Piston speed and Rpms

[buster]

I always thought that my engine running at 80mph or 3k rpms was hard on the engine. But VaderSS said I should look more at piston speed rather then rpms. For my engine this is what I found:

I found this about my car:
4qt. oil capacity.
3.60" stroke or (91.5mm)

So at 80mph, I run 3k rpms.
This comes to 1,800ft./min. Is this good? It falls within the 3,500 range, however, this is a constant highway speed, not an engine rev to accelerate.....not sure if this is too much to sustain..I always thought 80mph was pushing a little 4cyl.

This is the link for Piston speed calculator:
http://www.prestage.com/carmath/calc_pistonspeed.asp

[ January 01, 2003, 05:52 PM: Message edited by: buster ]

 

[Soma07]

I dont think its good or bad, its just the way it is. I wouldnt worry about it, even though you're turning some decent RPM's its all fairly low load so its easy on the bearings.

Just for comparison purposes here's my car:

3.48" stroke
2200rpm@80mph

1276ft/min

FWIW most race engines that are really pushing (F1, NASCAR, etc) it are doing ~5000-5000fpm.

Jason

 

[DABEAR95]

The Engine in my Vibe GT has a stroke of 3.349 (85mm) and runs to 8,200 RPM at redline. Based on that calculator the piston speed exceeds 4,500 fpm.


Hmmmm...

Jason

 

[mkosem]

ok, given a stroke of 3.45"(90mm) and my engine being around 3100-3200rpms at 80mph. My saturn has an average piston speed of 1829feet/min. At the redline of 5500rpm that puts me at 3245feet/min. What kind of car do you have? I'd imagine 80mph was probably not in mind when the manufacturer was building the car because normally people don't drive that fast. But I'd also have to go and expect you're probably fine even given that piston speed.

--Matt

 

[DABEAR95]

Its a Pontiac Vibe GT with the 2ZZ-GE engine tweaked by Yamaha.

 

[dragboat]

Piston speed is also effected by the rod ratio. Rod ratio is determined by taking the center to center lenght of the connecting rod and dividing by the stroke of the crank. Increasing the rod lenght will reduce the rod angularity and lessen the amount of force on the thrust side of the cylinder "inner on a V-8".

Other benefits of a longer rod with a better ratio is longer dwell time near or at TDC for combustion efficiency but this is all out of control for the consumer to pick a crank,piston and rod lenght for his/her

desired results in a performance motor.You get what the makers design which is not always best for engine longetivity but these new near square engines produce good low end with a automatic transmission and the improved cylinder head flow for ample top end and still pass emmisions.

Lets take a look at two big block Chevy's
the 454 with a 4.00 stroke piston moves at
4500fpm @ 6750 RPM
5000fpm @ 7500 RPM

The old 427 with a 3.760 stroke
4500 fmp @7181 RPM
5000 fpm @7979 RPM

These engines both have the same std bore size but in 50k miles one can tear both motors apart,all else equal the 427 will show less wear on the piston becasue of the better rod ratio of that motor.The higher numeric number rod ratio the better rod angularity. The rod lenghts are the same 6.135 for each motor.

Same for the 327/350 Chevy engine series the 350 is a stroked 327 . Now take these 383 strokers built from a 350 block and start stacking the ring pack together and this tells the tail of why they burn oil for the most part.There is more,but.....

Remember average piston speed is not the same as peak piston speed

[ January 16, 2003, 09:20 AM: Message edited by: dragboat ]

 

[mdv]

quote:

---

Originally posted by mkosem:
What kind of car do you have? I'd imagine 80mph was probably not in mind when the manufacturer was building the car because normally people don't drive that fast. But I'd also have to go and expect you're probably fine even given that piston speed.

--Matt

---

Matt, not sure where you are in OH, but in the Boston area it's not unusual to be passed when you're cruising at 80. When I lived out west, the cops wouldn't even look at you unless you were doing over 90 (75 mph speed limits there). I don't think 80 is that unusual anymore.