BurnOut

iTrader: (1)

Do any of you engine builders out there have a good rule of thumb regarding maximum (reliable) piston speed?? For instance, according to my calculations, a stock-stroke LS1 at 6500 RPM has a piston speed of ~65.4 fps, which I admit my be wrong...

Here are my calculations:

- 6500 RPM / 60 seconds = 108.3 power strokes per second
- 108.3 * 2 full strokes (2 crank revolutions per RPM) = 216.6 cycles per second
- 216.6 * 3.622" stroke = 784.76 inches per second
- 784.76 / 12 inches in a foot = 65.4 feet per second

Are there any good rules of thumb regarding piston speed limitations??

TIA

pimpmaro

my mind isn't working right now, but did you take into account that in those 2 crank revolutions, the piston is actually traversing the 3.622" stroke distance twice? (twice down, twice up)?

2001CamaroGuy

iTrader: (6)

thats what I get (65.4 feet/second) ....holy crap....I had never though about it before but 108 revolutions in ONE SECOND!!!!!!!!

2001CamaroGuy

iTrader: (6)

you need to reword that to say

- 108.3 * 2 full strokes (2 piston strokes per crank revolution) = 216.6 cycles per second

2 crank revolutions per RPM would mean that you are only counting every other revolution of the crank (could cause some confusion)

Cheatin' Chad

iTrader: (3)

cut and paste from a previous post of mine:

Conservatively using the stock stroke I would say to not move the pistons past 4,000 FPM. a 7,000rpm redline works out to approximately 4,225fpm/70.42fps/48mph (ignoring rod angularity). With upgraded springs,rods,and a properly balanced motor. You should not incur damages at that speed for short amounts of time.The limit I propose is based almost solely on mean piston speed and does not take into account valve float,puhrod flex,etc.

A complete list of specifications icluding:piston,rod,pushrod,lifter and retainer weights as well as rod length,spring rates,etc. would allow someone to give a more accurate answer.

2001CamaroGuy

iTrader: (6)

But what about a road race motor that gets run at 7000RPM for say 4 hours (just pulling numbers out of my head)? How do you make a motor last (like the 24hour Le Mans)?

BurnOut

iTrader: (1)

2001 Camaro- good point... the wording is a little confusing.

lerajie- Okay, so a little over 70 fps... however, consider that if you use a longer stroke to increase piston speed as opposed to increasing the redline, then what?? For instance, a 4" stroke at the same 6500 RPM redline is moving the piston at 72.2 fps. Obviously, valvetrain stability at 6500 is the same regardless of stroke/piston speed, so that's not really an issue. Rod angularity may well be an issue, but since these are already fairly long rod motors, it could be a lot worse than it is.

As for high RPM motor durability, the use of as light of a reciprocating assembly as possible coupled with short strokes (for instance, most NASCAR motors have strokes under 3.4") and the knowledge that the motor will be torn down after the race anyhow... I think that all of those things are contributing factors.

Louis

iTrader: (7)

they have short strokes, big bores.

Trans am race cars have a series limited limiter of 8200. 4.xxx bore, with a 2.9-3.0 stroke.

Vents

iTrader: (9)

Chris,

just to bend your brain a bit, the F1 cars and LEmans stuff uses these same MAX piston speed theories. they're built the motors to turn that RPM and heads to match, but the pistons dont exceed they're max or terminal piston velocity. even @ 15-18k RPM's read some of ol Smokey Yunick's, stuff or even Victory library's site. take a gander at some rod/stroke ratio math. great stuff. LONG rods + short strokes = less piston speed/ longer dwell time @ TDC, ect.. ect.. cool stuff.

robertbartsch

....why worry if Formula One engine rev to 16,ooo?

...is their ability to survive dependent upon lighter stronger pistons?

