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Calculating the Redline of Your Built Motor

Engine Building Fundamentals

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I just completed the Engine Building Fundamentals course, and I really enjoyed it but I had a question that came up while I was watching the videos. After you build your motor, how do you calculate it's new redline? Is there a certain formula that can be applied, or does it involve in-depth research about the materials used and other internal components?

Thank you,


It generally relates to the infinite fatigue life of rod bolts/rods, or valvetrain wear/control and potential for valve bounce or interference or valvetrain components being damaged or ejected. The airflow capability of head/cam will come into it too.

You also need to be mindfull of the limitations of components attached to the engine like flywheel and clutches.

Would fatigue life of different components, valve train durability, etc be provided by the manufacturer or aftermarket company? Is there a modeling software or course to help with determining those things?

You are probably best to ask the manufacturer of components what they suggest is appropriate for their parts, even software packages with 30k+ yearly licences don't generally do dynamic FEA which you really need to understand forces acting in running valvetrain, big end bolts/rods can be modelled statically well enough with a basic spreadsheet or hand equations for peak load vs engine speed.

Got ya, thank you so much for your time!

yes, unless you have a GOOD mechanical engineering degree, a lot os exacting test equipment, etc, I would go by the LOWEST figure your parts suppliers give you.

Or you can go 'old school'- increase rpm until something breaks, then set the red line 100rpm lower...

I thought I might chirp in with some maths, don't worry won't go to in depth. The guys are right, there are many factors when wanting to increase your rpm of an engine.

As discussed else where balancing or the best efforts to have the least amount of vibration possible are made! but there is a cap to rpm, or at least a theoretical one...Piston Speed.

Generally and in higher performance race engines the likes of F1 piston speed is kept to or below around 28m/s (meters per second) this doesn't mean you can't surpass this figure but more investigation as mentioned into the valve train keeping up, coatings to pistons, piston to bore clearance, oil supply and weight etc etc need to be taken into account. Race teams spend a lot of money and development trying to reduce fatigue and friction. Also race engines are normally lifed (end of they're use) after 5000km.

The equation:

Piston speed = 2 x stroke x rpm / 60,000

For example;

Honda K20 has a stroke of 86mm, factory red line of 8500rpm

Piston speed = 2 x 86mm x 8500rpm / 60000

= 24.36m/s

Increasing the rpm to lets say 10,000rpm, piston speed would equal = 28.6m/s!

In this instance factory parts would most certainly not keep up with that speed and more than likely a valve might meet a piston resulting in a mess! (to say the least)

If rpm is your game then a shorter stroke to bore size (over square) would be advantageous!

RPM limit is rather nuanced with quite a few factors to consider but as mentioned above calculating mean piston speed and limiting it to 24-26m/s is a generally accepted method.

to fully solve this we would then need to define acceleration, usually done by calculating instantaneous displacement then using the 2nd order derivative to calculate acceleration. Once acceleration and mass is known calculating force therefore pressure is simple. Then we can see if this fits within the elastic limit of the materials’ UTS.

There are however a multitude of other points to consider when defining rev limit such as valve float, oil pump cavitation, management of blow by gas, ring flutter etc.

I probably raised more questions than answers with that one

How about suggestions on how to test / detect valve float? What are the signs of oil pump cavitation in a data log? What does ring flutter actually mean and how do you detect it?

If you're suggesting they may be future topics, I would agree - the information is out there, but some may have trouble interpreting it and need a little help.

As was quite rightly pointed out each point is a topic in and of its self and there is indeed a lot of information out there and I think a comprehensive deep dive is beyond the scope of forum reply but I’ll try give you something to take away.

Essentially valve float occurs when the reaction force/time of the spring is overwhelmed meaning there isn’t enough time for the spring to return the valve to the seat before the next actuation. This is effected by the spring rate, material, shape, mass of the reciprocating components, rocker arm ratio etc. As far as detecting it there is a very fine window between valve float starting and major valve to position contact but the usual signs are a drop in power, missfire, lean lambda and it will be evident as the the engine note changes. This can be mitigated against by reducing valve train mass , ensuring the valve spring is operating in its optimal window by measuring the pressure vs. displacement, peak valve lift doesn’t go beyond the knee point in the force curve, the clearance from coil bind and ensuring or shimming for the correct installed tension.

Regarding oil pump cavitation in a data trace - this introduces air into the circuit which is compressible and the tips of the pump rotors can no longer ‘grip’ the oil so we would expect to see a drop or oscillations in the pressure trace outside of a corner usually at the end of a straight. There are pretty obvious whiteness marks created by cavitation so inspecting the oil pump is a good sanity check.

Ring flutter occurs at high rpm, when the engine is in overrun or spends too much time in torque cut and is due to the relationship between the inertia of the ring and the pressure differential across the ring. It can been seen in the data as an increase in crankcase pressure and a drop in power. If allowed to persist galling can occur between the ring and groove eventually resulting in the ring land collapsing. The generally accepted method to improve galling is to hard anodise the surfaces of the ring groove and by designing the piston with a pressure relief port the pressure differential can be reduced. Reducing the mass of the ring also reduces its inertia increasing the rpm at which ring flutter occurs and increasing ring tension can help.

Very well said, Scott, a few expansive comments, if I may.

Valve to piston contact can't happen with the intakes, unless there is a REALLY big valvetrain failure, as the piston is well down the bore - it's the exhaust that's the danger because the piston is chasing it towards TDC. As you say, it can be difficult to discern minor float, especially if it doesn't continue to rebound on the lobe, but sometimes there is evidence in the wear marks on the trailing face.

Another term for this is "lofting", which, IIRC, is described as uncontrolled separation of the valvetrain - basically this is when the spring cannot control the valvetrain and it can actually happen earlier than expected - all materials have a degree of flex, and act like a spring in their own right, the different components of the valve train will each have different, 'spring rates'. Normally all these are different, so you will have little interaction, but sometimes there can be components that get into the same resonant frequency and the affects are combined. One of these components, weirdly, are the springs themselves, so two springs may have the same seat and nose (open) force, and the same diameter, but have different critical frequencies that may cause loss of valve train control at some point but move outside it at higher rpm and still have control at those engine speeds.

There is also some confusion between that 'loft' and valve bounce - this is basically an extreme case where the valve hits the seat so hard it bounces back off it because of the elastic nature of the components. Basically the loft is so bad valve train control isn't able to return the valvetrain into proper contact with the cam which would normally lower it to the seat. This is the one that, IMO, is the real problem because of the very high forces that occur - common symptoms are valve train breakages, valve clearances closing up as the valve head 'cups' and/or stems stretch and, in extremis, valve seat inserts can even loosen.

As Scott says, it's usually a sign of the pump pickup sucking in air and as the air is compressible, the pressure drops. Even with that problem eliminated, it is not uncommon for some pumps to be able to move more oil than the pickup assembly can supply and, rather than air, there are pockets of vacuum that form in the feed side of the pump assembly - with many engines there are larger pickups available to eliminate this problem. This can also be a problem with some oil pumps' housing designs, and some careful porting can make a big difference - i believe Honda is an example of this with some pumps requiring porting to improve oil flow within the pump casting.. Sometimes there are chatter marks on the gears or even some pitting evident.

Ring flutter will also show as increased ring groove clearances on strip-down. There is quite a lot of discussion about contributing causes, the prevailing one seems to be excess gas build-up between the top and second ring, and so many builders use larger secong ring gaps than techically heeded for heat expansion, to bleed that trapped gas off.

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