So it sounds like the main limiting factor involved in running high rpm engines is piston speed. Exactly what effect does piston speed have on the engine. Can you get aftermarket pistons, rods, ect which can handle the higher piston speed? I am looking to keep the engine around 7 liters and want to achieve a red line of 8,000 rpm.

 

piston speed, as well as rod angle, are two very important factors when determining max sustainable RPM.

rod angle (as well as piston speed) can be increased/decreased by changing the relation of stroke/rod length and wrist pin location. ie. long stroke short rod creates a very steep rod angle, something to avoid in a high RPM engine.

as i am sure you know, this can also determine charcteristics of the engine such as how "torquey" it feels as well as it willingness to rev.

 

That's why people spend a fortune on lightweight cranks, titanium, and aluminum rods.
You really can't get around the speed to much unless you switch to a V12 or different engine design. It's the momentum that will kill you.

The only significant way to reduce piston speed is to run a shorter stroke, and there goes your displacement with a V8.

 

Piston speed is not really much of a concern compared with valvetrain stability. You will hurt stuff here much faster!

 

Average piston speed (PSa) is determined from stroke and rpm only. PSa = rpm x stroke /6, so a 4.0 stroke 427 @ 8000 has a PSa of 5333 ft/min. That is higher PSa than current F1 (~5200) and Cup (~5300) engines spinning 19K and 9700 respectively! You need to have everything spot on to live up there.

Max piston speed (PSm) involves rod length, but it doesn't make all that much difference. A 4 in. stroke with a 5 inch rod(!) has a PSm of 8950 @ 8000, and a 7 in rod version has a PSm of 8690 ft/min or about 3% difference. No big deal.

Mechanically, a PSa about 5500 ft/min (ProStock range) seems to be the current limit. Engines like F1, Cup and ProStock have valvetrains that live at almost any revs, but they don't exceed the 5300-5500 PSa range. The new F1 V8s with a max bore limit (therefore min stroke limit) will get ProStock PSa at about 21000 where they will get before the rules mandate a limiting rpm.

8K pushrod valvetrains, while not cheap and easy, are doable. You should be very concerned with g-force felt by the piston. At 8K, a 4 inch stroke 6 inch rod engine feels 4840 piston gs. The LS7 (4.00 stroke) at it's 7000 redline only feels 3710gs, and it has Ti rods and light pistons. Your parts should be at least 30% stronger than the LS7's, and VERY light. The force the crank and rods see is determined by mass (part weight ) and gs (acceleration). F=ma, from Mr. Newton, if you recall.

FWIW, 7 L @ 8K is going to take some big flowing heads to make power up there, and some tough, LIGHT pistons and rods to keep the thing together. 7500 would be a lot easier...and cheaper.

As far as rod angle or rod/stroke ratio, longer/higher is probably better but with a given engine, it won't make a ton of difference. F1 r/s ratios are around 2.0+. A lot of that is because that's about the shortest rod they can use to keep the piston from hitting the crank. Cup engines are around 1.9 r/s. You will be closer to 1.5 due to packaging limits.

My different point of view.

 

Those are breathing limits dictating by how big a valve you can get on any given bore. Therefore all valve train aside you will always top out at some piston speed because you can only get so much airflow per square inch of piston top. The breathing characteristics of an engine dictate where in piston speed an engine will lay over not mechanical limits.

Rpm is tons more destructive to the bottom end than piston speed. If I have two engines and one has 6 inches of stroke and I turn it 6000 rpm and then I have another engine that is 3 inch stroke and turns 12000 they both have the SAME piston speed but the smaller engine has TWICE as much load on it all else equal. Piston speed is not what you look at to determine a mechanical limit unless you hold stroke the same and again then you are looking at RPM again.

Too many people on this board are worried about turning "too much piston speed!" It just won't happen unless you have some incredibly bad *** heads and the that's a good problem to have! Rpm is what you look at primarily to determine bottom end loads and pistons speed is what you look at to see where your engine will lose power at due to the breathing you gave it at design time.

You put those two together as in airflow limited piston speed and the stroke you are running and you can guess pretty damn close as to where you will lose power if your valvetrain can keep up. In small stroke engine the valvetrain will again limit you instead of any bottom end problems. Then you see what rpm you are turning at that stroke and piston speed and you can start to figure out how much load things are really seeing.

Knowing the piston speed an engine is running at tells you almost nothing about the g-loads on it unless you know everything else like how you got that piston speed.

 

So if you add a supercharger you should be able to increase piston speed significantly. I don't see that happening regularly.

You make some good points even though we don't always agree. It seems strange that the big dogs (F1, Cup, ProStock) who have quite different breathing abilities all end up with with piston speeds within a few %. Their g loads on the bottom ends are vastly different due to rpm as you said. With twice the gs of a 4.18 in. diameter Cup piston, the 3.8 in. diameter F1 piston must be a sub 200 gm piece, don't you think?

