Provided there is no detonation...In terms of stress on engine components...whats the difference between 600ft/lbs at 3000 rpm VS 600Ft/lbs at 6000 rpm. Since TQ is really a direct measurement of cylinder pressure it seems for the pistons and rods the stresses should be about equal during the power stroke. I understand piston speed will be much higher at the higher rpms which of course will raise loads. But during the power stroke it seems the stresses/pressure pushing on the piston and rods would be similiar at 3000 vs 6000.

Now it seems the crank would be an item that would see more stress with 600 ft/lbs at 6000rpm than at 3000 rpm since the force is almost continous and applied at a higher rate.

Any thoughts or experiances?

 

Hmmmm. You always seem to come up with topics that make you think lol. Did you ever come up with a final decision about what causes the nitrous backfires?

 

Lol...I dont think we are going to solve that one any time soon without some extensive testing. Either way the basic rules that prevent nitrous backfire remain the same.


So any input on the Tq and rpm idea?

The reason Im asking is looking at Turbo dyno graphs...they make incredible HP #s mostly because with a good setup they make all thier TQ at higher RPM...of course HP is just TQxrpm/5252. Turbo motors that are tuned well seem to make that incrdible HP and have a bit better lifespan than equivilent nitrous motors. It seems to me that if someone could build a better nitrous system...something capable of very tightly controlled rpm based delivery of nitrous...you could reap all the benifits of a turbo setup at much less cost.

Hence the question of TQ and rpm. If my bottom end is easily capable of handling 750 rwtq at 3000 rpm...then it stands to reason it should hold 650 rwtq at 6500 rpm, which is 804 rwhp btw.

 

I believe the reasoning for turbo motors longevity has to do with the gradual increase in power the turbo produces. As you well know the instant increase in cylinder pressures from nitrous is the main stressor on engine components (aside from detonation) I do however fully agree with you on the idea of a tightly controlled nitrous setup producing more power safely.

I am EXTREMELY interested in NX's new progressive controller they have been working on, I have talked with Randal on a few occasions about it and have been VERY impressed with what he has told me (which unfortunately isn't much) I do know however it will be controlled via laptop and you will have more control of the kit than you could ever imagine. One example would be having the ability to "Draw" your horsepower curve, giving total control of how you want the kit to bring the power in at given RPM's.


Having complete control of the kit should dramatically increase the lifespan of an engine. That is of course assuming you have every thing tuned properly.

As for the torque vs. RPM question. I believe you are right about the crank seeing the most stress given the higher cylinder pressures at higher RPM's. The crank is seeing the same about of stress as it does at 3000 rpms but it is applied continuously as you stated. Now, I'm not sure how or if this would factor into the equation but would engine efficiency at a given RPM have any affect on torque at certain engine speeds, and how detrimental it may be to the rotating assembly?

 

Piston speed would be half, so pressure would have to be double or more. Looks a lot more brutal to me. Just thoughts, no experiences. But it sounds like a big diesel guy question.

 

Thoughts:

We just computer simulated an NA big block engine that had about 600 lb-ft at 4000 and 6000 (peaked torque at 5000). Yes, the cylinder pressures are the same, but in your example the hp is double, and the piston g loading @ TDC is 4 times as much at 6000 (about 2700 g's on a 454 BBC). Those g's are out, not in, of course. The inward loading is about the same @ 3000 and 6000. The crank sees much more load at 6000 than 3000 with similar torques.

I agree with your thinking.

 

RPM = Ruins People's Motors

Everything mechanical will eventually wear out, or wear enough that it becomes weak and breaks. More rpms mean more wear. But that is just a simple linear relationship, and in the motor it doesn't work quite that easily. If it did you could just measure engine life in RPMs turned.

Now do you know when most rod failures happen (now this is all naturally aspirated speak, I have no clue when most nitrous engine failures occue, forced induction motors usually fail on the burn and power stroke)? If detonation isn't present, then its usually not when the engine is under its highest load (if it was, then almost all engine failures would be at the torque peak). It often happens when someone is going WOT and then slams the throttle shut for some reason, meaning the engine is turning some huge RPMs but all of a sudden its under no load. Even if this isn't the case, rods in naturally aspirated motors fail most often on the overlap cycle, where there is little to no compression resistance since the valves are open. This means the rod and piston are going up really fast with nothing to slow them down, then all of a sudden the crank decides it wants to reverse thier direction by 180 degrees and either they change direction real fast or something has to give.

You know I coulda skipped all that. This is all really just simple fisiks, M x V^2, mass times velocity (squared). Both the reciprocating assembly (mass) and the rpm or piston speed (velocity) factor in to this, but the velocity is squared. Of course to build a longer lasting engine you can either reduce the weight of the reciprocating mass or reduce the rpms. But reducing the rpms has a much greater effect due to that little v^2.

Not to mention its a whole helluva lot easier to take an engine designed to run 6000 rpms and make it reliably lubricate itself at 6500, versus taking the same engine and trying to lubricate it at 8000+ rpms.

So if you couldn't tell, I conclude that less rpm with more torque will be easier on the mechanicals. Not to mention periphials like it will be easier to lubricate, ignite, etc.

J.

