Cascade Crew

[blue89]

What about the 383 fits so well with the SBC? Seems like that is the thing to do. I'm just curious as to if its something cool like bringing the port velocity up enough to take advantage of the heads or if its just the biggest you can go without resleeving the block.

[Dewey316]

Can you post the data you are speaking of? I saw your other post, and was going to ask about it.

There is a ton of data out there about bore/stroke ratio's, and the 383 doesn't really fit what is "ideal". I think the key is, at the level most people are building engines at, they are not building combos designed for the "most horsepower", if they were you would see combo's more like the F1 cars. We build engines that will go 100k miles, and make power to move heavy cars. A 383 fits that bill well, because when balanced right it will spin up like a 350, so long as you use descent rods, but the extra cubes and leverage on the crank gives it a good torque curve.

--John

[blue89]

I don't follow when you say "spin up like a 350". And what head flow numbers do you have to work with? Seems like the larger displacement will only choke the heads sooner. While I agree that "there is no replacement for displacement", I also see different chamber fills and swirling possible when you increase bore/stroke. Being that I'm all for over-square engines.

cuin stroke bore
350 3.48 4.00
383 3.75 4.00
400 3.75 4.15

Does this seem right? Looks like they are all over-square to me. I guess you would also need to see how far you can bore/stroke a standard GenI 350. Then the next thing to consider is the hp number and how fast you need to rev it to get the number you want?

Is the 383 just popular because its the biggest you can go with a 4.00" bore? To go to 400 looks like it would need a .150 overbore. Can you even do this on a 350 block?

[Dewey316]

My question is, what details did they use to determine that the 383 more more HP/CI than other combos

The shorter stroke motors with everything else being equal, can spin higher (IE same rods, the rods would fail with the longer stroke first, as there is more piston speed at the same engine speed, if the pistons weight the same, then the tensile loads on the rods will be higher). Since HP is a funtion of torque and rpm, even if you lose torque from the shorter stroke, you can make up HP from the revs.

I like the 383 combo, don't get me wrong, but I would GUESS that if you wanted to, a 327 or 302 could yeild a higher peak HP per CI number. That is why I was asking if you had a link to more information on the article you are refrencing. I would like to read it, and see what method they used for this.

[blue89]

NP, let me search around and see what it said exactly. I think it was on TGO.

With all that said, what crank has the shortest stroke? I would agree that the smaller displacement could produce more HP/cu.in. simply because you can rev them higher without mechanical failure. I may have miss-quoted someone, I'll look into it. But since we're on the subject......

[Dewey316]

Not that simple.

There is more there, like piston dwell and so on, a long rod short stroke combo, may be more detonation resistant because of this, letting you run a hair more compression. There are also other factors, such as the cams needed to make them run at the RPM given. along with the needed valvetrain. The stroke is just part of the equation. Torque is a function of force, so your bore size has an effect, your stoke also has an effect as that is what the force acts on, to create the rotating force (torque). It gets really complex.

But, here is the thing, look at the forms of racing that create the highest HP/ci, they are all small stoke, high RPM motors.

--John

[83Z28BlackBetty]

so if the best is to have gobs of torque up at a high RPM, a big block v8 that makes tons of torque but can only rev to 4500 RPMs will not be any faster than a small block with less torque but can rev to 6000. (a billion other factors involved i know, but for simplicity's sake)?

[Dewey316]

It depends on the shift points, the key is to keep the motor where it makes power. 500hp is 500hp, the car doesn't know that it is coming from a big block or small block. Look at a diesel pickup, they may make 800lbs/ft of torque, but only 200hp. That is great for pulling a stump out, but doesn't get you down the dragstrip as fast as a car with 400lb/ft of torque, that makes 500hp. There are advantages to both, this is the fun and frustration in building cars, there are a lot of options, and lot of trade offs, there isn't always "better", there is often which one works for what I am doing, and what I want.

--John

[83Z28BlackBetty]

ok so i'm starting to understand all this. sorry i'm such a noob

but 2 engines that produce the same torque, but one's curve revs higher, it will produce more HP and can therefore take advantage of a higher gear ratio?

