F = ma

At 10K rpm a Pro Stock engine experiences almost 6400 gs of piston accelertion (a in the formula) at TDC trying to pull the rod apart. With minimum piston weights (m), the inertia loads (F)---loads on the piston pin, con rod and crank due to acceleration, are the limiting factor.

According to Mario Illien, technical chief for Mercedes F1 engine, con rod life was the limilting factor on how high they could turn the 2005 engine. They kept it to 18,700 on a 18,300-18,500 power peak rpm.

 

If pin can be made rather strong, in some applications in is an 1" in diameter. However, if the the rod bolts are the limiting factor, why aren't they using 1/2" bolts or changing the design so all the primary loads aren't through the rod bolts in tension?

 

Its not necessarily the rod bolt. It can be the rod, it can be the "hinge point" where the rod moves from the big end and transitions towards the small end.

Keep in mind "most" rod failures are not as a result of compressive load, but rather as the rod is leaving TDC on the exhaust side, it is pulled apart.

One of the main issues for instance IMHO on the Ls1 when folks scatter a motor is not the stock rods, it is valvetrain control. They over-rev the motor, and the valve float removes the "cushion" in the combustion chamber. Couple that with a rod near its limits, and the rod comes apart.

Had they had proper valvetrain control chances are the motor would survive.

I know there was some testing done down here, and some F1 guys were visiting a local PS team, and the main focus of their testing was all valvetrain, and spring development.

 

inertial resistance to acceleration is the difference in potential energy from the starting state (start rpm) to the final state (shift rpm). If you start at one rpm and end at another you can do the math to see how much power was diverted into the rotating assembly. This is also why the inertial dynos read different in different gears at different step rates.

An engine going from 5000-10000 rpm has much more inertial resistance to acceleration than one going from 1000-2000 rpm. The higher you go the worse this gets as well irreguardless of whether or not the rotating assembly or con rods are reliable or not. Another reason that the larger engines always make more power and accelerate faster.

The fact that Mario says that con rod strength is a major issue on his engine at that time shows how little piston speed affects loads compared to rpm as well. I know Mario has done NASCAR and PS stuff too and he says it's valvetrain limted. It's very hard to open big valves as far as they do and at the rpm they do. Without that lift the heads do not work.

 

Talking about OHV V8s (NASCAR and PS), It should be possible to add meat to the rod, pin, and piston to handle the loads. The trade-off is in strength vs weight, where weight increases the load requiring the strength.

The cross section of the rod is large compared to the cross section of the bolt, so it was my understanding that the bolts were the weak point.

Erik is stating clearly that he believes it is the valvetrain. If it is the bottom end, what in the bottom is the weak point?

 

The reference was to loads on the pins, rods and crank due to piston gs @ TDC. These are proportional to rpm of course with max. loads at max rpm. I wasn't referring to rotational moment of inertia (MOI). Sorry if that was confusing.

I'm having a little trouble understanding "the larger engines always make more power and accelerate faster". I will subscribe to the generalization that larger displacement engines can make more power than smaller displacement engines. Any engine 6+ L (366 cubes) engine that doesn't make over 940 hp negates that generalization when compared to the 3.0L Merc F1 V10, I guess. How do the larger engines with more rotating inertia accelerate faster than smaller engines with less rotating inertia?


Ever wonder why Illien does Cup work? Who's it for?

Actually, the Mercedes F1 V10 used a smaller bore dia that the max or what some other teams ran, so they needed a slightly longer stroke which gave them more inertia loads and more piston speed for a given rpm. The titanium con rods gave out before the 1400 km point which they needed to run for two race weekends.That's why rpm had to be limited.

FWIW, this year some of the lower budget F1 teams are running the old 3L V10 with an air restricter and a 16,000 rpm max. They are having no trouble getting 2500+ km out of the 16k engines. Lower inertia loads are a lot easier on the parts.

Stay cool!

