Longer stroke... Better for fuel efficiency?

I have always heard that longer stroke engines were more fuel efficient, but is it really true?

Obviously longer stroke engines will tend to have a power band at a lower RPM (and opperating at these RPM's will be more efficient), but will the BSFC of the engine at a given RPM (say for example, 2200 RPM) be lower on a longer stroke engine than on a shorter stroke engine having the same displacement?

Any test data out there?

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

Quote:

Longer strokes mean more energy lost to ring friction (same ring tension, but has to stroke further) and more heat lost to the walls.

This (and it does make sense) would tend to mean that the OPPOSITE of what I was always told is true.
Where's the data that tells me longer stroke is more efficient and gives better fuel economy?

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Re: Longer stroke... Better for fuel efficiency?

RoadWarrior's picture

Well, firstly one has to be careful about what group of people one is talking with... there's some groups of performance enthusiasts stuck with obscenely long stroke motors, who will tell you shorter is better period, and on the other hand there's some folks stuck with really short stroke motors who'll tell you longer is better. In reality there's a happy medium. Much of it relates to piston speed in the typical RPM range, and it is also rather dependant on how valve timing is set up, and how high or low the dynamic compression is for a particular motor. At the extreme end of short strokes, one might say both that the piston and head area is too great, and that the combustion gases lose too much heat to those, also that the piston displacement speed is too slow such that pressure is high and the combustion gases are getting extra time to bleed away heat while they force inertia into the moving parts, although the rotational inertia is typically lower on a shorter stroke motor. Then on an extreme long stroke motor one can say that the wall area is too great, and that one loses too much heat there, and also that the piston moves such that it is accelerating away from the expanding gases, ergo it is subject to drag. Then bear in mind that combustion chamber shape is less than ideal on a short stroke, it has to be somewhere near pancake flat to get any kind of favourable compression ratio, having a well shaped chamber and a decent static compression ratio is easier with a longer stroke. That would be also where induction timing comes into play, having a higher static compression ratio with a longer stroke motor, and delaying intake closing, allows a greater expansion ratio for the combustion gases, due to reduced dynamic compression. (Atkinson cycle)

Then some other mechanical geometry can come into play, the rod ratio, which determines the acceleration of the piston at various points in the cylinder. If this is far off the piston will resist acceleration or try to outaccelerate the gases at various positions, this is bad and can cause inefficiency and NVH issues. However motors are around with these problems due to the convenience for manufacturers in offering varying displacements in the same block, with only a couple of bore and or stroke differences. Rod ratio and stroke also determine the lateral pressure that pistons put on the walls of the bores, a longer stroke motor that is more likely to have a longer rod ratio may not put much sideloading on the bores at all, whereas at the other end a short stroke with a short rod ratio would. It is possible to have a short stroke with a long rod ratio, which reduces sideloading, but if overlong, the acceleration profile would be out of shape, and the criticism would be that the block was "too tall" meaning it was carrying extra unnecessary weight for it's displacement and power output...

So anyway, before trying to "Fix" a motor, one should determine exactly what the problem is, if there is one and what you're trying to achieve... in some engines, putting a "stroker" crank in it, might be fixing an unfavourable rod ratio as much as it seems to gain more than a predicted amount of power from the displacement increase.

Looking at the situation for the 6G72 V6 motor in my voyager for instance, and available parts, I have come to the conclusion that what might be the ticket for better efficiency is a slightly longer stroke and a slightly shorter rod ratio. Bore wear on these motors is very slight, so a slight increase in sideloading probably won't hurt it, and I will find it easier to get compression up to about 10.5:1 given that skimming the head is limited to about 10 to 20 thou... this would be accomplished with a 2mm offset crank grind, 5mm shorter rods and 3mm taller pistons (with less dish).... and the motor may run a little smoother and rev a little easier despite going a little longer on the stroke, because I will have reduced harmonic vibration issues due to piston acceleration inefficiencies.

