View Full Version : WHY KEEP ENGINE RPMS under 7000rpm

04-01-02, 05:38 PM
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
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
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.

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.

04-01-02, 05:50 PM
Great info ... and ... well said ... THANKS!

04-01-02, 06:25 PM
Wow! That was a great explaination. Do you work in engine engineering?

Radar :s

Tom Bryant
04-01-02, 06:32 PM
Great explanation Grumpy. That sure makes a good arguement for the big torquey Americam V8.


04-04-02, 02:03 PM
thanks for the explaination.

I am going to swap my engine, what do you think of it?

I want to build a new 427 Cid small block with a compression ratio of 1:9, use special JE pistons and al the other needed stuff, and use a Procharger or Vortech supercharger with intercooler, witch will run on 8 psi. I want to make 7200 rpm max.
I also use my LPE SuperRam and Edelbrock Performerheads. And a special cam. TB 58mm.

I think that this setup will make a nice and smooth engine to do grocerys with it, but when I step on the gas, I will awaken a lion. I think with this setup I will make about 580 hp? And a relyeble engine.

Do you have a better idea?


04-04-02, 04:30 PM
Your posts PROVE that science & math ROCK!


Impressive as usuaul


04-04-02, 07:16 PM
read this







04-08-02, 03:52 AM
I will never forget the advice of an engine builder when I told him I was building a 7400 RPM motor. He asked what transmission I was using and what RPM drop there was when I shifted gears.
The drop was 1200 RPM. He said
"So, we are building a motor that will go safely to 8600 RPM ? " That's where you are going to be when you slam it into the wrong gear. Valve springs are real important for a high RPM motor,, and lots of money. As the RPM increases, the valvetrain maintanence increases, and lifespan decreases, in a big way. But there is nothing like it when you stomp the GO pedal . :_rock

04-17-02, 04:27 AM
The hell with those Japanese V-Tech's 8900 rpm's red lines :L they should look into making more HP/Torque than rpm's.
Well, Formula One engines have a little 3.0 liters/183 cu in V10 aspirated that rev around 18000+ rpm's :BOW and some 800/900 HP's.(God Like) :j
I like it simple 5600 rpm's red line is enough.

07-02-07, 11:11 AM
It is true that Formula One engines have small engines, rev idle at about 10,000 rpms, and generate 800/900 HP.
It is also true that those motors last 1 race.
It is also true that they spend hundreds of millions of dollars every year.

Good Post.

07-02-07, 03:02 PM
Well, Formula One engines have a little 3.0 liters/183 cu in V10 aspirated that rev around 18000+ rpm's :BOW and some 800/900 HP's

Actually, now they're 2.4 liter V-8's, electronically limited to 19,000 rpm (last season they weren't limited, and peaked at 20,000); the new 19,000 rpm limit was enacted to keep component costs down. Stroke is a little over an inch. Valve springs don't work due to dynamics over 12,000, so they use a closed high-pressure nitrogen actuator system that runs at 2,000 psi to close the valves. :)

07-03-07, 05:04 PM
talk about dragging up ancient posts......:upthumbs:L

07-03-07, 11:37 PM
Pretty cool stuff! See... recycling works!!


07-04-07, 08:10 AM
Pretty cool stuff! See... recycling works!!


first time I've seen it. Back when I was a Kid I prefered the 302; high RPM low torque screamers (back in the days before rev limiters) and had a stack of "broken parts" Now that I'm older I like to build for torque and keep it under 6000, usually closer to 5500 then anything and guess what; the 76 is running a short block I built back in 1990 with no broken parts. ;LOL