My car hits the fuel cutoff at 6000 RPMs or so. Obviously there’s a built in limit to how many RPMs an engine can turn, else they wouldn’t have the fuel cutoff.
So, what are they? Why can some engines turn faster than others? Can the performance of a previously limited engine be modified past the current limits?
Valve float is a big reason. When the valves move too quickly, they may not seat properly. Stronger valve springs can be added to reduce float. An interesting way that Ducati motorcycles handle this is using a Desmodromic valve system, where the valves are opened AND closed by a camshaft.
Piston inertia can also cause stress on the crankshaft journals and bearings at high RPM. Crank upgrades can contribute to a higher RPM limit.
Also, the ignition system may not be able to handle high RPM.
Sure. All those Chevy mouse motors that wound up riding the rails for a quarter mile hitting 8,000-9,000 RPM needed a little fixin’ up, but they got there.
A quick perspective on my part splits the answer into two parts: a.) you have to be able to produce the higher RPM and b.) you have to be able to handle the higher RPM.
The answer to a.) above would involve aspiration and combustion. This would involve all of the many modifications you can make to more efficiently, and quickly, deliver and burn fuel. Your intake system, camshaft profile, valving, compression and exhaust are all components that contribute to this, and can all be modified to do so.
Just as important is b.) above, which involves making sure your crankshaft, mains, rods, valve springs, lifters and, for that matter, drivetrain and suspension are up to it.
There’s a whole lot of stuff in an engine changing direction really fast. If it goes too fast, the inertia starts tearing things apart. This is why small engines (like in most Asian imports) rev higher than, say, a Ford 427-cubic-inch V8; less mass = less inertia = can go faster before it starts falling apart.
If you over-rev it, you could throw a rod (i.e., a connecting rod breaks at one end and goes out the side; not pretty) or in older pushrod engines like the 427 I mentioned, you could float the valves (valves out of sync with pistons = “bad” to “holes in pistons”, depending on the design of your particular engine).
Modern computer-controlled engines may have interchangeable chips for the rev limiter; consult a board dedicated to your particular model. I’d be careful with it, though – although I’m sure it’s capable of more than what the factory limit is set at, you’re looking at a new engine if you go too far.
The valvetrain was apparently what limited the big V8s of the '60s – in '64, Ford put overhead cams on a few 427s, which allowed them to rev to 7500 rpm (6000 rpm was as far as any sane person would take a normal 427). The “Cammer” was banned by NASCAR before it even made it into production.
Airman Doors: It would be my pleasure to attempt to answer your question - but forgive me if I use Formula One analogies to do it.
Quite correctly, Formula One these days cops a lot of valid criticisms over a variety of different areas - namely, the racing is boring, the strongest team always wins etc etc.
But in it’s defence, Formula One still has ONE extremely honourable point in it’s favour - that is - within reason, the sky is still the limit in terms of engine development.
As it stands, the current engine spec for an F1 engine is 3 litres, and it can be either 8, 10, or 12 cylinders. There are some relatively minor restrictions on the gasoline - insofar as it has to “nominally” adhere to “unleaded gasoline” in terms of it’s lead content - but other than that it’s all bets are off.
Accordingly, the F1 engine of the modern era remains the undisputed king of test beds in terms of horsepower per given engine displacement without atmospheric assistance. Obviously, many other forms of motor racing produce superior horsepower due to having a form of supercharging in the inlet manifold, but as normally aspirated engines go, the modern F1 engine is without doubt the current state of the art engine to examine.
And here’s why - nothing on the planet comes close to the astonishing power outputs and engine RPM’s that an F1 engine produces - at least in the context of an automobile engine. Yes, you can buy little 2 stroke 20cc single cylinder model airplane engines which rev at 25,000 RPM but I hardly think they count.
No, for a 3 litre engine producing 850+hp at 19,000 RPM without any form of super or turbo charging - that’s really, REALLY out there.
Apparently, the single greatest breakthrouhg in the last 20 years of F1 engine development took place in the 1986 season when Renault perfected their “compressed air valve actuation” system. Every F1 engine builder uses a similar system these days. Prior to the pneumatic valve system, F1 engines had kinda peaked out at 13-14,000 RPM for over 20 years. No amount of effort in regards to spring and valve metallurgy could make an engine head work much beyond 13,500RPM or thereabouts. Quite simply, the valves would start bouncing like all crap at rpm’s above 13,500 and horsepower losses would take place.
But in 1986, Renault perfected a computer controlled valve system which uses solenoids and compressed air from an air tank very similar to a scuba tank believe it or not - and the solenoids take turns in routing compressed air to push the valves either upwards, or downwards. The only moving parts - apart from the valves themselves, are the computer controlled solenoids which open 8mm apertures.
The scuba tank contains enough compressed air for the F1 engine to do a warm up, some extra laps, and a 300km Grand Prix - but not much more.
When the engine is activated, the computer’s “virtual camshaft” sets all the valves into their correct positions as though a true series of camshafts were in place, and then the engine ticks over.
What makes this system so impressive is that the software can think in terms of variable valve actuation - that is, the valves can open for durations way, way in advance of the cylinder’s movement to allow for the amount of time it takes for air pressure to begin moving air inertia downwards into the cylinder bore. This intelligent valve actuation, combined with incredible advances in ignition advance timing, and the remarkable advances in flame front propagation in fuel research allows your modern F1 engine to spin at well over 18,000 rpm these days - which means shitloads and shitloads of horsepower.
