why you should stay under 7000rpm

grumpyvette · April 1, 2002

theres been alot of discussion about building high rpm engines lately, lets look at the pros and cons of running an engine over 7000rpm

read the links then the comments please

http://www.iao.com/howthing/improhtw.htm

what they are saying is that the faster you spin an engine the less efficient the wet sump oiling system is and the greater the percentage of loss to friction from all sources , now remember this post,

ever wonder why your engines torque curve gets higher with the engines rpm level untill about 4000rpm-5500rpm(DEPENDING ON YOUR COMBO) but fades above that rpm level?

well it depends on several factors, first as long as the cylinders can fill completely you get a good fuel/air burn so you get a good cylinder pressure curve against the piston each time the cylinder fires,THE ENGINES TORQUE CURVE INCREASES WITH THE NUMBER OF EFFECTIVE POWER STROKES PER SECOND, at very low speeds theres not enough air velocity to mix the fuel correctly or produce a effective ram tuneing effect but as the rpms increase the cylinders fill very efficiently untill the rpms reach a point where the cylinders just don,t have the time necessary to flow

enough air through the valves to fill the cylinders , remember a 5000rpm the intake valve out of 720 degs in each cycle opens for about 250degs of effective flow even with a hot roller cam, now thats only about 35% of the time and theres 41.6 intake strokes per second , thats only 1/60th of a second for air to flow into the cylinder, I found this graph that shows the relationship between V.E.(VOLUMETRIC EFFICIENCY) and AN ENGINEs torque CURVE

http://www.n2performance.com/lectures/lect1/n2perf5.gif

WHAT THAT GRAPH SHOWS RATHER EFFECTIVELY is that its your engines ability to fill the cylinders that increases your power and the more efficiently you do that the higher the rpm level you can acomplish that at the more power your engine makes, remember the formula for hp is (torque x rpm/ 5252=hp)so moveing the torque curve higher in the rpm range increases hp but look at the curve on the graph carefully.....after the peak torque

is reached the efficiency of the cylinders filling drops off, and as rpms increase its a race between more power strokes per minute trying to raise the power and the increaseingly less effective percentage of cylinder filling dropping the power.

Volumetric Efficiency

The volumetric efficiency of a 4-stroke engine is the relationship between the quantity of intake air and the piston displacement. In other words, volumetric efficiency is the ratio between the charge that actually enters the cylinder and the amount that could enter under ideal conditions. Piston displacement is used since it is difficult to measure the amount of charge that would enter the cylinder under ideal conditions. An engine would have 100% volumetric efficiency if, at atmospheric pressure and normal temperature, an amount of air exactly equal to piston displacement could be drawn into the cylinder. This is not possible, except by supercharging, because the passages through which the air must flow offer a resistance, the force pushing the air into the cylinder is only atmospheric, and the air absorbs heat during the process. Therefore, volumetric efficiency is deter-mined by measuring (with an orifice or venturi type meter) the amount of air taken in by the engine, converting the amount to volume, and comparing this volume to the piston displacement.

this increases untill the torque peak then falls as the rpms increase.

engine red line

piston speed is about at a reasonable max with stock components at 4000 feet per minute, now rpms alone do not have as great an effect as stoke x rpms in figureing piston speed.

example 4000fpm(feet per min.) is 48000 inches per min. if your stroke is 3.48 like in a 350 chevy the piston must go up then down in each dirrection once for each rpm(revolution per min) so 3.48x2=6.96 so 48000/6.96=6896 rpm, in this case the valves are more likely to float before the rod bolts snap from inertial stress, but lets try a 383 that has a 3.75 inch stroke,3.75x2=7.5" so 48000/7.5=6400 rpm max for the rod bolts(about the same as the probable valve float rpm.

btw long term highway cruiseing speeds should be kept to 2000fpm piston speeds for best engine life.

http://www.melling.com/engoil.html

what this is saying is that the faster you spin an engine the harder it is to maintain proper oil pressure and oil volume, thats why dry sump systems are normally used on engines that need to turn over 7000rpms on a regular basis.

now air flow through the cylinder heads also peaks at about a lift equal to 1/3-1/2 the diamiter of the valve and at about 4000-6000rpm depending on the cylinder head flow numbers/engine stroke and displacement and cam timeing, so with all those factors working against spinning an engine to over 7000rpm its realy better to concentrate on building an engine that has a greater efficiency in the 4000rpm-6500rpm range as the parts necassary to run at the higher rpms will double or triple the costs involved while just increasing displacement of efficiency in the 4000rpm-6000rpm range will cost less and will result in an engine thats under less stress and lasts longer.

April 1, 2002

Good reading...

can you say racing fanatic?

Grumpy, you always got all the tech stuff down huh???

Thanks..

April 1, 2002

Great reading as always Grumpy, I've never really saw the sense (yes I did read the other article about 'HP at a higher RPM is a good thing') of spinning a Chevy that high, but thats a personal preference.

IMHO if you want the tach to be buried and dig seeing it over 7k RPM's, then either get a Wankel or a high strung turbo 4cyl, just don't let me catch you in a wrong gear where you have no torque or my low winding low tech V8 with low-mid range torque will have you for breakfast. Just depends on what you like I guess, but for a street car, light to light , those RPM's are about as useless a priests bits. Opinion of course.

Regards,

Lone

pparaska · April 1, 2002

Maybe I thought that way before, but the more Grumpyvette talks about this stuff the more I become swayed to thinking my next SBC will be 400+ cubic inches. Heck, the big bore of the 4.125+" block has breathing advantages alone.

Or maybe do a big bore truck Gen III sbc block and build a 400+ cube LS6 type motor.

The Honda S2000 makes a sweet track car, but I'd probably hate it on the street. Sure, I want to cruise around at 4500+ rpm all day, just in case I want to be able to really accelerate - NOT.

grumpyvette · April 1, 2002

the 400 can be bored and stroked to about 427-440 cid look here,and NOT ONE OF THESE ENGINES NEEDS TO SPIN OVER 7000rpm,BTW some of these guys are running over 1200hp

http://www.theengineshop.com/engine5.shtml

http://www.amerspeed.com/engine_prices.htm

http://www.shafiroff.com/434_sportsengine.html

http://www.lingenfelter.com/packages/427TT.htm

http://www.golensengineservice.com/engineDetail.asp?engine=85

http://www.geocities.com/monty_williams/

http://www.skulte.com/turbo.html

http://www.grapeaperacing.com/GrapeApeRacing/

http://www.turboclub.com/aturbo.htm