high reving in large displacement engines

[MetalHeadZaid]

http://www.supercars.net/cars/3448.html

im not sure how accurate that website is but as you can see, 12.9 liters and 8000rpm. how exactly does an engine of this size rev so highly? what factors usually prevent such large engines from reving highly. i dont really understand the relationship between displacement and engine speed.

[Moppie]

Don't believe any, or everything you read on Supercars.net.

Second, how high an engine revs is determined by how strong it is, and how much air and fuel can be pumped in through in the carb(if used)-inlet-cylinder head, then compusted, and pumped out through the head and exhaust.
This is depentant on manifold, cylinder head, fuel system and ignition system design.

Of course the bigger the engine the stronger it needs to be, and the more air it needs to be able to pump.

[534BC]

Mechanically, some factors are reciprocating weight or piston speed or more accurately the accelleration and deceleration of the pistons along with the mean piston speed. In normal engines this translates into

stroke, rod length, all weights, strengths,ect.

Along with the top-end stuff mentioned above to be able to feed the engine the crank should be strong and the rods strong, long and liteweight. Pins should be short, strong and very light. Pistons shall be very lightweight (large diameter and short height.

The best way to get power (since piston pressure is limited) is to produce the high piston pressure on a very large bore (for total surface area) along with long stroke (for higher leverage) and then make it stay together at a high rpm (works at a fast rate)

ps Check out a bourke engine or a scotch yoke with slipper bearing to see a prety long stroke engine that can rev real high. It is still reciprocating pistons

[GreyGoose006]

i dont know if i believe this.
first of all, who builds a 12.8L/780ci engine in the first place.
that would have to be one MASSIVELY strong bottom end to rev to 8000 rpms. either that or a really short stroke and huge cylinder bore

[MetalHeadZaid]

haha, i didnt think supercars was very reliable. so in conclusion, bore is more desirable than stroke (generally speaking, depending on weight and strength of rods, pistons ect) because longer rods will take a bigger beating. is there any downside to increasing the bore? i mean, you have all of these variables when playing with rod/stroke length, but it seems that bore cant harm any other aspects of the bottom end, other than thinning cylinder walls and the strain of more power. i would think that the fuel system is the least of worries when it comes to building a high reving, big engine.

[534BC]

You got it, "the power is in the bore"

[534BC]

That engine reminds me of an eagle engine that was about 700-800 cid. It looked real nice, but never heard another word about it. This one appears to be Chevy based mains and it is possible to get the displacement on a Chevy based crank.

EDIT my bad , it is probably a big Ford block. duh.

[MetalHeadZaid]

i would love to see this kind of engineering in a v or even flat 12. obviously nowhere near as plentiful or practical as the good ol v8 but with the resources whoever built that engine must have, there is potential for an awesome engine. i would pay to hear the sound of a 12.9 liter flat 12, at 10,000 rpm.

[534BC]

The allison v-12 sounds pretty good (when they run right) They seem to be way ahead of their time in engineering, but I think were 1710 cid and maybe run 5000 rpm.

[TheSilentChamber]

Bigger isnt allways better. Look at f1 cars, 2.8l v8 that revs to 19k and makes 900hp.

[UncleBob]

hehe, I like some of the claims there: "worlds most powerful ....road legal car"

not even close guys.

"world record torque of 1300 ft/lbs"

yawn

But anyway....that is a rather impressive redline for a motor that big. One of the basic fundimentals, no matter how light weight the pistons, no matter how strong the crank and rods, the piston average speed can only go so high. For race application, 5000 fpm is cutting it close. Most stock applications keep it under 4000fpm. Even the most high tech engines in the world, such as motorcycles, never get over 5500 fpm.

The formula isn't too hard, all you need is the stroke. Lets take a 3" stroke for example: 3X2/12 = .5 X RPM = fpm. 3" stroke at 10000RPM = 5000 fpm.

Take a 6" stroke, and as the simple formula shows, 5000 RPM's is playing with fire no matter what you got.

Since we don't know what the stroke is on this 780ci engine, we're guessing, but I assume its going to be a large bore AND a large stroke engine. 5+ inches. In that case, I have to say, its either a short lived time bomb, or its a big fat lie.

But large bore does have its disadvantages....the bigger the bore, the less efficient the flame front is, the harder it is to get enough valve flow to support the cubes. The general rule is, anything over 4" bore is losing efficiency.

[MetalHeadZaid]

Originally Posted by UncleBob

hehe, I like some of the claims there: "worlds most powerful ....road legal car"

not even close guys.

"world record torque of 1300 ft/lbs"

yawn

But anyway....that is a rather impressive redline for a motor that big. One of the basic fundimentals, no matter how light weight the pistons, no matter how strong the crank and rods, the piston average speed can only go so high. For race application, 5000 fpm is cutting it close. Most stock applications keep it under 4000fpm. Even the most high tech engines in the world, such as motorcycles, never get over 5500 fpm.

The formula isn't too hard, all you need is the stroke. Lets take a 3" stroke for example: 3X2/12 = .5 X RPM = fpm. 3" stroke at 10000RPM = 5000 fpm.
Take a 6" stroke, and as the simple formula shows, 5000 RPM's is playing with fire no matter what you got.

Since we don't know what the stroke is on this 780ci engine, we're guessing, but I assume its going to be a large bore AND a large stroke engine. 5+ inches. In that case, I have to say, its either a short lived time bomb, or its a big fat lie.

But large bore does have its disadvantages....the bigger the bore, the less efficient the flame front is, the harder it is to get enough valve flow to support the cubes. The general rule is, anything over 4" bore is losing efficiency.

very informative post. so basically, what you're saying is that a certain ammount of stroke is required to take advantage of bore, and when we increase bore, we get to a point where the engine simply needs more stroke to be effective, in the way of physically pulling, pushing, and igniting fuel.

[drunken monkey]

wouldn't an over sized bore also affect the efficiency and consistancy of the swirling of fuel you get injected into it or is this part and parcel of less efficient flame front?

[534BC]

Going to a bigger bore will net more hp even if cylinder filling is not enhanced.

Going to larger main spacing and larger bore centers assumes a larger valved head.

As far as stroke goes most engines are relatively square for easy figurin' , but an oversquare (larger bore than stroke ) is almost always better for power.

Another way to say it is that increasing bore size increases power and increasing stroke does not (assuming a mean piston speed constant)

[534BC]

I have built a number of 903 cid v-8, they were very heavy components because they were made for diesels . I don't remember the stroke, but it was an over square with 5.5 " bore and we cranked 4000 rpm max with out of balance assemblies. The bobweight of one single crankpin probably weighed more than 4 bobweights on a big block chevy. Definetely would not want to drop a piston on your foot, ouch.