 

"RPM = Ruins People's Motors "

Man, that hits it right on the head.
As RPM's go up the loads increase at a much higher rate.

Power down low is better for long motor life if everything is equal... but to hit it with a 300 shot at 3000 is brutal on a motor....especially a stock one.

Dang good thoughtful question.

 

Another thing to consider is that the shock loads on the drivetrain increase in a quadratic function with rpm. That is to say that the twisting load applied to your driveshaft QUADRUPLES when going from a 3 grand to 6 grand clutch dump!

 

Fenrus...good point.

But it seems the concensus is that its RPM is a big killer, and seperately...cylinder pressure is a big killer...but the two together dont necessarily combine to increase stresses. So with that in mind...let me run some logic by you guys.

If a motor can handle 7000 rpms NA without a problem...but can also easily withstand 700 ft/lbs of tq....having that 700 ft/lbs at or around that 7000 rpm doesnt really add that much stress to the rods and pistons since those stresses seem to be different events?

Another way to look at it would be like this.

If you had a nitrous motor hit with a decent shot at 3000 rpm and Tq was always at 700ft/lbs@3000 rpm and then steadily dropped off...but continued to run the engine all the way up to 7000 rpm(at 7000 rpm you would see all those high reciprocating loads due to high piston speed) .........How would that differ in loads and stresses on the motor if you tailored the nitrous delivery so you saw 700 ft/lbs at 6500-7000 rpms.

 

Why do I have the feeling that someone is looking for the okay to try this?

 

Shortblock limits are usually described in terms of HP. This is because it IS the combination of torque and rpms that determine the stress level on the motor. With 700lb/ft at 7000rpm what kind of HP will you be running? It will be HUGE! Your motor will blow up!
to achieve the same torque at a higher rpm requires MORE cylinder pressure because the pressure has much less time to act on the piston (torque comes from the average of the cylinder pressure through the power stroke). This means that the peak cylinder pressure is a whole lot higher to see the same torque value when the rpms are at 7K or even 6K. Not to mention that burn rates taken into account require a greatly increased amount of nitrous at that rpm to even build that pressure.
I just don't think you're heading in the right direction.

 

Its true that there is less time for the cylinder pressure to act in each event but those events are occuring at a much greater rate and applying that to the crank. I believe thats why Tq is roughly the same for similiar cylinder pressure throughout an rpm range.

Everything Ive always read said cylinder pressure is greatest at peak TQ. I havent seen anything say cylinder pressure is greatest at X tq and Y rpm.

Faster burn rate with RPM is true but mostly due to increased motion in the chamber.

Quote from someone important:
"Flame speed often increases nearly proportionally with engine speed-primarily due toincrease in chamber turbulence as engine speed rises."

Yes packing in more nitrous will increase the combustion process...but it seems the amount of timing needed to be removed for a certain shot size is about the same wether its 3000 rpm or 6000 rpm...the most timing retard needed is usually at peak TQ.

 

What kind of simulator was this. This was the kind of concrete engineering type of info I was looking for...thanks...Al

 

If you didnt know by now...the goal here is to use a multi staged tightly contolled rpm based nitrous setup to tailor the TQ curve the way i want it. If you noticed already wth a simple two stage I made 665 rwhp but at a modest Tq level. Its that way by design. My thinking is if i can get a stable tq load of 600 ft/lbs +/- 50 from 3000 rpm to 6500 I could get a very usuable tq range and yet the pistons and rods will not see huge cylinder pressures/stresses provided its tuned correctly. 600 ftlbs at 6500 rpm is 742 rwhp btw.

My motor has seen Tq ranges in the 750 ftlb range with ease...but at 3000 rpm.

 

what causes your tq curve fall off at higher rpm how do you make a motor gain or lose tq

 

in what way does compression play its role

 

Thats because peak torque is when the engine has the greatest clyinder pressure, or if you prefer, when volumetric efficiency is highest.

Now we're all mostly polite grown-ups on this board and no one is going to tell you yes you can or no you cant do something since its your own motor and your own money. Using progressive control is a great idea, it lets you run the big shot up top without overjuicing it so much that the motor pops at 3000 rpm when the cylinder pressure is so high.

Dont forget that you're not just wanting to build a fast motor, but a fast drag car. As you cut the torque and raise the rpm, you need to gear the car deep to get up into the power. With less peak horsepower but a flatter torque curve, you can run a milder gear and stay in each gear longer. And the engine with the most transient power (the power to accelerate under load over an rpm range) will be the one that wins the race, not the engine with the most peak hp.

J.

 

To everyone involved here, Great thread and replies!

 

As the piston speed increases, the engine doesn't have enough time to fully fill the combustion chamber, when this happens torque begins to fall off. This is one reason why supercharged and turbocharged motors make so much torque, because they literally have an air compresser packing the air into the chamber versus a naturally aspirated motor that has to use vaccum and atmospheric pressure to get air in the chamber.

There are tons of things you can do to gain or lose torque, and this will also effect your hp. Changing the velocity of the air intake charge by using different carbs, intakes, valves, heads, head porting techniques, etc. Changing the exhaust ports, header styles, exhaust, etc all effect velocity and torque as well. Timing, compression, they all effect torque. To answer your other question, higher compression usually equals more torque as well.