Limiting factor for RPMs?

[OldMachinist]

That is true, but that doesn't mean you can't RPM the other motor even more......

IIRC you said earlier it is valvetrain that limits RPM. Has it changed to stroke limits RPM? Sounds like our president who doesn't remember what he said last year.

[Barry Donovan]

That makes some sense if you're endurance racing with a restrictor plate and ancient 2-Valve Kettering technology. I suspect the answer would be a tad different down in the Porsche pits (and they've won a lot more endurance races than "Government Motors.")

The Vette mill supports the oversquare theory for competition engines. BTW, the only reasons I can think of that you'd want to run a small bore/long stroke are: (a) increased detonation resistance and reduced emissions on cheap pump gasoline; (b) the rules require it; (c) tooling/bore-spacing requires it; (d) You're trying to optimize the lever-arm length for maximum low r.p.m. torque and efficiency; (e) you're building a vintage engine.

low octane and boost go with long stroke and regular fuel..

today avoids nature, that does not mean evolution.
that n/a boxer in the little BRZ is at 86x86 because asians worship fairy math. that is not going to be a hero as n/a or boosted.

there is a gas math, it is still around. GM to me is the hero of it. Ford finally gets boosted, and dodge is like a giant bird.

There is too small and too big..but you would have to mention 87 octane or whtver to decipher it.

I am at 2.65 inch stroke, and it somehow is not too small..only if it stays in the 30s mpg. that is the biggest function.

all kinds of stuff gain or lose rpms. Respecting the past is a good foundation.

[TC]

IIRC you said earlier it is valvetrain that limits RPM. Has it changed to stroke limits RPM? Sounds like our president who doesn't remember what he said last year.

The problem with to long of a stroke and RPM is piston speed..... as RPM increases so does the speed of the piston and once the piston reaches a certain speed the piston starts moving faster than the flame front which would start costing you power...... Or at least that is one of the negatives that was discussed about running a long stroke that I read, which I guess would be a limiting factor for RPMs.........

[OldMachinist]

As has been repeatedly proven over the past 100 years, 4-valve engines tend to have drastically more valve curtain area and low-lift flow, relative to bore size.

`Nobody is going to deny the flow advantages of 4-valve heads or the RPM potential of OHC engines with their lighter valvetrain.
There are significant disadvantages though regarding cost ( 2-valve 5.4 engines cost $800 more at the factory level than 5.3 LS engines) complexity and overall dimensions. OHC engines were produced in the 20's or 30's so they must be vintage also. I believe in the KISS principle so if the simpler cheaper and smaller dimensionally engine does the job why change? The hated LS is so much more popular in swaps than the Mod because it is easier to fit. With the LS3's redline at 6600 you would think the 5.8 Mod would redline at 9000 being so superior and lighter in the valvetrain instead of 7100. I suppose if Ford had designed its 4-valve heads with a direct acting camshaft position over the valve instead of rocker arms with hyd. lifters it would RPM higher but that would require more maintenance via buckets with shims or some such arrangement.
According to Chevrolet advertising the ALMS Corvette engine had to be de-tuned to be accepted in its class which explains why its RPM is kept so low. Maybe the 2-valve is still viable in high performance applications

[OldMachinist]

The problem with to long of a stroke and RPM is piston speed..... as RPM increases so does the speed of the piston and once the piston reaches a certain speed the piston starts moving faster than the flame front which would start costing you power...... Or at least that is one of the negatives that was discussed about running a long stroke that I read, which I guess would be a limiting factor for RPMs.........

What RPM is too much piston speed for 4" stroke? What is the RPM you are going to turn your turbo engine?
IHRA pro stock engines have 5.750 stroke and turn 8100.
Damn sucked in again.

[TheSilverBuick]

The problem with to long of a stroke and RPM is piston speed..... as RPM increases so does the speed of the piston and once the piston reaches a certain speed the piston starts moving faster than the flame front which would start costing you power...... Or at least that is one of the negatives that was discussed about running a long stroke that I read, which I guess would be a limiting factor for RPMs.........

And yet people are running long stroke cranks in small block chevy's well into 8,000rpm territory, with more displacement then your engine will have. So when does this negative effect start happening? 12,000rpm? 20,000rpm?

Theoretically if you get the piston speed up fast enough it'll reach a limit that at which point it'll start getting heavier rather than quicker.

[CDMBill]

The problem with to long of a stroke and RPM is piston speed..... as RPM increases so does the speed of the piston and once the piston reaches a certain speed the piston starts moving faster than the flame front which would start costing you power...... Or at least that is one of the negatives that was discussed about running a long stroke that I read, which I guess would be a limiting factor for RPMs.........

