What stops an engine revving?

[2F45T4U]

My cars got fairly good torque low down but doesn't want to rev above 5k. It's like driving a diesel. I know I could not screw the balls off it but really thats not an option

[MarkVeryard]

May seem a bit silly, but are you sure that your carbs are opening fully?

I had major problems with my Sprint being overtaken by Ford KAs early last year (only had 60bhp when measured on the rolling road in Fairfach) - it was a real struggle to get to 5000rpm and even then it was gutless. After posting a question on the old discussion board, Jon Tilson suggested that the carbs weren't opening fully, so I investigated...

Turned out that the carbs were only about 40% open when the accelerator was pushed to the floor! This was due to:
i) parts of the throttle linkages fouling each other
ii) two big splits in the bulkhead where the accelerator bracket was mounted - this meant that when I put my foot down, most of this was taken up by the accelerator mounting bracket just twisting and only a small amount was left to move the throttle cable!

Thanks Mr T! - sprint now goes like stink!

[2F45T4U]

Hmm yer I've got a new throttle cable, Thanks George!, to fit so will have a look. But as Richards Sprint showed us with the throttle not opening fully you have a lack of performance through out the rev range. Mine seems to hit a wall.

The previous engine I had would pull the car up to 6000 in top gear! Thats impressive for an unknown engine (that later blew its gasket ha!) and a car shaped like a brick. Jonners once said to me, to see if your engines pulling well see if you can do 60 in second. I havn't tried that but I did have a pop at what it could do in first. Started spitting and popping at 35mph = 5000rpm


Points gap is good and carbs have ATF in them

[MarkVeryard]

Ok, what about the obvious... knackered alternator ? This would cause that 'brick wall' syndrome.

[2F45T4U]

Would it? My car always starts in the morning so does that rule it out?

[MarkVeryard]

Well, I'm not really an expert on how alternators work, but I have experienced a few failures. If the alternator fails, basically meaning that you are running the car off the battery alone, you find that the engine runs fine up to a certain rev level, at which point it is a bit like hitting the rev limit on a diesel - coughing spluttering, 'brick wall' syndrome. As the juice in the battery starts to run out, you start to hit the brick wall at lower and lower revs.

I guess that the level that you hit your brick wall is consistently 5000rpm, which would suggest that you aren't running off the battery alone, but I suppose it might be possible that the alternator is 'a bit faulty' or is under rated, and is therefore not supplying quite enough juice to the battery...

Just a theory, like I said, I'm no expert - I could be talking bollox

[dailysprinter]

could be fuel pump not delivering enough fuel?

[DavePoth]

All the ignition bits in good condition? if the points are knackered it would be worse at high RPM.

[xvivalve]

What stops an engine revving?

An old lady in a Nissan Micra!

[2F45T4U]

Jeez it's like chinese whisper It was a Vauxhall Nova

Richard is the old lady with a Nissan Micra

[2F45T4U]

http://www.youtube.com/watch?v=Y5sqRRBlsuU

Ok I'm a liar

I adjusted the points a day or two ago and although performance was improved I hadn't tryed revving it that hard

[DavePoth]

Oh well, at least you didn't need to change another engine.

[ALGIK]

VE

[2F45T4U]

VE?

[Lewis]

Haha.......he's going for the engineering answer.

VE stands for Volumetric Efficiency - it's the definition of how efficiently the engine can pump the charge in and out of its cylinders (after all, an engine is just a big air pump when you get down to basics!)

On a technical level, VE is the ratio (or percentage) of the volume of air/fuel mixture that enters the cylinders during induction to the capactity of the cylinder itself

Volumetric efficiency. VE is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency.

On the other hand, turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%. However, if the cylinder is pulling in a vacuum, then the engine has less than 100% volumetric efficiency.

Normally aspirated engines typically run anywhere between 80% and 100% VE. So now, when you read that a certain manifold and cam combination tested out to have a 95% VE, you will know that the higher the number, the more power the engine can produce.

What stops the engine revving is that the volumetric efficiency falls off - the faster the engine spins, the less chance there is for the cylinder to be filled with air/fuel mix, so the power decreases and the engine's ability to rev starts to decline.

The physical properties of an engine also entail how high it can rev. The rod/stroke ratio can alter the engines ability to breath at high revs - which can give increased linger (the piston reaches the top of the cylinder and pauses, as the combustion chamber is filled with fresh mixture and distributed effectively) at TDC to allow adequate filling, improving VE.

Bore/stroke is also another important factor which determines a bottom ends ability to sustain high rpms (the forces of the whole assembly rotating)

To get an engine to rev higher, with quick valve opening and closing, require a cam or cams which have long duration, high lift, and alot of overlap (basically), allowing enough mix into the cylinder whilst getting rid of the burnt-off gases.

After having a cam designed to make high rpm power, intake and exhaust design become important. Air has mass and anything with mass has the ability to have inertia, if and intake or exhaust system is "tuned" it has the ability to take advantage of this inertia and can increase volumetric efficiency. This will aid in getting cylinder filling at those high rpms and in turn make power...so long as the head and cam will allow it.

As Darren has proven also - turbocharging and supercharging can give a VE over 100% (also - some NA engines can attain VEs over 100% with enough work). These systems cram the cylinders full of air, even with short valve opening durations, keeping the VE up and allowing the engine to rev faster, whilst still generating power.

There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use a larger number of valves, see multi-valve, which cover a greater area of the cylinder head. Carefully streamlining the ports increases flow capability. This is referred to as Porting and is done with the aid of an air flow bench for testing.

Today, automobile engines typically have four valves per cylinder for this reason. Many high performance cars in the 1970s used carefully arranged air intakes and "tuned" exhaust systems to "push" air into and out of the cylinders through the intrinsic resonance of the system. Two-stroke engines take this concept even further with expansion chambers that returns the escaping air-fuel mixture back to the cylinder.

A more modern technique, variable valve timing, attempts to address changes in volumetric efficiency with changes in RPM of the engine -- at higher RPM the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.

In a four-stroke naturally aspirated engine, the theoretical maximum amount of air that each cylinder can ingest during the intake cycle is equal to the swept volume of that cylinder (0.7854 x bore x bore x stroke).

Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE). An engine operating at 100% VE is ingesting its' total displacement every two crankshaft revolutions.

There are many factors which determine the torque an engine can produce and the RPM at which the maximum torque occurs, but the fundamental determinant is the mass of air the engine can ingest into the cylinders, and there is a nearly-linear relationship between VE and maximum torque.

For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can exceed 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components.

There are so many factors that limit an engine's ability to rev - electrical, mechanical, physical and chemical properties....but there's a few

I think I'll stop now before I bore myself/everyone else to death I think I've got it right but it's something that I still need to cover in a bit more depth! Any corrections/omissions greatfully recieved!