around 80 hp. Something like 40 hp less than peak. (At the wheels)

 

[Scott_Tucker]

Originally Posted By: Scdevon
Redline is determined by the maximum usable RPM in an engine's power band, not necessarily the speed that an engine would blow to pieces.

The stock rods / crank / pistons in most gasoline passenger cars would survive 7000 RPMs easily. The risk of valvetrain damage at this speed would be high though and most cam profiles are designed to make peak HP / torque below 6000 rpms,


That is true but mean piston speed can be used to your advantage. Engines that have small bores with long strokes make torque in the lower rpm range. Engines with large bores and short strokes will make their torque in the higher rpm range. The large bore/short stroke engines are built for horsepower, which increases with rpm.

Mean piston speed increases with stroke length so the long stroke engine can't rev as high before it fails. Since the short stroke engine has lower mean piston speed it can rev higher before failing.

 

[SteveSRT8]

There are exceptions.

There are many. Chrysler's 6.1 engine has peak HP at 6400 rpm and shifts at 6400 rpm. Simple reprogramming to a higher shift point actually benefits their performance rather dramatically.

I have also been told that GM established the Duramax redline based on Allisons recommendations since it was their gearbox designed for low revving diesels.

 

[rslifkin]

Keep in mind, on the topic of pickup (and small car) diesels, that they tend to rev them higher than many diesels, simply for more HP, even though the torque is about the same. For example, a 7.3 Powerstroke revs to over 3000 rpm, while the International T444E version of the same engine (same internals) is governed at 2600. It can be revved higher without damage, but in stock trim, it won't pass 2600 under power.