What factors would considered for maximum engine RPM? Components quality, component weight, valvetrain components, power production? Will a billet crank & rod allow a 7000rpm redline? Will that depend on the the power around that RPM? I know if you make peak power at 5500rpm it is pointless to rev to 7000, but what about a 6200rpm peak? Maybe valve timing in risk to making contact with the pistons? Valve float of course? I really don't want to hear piston speed because I have seen mountain motors with really long strokes rev to 8000rpm. I am hope to hear from experienced engine builder here.

 

well if you were hoping to hear from an experienced engine builder then im sorry.

but hydrolic lifter limit an engine speed also.

 

Mostly the valvetrain is the limiting factor. Valve weight, spring strength & harmonics, retainer weight, lock weight, pushrod strength, timing chain stretch, rocker stud strength, rocker weight & strength are all issues that need to be covered when building a high-rpm engine. Other non-valvetrain issues, would be things like rod bolts, oiling issues, main cap bolts, BALANCE, rod strength, ring gap, piston ring groove strength, and several others that I'm sure to be reminded of by someone.

You want to make the valvetrain as light as possible without sacrificing strength. Titanium valves & retainers, sodium-filled valves, aftermarket roller rockers, chromemoly pushrods, and dual or triple valve springs should definitely be in your plans.

Bottom end of the motor, go 4340 crank/rods, high VOLUME oil pump, and find the stoutest rod and main cap bolts made to hold it all together. Make sure everything is balanced to keep it all from flying apart.

This is just a good start, so don't think I've told you all you need to know about handling high rpm, aiight?

 

Sorry, but piston speed can be the limit. It's the loads that kill the parts. The loads result from the gs and the mass of the parts. The gs come from rpm and stroke length just like the piston speed.

It depends on how long you want the engine to live. A drag engine only runs a few hundred revs per race, so a 4.5 stroke 8000 rpm engine can live for a while @ 6000 ft/min piston speed. The 2004 Mercedes F1 engine (18,500 rpm) was rpm limited by con rod strength at about a piston speed of 5000 ft/min. Of course it had to live for two race weekends and over 4 million revs.

Unless you are building a very high-end race engine breathing may well be what limits your usable rpm. You can do a valvetrain that will support about anything your heads/intake will allow. Cup and Pro Stock engines get stable 10,000 out of their valvetrains.

So if you design your engine around the airflow potential you find the usable rpm range and specify a valvetrain that is stable in that range. You then specifiy rotating/reciprocating parts that will live at the rpm. Personally I'd keep mean piston speed below 5000 ft/min using lightweight parts and the best forged crank and rods. That's 7500 with 4 inch stroke.

 

The answer is something. It could be the hydraulic lifters. The mass of the valvetrain. The stiffness of the valve spring. The profile of the cam. All things that could limit at what speed the valves no longer follow the cam as intendeded.

Then there is breathing. The TB. The intake. The heads. The exhaust. The cam timing. Big revs require big breathing in a short time.

Then there is fuel. More revs mean more power. And shorter times to deliver the fuel. So injector size. Fuel pressure. Etc.

Then there is the mechanical strength of the parts. The crank. The rods. The pistons. The rod bolts and the wrist pins. The valve stems. The block (it is amazing how flexible those things are!).

All of these can be beaten to death with money. However, some of that money also comes in the form of reduced duty cycle. As Old SStroker said, going from one race to two caused issues for F1 teams.

And there is also a multi-use issue. An engine that only has to run 6k-10k (say a Cup engine) is different than an engine running 9k and running 2k in traffic (say a Honda, M3, RS4). What you can do with variable valve timing and variable intake length is not really possible with a single cam and fixed rockers. You can make an 8500rpm LS1, however you may find it less enjoyable in traffic.

Any specific engine deals with all of these factors to make a solution. Overwhelmingly, the people on this forum stick with hydraulic roller lifters, limiting their engines to under 7000-7500rpm. At those speeds most of the mechanical issues go away (or are at least minimized), breathing is fairly straightfoward, and the valvetrain is manageable. Engines working best at 7000 work a little worse at 2000 though.

Note, just making peak power at 5500 doesn't mean it doesn't pay to stretch the revs. Intake limited oval track engines typically power peak around 6k but are regularly taken to 7000 or just beyond. The deciding factor is your power curve. If their is more hp at 7000 than at 4000, you rev to 7000. To make 400hp at 7000rpm requires 300lbf-ft of torque. You would need 525lbf-ft at 4000rpm for the same power.

 

If you start with a total clean sheet of paper for a new engine, then the tension on the connecting rods is almost always the starting point. In general, the rest of the engine is then built to support that. That is true in everything from model airplane engines to massive ship engines to 20,000rpm F1 motors.

 

With most American street car engines that feature pushrods the limit determining a redline is usually the valvetrain. I have yet to see many engines that blow up from excessive rpm lose a rod ever where as tons drop valves and or lifters etc.

On a super high rpm OHC engine everything is at stake. RPM kills parts in the top and the bottom since with the much lighter valvetrain of the OHC race engines both are fairly highly stressed but its the rpm and not the piston speed that creates these loads.

There are super high mileage engine that can turn the piston speed of an F1 engine but none that can turn the rpm of the F1 engine. Loads in the bottom end go up at the square of the rpm and only linearly with stroke so piston speed gained with stroke and lower rpm is MUCH less hard on parts tham piston speed gained solely by rpm.

So basically it depends but on our pushrod stuff the valvetrain is the limit 90 per cent of the time earlier than the bottom end is. Remember as well you only turn an engine to the rpm that it makes good power in so even in the absense of mechanical damage you might as well still have a redline for logical reasons when shifting gears anyway.

