cylinders vs rpm

Topic: cylinders vs rpm (Read 2563 times) previous topic - next topic

cylinders vs rpm

December 02, 2004, 12:21:30 AM

Maybe I've never actually thought about this, but I see all the time people talking down upon larger engines just because they generally don't spin at as high of rpm's as smaller imported engines.

Why this comes up now is another thread on turbochargers. Isn't a decent v8 capable of burning through just as much if not more oxygen as a smaller engine, all while keeping the rpm's down low? I haven't put thought into how numbers of cylinders or displacement amount affects the rpm figure of the engine, but I'm guessing those people who love imports for their "high rpm's" have nothing over the v8's in airflow (and in turn, amount of exhaust gases).

Weight on the other hand

Re: cylinders vs rpm

Reply #1 – December 02, 2004, 12:29:36 AM

There is nothing that says a big v8 can't run high rpm.
They just can't run high rpm from the factory. If you put money into them and buy all the light weight rotating parts you can run just as high if not higher rpm.

But that is just my opinion.

Re: cylinders vs rpm

Reply #2 – December 02, 2004, 12:33:42 AM

well yeah, but typically, many stock imports are able to spin faster than the larger domestic stock engines

I just don't see how rpm equals higher exhaust pressure when comparing two completely different engines

Re: cylinders vs rpm

Reply #3 – December 02, 2004, 12:36:38 AM

Look at the size of the turbos on an import campared to the size of a turbo on say a Grand National or say a super duty ford.

Re: cylinders vs rpm

Reply #4 – December 02, 2004, 12:54:17 AM

Ford smallblock 289's are known to spin up to around 12 grand with some work done to them.

Re: cylinders vs rpm

Reply #5 – December 02, 2004, 12:54:56 AM

I think it's more about power:weight ratio rather than rpms :shakehead silly ricers, they probably brought up power per cubic inch too

Re: cylinders vs rpm

Reply #6 – December 02, 2004, 01:23:32 AM

I'm just asking, or rather asking for people's thoughts on, if rpm means anything when comparing the two different engines. With how little I know about how engines compare and how they're engineered, I wouldn't be surprised if what I'm thinking is even close to correct. For all I know, you could have an engine that is terrible at spinning the crankshaft efficiently, yet the same amount of mixture could be burned. Extremely low rpm yet a ton of exhaust flow.

So what is it based on? Efficiency? Everything? :shakehead

Re: cylinders vs rpm

Reply #7 – December 02, 2004, 08:19:45 AM

Usually a longer stroke means a lower RPM (and more torque), and a shorter stroke means a higher RPM (and less torque). Number of cylinders has very little to do with it - just look at the V10 Formula 1 or Indy engines that spin at 15-20k RPM. The small puppiesanese engines have to spin at high RPM's to make any power because most of them, including VTEC engines, have very little low end torque. High RPM has little to do with technology, it is more about making small engines produce acceptable power levels. For everyday driving, IMO, the large displacement/long stroke/low RPM approach is better because the car doesn't feel "wound out" at highway speeds. Just take an econopoopster out on the highway and listen to the hornet's nest buzzing away at 3000RPM at highway speeds and tell me you'd like to listen to that for hours on end. Diesels, which typically have a very long stroke, usually have a very low redline and a whole bunch of torque.

Horsepower sells cars. Torque moves 'em. To put this into perspective, just look at our "underpowered" 150-horse 5.0 SO cars. Sure, they're dogs, but they are acceptable for everyday use. Take something like a 150-horse 2.3 Zetec engine and put it in one of these boats though, and see how fun it is to drive. Same horsepower, but the Zetec would barely be able to move the car for lack of low end torque.

Re: cylinders vs rpm

Reply #8 – December 02, 2004, 02:02:31 PM

well yeah, but even when, I don't see how any of it has to do with exhaust flow numbers. You can't (well the people that do) compare two completely different engines and say the ones spinning upwards of 8-10k rpm's will work better with a turbo than a larger engine with, yes, a longer stroke and/or higher displacement per cylinder. Thats just what I'm trying to figure out. Is there any single thing that affects an engine's turbo capacilities (not taking into account weaknesses of the internals). RPM generally means squat, does it not(once again, only when comparing different engines)?

Re: cylinders vs rpm

Reply #9 – December 02, 2004, 07:09:53 PM

Compression ratio would go a long way toward limiting your turbo capabilities. If your CR is too high you can't run as much boost.

A high RPM engine may work better on the race track, but on the street the larger engine will work better. A boosted, high-RPM sewing machine motor will have little off-the-line power (turbo lag), while a large displacement V8 will have power as soon as you mash the gas.

Exhaust flow numbers generally affect torque by moving the torque peak around. When you add a set of headers to an engine you are improving the exhaust flow, and thus you are improving power, BUT you are moving the power peak higher on the tach. If you go too large the car will actually feel slower because you lose that low end grunt. The car is not making less power, it is just making it at higher RPM's. Just look at the 87 and 88 5.0 T-Bird. In 87 it produced 150 horses and 270 tq @ 2000RPM. In 88, with no changes other than (fake) dual exhaust, the HP went up to 155 but the torque went down to 265 @ 2200 RPM. The torque peak went up by 200 RPM. The number went down by 5 lb-ft as well, but this is more due to the fact that the engine can't breathe well enough to use the extra flow the duals provided. Backpressure is imortant - you do need a certain amount (this is why exhaust valves are smaller than intakes), and the key is to get the magic number when compared to intake airflow.