But it has to be said - every other part of the engine has been engineered within a millimeter of it’s life too. They are incredibly small and light blocks - and their cooling systems are just amazing. The crankshafts are awesome - capable of spinning at 19000RPM and also moving oil about to lubricate everything.
Very short strokes and way, way oversquare pistons. Magnificently tuned exhaust systems with zero silencers.
All up - very impressive technology I must say.
And you can get an original never been run “cammer” in Oklahoma City at a place that sells old/reproduction Ford parts, (forgot the name of the place.)
That’s one great big ass motor fer sure!
Stock trim, 600 normally aspirated factory horsepower can be yours for the paltry sum of $6000.00.
CedricR.
And you can get an original never been run “cammer” in Oklahoma City at a place that sells old/reproduction Ford parts, (forgot the name of the place.)
That’s one great big ass motor fer sure!
Stock trim, 600 normally aspirated factory horsepower can be yours for the paltry sum of $6000.00.
CedricR.
There’s also some other simple reasons, while all the above are still valid: there is no reason to rev higher if the engine can’t breathe as well and the horsepower curve drops off. So, why squeeze out anothter 1,000 rpms - even in an engine capable of it - when it gains you nothing? Sometimes the ‘redline’ is actually lower than the engine’s potential, simply because the HP curves drops, along with efficiency.
Sometimes, as in the case of the Honda S2000, high revs means Honda gets to hype 230plus ponies out of a small 4 cyl normally aspirate engine, but the 230 is the peak and not very reflective of the actual power curve (I’m not knocking Honda! Believe me! But all the revs and the high HP rating are just a tad deceptive in terms of real world power). Still a cool motor/cool car.
As a followup to my original question:
If efficiency is the goal for car makers, why do they detune engines, thus making them less efficient? If, as Philster says, there’s more there, why don’t they maximize the efficiency right away. I read about “detuned” engines a lot. What for?
I’m not making claims about ‘detuned’ engines. I think what you might be referring to is lower HP output for the benefit of efficiency, not to ‘hurt’ efficiency.
Many engines can get more HP from more aggressive cams, or higher flow injectors (among a host of things).
Key: A mfgr has to put out a product that balances the cost/power/efficiency/reliability/drivability for that specific product.
For example, my 2001 Dakota has a 4.7 litre v8 making 235 horses, but this thing is very capable of getting up to 285-300 horses with no turbocharging. The issue would be: more power from higher lift cams = worse fuel mileage, and different drivability (performance gains at the cost of daily driving. Engine might be happier at higher revs, etc). Might create emmision challenges that need to be countered with better breathing/exhaust, which creates more production costs, etc. Could me more load on tranny, requiring better tranny cooling, bigger radiator.
That’s just an example.
For durability, in many cases. Often, engines that are set up to produce power to their maximum capacity are on the edge of reliability. I recall reading about some military aircraft engine that had an exceptional durability record when producing about 90% of the power it was capable of. When some things were modified to get the power output at or near 100% of capability, the planes were noticeably faster. And engine breakdowns became common.
Generally, when engines are commercially detuned the idea
(especially on high revving engines such as motorcycles) is to improve the power delivery, increase torque, improve gas mileage and increase reliability.
High revving engines tend to have a ‘step up’ in their performance, where the power suddenly comes in. This is partly to do with exhaust and inlet tract design which can be used to scavenge spent gases and provide pressure waves of air for increased air charges into the cylinders. More air in means you put more fuel in and thus get a bigger bang.
The problem here is that exhausts and inlet tracts in most engines have fixed length, which means that they have frequencies of resonance for waves of gas passing through them, detuned engines are designed usually to have fairly low resonance across a wider range of engine revs to try and smooth out the power curve.
The lower the resonance, then the flatter tends to be the power curve(ok so there are other factors here) but this also has the effect of reducing the peak power too.
Having a broad even spread of power across an engines rev range is generally desirable in town cars as revs are generally low, you don’t have to change gears to stay within the best power available and you have less chance of stalling.
When engines are highly tuned their power is often produced within a relatively narrow rev range, as little as 3000 rpm wide(say from 10000 to 13000rpm) on very highly strung engines, it means you have to match your engine speed and gears supremely well or you drop out of the big power and bog down.
Race engines are usually operating within a narrow power band, but the peak power can be immense.This, of course, does not make for a relaxed drive.
Newer technology means that changes to inlet tact length and exhaust length(largely borrowed from F1 racing) mean that highr power can be obtained across a far wider range.
For example Yamaha have been using a method of diverting exhaust gases into differant parts of the exhaust system, effectively changing its length, dependant upon the engine revs, this was patented as the EXUP valve and was first found on the Yamaha FZR 750RR and this broadened the power spread making drive out of corners more torquey, smooth and predictable, and without the sudden power ‘step’ it was far less likely to break tractions and slide in exteme cornering manoevers.
Lately Suzuki and Ducati have both been looking at ways to include varible injector inlet tracts, basically at low engine revs the fuel does not atomise well because the airflow is too low, and air pressure waves from the airbox are also low. Secondary throttle valve restrict the air path and speed up the airflow, and increasing the inlet tract length allow longer for the fuel to atomise as well as tune the pulses of air into the engine far better.
At higher revs inlet tract length is shortened to get bigger chunks of air into the cylinder as possible.
This sort of stuff has been in top flight racing for some time but F1 designers are now being called in to desing bike engines more and more.
The main problem is price.