There is never, ever a situation where the piston is actually moving faster than the flame front. I'm not just talking flame front while the piston is at its slowest as it comes up to and leaves TDC, it's never moving faster even at mid stroke when the expansion of the combustion plume is beginning to slow. To think otherwise is to conclude that the pistons power stroke finishes at some pressure far below residual exhaust pressure.

[TC]

There is never, ever a situation where the piston is actually moving faster than the flame front. I'm not just talking flame front while the piston is at its slowest as it comes up to and leaves TDC, it's never moving faster even at mid stroke when the expansion of the combustion plume is beginning to slow. To think otherwise is to conclude that the pistons power stroke finishes at some pressure far below residual exhaust pressure.

I'll agree that saying the piston out runs the flame front is kind of a loose term, your right it doesn't actually out run it, but what does happen is a drop in average cylinder pressure...... I guess Darin Morgan(Reher-Morrison) explains it best.......

(in one post)
In my opinion it has less to do with rod ratio and more to do with piston speed. Average, not instantaneous. Endurance engine builders try and maintain a mean piston speed of 4500 to 4800 ft/min. An endurance engine that operates above 5000 is considered HIGHLY stressed. The current NASCAR engines operate in that range and they are pushing the limits of metallurgy in order to achieve that. It has everything to do with longevity due to the inertia loads on the parts over time. The pistons do not start to" outrun the flame front" until the piston speeds exceed 6800 ft/min. The Asiatech engine has a mean piston speed of 4977 ft/min and the Corvette Le Mans motor has a mean piston speed of 4333 ft/min. That's not even close to 6000 ft/min.

IRL engine = 4225 ft/min
Pro Stock = 5991 ft/min
average comp eliminator engine = 5800 ft/min
IHRA Pro Stock = 7400 ft/min OUCH! They don't last long at this speed and they nose over after the power peak big time!

(in another post)
The piston does not actually "out run" the flame front because that would result in a -HP reading but the amount of average cylinder pressure BMEP is falling off rather quickly.
" out running the flame front" is a loose terminology used to explain a quickly decreasing BMEP specifically caused by the rapid expansion of volume in the cylinder due to excessively high piston speeds. The jury is still out on this one although I personally feel it is a factor in engines that operate at or above 6800 ft/min.

http://speedtalk.com/forum/viewtopic.php?t=406

[JeffMcKC]

Actually, forced induction doesn't alter the advantages of greater displacement, you just need a bigger pump to match.

All engines are forced induction........................ some just have less force.

[TheSilverBuick]

I'll agree that saying the piston out runs the flame front is kind of a loose term, your right it doesn't actually out run it, but what does happen is a drop in average cylinder pressure...... I guess Darin Morgan(Reher-Morrison) explains it best.......



http://speedtalk.com/forum/viewtopic.php?t=406

Using this calculator a 4" stroke hit's that magic 6,800 ft/min a bit over 10,000 rpm. If you're going to only go to 8,000rpm you are leaving displacement on the table if you are using the flame front arguement.

http://www.wallaceracing.com/piston-speed-velocity.php

[jcharliem]

I'll agree that saying the piston out runs the flame front is kind of a loose term, your right it doesn't actually out run it, but what does happen is a drop in average cylinder pressure...... I guess Darin Morgan(Reher-Morrison) explains it best.......
http://speedtalk.com/forum/viewtopic.php?t=406

LOL. TC is so predicable. As always... he comes on with some half-baked theory (usually it's a hypothosis with absolutely no facts to back any of it up). Then when he paints himself into the corner, he'll come out swinging with non-related links pulled out of Google's bung hole. Then, he'll do a little side-step and try to run down another path that has little to nothing to do with his original lame-ass theory. It's no wonder none of his "projects" are ever completed. A waste of time.

[JeffMcKC]

[TC]

Using this calculator a 4" stroke hit's that magic 6,800 ft/min a bit over 10,000 rpm. If you're going to only go to 8,000rpm you are leaving displacement on the table if you are using the flame front arguement.

http://www.wallaceracing.com/piston-speed-velocity.php

Your just stating what I already know, you didn't think I didn't do the math did you??......

[TC]

LOL. TC is so predicable. As always... he comes on with some half-baked theory (usually it's a hypothosis with absolutely no facts to back any of it up). Then when he paints himself into the corner, he'll come out swinging with non-related links pulled out of Google's bung hole. Then, he'll do a little side-step and try to run down another path that has little to nothing to do with his original lame-ass theory. It's no wonder none of his "projects" are ever completed. A waste of time.

Sorry twinkle toes, this was not my theory, it is someone elses that I read, I was just relaying the info...... I have to reread that article from DF, I think he mentions it in that to.......

[dieselgeek]

Sorry twinkle toes, this was not my theory, it is someone elses that I read, I was just relaying the info...... I have to reread that article from DF, I think he mentions it in that to.......

What a load of BS. This is why every single person on this entire forum thinks of you as a total fool.