Old SStroker

Average piston speed, which is what is generally discussed seems to be about the same upper limit for highest end race engines used for hours rather than seconds. Cup engine with 3.30 stroke at 9000 has same piston speed as F1 engine with 1.65 stroke, which is about what the best guesses I've seen for them. (3.74 bore x 1.65 stroke gives just about 3.0L)

That's pushing 5000 ft/min, which is the units I'm used to. OK, that's about 83 ft/sec. or about 56 mph for a comparison.

Another forum had some good discussion recently on piston speeds relative to airflow, wear, loads, stroke, etc. Not everone agrees on why there is a "limiting" piston speed.

As lerajie said, 4000 ft/min mean piston speed (MPS)is quite safe for our type of engines, at least for street/strip driving.

(2 x stroke x rpm)/12 is the usual formula for MPS
(2 x 3.75 x 6500)/12 = 4062.5 ft/min

My $.02

Louis

iTrader: (7)

F1 cars have an undisclosed Bore:stroke ration of ~ 3:1.

BurnOut

iTrader: (1)

I guess that what I am trying to get at here is that let's say I build a stroker motor, and twist it to ~6500 RPM. This would be a street/strip type of motor, so we're not talking about hours (or even more than a couple of minutes, TOPS) at high RPM, just short blasts up to that point while running through the gears.

Would the longer stroke motor be living on borrowed time being twisted to that RPM??

J-Rod

http://www.slowcar.net/shared/pistonmotion.xls

Old SStroker

Where did you read/hear that? That seems rather difficult to achieve.

I really don't think it's anywhere near 3:1. Using even mm that would be 105 mm bore and 35 mm stroke, and would put piston speeds maybe 15% below Cup engines. While F1 folks don't disclose exact figures, a 105 mm (4.134 inches)bore with 5 cylinders per bank would make an engine almost 2 inches longer than that which can be extrapolated from photos.

F1 engine designers admit to 93-96 mm bores. Even giving them 98 mm, that's under 2.5:1 B/S.

FWIW, at 18-19000 rpm I think you need to worry about flame front travel, which is pretty well fixed in a gasoline/air mixture. At less than 1/2 that rpm, I believe a 4.5 inch bore with one plug located approximately where it is on a BBC is aproaching the limit of flame travel for good combustiion. 4.134 might be a problem @ 19 grand.

IMO, packaging, piston speed, valve area and shrouding and maybe flame travel determine F1 B/S. Just my $.02.

Cheatin' Chad

iTrader: (3)

One way is to use higher quality parts,proper machining and proper tolerances.

No one on this board has 163 million (Ferrari's budget for '02 I believe)per year to maintain and produce a vehicle that houses and engine that can move at that speed.

Their ability to survive at that RPM depends on many of things #1 being money and #2 being the absolutely unobtainable (to average people) materials and knowledge they posses.
In all seriousness they survive at 18k RPM due to many things one of the most signifigant being pneumatic valves.

Well that's a loaded question in a sense. There are far too many variables for anyone here to just say yes or no. What I can tell you from my experience is that if you do choose to use a longer stroke crank and it is **properly** machined,balanced and assembled than you should not have to worry about it being a time bomb.
I have seen longer strokes move at greater speeds and be absolutely reliable. It's a chance you take when you fiddle with engines. I personally would run a 4in stroke without too much concern.Actually, I plan on using that exact stroke when I build the LS1 that will wind up in my Trans Am.I will use as light a piston and rod as practical to do so. Weight reduction is good whenever you can pull it off without compromising strength.
This is all assuming you start with high quality parts from a reputable manufacturer to begin with. If durability is your #1 concern do not for a minute think you can get off cheap.




"I used to think I knew alot about engines" - Smokey Yunick

Cheatin' Chad

iTrader: (3)

Well, it was a cut and paste so it didn't apply exactly to the question you were asking.However I felt some of the information in that post is still applicable.

BADZ

iTrader: (5)

Sent you mail BurnOut!!!

Please read,

https://ls1tech.com/threads/showflat...14&fpart=1