Of course they all have BMEP @ power peak that are very close, so they are all getting about the same torque/cubic inch @ hp peak rpm (~1.4 to 1.6 lb-ft/cubic inch) even though the hp/cubic inch varies from about 2.4 (Cup) to over 5 (F1). BMEP is pretty much about breathing, so they are all doing about the same (excellent) job.

Always fun hearing from you, E.

 

I think some people who have shifted down at WOT instead of up might not agree with you about the valve size limit.

 

ok, so it sounds like, as long as the engine components are capable of handeling it, the engine does not care how fast it is spinning. Correct?

Jesel J2k valve trains are designed for any engine speed. Crower makes 10,000 rpm rotating assemblies. So what is the largest stroke you can reliably run at 8,000 rpm?

 

Well something else to consider may be the wet sump abilities of the gen 3 motor which may not be adequate for anyone spinning that high on a somewhat regular basis. This is why my motor will be limited to 7500 or less. There are many other things to consider, some of what has ben discussed here, that will answer that question for you as well. Good luck! 8000rpm big motors are always cool!

 

How much can you afford to spend? Define "reliably". More than 100 miles in anger (at the high revs)? 10 miles? or ???

If you can equal the weight/strength components in Cup or ProStock, use the 5300-5500 ft/min PSa they use. If you ain't that rich (or connected), back it down to 5000 ft/min or less.

PSa = stroke x rpm / 6

so: stroke = 6 x PSa / rpm

You can do the math.

Can you get enough wind into that thing to be useful at those revs? Maybe you don't care if it peaks power up there, and just want to wind it. Different strokes for different folks. (Pun intended)

FWIW, if I were designing a 7L V8 for 8000 rpm, I'd be in the 4.4-4.5 inch bore range. You could probably find some heads to get enough wind for engines with that kind of bore capability. I'd want someone else to be paying for it, however.

My highly-opinionated $.02

 

Superchargers only increase the air density in the runners but not really how much they flow in CFM or the velocity they flow at. The ports act much the same under boost and the engine will still lose breathing effeciency whether blown or not after a certain rpm plus you will start clogging up the exhaust port as well with the supercharger. It's like the people that think you have a lot more velocity when you are running boost! You don't. People measure it. It's just denser.

Also it doesn't matter what type of 2 or 4 valve head you run you can still only run so big an intake valve in that bore before it's overly shrouded and before you run out of sufficient exhaust capability and that's what limits the airflow is that intake cross section. After that the stroke or piston speed simply ends up being limited to whatever stalls that port out. Basically you are limited on any bore to a certain percentage of intake valve size before it stops working.

A good 4 valve head like an F1 head can make better IMEP than the 2 valve but in a SUPER HI RPM engine like Formula One it is also saddled with an incredible frictional load that reduces the actual BMEP you really see at the back of that crank so they appear closer than they are if the F1 head could be run with a stroke to end up in NASCAR or PS 2 valve rpm range. Then the 4 valve head would also have a BMEP advantage as well.

As per our previous F1 piston discussion people weren't really running pistons that light in the real engines just like they weren't running the superlight PS pistons and making good power but they are real light. I think your son mentioned a certain companies piston that started with a C and that was a fully machined top to bottom billet piston of low weight in PS but I know that piston was 25 hp down to the WIseco PS pistons at two shops and wouldn't run down the track. Often what is being talked up on these boards isn't really what is run in the real deals. They had ultra light Cup pistons too but they would only qualify with them back then and couldn't run a real race or anything.

 

Superchargers and Turbochargers don't increase airspeed, velocity, or the volume of air entering an engine. Only the density of the air. Which means it has zero affect on where the engine will operate in terms of RPM.

 

You should think before you post stuff like this.

 

That's kind of a loaded question. You haven't given anybody enough absolutes to say for sure.

 

IHRA Pro Stock has piston speed in the 6,000 plus area now right?

 

Piston speed is only a rule of thumb its the piston G forces that are the most troublesome.

Stroke is the number one factor in piston G forces. Rod length is the second.

EAPro... Engine Analyzer pro has a nice demo you can download that has a LS1 file in its demo files already.

These graphs are from another application but still a good idea as to the effects of engine stroke.

88mm vs 100mm stroke
http://www.dsmtuners.com/forums/atta...9&d=1105679255


HP loses due to friction..
http://www.dsmtuners.com/forums/atta...2&d=1105680236

 

RPM is the number one factor in G-forces.

 

These are cool but the 88mm stroke engine will HAVE to turn higher to make the same power so the G forces will be higher on it and so will the friction thus part of the reason why when you destroke engines you always lose power. At 7000 rpm the 100mm engine would be losing like 30 friction hp but at the matching piston speed the 88mm is turning 8000 rpm and losing 33hp. Also this drop in stoke cuts loads less than the rpms raise it. But at the same rpm of course the shorter stroke engine has less friction AND less air going through it and LESS hp as well.

