Stroker physics - where the differences are?

[Moparmal]

I read a lot of comments on the character of SB strokers on this and other boards from pretty knowledgable people - but I don't always completely understand why SB strokers behave the way they do...

Im hoping someone can deal with the following questions/comments and it might give me (and others) a better understanding of how they are different from the Standard Stroke engine.

1st/ -

The piston moves away from the valve more quickly

Why?, and
What effect does this have on induction ie Do we need bigger carbs? Do we need bigger plenums?, do we need bigger valves? than as for the std stroke engine?

2nd -

Strokers really soak up the duration

Again - Why? Also,

What causes a choppy idle and why does a stroker smooth it out?

3rd - Ignition timing -

You need less total advance for a stroker

.

Why?

I hope these questions don't require too much time to answer - but Im sure it'd be useful info to know.

 

[MopaR&D]

In a stroker engine, the stroke of the crankshaft is increased while the length of the connecting rod stays the same. Because of this, the connecting rod makes a greater angle with the cylinder bore axis at every point except TDC and BDC. This, in turn, moves the piston away from BDC and TDC more quickly.

Strokers can run more duration without having poor low-speed characteristics because the piston moves farther down the cylinder bore during each stroke. This creates a stronger and longer-lasting "suction" effect which increases the speed (and thus the inertia) of the gases flowing into and out of the cylinders. In shorter-stroke engines with long-duration/high-overlap cams, the gases don't have enough inertia at low engine speeds to properly empty and fill the cylinders between the exhaust and intake cycles. This "dilutes" the incoming air/fuel mixture at low engine speeds with already-burnt exhaust gases to the point where the mixture can't burn properly, causing misfiring at low speeds and the choppy/lopey sound that comes with a radical cam.

Stroker engines require less total ignition advance again because of the increased piston speed towards and away from TDC and BDC. This produces less "dwell" time where there is little/no movement of the piston at the top of its stroke and gives the air/fuel mixture less time to burn, which reduces the amount of ignition advance that needs to be run for optimal combustion efficiency.

 

[Moparmal]

Wow thanks for that! Makes perfect sense!

Cheers

 

[rumblefish360]

1st/ -
What effect does this have on induction ie Do we need bigger carbs? Do we need bigger plenums?, do we need bigger valves? than as for the std stroke engine?

Since the engine is now larger in it's total C.I.D. and because of the longer stroke, the draw on the carb through the intake is longer in duration time only shortened by the cams timing events. This longer draw time can create alot of velocity to your bennifit.

Bigger carb?

Maybe, it's a combo dependent thing. You could run a 600 cfm carb on a 400 inch stroker just as you could run a 600 cfm carb on a 400 low deck big block. But this is also combo dependent just as the rest of the question like plenums and valves. The real question to any engine build is what are you doing with the engine, it's intended purpose?

2nd -
Again - Why? Also,
What causes a choppy idle and why does a stroker smooth it out?

A cams timing events will give the choppy idle. The centerline of the cam and it's overlap (The amount) play a part in this. A larger engine can use more cam without adverse effects that the smaller engine gets with the more race like cams.

3rd - Ignition timing - .
Why?

You can reduce your ignition timing on a standard stoke engine with detonation resistant ideas built into the engine and coatings on the piston and heads.
For example;
A tight quench zero deck piston with a heat resistant coating on top, closed chambered head also coated in the chamber, with a wider centerline cam (110 - 112) and reduced overlap cam will require less timing and be more ping resistant.

If you take this combo and decide to take it to the track for race purpose's, you can take this tight queezze engine and give it a ratherly large cam and still run it on the street. A race cam with a C-line of 110 - 108 would work well.
A reduction in head cc for a very high ratio over 11.5-1 is possible and would like to see a cam with a 108 or less centerline. This would be a very powerful type of cam long in duration and high in lift needing a big stall and steep gears for max performance type build but still get away with less timing than an open headed combo with a as cast/forged piston un preped with ruff edges, wrong size gasket, untouched head chamber, etc........

