What exactly does increasing the stroke of your engine do?

[Gravitom]

I understand the basics of this. You put in a bigger cranks to the engine has a longer stroke, therefore a larger displacement. But what does it do to your hp/torque ratio as well as your redline. Since the piston is moving more I assume you would get a lot more torque but minimal horsepower. And since F1 engines have such a short stroke so they can rev to 18k, I would think a longer stroke would lower your redline.

[SaabJohan]

Yes, longer stroke means more torque. But you can't use so high revs as with a short stroke engine. F1 engines have around 42-45 mm stroke and 92-96 mm bore.

[911GT2]

It increases displacement as well as compression ratio, usually wreaking hell on your related components unless they're well reinforced and increases torque and smooths out your powerband.

F1 engines use all bore to increase high rev horsepower, which is why they require 7-speed gearboxes, to keep them in the 12K+ rev range at all speeds.

[ivymike1031]

911gt2, perhaps you could describe how increasing the stroke affects the compression ratio? Will it always have that effect? What are the limits to which you can increase stroke? What actions can you take (if any) to keep CR the same when you increase stroke?

[Polygon]

Well, if you increase the stroke the piston is moving farther up the cylinder thus higher compression. To counter this you can use a spacer between the block and head (not recommended). The best thing would be to use lower compression pistons. If you increase the stroke without doing anything to counteract it, you will always increase the CR.

[911GT2]

Fuck, i just wrote a really long response but the fucking servers couldn't handle my fucking request. Anyways, to rewrite it...

Stroking increases the CR because you don't lower the starting point (sometimes in some stroker kits, but not by as much as you increase the endpoint) and increase the ending point, the air trapped inside the cylinder is forced into a smaller area, compressing it more.

Yes, it will always have that effect unless engine components cannot withstand the compression increase or until the air is at its maximum pressure (which will just never happen). Otherwise, it'll always happen.

The only limits are the physical limits to which air can be compressed or more likely, the limit at which your engine components will go through severe damage. Engines have been known to blow under extreme stroker kits, so that's not a non-issue.

And aside from modifying your engine by lengthening the cylinder itself (note: EXTREMELY UNLIKELY) or by making the starting point lower (which voids the usefulness of your stroker kit which makes this option also unlikely), there's no way to counteract the effect. I guess there's also injecting air that is less dense, but that would also require a ridiculously expensive system which is totally useless.

And thanks polygon, but i thought it would be best to explain to mike myself. Your help is appreciated. And mike, i know you;re a smart guy as far as engines go, why are you asking me these things that you obviously know?

Are you testing me? And if you are, please correct me if i'm wrong. I'm attempting to take this path in education, so anyone willing to teach me is welcome

[ivymike1031]

Well, I just wanted to see how much you already knew about it before I started blabbering on.

Here's my take on it:

* Changing the stroke directly affects how far the piston moves (the stroke). When stroke increases, if nothing else is changed, the piston pin will start lower in the cylinder and end up higher. In most engines, changing the stroke will have exactly the same affect on the TDC position that it has on the BDC position, ie top gets 2mm higher, bottom gets 2mm lower. (To calculate the bottom and top positions of the piston pin relative to the cylinder, take the conrod length and either add or subtract 1/2 of the stroke, depending on whether you want top or bottom)

* Changing the stroke directly affects the engine displacement (stroke * piston area).

* If you change the stroke by more than a little amount, you'll have to change something else too. Otherwise, the piston will run into the cylinder head (there's typically very little clearance). Common things to change are the length of the conrod and the compression height of the piston. There are, of course, limits to how far you can change those (you don't want too much conrod angularity, and there are obvious limits to how high you can put the piston pin on the piston). You can also get a cylinder block with a higher deck, but you're basically talking about a new engine at that point, so let's stop here. Note that noe of the above will change engine displacement.

* If you change the stroke and implement one of the above methods so that you still have some TDC clearance, you'll probably end up with a higher compression ratio than you'd like. You can do a few things to offset this as well, such as using a thicker head gasket, or using a piston with a bit of a dish in it. If your pistons already had dishes or a bowl, you'd make them bigger. Note (again) that none of the above will affect engine displacement.

I probably forgot to mention something, so feel free to ask.

[911GT2]

That's about right. What i said plus your take on it (a little more scientific though).

[V.S.]

Originally posted by 911GT2
I guess there's also injecting air that is less dense, but that would also require a ridiculously expensive system which is totally useless.

It would still be totally useless, but couldn't you just obstruct the air intake enough to decrease the pressure of the air going into the cylinder?

[911GT2]

It wouldn't decrease the pressure of the air but lessen the amount going into it, which is the principle that rally cars as well as NASCAR racers use to limit power.

[ivymike1031]

911, you'd better check your figures. You're pretty much stuck with a pressure once you know density and temperature (and I assume you're not planning to heat the intake).

[911GT2]

Yeah, i'd thought of that after i posted but never bothered to edit. It would result in overall less density, but would not take in less dense air. Just less normally dense air.

[Chris]

Originally posted by 911GT2
Fuck, i just wrote a really long response but the fucking servers couldn't handle my fucking request. Anyways, to rewrite it...

Ctrl-C, my friend, Ctrl-C

[smenders2@att.net]

@ Gravitom. Increasing torque will always increase hp because hp is a mathematical calculation of torque x rpm divided by 5252 (torque and horsepower will always be the same at 5252rpm).

[Dave B.]

Without going into a lot of detail in this post; but when changing the stroke of an engine, the relationship between crank throw, connecting rod length and piston/pin design all impact the process as well as the end compression ratio.

A good read on the subject is:
http://www.hotrod.com/how-to/engine/...ke-any-engine/
A bit heavy on tech, but it does answer most of the questions someone new to the process would usually ask.

DB