S2000man01

iTrader: (1)

I posted this in another thread, but thought it would be a nice lesson for those who linger in our tech forum. Feel free to ask questions or add anything you'd like!


Alright, here's a short lesson in the mathematics and all that is involved in de-stroking an engine.

When you destroke an engine using the method of replacing the crank shaft, you are shortening your stroke. Stroke is the distance a piston travels from top dead center to bottom dead center (TDC and BDC). The bore is the distance from edge to edge, also known as diameter, of the piston head. The crankshaft spins in a circle, as do the rod connecting points on the crank shaft. These connecting points will make a diameter (when spinning) equal to that of the stroke. So, getting a smaller crank, makes a smaller diameter, and makes the pistons move up and down over less ground, thus de-stroking or shortening the stroke of the engine. This, in turn, will change the displacement of the car's engine.

A stock D17 has a 94.4mm stroke, and 75mm bore. The formula for calculating displacement is: N * stroke(mm) * bore(mm) * bore(mm) * .0007854 = displacement in CC's. (N is number of cylinders)

So for a stock D17: 4 * 94.4 * 75 * 75 * .007854 = 1668.1896 or 1668CC's.

When you destroke the D17 to a 1.6 liter engine, you are changing the crank, and making the stroke shorter. The connecting points on a 1.6 liter crank make a diameter circle when spun, of 90mm. So they also move the pistons 90mm from TDC to BDC.

So for a 1.6liter crank: 4 * 90 * 75 * 75 * .0007854 = 1590.435 or 1590CC's.

Now, here's where you come into a problem.....

When you change the crank, you are lowering the height at which the pistons sit in the combustion chamber. (the pistons are lowered approximately 2.2mm, or half the change in diameter from one crank to the other) What this does is lower the compression your car is running quite substantially. There are 3 things you can do to solve this.

A. you can buy longer rods, which will restore the pistons to correct height. (you'd need rods that are 2.2mm longer than stock)

B. You can buy taller pistons from rod connecting point to top edge of the piston. (again, must be 2.2mm taller than stock)

C. You can shorten the deck by 2.2mm, thus reducing the extra volume gained in the combustion chamber.

There is de-stroking of an engine in a nutshell.

Ike

iTrader: (2)

You're the man

4drcivic2k1

iTrader: (2)

or you have effectively lowered the compression ratio and can run a turbo etc

fraga

iTrader: (3)

Wow, man that's ****in awesome. And who says there's no more useful information on this site anymore.

AFlyer

so why would you want to destroke your engine, making the displacement smaller? other than the fact that it's a safer (maybe?) way to reduce compression?

S2000man01

iTrader: (1)

de-stroking an engine is not a good way to lower compression, since you would have to go through the trouble of measuring out what your new compression would be. (it's a pain, but I will explain how it's done in a future thread... though it's also something a shop should/has to do. [IMG]i/expressions/face-icon-small-smile.gif[/IMG] )

de-stroking is a good way of making it so you can rev higher (shorter stroke is more stable at higher rpm).. that's one benefit off the top of my head.

2ndScratch

its not hard to figure out the difference in compression. say you had a 2.0L engine(not in honda's a iknow but it makes math easier), that would make it .5L per cylinder, lets say you have a 50cc combustion chamber. when you take the .5L,or 500cc and have the piston smash it into 50cc you will have 10:1 compression. so if you could destroke it to a 1.6L(probably not very likely but again, easy math) that would give you .4L per cylinder and when the piston compress then 400cc into 50cc you will get 8:1 compression. i don't know if they do it for imports but companies like fel-pro and mr. gasket make thinker gaskets which would raise the head giving a bigger compression chamber. also, with buying a smaller crank it will allow you to rev higher because there is less travel and it keeps everything a little "tighter".

JDog2001CivicEX

iTrader: (1)

Are you posting this in rebutle to black99vtec's earlier post where he said how he de-stroked his engine?

Joe

S2000man01

iTrader: (1)

The S2000 has a 2.0liter and that's a honda. [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

and yes, this was response to black99vtec, but i posted that full response in his thread.

by the way.... Let me show you how drastically you are changing compression with that mere 2.2mm.....