 

The problem is that there are minimum weights required for pistons, rods, etc. imposed by the sanctioning organization. The goal to is increase the strength of a given minimum weight (and restricted material) part so it can resist the inertia loads. This is being done by where the material is placed in the part. F1 pistons, which are about the diameter of an 5.7 LS1 piston and weight in the low 200 gm area, were having trouble living for two race weekends. The material was not changed, nor was the weight, but there were detail limprovements in the shape. My guess is that Cup pistons, with their 400(?) gm min weight have gone thru the same cycle.

Rod bolts probably aren't the weak point. More than likely it's the cap end with its high hoop stress when the tension loads are worst (at TDC on the exhaust stroke). Think about what happens if the rod big end goes out of round.

In some applications pistons have been the weak point. The cranks probably aren't unless you lighten them enough or make the bearings small enough that they become the weak link. In the pro-world you probably keep trying to lighten or "smallerize" anything you can to get more rpm, more power, and less weight. It's a balancing act.

With NASCAR effectively limiting max rpm on Cup engines, the valvetrain isn't the RPM limiting factor. What may be limited is the lifter velocity (lift/degree of rotation) which is pretty much controlled by the flat lifter diameter and max lobe size which is limited by cam bearing diameter. That may be what folks say when they say Cup engines are "valvetrain limited."

How about this idea? Conventionally we limit cam velocity to the point just as the edge of the solid lifter is digging into the lobe flank. Diamond Like Coating (DLC) is now being used on lifters and perhaps cam lobes, and it's so hard and DLC on DLC has almost NO friction. Could we actually put a very small radius on the edge of the lifter and allow it to try to "dig into" the cam lobe flank, but because of DLC-DLC it might not and we could thereby increase the cam velocity (aggressiveness) of the lobe and therefore get more area under the lift curve? This would have the same effect as a larger diameter flat lifter. I don't have any secret info, but my guess is that this has been tried.

I read where the "Dodge Boys" said they are using tool steel Cup cam blanks in place of the traditional stellite-welded lobes. This might help provide the strength or hardness necessary to use DLC on the lobes. Who knows?

 

Strictly speaking, it is the change in velocity,
acceleration is the rate of change of velocity.
, Momentum is = Mass x Velocity and
(change in momentum)/time = Force
Provided that they are doing it in the same amount of time.
If the engine rev's from 1000-2000 in 0.1 sec it will require more force than the one going from 5000-10000 in 1.0 sec. ie (1000x10)vs(5000x1)

if we disregard resistance, then the relative RPM is not a function of the required force for the rate of change of RPM, acceleration.

But then again I could be wrong.

 

What's the source on that quote?

-Ben R

 

Jason Line:

http://www.racingone.com/driver.asp?...2&subseries=17

 

Ben,

Race Engine Technology, May 2006 Pg 67

Bret

 

Is that a pretty good magazine? I've browsed through it a few time but I haven't actually read anything in it.

 

Yeah, Ian is finally getting into Pro Stock stuff. I saw him at PRI and mentioned that he should look into what they are doing, thank god they started too. Lots of good articles on F1 and Cup motors as well and of course the Audi V10 diesel is a big story lately. Only problem is a 2 year subscription is $257!

Bret

 

Sorry, I don't mean step on any toes in saying this...

ignoring any losses from friction/compression and any gains from combustion, piston g's (acceleration) are proportional to rpm squared.

 

Ouch!

 

When you destroke something and LOSE inches you must now turn more rpm thats what I was talking about.

When you destroke an LS1 or any engine you do not lose almost any mass at all and you only slightly change moment of inertia but now you are in an rpm zone that is HIGHER to hope to make the same power as the larger engine.

The loads and inertial resistance to acceleration change at the square of rpm and are not in proportion to it at all. You still need to brush back up on your physics big time.

You are trying to accelerate through piston speed and not rpm to go fast. If you have to spin the engine extremely high in rpm to make that piston speed then you are loading tons of stress on the engine and you will also have more inertial losses. If I have to span double the rpm and double as high I would have to have 1/4 the moment of inertia to make that loss the same as the much heavier engine with the much worse moment of inertia but that spins only half as high.

In reality you don't even lose much mass when you destroke anything and the rods usually have to become longer and heavier as well AND they have to be spun higher.

