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.

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.

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

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

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.

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

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.

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.
(http://psp.nixlabs.net/08%20Tommy%20The%20Cat%20%28Live%29.mp3)

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.

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

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.

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.

(http://psp3d.com/penguin.rar)

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?

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)

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.

i've helped friends work on a 20.9-litre Mercedes V12 diesel (1,221ci) and that revved to something like 2,500rpm and no higher. i think it made peak torque at about 1,900rpm and peak power at 2,400rpm.

cany anybody say LONG GEAR RATIOS!

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)
this is the problem with such generic discussion. Going with larger engine (bore and/or stroke) will definitely increase power, both torque and HP, but it moves both peaks lower, due to head flow (assuming it stays the same).

If you increase bore AND valve size/ports to take advantage to the increased combustion chamber size, then you can definitely net a more linear power increase (more power than just cc increase accounts for)

If you only increase stroke, there is no valve size increase option that you didn't have before.

The most basic principle to motors is, the smaller the engine, the more power per cc potential. This is why, per cc, radio controlled engines have unbelievable power to cc ratios (even after taking into consideration they are 2 stroke alcohol/nitro engines). The smaller the engine, the better air flow dynamics potential. Going another step up in size, motorcycle engines are breaking the 200hp/liter in stock trim, NA. very oversquare engines, lots of head flow, lots of RPM's. Very small cc per cylinder. Much further down the scale is such huge engines as the one mentioned in the beginning of this thread. Huge cc/cylinder, probably undersquare (assuming), very bad head flow for its size. (realistically) low RPM's. It all would spell out less impressive per liter numbers.

But that is way too simplistic of a comparison also. When you start comparing severe oversquare to severe undersquare, 2 valve to 4 valve, multi cylinder to less than multi cylinder....lots and lots of variables start coming into play.

To put it simply. As far as peak power is concerned, its cc's, head flow potential, and RPM's. Those three variables are the nitty gritty of performance. If you have good specs on 2 of those items, and lack luster on the third, you will have a three leg stool with a missing leg compared to what you COULD have with simular, but even distribution between the variables

to what extent does the extra weight of having a larger bore (and thus larger/heavier piston) play a part?

effects the realistic fpm the pistons can acheive before bad things happen.

to what extent does the extra weight of having a larger bore (and thus larger/heavier piston) play a part?

it's like having a weight on the end of a rod. if you took such a thing, and used one hand as a guide and the other to move it back and forth you can do this easily up to a point. if you go too fast the sudden change in velocity (velocity being speed with direction, not speed.) puts strain on the rod. if this change is too sudden and too violent then the rod snaps. (it can snap on both the outward and inward stroke)

so it's the same as a con-rod breaking inside an engine (except without the side-to-side movement of the crank's rotation)

i hope that helps.

this is the problem with such generic discussion. Going with larger engine (bore and/or stroke) will definitely increase power, both torque and HP, but it moves both peaks lower, due to head flow (assuming it stays the same).

yes, torque will definately increase. horsepower won't always. horsepower being calculated. and it's the work done as opposed to a force that's measured. a high revving engine with less torque can make more horsepower than a bigger engine with more torque that can't rev so high.

so you may increase engine size and see a gain in torque, but if it can't rev as highly (or there's a big drop in torque higher up in the rev range) then it may not produce as much horsepower.

yes, torque will definately increase. horsepower won't always. horsepower being calculated. and it's the work done as opposed to a force that's measured. a high revving engine with less torque can make more horsepower than a bigger engine with more torque that can't rev so high.

so you may increase engine size and see a gain in torque, but if it can't rev as highly (or there's a big drop in torque higher up in the rev range) then it may not produce as much horsepower.

that is true, but its unlikely that you'd have a situation that was that extreme. Lowering your redline slightly on most engines, won't effect the peak HP point, since peak HP isn't usually at peak RPM.

well... i was aware of the efect of extra weight on the piston but was trying badly to allude to the use of balancing shafts in certain "regular" engines.
i know that in the context of this discussion it is largely redundent (refering to the 780ci engine) but well, what i'm now trying to ask is; is it possible to engineer from scratch a big bored engine to rev high?

also how big can you go before a carefully designed injection of fuel/mist becomes a rough squirt (if that makes any sense)?

counterbalancing only reduces vibration, it doesn't reduce the mass of the recipricating components (it actually adds to it) which is what limits redline.

injectors (assuming you use them) are limited by opening time, as far as what RPM's they can handle, which gives them a smaller window to achieve the quanity of spray they need.....but this can be overcome by fast injectors, large injectors, or even multiple injectors. Bikes rev to 16.5K RPM's these days using injectors...and then there's F1 cars over 23K RPM's

well... i was aware of the efect of extra weight on the piston but was trying badly to allude to the use of balancing shafts in certain "regular" engines.
i know that in the context of this discussion it is largely redundent (refering to the 780ci engine) but well, what i'm now trying to ask is; is it possible to engineer from scratch a big bored engine to rev high?

also how big can you go before a carefully designed injection of fuel/mist becomes a rough squirt (if that makes any sense)?