 

[moper]

The only thing I can add is that the piston speed has both bonuses and caveats. On the up side, Kid mentioned the timing and "suction" deals. But there's also mechanical leverage from the longer crank throw. These all come into play at relatively low rpm. Meaning it's torque that is boosted by these bonuses. The caveats are: The piston speeds place a much higher stress (exponential) on the rods and pistons so materials and choices and rpm limits become more important. The ports reach choke faster because there is only so fast a port can flow before it runs into issues and stops flowing more. The added mechanical leverage also means there is more friction on the cylinder walls and piston skirts and more liklihood of the walls crackign under the added stress. The isues mostly affect the higher rpm levels where friction becomes a big deal and the larger support parts are required to maintain the power level. (Carb, heads, headers)

 

[rumblefish360]

Good addition Moper. Mal This is also why many racers tend to stick with short stroke engines. Long stroke engines are great dual purpose engines and do make excellent racers since there lower on the RPM stress. But pistons speed and extreme torque with the sidewall loading are the backlash to this long stroke combo. With a short stroke combo, it's tuff on the engine due to RPM's needed to get where your going which is a very high stress thing for an engine to do. Constantly live between 3500 - 7500 RPM's.
Things tend to bend, break or just simply fail.

 

[moper]

if a race engine is being put together.. meaning the package is being designed to race with none of the compromising for street miles... Then the best way to make more power is to rev it. Simply put, if a 340 makes 320hp or 40hp/cylinder/power stroke, the best way to get more power to the wheels is to get more strokes per minute... or more rpm. So more power gets to the tires in the same amount of time, thus accelerating it faster. A longer stroke focuses more power per power stroke but they get harder to hold together and feed at higher rpms.

 

[needsaresto]

Not that it's a small block,but thats whats beautiful about the 451 stroker. Stock 400 bore is already biggest of the big block mopars,the 3.75 440 stroke is all stock parameters, and if you use the 440 rod you end up with a very light piston. You then have a quick revving big block without the usual stroker issues associated with a long arm. The crank also gets lightened some during the build as well. It's really a win win combo.

 

[rumblefish360]

And such is the case with a 500+ stroker but even better, then again, mountain motors excel at this and the B.S. just gets piled on top with more cubes and the talk continues on & on.

 

[Moparmal]

Thanks for the info on the +s and -s .

I was aware of the rod / stroke frictional issues - but I was not up to speed about the flow characteristics etc.

Another term I have heard used - the comment that it

"allows the cam to think its bigger than it is"

- is that relevant to strokers and I'd like to know what it means anyway?

 

[moper]

It is not nly relevant in strokers... it applies to any design. It means that when you look in a properly matched dynamic engine... To make sure those are defined well... by "matched" that I mean matched in parts and matched to the particular performance window it's built for and by "dynamic" I mean as it's running where it's supposed to be in terms of load and rpm... The right combo will exceed the performance it should be capable of on paper. The phyhics comes into play and things like pressure waves and wet flow characteristics of the head and chamber and flame propogation and that sort of stuff work together to develop more power than it should be able to. We see efficiencies in most drag racing engines in the 115-120% level at peak torque because of these unseen forces workign for us instead of against us. So, the better matched the parts are, the more the cam "will think it's bigger than it is" in terms of power output. The really short version is... poorer ports need more cam to keep the valve open longer when good ports will make the same power with less cam.

 

[rumblefish360]

The really short version is... poorer ports need more cam to keep the valve open longer when good ports will make the same power with less cam.

LOL, great short version Moper. Mal, this also applies to everything else about the engines flowing parts, not just the heads. Intakes, carbs and spacers, headers and there collectors and diameter pipe size and length as well as after the collector. Even the collector itself has see jumps in power with odd designs.

(The problem here is you can test your wallet out of existance. The choices can really be a long list.)

This also applies to cams. Everything about a cam is subject to improvement for that particular engine and the combo as run. Cahnge an intake and the once perfect cam is not as perfect as it could be. Swap headers out for a longer tube version, the cam dynamics change again.

Rate of lift is often spoke about as quicker the better. This is also not allways true. A few times I have seen rags test a certain combo. End up with *** HP and TQ figures. Then they, the builders, figure, well, the cam is a extremely fast rate ramp, lets try 1.6 rockers. Power fall like rock!

Best case for anyone building a engine/car.....know what you want out of it first, then the target can be had.