akumabito
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| posted on 6/5/08 at 10:29 PM |
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Rev range - limiting factors?
Maybe a dumb question, but I am wondering what the limiting factors are that decide an engine's red line.. Why do some engines max out at
6,000RPM while others are quite happy doing 9,000? I don't think it's just about the rotating mass.. what other factors play a role in
this?
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dhutch
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| posted on 6/5/08 at 10:40 PM |
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quote:
Originally posted by akumabito
Maybe a dumb question, but I am wondering what the limiting factors are that decide an engine's red line.. Why do some engines max out at
6,000RPM while others are quite happy doing 9,000? I don't think it's just about the rotating mass.. what other factors play a role in
this?
The main limit factor of how high you can tune a partiaular engine can be simply down to the strenght of the core parts. The weight of the piston and
strength of the conrod.
- The forces involed go up with the square. So double the speed, quadruple the force.
Also things like gas flow, hence opening our inline/exhast and high lift cams etc.
Smaller engines therefore naterally rev harder. And two strokes the same.
I look forward to others comments however.
Daniel
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DIY Si
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| posted on 6/5/08 at 10:43 PM |
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One major factor is how square the engine is. A square engine is one where the bore and stroke are the same. Higher revving engines tend to be over
square, this is where the bore is larger than the stroke. The S2000 for example has, with it's 9,000 rev limit, an 87mm bore and an 84mm stroke.
The blackbird engine in my car is even more over square, with a 79mm bore and a 58mm stroke giving it a 10,750 red line. Lighter, well balanced
internals also play a large part. There are other things, but those two are pretty big ones.
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nitram38
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| posted on 6/5/08 at 11:38 PM |
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Things like valve bounce come into play which therefore limit power and revs
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Simon
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| posted on 6/5/08 at 11:51 PM |
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It also comes down to simple things like airflow. I seem to remember an article about (iirc) Bogg Bros putting bike carbs on a Nissan Micra engine and
getting it to rev to nearly 11000rpm. Yes, 11 thousand.
Scary hey
ATB
Simon
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BenB
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| posted on 7/5/08 at 07:42 AM |
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quote:
Originally posted by Simon
It also comes down to simple things like airflow. I seem to remember an article about (iirc) Bogg Bros putting bike carbs on a Nissan Micra engine and
getting it to rev to nearly 11000rpm. Yes, 11 thousand.
Scary hey
ATB
Simon
You can rev most engines to 11k.
Once
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Mr Whippy
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| posted on 7/5/08 at 08:02 AM |
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the real limiting factor is how quickly the fuel can burn.
Fame is when your old car is plastered all over the internet
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CaptainJosh
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| posted on 7/5/08 at 09:24 AM |
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And what cam/s you have in it, no point revving an engine to 9000rpm when it makes its max power at 5500rpm.
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MikeR
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| posted on 7/5/08 at 11:58 AM |
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as well as the rotating mass of the piston / crank etc you have to consider the valve train (not just the springs). I believe a weak part of the
crossflow is something to do with the rocker gear - although i can't think why i think this so its just as likely to be complete tosh.
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Mr Whippy
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| posted on 7/5/08 at 01:24 PM |
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quote:
Originally posted by MikeR
as well as the rotating mass of the piston / crank etc you have to consider the valve train (not just the springs). I believe a weak part of the
crossflow is something to do with the rocker gear - although i can't think why i think this so its just as likely to be complete tosh.
perhaps the rockers just snap
Fame is when your old car is plastered all over the internet
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DIY Si
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| posted on 7/5/08 at 01:36 PM |
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The valve train can often be a limiting factor. It often is on the A series. Although they can, with the right bits of fiddling be made to spin to
9,000. Mine does, but it won't do it for long, as the crank should snap at just above 9,000! The self imposed red line of 8,5000 hasn't
broken anything yet. That's with a later offset ground crank, and cooper rods and cooper pressed steel rockers. Otherwise nothing special
really. You'd be amazed at just how much affect little things can have.