A larg bore engine rev limit will still be determined by the previous items mentioned. A grossly oversquare engine may be a high revver, but a short stroke may be hard to get some adequate compression ratio.

If you took my 903 cid v-8 as an example and cut the stroke in half theoretically you can double the rev limit. The piston speed would be identical and amid a bunch of variables theoretically would produce the same hp as the 903, but do it at twice the rpm and be 452 cid.

hope that came out right,. :eek:

basically, if you want a torquey engine, concentrate on lower rpm's and very long strokes.

if you want high horsepower, focus on high rpm's and large bores and short stroke

this can be seen with a 383 chevy versus a 377 chevy (smallblocks) the 383 is a 350's 4 inch bore with .030 more bored out, and a 400's 3.75 inch stroke. it is great for low rpm torque and is an easy 500lb/ft of torque.

the 377 is a 400's bore (4.125 inches i think) with a 350's stroke (3.48inch) this motor cant make as much streetable torque, but it makes a lot more actual horsepower because the shorter stroke lets it rev much higher than the 383. i have heard that a 350 bottom end (3.48inch stroke) is good for about 6500 rpm with no modifications.

the problem with huge bores is that it takes longer for the flame to spread and ignite. if you want to engineer an engine from scratch and use a large bore (over 4.5 inches) you would be looking at huge valves and probably twin sparkplugs.

i have heard that a 350 bottom end (3.48inch stroke) is good for about 6500 rpm with no modifications.

350 is capable of way more RPM's than that...but its a question of good parts and smart engine building. But back to my earlier formula, 3.48 x 2 / 12 = .58
1/.58/5000fpm = 8620 RPM's at a fairly aggressive build.

Now....a 283 (3" stroke) would do 10K RPM's. An old fart I work with used to race a 283 he would spin up to 10K every run. Motor held together for years with nothing breaking on him....very impressive!

yeah them little 283's are rockets, but anyways, were you saying that a stock bottom end 350 with no mods can hit over 8000 rpm? or is that with h-beam rods, forged cross-drilled crank, and forged pistons?

also, i have heard of people making the main bearing size smaller to allow more rpm... its like being farther out on a merry-go-round, you are moving faster than a person farther in towards the middle. i think that might be the principle behind that main bearing re-sizing.

That is indeed reducing the bearing surface speed and is one reason why the 283 and 302 can rev higher. Your redline of 6500 is pretty acurate for a "stock" bottom end 350. I think of stock as iron rods, crank and block with stock bobwieght. I always considered about 6200 as a max rpm as most "stock" springs will allow bounce or float above 6500 anyways.

Another thing you reminded me of when talking about reducing main bearing size is the overlap between the two crank journals, the larger the shaft size the stronger it is. I think the short stroke chevy also had small rod pins.

How about a formula for bearing surface speed? It must be circumfurance times rpm or something like that? lol. I've always wondered about ball bearings in a car engine? probably won't hold up?

I've always wondered about ball bearings in a car engine? probably won't hold up?

they've been used and they hold up. but they require a multi-piece crank if you're using them, and they don't offer a huge advantage over plain bearings in most engine applications.

the old Auto Union V16 engines used roller/ball bearings. but they were very complicated engines, the cranks alone had over 1,100 individually machined pieces.

ill put my 2c in even though its all been probably covered just in different words.

bore is where power is, stroke is where potential is. very basically. it was mention how you need some of one to make best use of the other, which is true. stroke actually doesnt really matter as far as engine durability goes, as long as you can increase the size of the rod along with the stroke. also high revving engines need to have efficient rotating assemblies in the block in order to make high end torque, and need high flowing heads in order to make high end hp. but one thing about hp/tq is that, say if you make 200ftlb at 4000 rpm with your car, and do some mods, dyno it, and now you make 200ftlb at 5000 rpm, you will see a big increase in hp. the lower the torque peak is in the rev range, the lower the overall hp will be. some forced induction engines get partially around this fact with things like twin turbocharging, but for the most part it holds true. also, im not sure why diesel engines were included in this, they have low rev limits and huge torque figures for different reasons than gas engines.

yeah them little 283's are rockets, but anyways, were you saying that a stock bottom end 350 with no mods can hit over 8000 rpm? or is that with h-beam rods, forged cross-drilled crank, and forged pistons?


i think what you're asking is, are the bottom end components on a dead stock 350 (which one?) capable of reving to 8K, but then the question would be very theoretical, since the valve train wouldn't make it there anyway.

I heard once (and I have no reference to this) that stock 350's were TESTED to 7200 RPM's. IE, chevy feels thats a safe RPM for them. I feel confident saying that they could live at that RPM, but for how long, I wouldn't take a guess at. I would assume anyone that is really interested in making a 350 spin to 8K RPM's, and taking advantage of those RPM's, would be willing to spend a little money on the bottom end. Because they'll surely be spending money on heads and valve train!