Also remember that peak HP numbers sell cars, so things might be tweaked away from midrange efficiency to put a bigger number on the top end, but remember, something that makes 110HP from 2000 to 5000 RPM will probably outdrag something that makes a spike of 140 at 6000 RPM and barely anything until you get to 4K.

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

OK, so maybe there isn't a 'general rule of thumb' on stroke lengths and rod lengths and side loading do make sense to me. Seems like it might be a bit of a daunting task to analyze all that on my GM 454.

What I am doing:
More than likely I am inheriting my dad's pre-smog motorhome (FMC) with carbureted 454 engine. It turns 1850 RPM @ 60 MPH, and pulls 9-11 inches of vac at that speed.

I really like the unit and consider it worth investing in a motor. If I am going to build a motor, I want to do with efficiency in mind, especially since there is an abundance of aftermarket parts for the big block Chevrolet engine, including tall deck height blocks (to accomodate longer rods or longer strokes).

The range of options is pretty impressive with bores from 3.935" dia to 4.5" and strokes from 3.76" to 5.3"

For fuel economy, it might make more sense to go diesel, but part of me wants to prove that properly built and tuned, a gasoline engine can run efficient.

Is there an "ideal" ratio between rod length and stroke?

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Re: Longer stroke... Better for fuel efficiency?

Pinhead's picture

Personally I'd leave the block crank and pistons alone and work on the cylinder heads. There's plenty of improvement to be made in those old heads, especially with the cylinder head modifications that can be found on this forum. IMO all you should worry about in the bottom end is getting the compression up. After that, the cylinder head mods will eliminate the detonation that you'd normally be worried about in a high compression engine.

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Re: Longer stroke... Better for fuel efficiency?

ssheen's picture

Along the same lines as Pinhead. Though I think pistons do have a role to play to help squeeze more mileage and whp out. At the very least, rounding of all the edges. Think Coandă effect.

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

I saw some of the ring/grove mods. Flow is very small in MPG mode (high man. vac, low CFM, etc.).

Are the cyl. head mods you are refering to an effort to keep velocities high? Turbulence up (I thought my 'as cast' ports would do a reasonable job of that) or something else?

Logic for longer stroke giving more efficiency at low revs is that the ATDC burn has more linear travel to work with (possibly burn more completely) before getting to (toward) BDC. This should especially be true with water injection, where the burn is even slower.

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Re: Longer stroke... Better for fuel efficiency?

Pinhead's picture

journeyman wrote:

I saw some of the ring/grove mods. Flow is very small in MPG mode (high man. vac, low CFM, etc.).

Are the cyl. head mods you are refering to an effort to keep velocities high? Turbulence up (I thought my 'as cast' ports would do a reasonable job of that) or something else?

Logic for longer stroke giving more efficiency at low revs is that the ATDC burn has more linear travel to work with (possibly burn more completely) before getting to (toward) BDC. This should especially be true with water injection, where the burn is even slower.

The Powre Lynz (screw threads in the intake ports) help keep velocity up at low lift and low throttle opening. However, they help most with wet flow, keeping the fuel vaporized and homogenized better than any factory "as cast" port could ever do. As SSheen mentioned, the combustion chamber can always use some work, "Edging" as it would be called on the forum. Also, Singh Grooves (if your chamber has squish) have shown to lower lean burn limits and make the engine run cooler.

As for the stroke making more torque... It's only partly true. The rod ratio has a lot do with piston velocity at various degrees of the engine and greatly effect efficiency/power output. The problem is, I don't know what is what... I read something about it somewhere but I can't pinpoint it right now. :oops:

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

So is there a rule of thumb with rod lengths compared to stroke?

I always assumed that the longer the rod could be, the more efficient the transfer of power from linear (piston moving down) to rotary (crankshaft rotating).

To me this means that the higher the rod length/stroke ratio the better... This is correct isn't it?

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Re: Longer stroke... Better for fuel efficiency?

paul1orr's picture

journeyman wrote:

So is there a rule of thumb with rod lengths compared to stroke?