 

That's because it's far more desirable--from a repair/cost standpoint--to lose a part in the top end than the bottom end. You ALWAYS overdesign the part that is hardest/costliest to repair. That's basic engineering and it gets missed a lot by modders. The manufacturers and better build shops know this quite well and that's what keeps them in business.

Pushrods/lifters/springs/etc can easily be made as strong as you need them to handle the forces involved. If you beef up conrods and pins too much, you aren't going to spin anything else very fast.

I don't disagree with anything you said, but there's a reason your experience with valvetrains has been what it is.

 

If you break something in the valvetrain on a race engine you will usually trash the entire engine. Bottom end engine parts in the aftermarket are extreme overkill for 90 per cent of the apps we will see. They just don't fail in general unless installed wrong.

Outside of running the oil dry you just don't see bottom end failures almost ever on the American pushrod stuff. I haven't had one in 200 engines here and some turn some pretty high rpm. At the school we have 100% duty cycle 18 degree chevys that see continuous 8500 rpm and yet never have bottom end problems either outside of a dry sump belt falling off etc.

 

I watched a big block Chevy 454 blow a connecting rod out of the bottom of the engine at 8000rpm on an engine dyno, so it can happen. Don't take this as me trying to contradict what you have stated, because I pretty much agree with all of it, but it does happen once in awhile.

 

It probaby was not the rod's fault though. We have guys like that here in Houston too unless you are talking about stock engines.

 

I couldnt agree with you more. Bottom ends of cars rarely die from raw HP abuse, they usually die from other problems... I see people every day throw $1000 crankshafts at 400 rwhp daily driven cars and it blows my mind. Spend that money on a decent piece and get is balanced more accurately and you will get a better motor.

Now forced induction adds a whole different spin on things, but point is... most motors die from fuel starvation or inadequate valvetrains.

To answer the initial question, as long as you arent reaching the breaking point of the bottom end (TQ or RPM wise) your #1 limiting factor on RPM of a motor is airflow. Even if you throw the biggest cam and intake you can find on a motor... you will be limited by the cross sectional area of the intake port.

If you increase the velocity of the incomming air, you reach a point where the air becomes unstable. As the mach number of the air increases you increase the chances of boundary layers and flow separation. You can also see local shocks inside the ports as the flow is accelerated (locally) to a point near mach 1. This will significantly drop the efficiency of the port and cause turbulent flow, or even adverse pressure gradients.

Most of this comes from study of subsonic inlet diffusers or fluid flow around them... if you study airframe design, there is always trouble integrating engines to an airframe because of shocks around the nacelle as they aproach the transonic range due to mach speeds around .85. Even though the airspeed is below mach one, the airspeed will accelerate as it turns or is forced to flow between various orafices. This can cause local velocities approaching mach 1 and induce shocks.

If I am wrong, someone please correct me as I am still learning all of this, but isnt the max mach speed in a port supposed to peak around .8 or .85 ?

 

The limit on the redline of an engine is the depth of your pockets or the bounds of your credit.

 

Don't forget that I'm talking about what determines the redline on a clean sheet of paper design. That's not the same as building a race-ready LT1 or LS1. All new engine designs start with 2 fundamental elements: how big can it be and how much power do we want? The first answer to that is fixing the angle (90deg/60deg/etc) and stroke. That sets the piston speed, which sets the max rpm. I can make a valvetrain work from 400rpm to 22000rpm, but I can't make a piston move significantly faster than what we see in engines like the Integra's 4-cyl or some very high strung race engines.

When you limit the discussion to SBC/LT1/LS1/etc, the piston speeds are pretty much set. I'm trying to answer a broader question. I'm pretty sure your points are more of what the original poster wanted though, so I'll shut up now.

 

You're right, but it's very very easy to slow down the incoming air to appropriate levels with testing. Remember, what we really want is volume and pressure, not velocity. High velocity reduces pressure, so it can totally defeat the purpose of forced induction if done wrong.

If you understand shock cones and diffusers, then you know how to slow air down and why it's necessary. Even an F-15 at Mach 2.5 is running subsonic air through it's engines. It would flame out if it didn't.

 

Well, the whole purpose of diffusers is to slow down the incomming air to appropriate mach numbers. Typical inlet mach numbers are typically in the ballpark of .4-.5 for either turbofan or turbojet engines. At mach 1.4+ you need to very carefully design the inlet to give you multiple oblique shocks instead of one large normal shock. I believe an F15 uses three oblique shocks. It is actually a cool variable design that curves the incomming air quite a bit at supersonic speeds.

Well, volume and pressure is one way to look at it... another is mass flow rate, which will be determined by area, density, and velocity... mdot=rho*A*u

You also have pressure gradients and fluid momentum acting on the air.

So yea... it all depends.

 

Well with a clean sheet of paper design you guys are right that the connecting rods and the bottom end are one of the limits.

On the existing small blocks with the rods we have the rods are actually overkill right now in general though so we are coming from two different angles.

With existing two valve pushrod engines the valvetrain sets the limit 90 per cent of the time. It's the weakest link in general nowadays 90 per cent of the time but as people have said not always.

 

Another factor would be driven accessories. A/C, water pump, alternator,
etc. These parts all have breaking points as well.

I recall an episode of American Muscle Car paying tribute to the ZR-1 stating
the engine RPM limit was set to save the belted accessories.

Probably a little off topic, but it might explain some of the reasoning for RPM
limits on factory built engines.

 

The Mercedes F1 engine program has pockets as deep as almost anyone in the motorsports world and Mother Nature still determined their max rpm with connecting rod strength.

 

Rod bolt let go. Forged I-beams, forge pistons, forged crank, the whole 9 yards. Think they were ARP bolts. There has been some speculation as to whether or not some idiot over torqued them during installation.