Hi Guys,

Normally I dont crosspost as its annoying to responders... But I wasn't getting any joy on EXF, so here goes:

Right, so I wanted to post this and get some thoughts, before I blow something up

As far as I can tell, AMS recommend a maximum rev limit of 10,000rpm on their built motors (that utilise a stock stroke):

According to this website, this represents a maximum piston acceleration of 200984 fps^2

Now, given the tacho only goes upto 9k, and I would like to build a safety margin into AMS's limit, the 9000rpm limit results in a 162797 fps^2 max piston acceleration.

Now if we recalibrate the settings (say for a 2.5L 97mm stroke, 90mm bore) we get 162730 fps^2 using a limit of 8350rpm

So is it safe to assume, that if a built 2.0L can rev upto 9000, then a 2.5L can rev to 8350?

A lot of people in the past have been quoting mean piston speeds. Which in my opinion doesn't give the whole story. I think acceleration/deceleration is what breaks things, I assume that the acceleration = deceleration in a motor...

Thoughts?

Comments?

Thx!
D.

 

Sub'd.

Tehpra I'd think you'd be safe at 8300 as most motors with that stroke say 8500 is safe.

I doubt you'll be able to hold power much past 8300 anyway with that displacement.

 

Yes you are probably correct on the power thing.

But I still like to work out the numbers in a scientific method...

Just interested in what people think, and what I may have missed

 

Have a look at this:
http://www.motoiq.com/magazine_artic...part-deux.aspx

Page 2 has a comparison between an F1, Nascar, motorcycle, and Honda K20 engine with regards to piston speeds and acceleration.

 

nice article..

So I typed the F1 cars details into that website, and they gave me 277015 fps^2 or 84434 mps^2

So your chart on page 2 show that the piston ACCELERATION is different to DECELERATION.

I gotta wrap my head around that one, but I wonder which is more important, I would guess DECELERATION...

 

I'm no physics guru, but the easiest way to think about this is in phases right?

Acceleration on compression stroke.

Deceleration on compression stroke.

Acceleration combustion stroke.

Deceleration on combustion stroke.

The accel/decel phases are equal, but the stress seen by almost all components would be greatest at the moment of combustion I would think? Since there is a possibility that the timing could not be perfect, and the combustion process begins a millisecond or even microsecond before all momentum is lost from the compression stroke. I know a few tidbits about timing, but I don't know how exact a timing chain or belt is. But if what you aforementioned is true about the piston acceleration, at those kind of speeds, even a tiniest timing error could yield tremendous amounts of momentum, when weight is tossed into the equation. I guess the exact stroke length and component weight would be a great factor into your equation as well.

I would say the opposite of what you say is true. But then again, I'm just a lurker pitching his 2 cents. I've never read anything from you I disagree with though. Just felt like chiming in on something interesting.

 

appreciated.

I am still trying to workout how the acceleration and deceleration figures are different.

maybe its because at the bottom of the stroke the piston is closer to the crank which means the angle is a lot larger.

I think thats right, but I cant quite explain it...

 

This makes sense, but I'm not knowledgeable enough on the subject to provide you with an explanation. Sub'd for more info on a very interesting topic

 

I haven't ever thought about that before... but after you pointed that out Tephra, I could see what you're talking about.

 

It was my understanding that the weight of the piston was as much a factor as the acceleration, and I see no mention of that here.

 

its sure is..

to be honest I dont know what "lets go" when you overrev a motor..

 

Rod bolts can, rods can stretch (unlikely with strong ones), but I think the most likely is a problem with a valve spring where maybe you contact the piston to the valve or the valve is dropped into the cylinder.

Those seem the most prevalent to me.

 

Tephra, please keep in mind that the 10,000 rpm limit is in extreme situations. We don't suggest it as an every day thing. To keep a 2.2 safe I would keep it around 9200-9500.

 

I look at it like riding a bicycle. when you pedal, your leg only can exert force (accelerate) while pushing on the crank through an arc on the circle of motion.

The same thing is happening on the crankshaft. The piston accelerates as the crank pushes it. When the crank lob has reached it's maximum reach, the piston stops and changes direction (the deceleration and acceleration in the other direction)

The acceleration could be different from the deceleration because of drag, and possibly because the length of the arc is different for acceleration and deceleration.

 

Yeh valve train aside, I think it's probably the bolts that's the main concern... I should probably look up the weight of the rod + piston and work out the gforces on that bolt..

Yeh, thanks! I took your 10,000 and rounded it down to 9,000 as a bit of a safety factor