any advice on a combo i should do to acomplish this???

start with a bare block (a 400 SBC mabey) and destroke it with what size crank??? any part numbers from summit of anything? i am lookig around a $5,000 buget...

as you can tell i don't know much about this but i want a motor that has a very high power band, a road race motor for the street...


THANK YOU, Tim

 

Have a 400 block built with a 3.75" crank, perfect combo.

 

He said he wanted to de-stroke it...3.75 is the stock stroke for a 400 block

dropping a 350 crank would give you that combo but from everything Ive seen a 383 usually is an all around better engine to have. Big bore short stroke combo`s are usually high winding fast accerlerating engines...bush/nascar teams use big bore short stroke combos for short tracks. There is a guy around here that put together a 327 crank with a 400 hundred block to make a 358 and it ran fairly well, kept up with my 454 till mid seconed gear, it really lifted the nose of his 86 camaro High..pretty good for having stock intake and exhaust manifolds

 

Ummm, yeah, not to be cruel, but....

It's the wrong thing to do. People somehow get this idea that you can make a motor produce more power by taking out cubic inches. It's wrong. You produce less power by reducing the inches. In other words, you will be spending extra money in doing extra machine work and/or buying special parts, just to make yourself slower. Using your own money as the ammunition to shoot yourself in the shorts.

Where that works, is in situations where you are CUBIC INCH LIMITED. Busch/NASCAR being a great example. They run a 358 inch limit; there's no rules on how you get to that limit, just, that's all you can run. Their combo is 4.155 x 3.31. But I guarantee you, if the rules changed, and they could run 3.32", or 3.48", or 3.75", or even 4" stroke with the same bore, every single one of them would have that longer stroke in there by next weekend. They don't run that short stroke because it's faster than a long stroke; they run it because in their specific application, which is continuous high RPMs, the big-bore/short-stroke combo goes faster than a smaller bore and longer stroke. The same is not necessarily true on the street, where you are NOT CUBIC INCH LIMITED. The bigger motor will spank the smaller motor, dollar for dollar, every time, no questions asked, no further argument, no other possible outcome.

The way to get a motor to turn high RPMs, involves the cam, heads, valve train, etc. You're not going to create some instant high-RPM screamer just by shortening the stroke alone. That's only one tiny piece of the whole picture. A common misconception among people who have never done it.

To put it in perspective, imagine taking a 350 block, and boring it out .030". How much does that change the RPM capability? Now imagine boring it out .060". How much did that change it? Now imagine boring it out .125". How much do you think that's going to change it? Well, that's all you're proposing to do; is to build a .125"-over 350. Except that instead of upgrading a 350 block, you're downgrading a 400 block to get there. But the end result is identical: 4.125" bore x 3.48" stroke.

But, to answer your question, in order to make it easy for you to downgrade your motor and for everybody who did it right to hand your a$$ to you, the easiest and cheapest thing to do, is to use a stock 350 crank; stock 350 rods; stock 400 pistons (or the dimensional equivalents of those 3 stock components); and these right here http://store.summitracing.com/partde...?part=CCA-5620 That way, after you discover that it wasn't such a good idea after all, you won't have so much money tied up in it, and you can more easily and cheaply put the cubic inches back to get the missing power that you left out.

 

One such displacement limited competition: http://www.popularhotrodding.com/eng...ge/2004/rules/

The approaches of the chosen competitors: http://www.popularhotrodding.com/eng...0410phr_nifty/

Significant details: 410 cid max (410.1 cid is illegal), 92 octane unleaded, normally aspirated, small block-type, commercially-available parts. Dyno pulls scored by averaging and summing torque and horsepower between 2500-6500 RPMs.

The approaches being taken are interesting. Some subscribe to small bore/long stroke to improve torque while others prefer the large bore/short stroke to improve breathing.

The most interesting, I thought, was the use of "short" rods by some to reduce detonation with high compression and 92 octane fuel.

 

No kidding, but destroking and engine to make less CID is just stupid. Why would you ever want less power, unless rules dictated it?!

 

like others above have said, you don't have a limit for CID.

it seems like some people hear about some racing engines being short strokes and just assume it's some "trick."

there are 2 basic oversimplifying factors to making power, torque and RPM. Increase torque at any RPM, you have more power. Keep torque from falling off in higher RPMs, you have more power.

torque essentially comes down to how well the cylinders are filled, or volumetric efficiency. the mass of air you take in dictates how much fuel you can burn, how much pressure in the cylinder, and therefore how much torque you make. RPM is dependent on how strong/light/quality/expensive your components are.

people can talk about how neat torque is and misunderstand the difference between HP and TQ; TQ is how hard it pulls. HP is how long it can pull hard before you shift, and therefore how fast you will be.

it's in the definition: torque is just pound-feet.
HP is pound-feet per second .

since torque has nothing to do with time, it can't be the deciding factor in a rate, which also by definition is with respect to time .

Once you have the perfect breathing system and cams for the intention of your engine, your TQ is basically at the limit.

So when there is a displacement limit, and the cylinder filling is maximized, all you have left to go maximize is RPM; and if the valvetrain can handle it, it comes down to how much stress is on the crank/pistons/rods. So they run more bore and less stroke.