[Edited on 7/5/08 by DIY Si]
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dhutch
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| posted on 7/5/08 at 08:16 PM |
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quote:
perhaps the rockers just snap
Prehaps!
- Had that happen on a kart mid race. Honda GX-160 engine.
- You uprate the valve springs to prevent valve bounce, then rev it hard...
There only little pressed steel things.
Daniel
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ChrisGamlin
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| posted on 7/5/08 at 09:00 PM |
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I suspect the main limiting factors with a modern multivalve engine with fuel injection where breathing and fuelling is fairly good, is going to be
the stresses on the rotating parts. For a given RPM, a smaller capacity over square (big bore, short stroke) engine will be capable of revving higher
because the distance the piston has to travel per revolution is smaller, the the acceleration forces exerted on the parts are lower, and the the
average piston speed is lower, so although an average car engine may only be revving to 7,000rpm and a bike engine say 12,000rpm, the actual forces
exerted on the pistons etc will be very similar in both engines.
[Edited on 7/5/08 by ChrisGamlin]
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akumabito
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| posted on 8/5/08 at 06:38 PM |
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Thanks for the info!
Hmmm, I guess it's gonna be really expensive to get an Alfa V6 up to 9,000RPM without it shaking to bits then?
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David Jenkins
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| posted on 8/5/08 at 06:45 PM |
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quote:
Originally posted by MikeR
as well as the rotating mass of the piston / crank etc you have to consider the valve train (not just the springs). I believe a weak part of the
crossflow is something to do with the rocker gear - although i can't think why i think this so its just as likely to be complete tosh.
It's not tosh - the crossflow rocker shaft is not exactly rigid or strong. It's fine for its intended use, but a 'rockers and
rocker-shaft' upgrade is high on the list if you want to push it hard. Last time I looked, the kit was £450 inc. VAT from Burtons. 
On a cheaper note, upgrading the alloy rocker-shaft pillars to some made of steel is also recommended - and far cheaper.
[Edited on 8/5/08 by David Jenkins]
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Trems
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| posted on 8/5/08 at 06:56 PM |
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It depends if youre talking about having an engine that will reliably run at high speed or just physically getting it to do it.
Many of the above comments are true, just look at a current formula 1 engine, most manufacturers could have the engines over 20,000 as they dont have
valve springs to worry about and the acceleration of the internals is relatively low compared to road cars due to the massively over-square design
(current 2.4litre V8's run <45mm stroke with around 100mm bore)
But then again with F1 you are doing well if the engine does more than 500 miles! 
If you want to run higher revs without too much hassle many well designed engines will benefit from high tensile rod bolts, harder and/or dual
compound valve springs and some flow work on the head along with enlarged throttle(s). Of course if you run Efi then you will need to alter the
rev-limit.
Regarding crossflows, back in teh day these were the N/A engines of choice, if youre willing to fork out on the aforementioned valvetrain mods, forged
pistons, rods, new bolts and a billet short-stroke crank i belive people have had them over 13000RPM.
www.tR3M5.com
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DIY Si
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| posted on 8/5/08 at 06:56 PM |
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quote:
Originally posted by akumabito
Thanks for the info!
Hmmm, I guess it's gonna be really expensive to get an Alfa V6 up to 9,000RPM without it shaking to bits then?
Go and talk to these people. Some steel rods should go a long way towards upping the red line.
Other than that, lighter internals and better flowing heads will go a long way.
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akumabito
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| posted on 9/5/08 at 02:21 PM |
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Thanks for the link! I've bookmarked 'm for later use.
I have started the final countdown to my move back to Europe, and all of the ideas for Locosts and other wheeled madness start popping up again..
Still not entirely sure what I want to build after I get back, all I know is that the Alfa V6 is one of my favorite engines (I'm a bit of a
masochist I'm afraid - I like Italian tech! )
Does anyone know if 250 more-or-less-reliable HP is realistic on a 12V Alfa engine? ...without astronomical budgets? It'd be a pretty big step
up from the original 190Hp..
A set of turbo's would be tempting, but these engines jus sound so good by themselves!
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