The compression is figured out by volume of the cylinder before compression over volume of the cylinder after compression. (the cylinder is the combustion chamber) Let's say that you have a 550cc combustion chamber. Your volumetric displacement is 500cc (meaning you have .5 liters or 500cc displacement in each cylinder). That leaves you with 50cc once compression is completed. Soooo.... 550cc/50cc means 11:1 compression.

Now let's change that by adding that 2.2mm drop. That means that volume is added to BOTH numbers since the entire piston dropped 2.2mm. Soooo..... what is the displacement of 2.2mm in a car with a 90mm stroke and 75mm bore? 1 * 75 * 75 * 2.2 * .0007854 = about 10cc's. So now you add that extra 10cc's to each number.... this gives you 560cc/60cc. That equals 9.3:1 compression.

DAMN!!!! that tiny little 2.2mm that was lost from de-stroking dropped compression almost 2 whole points!!! Now you see why it's so important to make up for that 2.2mm by either getting longer rods, taller pistons, or a shortened deck.

coolness5545

iTrader: (1)

ive heard of companies that make thinner gaskets, so as to reduce to amount of space in the combustion chamber. i think this would be the easiest and cheapest way of doing this project, but unsure as to how effective it is.

chris

1drunkmunkyindatrunk

Wow, what a coincidence. I've been doing calculations in my head the whole day, and all of it dealt with this. I forget the calculations now, but I think I have a bit of an insight to why people would destroke.

Feel free to make corrections and replies. I'm constantly learning, and here's what I think:
-Let's consider a destroked engine, with other variables being the same, if possible. A destroked engine directly lowers the compression ratio. Lowering the compression ratio will lower the overall torque of the engine per displacement (that is, the area of bore multiplied by stroke). Why this would be done, is like what most of the other peoples said: A higher rev and an easier forced induction (consider the octane requirements too). Coincidentally, a higher rev directly increases power (horsepower). Add to that the capability of a turbo or whatever, and you've got some amazing power potential.
-My prediction would be that it would lower torque over the whole curve, but with a higher redline (revs). This would make the powerband longer, and potentially peak higher, but the slope of the power curve will not be as steep.
-From my understanding, it would give less performance at low to midrange revs, but would be better at high revs.
-Add to this kind of curve the boost of a turbo. I think turbo increases the torque constantly as revs get higher. This means that power would increase at an exponential rate as revs increase. Therefore that higher redline would get an even higher peak torque.

So I think destroking would only help if you're preparing for forced induction.
If absolutely none of this makes any sense whatsoever, please put me in my place, but if you do, please give an answer I can learn from.

Stone

Well I guess the power increases with RPMs, but you have to match the turbo to the engines owerband, and also where you want the power.

bigger turbo = slower spoolup + more power

smaller turbo = fast spool + less power

so the bigger turbo will take time to produce full boost, but will in the end produce more power in the top end.

the smaller turbo will spool up qucker, but the amount of power will be less in the upper RPM range.

So say you had a destroked 2.0, down to 1.6

The engine would rev higher, faster, but the powerband will be moved higher also.
So you throw a turbo in there, and then I assume the turbo would be spooled up quicker, because of the faster rise in RPM.

BUT, what I was thinking about was, if you destroke the engine, it is moving less air at the same RPM, so would it move so much less that the turbo isnt actually being spooled as fast, or would the increased rate of RPM rise offset the fact that less air is being moved(at the same RPM)

I am kinda picturing a graph, where the airflow numbers of the two engines intersect, and at that point one engine is producing more power.

The question I would like answered, is would higher RPMs(1.6L) with the same turbo as the non-destroked engine ultimatly produce more power, since the number of RPMs would (i guess?)pack more airflow "time" than the 2.0L would at less RPMs. Or would it equalize?

Or does the old saying "replacement for displacment" hold?

Keep in mind I'm am picturing two motors, with the same bore, just a different stroke.

I'm not really thinking hard on this, just letting out some stray thoughts, so if anyone can correct me, please do so.

Its part of learning.
peace

GLi4dr

Higher RPMS were mentioned. How do you calculate your new redline?
When you jump into your stk Civic its there in front of you, but when you finish building up that dream motor youve been working on, how do you know when enough is enough?

ImportRacer2123

iTrader: (13)

If you destroke a d17 to a 1.6 wouldn't the 1.6 civic parts work for our car? Say a Greddy Turbo? And all the other internals?

dKj

no, they're still two different engines