Why do you think they don't just have a bore limit on racing engines and let any stroke fly? According to you the bigger stroke engine would be slower.

Why can't you run unlimited stroke but with a strict bore limit in F1 then?

Why isn't PS just a 4.700 bore and ANY stroke?

Why do IHRA engine with almost identical bores run 6.3s with a 5.7+ inch stroke and turn low rpm?

Why don't they destroke F1 or PS engines?

Why do they still run all the stroke they can to get right to their displacement envelope?

 

Yep, rpm squared. (That's still proportional, just not directly proportional, right? ) Thanks. My toes don't hurt at all. You are a little more subtle than my friend Erik.

FWIW, aren't the pure inertia loads separate from those mitigating loads? Worst case scenerio for tension loads in the piston/pin/rod/crank is probably at max rpm at TDC, closed throttle and a very high vacuum above the piston. Did you ever see a short track engine blow just before turn entry just after the driver lifted?

 

Bill,

You are figuring your stuff backwards.

We are assuming that the two engines have the same average power and are geared appropriately so the engine going from 1000-2000 rpm would occur in the same time increment as the engine going from 5000-10000 rpm.

The piston speed change on them both is the same. Assume the first just has a tremendous stroke advantage. It would carry a gear 5 times taller.

 

Keep in mind that I don't think any of us are arguing the fact that to make maximum power out of any engine we must try to make as much tq as possible at as high an rpm as is possible.

The thing is that we are talking POWER here!

NOT just TQ or just RPM but BOTH.

RPM is NOT more important that TQ (and TQ is NOT more important than RPM) and I think this is what people are thinking and it is not.

The reason people more into engines and heads talk piston speed is that then they know where they are as rpm never really tells you anything unless you know stroke as well with an engine or a set of heads.

Some people get this and some don't. As high an rpm as possible is relative. 6000 RPM might be absolutely awsome if it's on a 6 inch stroke engine and 10000 RPM might suck if it's on a 2 inch stroke engine. Piston speed tells you where you engine will hell over at vs your heads.

 

Thanks for the reminder. Erik, meet joecar.

Yep, larger displacement engines can make more power if you do it right. No argument there.
I look at high piston speed as an enemy, not as a friend. Perhaps we differ there, EK.

With half the rpm wouldn't you need to make twice the torque to get the same power?


Are not mandated bore limits for a given displacement put there because with larger bores for a given displacement, the racers shorten the stroke so they can twist it higher and keep the inertia loads in check and not lose so much hp to ring friction? So why not just limit displacement and max stroke if stroke is where more power comes from?

Bore (or stroke) limits are often instituted when one group of guys make more power and/or rpm because their engine allows a larger dimension. eg. Chrysler's Cup block allowed 4.200 or so bores and made more power, but the other guy's blocks wouldn't allow that much bore, so 4.185 was the new max. allowed. You could run a 3.77 bore in a Cup engine with a 4.00 stroke and it would fit in all the blocks and NASCAR wouldn't say anything against it. You might have to notch the bores for the valves, of course. My guess is that it wouldn't be much of a threat for the pole, either.


With a displacement limit and a bore limit, you don't have to limit stroke; it's self limiting.

Wild guess here: 500 cubic inch NHRA maximum would be exceeded as stroke got much over 3.60 with the 4.700 bore.

791-815 cubes vs. 500 can make lots of torque if you do it right. You should be able to make lots more power with the larger engine, but not run into the huge inertia loads which are proportional to the square of the rpm but proportional only to the first power of the stroke. NHRA PS engines are horribly expensive, partly because the do run such high rpm for the size of the engine.

Let's see, maybe they want all the cubes the rules allow using the biggest bores they can.

Some of the F1 engine guys weren't real happy with the 98 mm max bore limit on the V8s. They had room for more bore spacing than the V10s had and still have a shorter engine. (Perhaps) they were looking at larger bores, shorter strokes, more rpm and more specific output, and Max Mosley got wind of that testing.

It's the "monkey see, monkey do" mentality in F1 and Cup. How boring (pun intended). The all run very similar bores and strokes. No inovation anymore.


Don't ask if you don't want my opinion.