[Dewey316]

HP is (Tq*RPM)/5252, so in this example, if one car makes 300tq at 3000rpm, and one makes 300tq at 5000rpm, you get the follow.

300*3000/5252 = 171hp

and

300*5000/5252 = 285hp

That is horsepower on a BASIC level. There is more to it, see the 383 will make more torque than a 350, with everythign else being equal, since it has the longer stroke, it can create more torque from the same force. YOu see torque is a funciton of force and distance (t=f*d). So it gets a little complicated because it is not a straight comparison. Since with the same explosion in the cumbustion chamber, the longer stroke makes more torque. There is so much involved here, because the longer stroke will effect more than just the toque output, and you can't really compare two engines, because there are so many diffrent factors.

So, the comparison would be more like.

383:

400tq @ 3800 rpm

400*3800/5252 = 289hp

350:

350tq @ 4400 rpm

350*4600/5252 = 293hp

See, in the real world, it gets to be a lot closer comparison.

--John

[blue89]

Thats exactly right. 500hp is 500hp. So if you want to run x.xx time in the 1/4 with xxxx weight car, you need xxx hp. Period. And its easy to get hp by displacement alone. Or a turbo. But to go fast with a small motor costs a lot simply because you need to rev the crap out of it to produce the same hp you could have netted from the large engine.

Jake, sounds like you read the hp vs tq site? Good info in there. Kinda puts it all in perspective huh?

But still, since the head design of the sbc is still 2-valve and we need to actually have it run at 2000rpm, the 383 crank works great and is a better option than overboring the 350 block to accomodate the 400 4.15 pistons? 33 more cu.in. should produce almost that amount of tq, so I could see it being common, cheap, and easy (ish).

[blue89]

yes, but still, 300hp is 300hp. And since HP is a function of time (rpm) and force (tq) it is a measure of work. So it is important to give tq with an RPM, but hp stands good by itself. Did I do a post on this already? Maybe I just thought it through on my own.

What I concluded is that wheel toque is what moves you. Its not uncommon to produce 2000ft.lbs of wheel torque. So if you have two cars going 10mph with one producing 100hp at 3000rpm (1) and the other 100hp at 6000rpm (2). You can obviously see that the (2) has to have a final drive ratio of twice of car (1). But since the engine revs twice as high, it has half the toque at that RPM. This all equals to the same wheel torque since one produces twice the torque as the other (1), but the other has twice the final drive advantage (2). But to keep up, the car with a higher hp RPM (2) needs twice the useable power band to keep shifting at the same point.



So it all comes down to HP.

Dew, if what you posted is true, the 383 still won't be as fast as the 350, but the 383 will still "feel" faster cause it would launch harder if all you did was change displacement with all else equal? I have no real world experience with sbc performance.

[83Z28BlackBetty]

but because HP is a funciton of torque and Revs, a high reving torque monster with good gearing = lots of HP = fast?

[blue89]

Of course.

Dew, because a stroker crank has more mechanical advantage on the crank, would you see greater than +33ftlbs of gain?

[Dewey316]

[blue89]

supprisingly, I found it:

The reason why the 383 makes so much power is that the motor is 10% bigger than a 350(or 10% more hp),and the fact that it makes peak power at 5500rpm,instead of 4500rpm like a stock TPI does(22% more power).If a stock 350 makes 250hp,and multiply it times 10% and again by 22%,you get only only 335hp,those 383 make 430hp.How?by making the motor more efficient,you make more torque(and therefore hp) at all rpm.The stock TPI motor is about 80% efficient on its best day,the 383 pushes 95%-which is the 95hp difference.I don't expect too many novices to understand efficiency,but the following areas I'll cover will help you get the ideas.

What do you make of this? What I meant by the +33 is you generally get 1ft.lb torque per cu.in. displacement for a 2 valve motor. Just a rule of thumb. 4 valves are 100hp/L. Not true for all cases, but most of them.

[Dewey316]

He is comparing a lignenfelter built 383 to a stock 350 TPI, its a rubish comparison. They have nothing in common. Get the right cam, heads and intake for a 350, and you can make those numbers.

The choice part is....

[blue89]

Well, I guess that killed that huh? Bogus write-up. Lame. Myth was definetly BUSTED