The perfect stroke?

Is bigger better or how much is too much? Is there a perfect balance point between the too? This is a question that can be used for small and big blocks and ones opinion vs the science is welcome, what do you think the perfect stroke is for a given cid / combo & why?

340 i think is the best, pound for pound stock 383 or 440 car would get beat by a 340 car.

Well there is no replacement for cubic inch displacement. But I understand (could be wrong) for whatever cubes you want to hit bigger bore and shorter stokes is best Or so I hear.
Then a high RPM motor would likely be a bit better with a shorter stroke (7500 shifts) but an low rpm motor (6000 shifts) would likely benifit from the longer stroke. So If you were to build a 540 to rev 4.25 stroke and 4.5 bore. if you want a street thumper and your going to the same 540 4.5 stroke and 4.375 bore.

Generally for a given/desired displacement)modern V8 engines seem to work best (at least in terms of highest average toruqe/broadest overall powerband) with between a .85 and .9 stroke to bore (not bore to stroke which would be the inverse) ratio and at least a ~1.7:1 rod ratio.

Big bore short stroke (like a 327 chevy and 340 mopar) used to be considered the hot ticket but with 40+ years of modern cylinder head technology you don't really have to rely on allieviating bore shrouding to keep the VE up at high piston speeds. Case in point look at the modern LS1 motor

This is over-simplified of course but in general I believe it to be true that this ratio seems to yield be the best balance between adequate torque, wide powerband and optimum power for a ~6500 rpm valvetrain. But as you spin the motor higher with higher compression, better flowing heads and racier valvetrain, generally/ideally the stroke gets proportionally shorter and the rod gets longer.

For every desired torque peak there is also an optimum runner length and cross section, but that goes out hte window whether we're talking Carb, EFI or modern Direct port injection (which doesn't have to carry any fuel in suspension in front of the intake valve. Just as first mass flow and later Direct port injection altered the "rulz", DPI is likely to re-write them yet again because it can alter/vary fuel volume and spray pattern (without vlave interference) as it injects INTO the cylinder.

i would look for Modern DFI and V8's moving to 180 degree crankshafts (to reduce rotating mass and balance intake pulses) to be the next big moves in V8 development. Flat cranks have been inherently low on low/middle RPM torque with carburation but I look to DFI to help offset that in order to take advantage of the better high RPM breathing and lower mass. Another trend once they get this sorted will likely be smaller displacenets (3.5-4.0 litre motors making roughly the same useable power as contemporary 5.0-5.5 litres)

That should be enough to get some debate (but hopefully not ) going!

Long strokes are nice but I like a longer rod to give
a better dwell period at the top..... JMO

One that leads to a happy ending!

Ok Fred, you have run lots of different combos bore to stroke, so whats your opinion? keeping focused on the subject at hand and not whats in your ..... is there a combo that performed the best?

Sorry Feed, that looks meaner after rereading then it sounded in my head, no harm meant

I,m all ears on this one. 390 cfm airflow, 4.5 bore, fuel injected. Best stroke? By Streetwise formula, 4.05 to 3.825 stroke for my setup? I run a 4.15 crank now.

It depends on way more parameters than you gave us.

Desired RPM
Bore spaceing
Valve size
Port effeciency relative to valve size
Flow

Ideally you want to make sure you have enough flow to feed your desired cubic inches enough to make your desired power and tq.

Example if you want to make 500 hp and 500lbs tq you need to adjust the bore size to unshroud your valves enough to get about 250+ CFM without shrouding the valve then you make the stroke long enough to get your cubic inches up to where it is big enough to make the 500 lbs tq. If you want the same motor to make 600 lbs tq add more stroke. If you want the same motor to make 600 lbs hp you need to make more CFM air flow, if you can do that with out a bigger bore then don't worry about it. I think ultimately you make sure you can get the flow for the power you want, if you need a bigger bore to get a bigger valve then thats what you do, if there is a cubic inch limit then bump you stroke to get the cubes.

Also depends on if you have a hemi or wedge or 2 3 4 or even 5 valves. More valves and valves that open away from the bore can make the same power on a smaller bore then a wedge can, in that case you can use a smaller bore that burns more completly and then use a longer stroke.

Bottom line is more cubes give you more tq, more flow gives you more HP (asumeing flow quality is not trashed). I don't think there is an ideal ratio as long as you can get the flow you need.

Some of my thoughts on this post are, is there a point of no return from a long stroke, seems like lots of rod failure posts from stroker combos lately, and if the stroker guys are pushing the rpm way past the peek torque and pushing up in the hp curve , where is the balance point where a shorter stroke with a better rod ratio could be better, given that the heads are not the limiting factor <exanple small block 3.79 vs 4 inch stroke> both fall outside the numbers that wise posted> but it seems that the 3.79 stroke turns much higher in the HP curve than its safe to turn a 4 inch , Does the increase in torque offset from the 4 inch make up for the HP up high? this reasoning can be applied to BB the same

a longer rod to give a better dwell period at the top

Depends on your definition of "at the top".
If it's 10° each way B/ATDC, the difference between a 5" stroke with a very short rod (7" = 1.4:1) and a long rod (10" = 2:1) is very small.
By 10°, the long rod has traveled .0474" down, while the short rod has gone another.... .0032" (three thousands and a bit).
Putting it another way, between the 2 engines the time to travel down from TDC to .060" is about 1°.

How much difference in velocity pulling down from TDC? Is this a factor in rod failure?

This exactly why I just put a flux capacitor in my car as it will adjust the jigowats on the fly to effectively balance the rotating mass and any rod ratio combo for maximum power with no chance of grenading the engine!

Thankx Wedgie, that was well said, How bout those pix from your secret mission? Any how how did mopar pick the 3.79 stroke crank instead of 4? There seems to be alot more to it than meets the eye

Quote:

---

Thankx Wedgie, that was well said, How bout those pix from your secret mission? Any how how did mopar pick the 3.79 stroke crank instead of 4? There seems to be alot more to it than meets the eye

---

The way all important decisions are made.