Remember, also, that HP is a mathematical calculation based on torque. You can not measure horsepower directly, you must measure torque, then figure out the horsepower from there.

Re: cylinders vs rpm

Reply #10 – December 06, 2004, 12:37:29 AM

Exactly, the little engines make very little toque at normal driving RPMs. I experienced this w/ my winter car, geo storm, made by Isuzu, w/ a dohc 1.6L. It had a peak hp of like 130, but that was at 7600 RPM. Which you don't get to see that often, and it had a hard time getting itself moving. It didn't even start making power until around 4000. To get the car into the where it will actually make any power they have to be geared down. Which then causes it to turn high RPMs. The high RPMs are great for racing, but doesn't work too well on the street. You really can't gear you car and just cruise at 5000 rpms all day to have power when you want to mash the gas. And you can only have so much overdrive in a transmision. More RPMs are also more wear on parts, and more gas. Which kinda defeats the purpose of a 4 cylider, economy. As far as turbos go, I dont really think a 4 cyl. turning high RPMs would be better than a v8 turning lower RPMs. Sounds like Ricer logic, just thinkin because there motors are spinnin faster then there car must be faster, or be able to spool a turbo faster. A v8 is able to flow the same amount of exhaust at a lower RPM. The turbo doesn't care about the size of engine or # of cylinder, only the amount of air flowing through it. Also, with a v8 you get off turbo power before the turbo spools, which would really help on the street. Also givin a 4 and 8 cylinder motors with the same turbo a v8 should spool it faster. More exhaust flow as soon as you hit the gas from a roll. While the 4 will have to climb up the RPMs until it finally makes boost. I saw a video of a turbo civic. It was a fast car, but he'd nail the gas and it accelerate pretty slow till it hit like 5000 RPM then break the tires loose and pull hard to redline, he'd shift then it'd repeat. Go over to turbomustangs.com, most those cars reach full boost by around 3500. Also there are many cars runnin 500 rwhp on the stock HO short block, that just amazes me. Ok, that said, I'm no expert, this is just my logic and opinion.

Re: cylinders vs rpm

Reply #11 – December 06, 2004, 03:10:07 AM

Quote from: xtbird302x

A v8 is able to flow the same amount of exhaust at a lower RPM.

thats what a figure, but I'm sure you could find an 8 cylinder SOMEWHERE out there that'd run a lot poorer than a 4, even at low rpm's. I have no idea how engineers build the things they do, it'd be a ton of math to figure everything out...but anyways, I'm just curious at what speed does say a 302 or 351 need to spin in order to burn through the same amount of airflow as a smaller 4 or 6, although the numbers would still be different when using different engine parts.

*sigh* I give up. I don't think there's a single accurate way to compare apples to apples when talking engines. There's always an orange

Re: cylinders vs rpm

Reply #12 – December 06, 2004, 08:46:32 AM

There is no accurate way, no, but there is a general rule: A 5.0 engine will move twice as much air as a 2.5 engine with similar head design at any given RPM. F'rinstance, a 2.5 HSC engine (pushrod, two valves per cylinder) will move half as much air as a 5.0 at 3000 RPM. The term "displacement" refers to how much air it takes to fill each cylinder multiplied by the number of cylinders. The more displacement, the more air will move through the engine, all other factors being equal.

Of course it isn't that simple - things such as camshaft lift & duration, number of valves per cylinder, valve size, port design, intake design, throttle body size, compression ratio and engine redline all play important roles. This is why slightly different engines of the same displacement can have entirely different power characteristics. Just look at the HO vs SO versions of the 5.0. With only slightly different heads, camshaft and intakes the HO puts out 50% more power than the SO. Just by adding a set of aftermarket heads, a cam and an intake you can nearly double the power of a standard HO engine. The aftermarket heads are very similar in design to the factory HO heads (and even the base SO heads), but slight differences produce big power. As you can easily see, it ain't all about displacement.

It's not all about number of cylinders, either. A 2.5 liter V6 will usually be torquier than a 2.5 4-cylinder, but it may not necessarily produce more peak power. At any given RPM the V6 will have 50% more power strokes per minute than the four cylinder, which will produce more torque. On the other hand, a V6 will also have more internal friction and pumping losses, limiting its high RPM capabilities (and thus limiting peak power output). Then again, a larger bore means that the air/fuel mixture may not burn as efficiently, so the 4-cyl loses some ground here. The 4-cyl will be lighter, so the car should accelerate better, but the added torque of the V6 should give it the extra kick.

This all bsaically adds up to one thing: displacement and number of cylinders are not enough information to come to any conclusions. Engine design plays just as much, if not moreso, of a role.