I always assumed that the longer the rod could be, the more efficient the transfer of power from linear (piston moving down) to rotary (crankshaft rotating).

To me this means that the higher the rod length/stroke ratio the better... This is correct isn't it?

Your stock engine should have a 4" stroke and 6.135" rods. Ratio=1.53. According to T.O.O. at Endyn the best all around ratio is 1.75 which would mean a 7" rod and matching pistons with relocated wrist pin holes. The OEM rods will make more low end torque and the 7" will make more HP at higher RPM but with lower BSFC (higher efficiency). The reason is piston dwell time or stated more plainly the number of degrees of crank rotation before the piston moves down the bore significantly. What do we want in an ideal world? We want the crank angle to move to a point where the most work can be done AND then stop right there while we light the fire and wait for all the mixture to burn and create the most pressure and then let the crank go. That point of rotation is dependent on rod length and stroke combined. Can't do that though since engine live in a dynamic world.

Since you said you want fuel efficiency, then the short rods need to go. The beauty of the short rods to move a motorhome is that they present a beneficial rod to crank rotation ratio early in the burn process to make that super high torque but then they sacrifice efficiency bcoz the piston velocities are too high part way thru the stroke and the mixture isn't expanding as fast as the piston is running away. The longer rod motor will come closer to allowing the mixture to produce max pressure over a GREATER number of usable crank degrees and so increase economy but will give up some drivability due to lost grunt. That is where compression and intake mixture quality come into play. Not only can you regain the grunt but also improve efficiency even further AND reduce the tendency to knock from what the stock motor experiences now.

Hope this helps to improve your understanding without getting too technical.
Paul

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

Since this is a low RPM application (4500 RPM absolute max, mostly run 1500-3500), "outrunning" the expanding mixture probably won't be such a critical problem (main concern is at 1800-2000 RPM), but one of the good things about the BBC engine is the availability of a "tall" block. It is .400 taller than the regular, allowing for .400" longer rods. I am all for getting the longest rod possible and still maintaining the integrity of the piston.

Stroker kits for this application sometimes require the use of this taller block to retain the same or similar rod ratio.

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Re: Longer stroke... Better for fuel efficiency?

mountainrich's picture

Shoot for that 1.7 or a little better for rod ratio. If you try to go above 2:1, you're gonna run into many other problems. A 1.5 rod ratio from factory, can easily be overcome w/ a longer rod and shorter piston (pin to deck). It'll mean shopping for pistons but there's thousands of 4" pistons- so be choosey. Keeping the skirt is not a big deal either.

Sometimes the longer strokes can get better mileage. That's if you know how to drive using vacuum. Most long strokes are very responsive to 2500 or so. Well, that's "lugging" to me. If you know how to drive, where your vacuum is, and stay away from too much gas pedal at one time, you can get good mileage out of a "stroker". Keep in mind- there is a point that just plain downshifting and getting the vacuum back is WAY better than trying to push the pedal deeper.

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Re: Longer stroke... Better for fuel efficiency?

journeyman's picture

Does rod length affect the low end (4000)?

Our application rarely if ever sees 4k and NEVER sees 5k.
90+ percent of the running is between 1500 - 3500 rpm.

Does the rod length make less of a difference at those low speeds?

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Re: Longer stroke... Better for fuel efficiency?

mountainrich's picture

Rod length does affect even low rpm #'s. It's not going to be a lot, but it'll help make the curve even flatter, maybe a bit broader. The whole idea is to get as good of a geometry as possible. Try not to deck the block- unless leveling it- get a modern, tighter ring package, then calculate the rod length. The longer rod w/ modern piston design will put less side-loading on the piston-equalling less friction- equalling some more power and durability. I am by no means saying buy a custom rod- not worth it for most builds- but for comparable cost, get the longer rod. You still may have to deck the head to top deck everthing, but you use as much deck heigth as possible.

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Re: Longer stroke... Better for fuel efficiency?

racprops's picture

Can I get you to cover a Chevy 350 to say a 383?

Rich

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