That is why in racing, with huge budgets for super-strong/light, high-RPM parts, they run more oversquare cylinders.

 

Not true.



True.

TQ is work and HP is power. HP is just the rate at which work from TQ per time is done.

 

I think there's been a great many discussions trying to figure out a lamen's term for torque and horsepower. The way that I like to think about it is torque how fast you can get your max MPH within a gear. Your HP determines your max speed. (This is lamens terms obviously, because suspension among with many other odds and end effect both these).

But in the sense that torque gets you to your top speed faster, is what makes our cars, stock, very fun to drive as a daily driver. We have what is it, 260tq, with only 170ish HP (some even less, like my Berlinetta). I love how I get thrown back in my seat going from a stop light at half throttle and am only going 30 (speed limit).

In that, I think that GM really achieved a great vehicle. It is practicle, they didnt make our cars to really go on an Auto-X, or Dragstrip, but a fun daily driver....

now, a 200HP 500ft/lbs of torque car would be a hell of a daily driver! That would be like running on nitrous all the time hah.

 

what did I get wrong?

I figure since a car's acceleration rate matches its torque curve, that's how hard it pulls.

I imagine a car with an impossible, flat torque curve (unless maybe it is an electric motor) for as high as RPM as you want to set. Say it has a non-CVT style, ordinary manual trans. A low-RPM limit car has to shift earlier, so it can't stay in it's faster accelerating lower gear as long compared to a high-RPM limit car. Since we assume a linear powerband, the high RPM setup is faster because it has more HP.

 

I think the main thing is "HP is how long it can pull hard before you shift". As stated, torque is work (force times distance), HP is the rate at which work is done (work divided by time).

Of course, the "right" engine in the wrong setup will not do the job properly. But, if each is optimized, a higher horsepower engine will move a car through the quarter quicker than a "torque monster", because it takes the same force to move the car through the quarter - it's the rate you do it at that makes the difference.

For the record: Only torque is measured. Horsepower is always calculated.

But, back to the original proposition: Removing cubes from an engine is not the right way to make more power. If you have a 400 that needs to be built/rebuilt for a particular go-faster purpose, you are not going to go faster by making it displace less. Regardless of how high you spin it.

(FWIW: A Mopar 440 has the exact same stroke as my 396. A fellow racer with such a 440 talks about his "big block torque" advantage - Huh? Why would he have more torque than me if the strokes are the same?)

 

Torque (ft-lbs) = cyl pressure (psi) x bore area (sq in) x ½ of the stroke (in) x convert from inches to feet

Note that if you multiply the 1st 2 terms, you come up with pressure (on the piston surface) in pounds, which when multiplied by the stroke and the inches<->feet conversion factors, gives you foot-pounds which is the unit of measurement of torque. Note also, that if you multiply the second pair of numbers, you come out with cubic inches (well, half of them), which means torque is proprtional to cubic inches and cylinder pressure alone; and therefore, it doesn't matter whether the stroke is long or short, as long as the inches are the same. That is, it doesn't matter whether you get your cubic inches from bore, or from stroke; assuming equal cyl pressure (= equal cyl fill, which is a function of the induction system, NOT the bore and stroke), torque remains the same.

The ONLY things that change as a result of leaving the CID constant and changing the 2 components of it, is the mechanical losses of the motor (inertia in accelerating and stopping the recirocating parts, and friction) and the "pumping" dynamics. Neither of those things will significantly alter the actual torque output of the motor; what they will alter slightly however, is the RPMs at which they occur. Within the range you might typically vary such things (such as the difference between a 377 and a 383) even that effect is worth only a couple of hundred RPMs; less than 300 in reality. The 2 motors make the same torque within a fraction of a percent, with the peak at a slightly higher RPM in the one than the other; and virtually the same peak HP, again at slightly different RPMs. In no case will either of them outperform an equivalent motor with both the bore and the stroke of the 400, instead of one dimension from the 400 and one from the 350. The 400 will outperform either hybrid.

 

Class dismissed.

 

the reason i said this was because i whant a motor with a really high power band and can rev ''to the moon''

like 7,500RPM and more

 

get a honda

 

ok comming from a guy who drives a car taht revs to the moon. rotary from a rx7.

lets talk about some of the disadvantages
first the launch

IT SUCK!!! if I dump the clutch at 3500 I get a chirp followed by a bog at 5000rpms I just sit and spin. to launch my car right it is more or less slipping the clutch
when you try to launch your car at high rpms you run into a nice little issue of shocking the tires with umpteen lbs of force spinning at umpteen thousand rpms. tires don't like sudden force like that placed against them. now with your v8 you are able to launch a lot lowoer in the rpm range which tends to make it easier for the tires and and a little more forgiving on the bog part being you have power down low

as far as once your moving sure straight line is fairly easy to deal with....
but you need to get gears to match the rpms. you also need to watch your gear spread cause with high wind cars genearlly you can get a nice power band out of them.... but if you hit the early part it drops off fairly bad unlike a most motors.

as far as road racing goes. picking your gears is a little more iffy. and can spend lots of time shifting


all in all they are not very forgiving motors.
sure you can make them fast but the forgiveness isn't there you mess up your done race over