PS. I know you aren't trying to say the shorter stroke has less actual friction or load when really running in it's respective powerband but just to keep it clear for everyone else. You have to correct for piston speed or the curves do not really mean anything since these engines wouldn't turn the same rpm in the first place anyway. If they could the 100mm stroker would really be putting the hurt on the shorter 88mm stroke engine.

 

Erik,

How is RPM the number one factor? In piston G's... that makes no sense. If you don't have the stroke then you can't know the G's plain and simple... both have equal weight.

When we got into this forever ago over on Z28, you flat out told me that my prediction of equal force on the parts in the Cup and F1 motors couldn't be right because of the G's and because the pistons in F1 couldn't have a 210g mass.... well, guess what that's exactly where they are.

The book V10 Formula One Engine Technology by Ian Bamsey states and I quote from page 71 "the bare weight of such a piston was likely to be somewhere between 210 and 250g. That put my F calcs right on track with a cup motor at the time as well.

Now with the new V8 F1 rules intact what do you think is holding them back now with a bore (96mm) and cylinder limit (8)?

I don't want to get on your case here, but piston speeds and in turn the lbs of F that they see (from the rods and crank from the mass hung on them and piston g's because F=ma and in this case a = g x 9.81m/s^2) are limiting PS and F1 motors and formerly Nextel Cup motors.

If these motors could live for the required life line, then why did F1 motors loose RPM going to a 2 race engine until they could develop the bottom end to get them back to where they were? They reduce the max engine RPM in practice and at the end of the race if need be to extened engine durability. So to me that sounds like it's what limits them and what worries them the most.

Why did Pro Stock teams work so hard to reduce the masses of their parts previously when they could? I would guess for both lowered inertia and to lower the max F on the crank and rods so they could extend the RPM range. Now they have to focus on component strength to increase the RPM of the motor since mass is now dicated by the NHRA.

Why in Cup motors do you see 2.18" valves in some motors and 2.150" in other unrestricted motors? They are all lifting the valve up as much as the other, maybe the 2.15" heads have more lift but it's doubtfull and it only needs about .010" more lift to be equal.... doesn't seem to be a airflow demand limit there. The carb is since the demand is well over what the rules allow it to flow.... this fact is the only case where the airflow requirements MIGHT limit the max rpm of the motors in high end competition. They still worked around it though.

One more.... why did Judd's LS1 car go boom when he revved the thing to 9K+ in a burnout? Sure as **** wasn't because their wasn't enough airflow, or valvetrain problems.

I know your a proponent of more cubes in applications... but you really need to specify if you are talking about longer strokes in equal displacement combinations or using increased stroke to gain cubic inches. In some circumstances the physical limitations of the induction system are what limits HP, and added cubes usually cause that HP to happen sooner which then means you run less gear, and in the end puts less TQ to the pavement which = slower. I know in situations when the cross sectional area of the cylinder head can't be increased more cubes can hurt a motor that is already at the velocity choke point.

In the case of the 88mm stroke vs. the 100mm stroke your going to gain rpm with the shorter stroke. Say you look at a 5000 ft/min piston speed on both motors (The piston g's will still be more at the same piston speed I know) Now the total force of the setup will bring that RPM gap down some because in the same block the piston and rod combo will be lighter in the longer stroke motor but still 800rpm (7500-8300rpm) is 10% more RPM, which means 10% more gear, so you have to make 10% more TQ with the longer stroke motor in the RPM range you run in to equal the output at the road, since you can gear the shorter stroke motor higher. Now if displacement is fixed.... then you have less bore and more stroke, there is NO contest.

So it really comes down to.... Do you want the 350 with 4.55 gears and the same heads, or do you want a 395 with 4.10 gears? Most drag racing classes also give you weight breaks for cubes as well.... so you gotta think about that too.

Ben did you follow the conversation at all.... OldSStrokers above statement was in reference to this...

He was being saracastic towards that statement... The old man knows how forced induction works. Obviously his point was if airflow limitations are what limits RPM ultimately in unrestricted catagories of racing then if you could add boost you could extend the RPM limit since you can now cram more air/fuel into the cylinder.... obviously that's not the case.

Which leads me to this...

Maybe you should fully read everything before YOU post stuff like this... I agree with white here, overspeeding the motor is going to break parts in both the bottom end and the valvetrain.... it's not the breathing ability of the motor that's limiting RPM in UNRESTRICTED forms of racing.

If F1 could stay unrestricted in terms of bore size for a given displacement then they would develop the ports to move the wind they need to increase RPM.... they have the capability to go over 20K but not the durability, and the V8's give them even more of a problem with increased RPM. I want to see if anyone else here knows why?

Bret