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optimum stroke for a small block

7K views 39 replies 15 participants last post by  davis 
#1 ·
this is a bit of an obscure question that i've been thinking about for a long time.

For an all out race motor after maximum power out of a small block, what is the optimum stroke?

We all know that increasing the stroke increases capacity of the engine and the torque exerted on the crank by the piston (T = Fr). Both of these factors increase torque. Increasing stroke with a given deck height also increases the rod angle which creates increased sidewall pressure (friction) and increases stress. Increasing stroke also increases acceleration of piston at a given rpm which means more torque is neaded to drive it (F = ma).

What is the general opinion on the optimum stroke, i'm pretty sure you can't squeeze as much power out of a 289 (no matter how high you rev it) as a 351. Do you get to a point where you have too much stroke in a given block? For instance can you get as much all out power out of a 420 as a 351 or 393?

I'm talking about hypothetical race motors where the power band doesn't matter because transmission and diff ratios can be tailored to suit. I know it doesn't have much to do with real life but i'm just wondering...
 
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#2 ·
It all depends on whether you want reliable power or high maintanence power. A machinist and I contemplated destroking a 302 I think it was. Worked out around 280ci I think. That engine would spin upto 11,000ish rpm. Problem is it'd go well but would require very high maintanence. Constant valvespring, bearings and ring changes.

The reason everyones going bigger cubes is more power with the same or less rpm.A 351 isn't a bad start point in regards to power stakes. They make good power if you have the right parts.
 
#5 ·
STROKEXD said:
Great question. I got told years ago that when it comes to Clevelands, a de-stroked capacity of 330 cubes gave the ultimate thearetical bore\stroke ratio. This was from a pro drag race outfit that was chasing the highest HP per cube record in a small block.
Most of the destroked Cleveland drag engines of the 70s were based on weight rules that added big penalties for going over a certain cubic inch displacement. It wasn't the theoretical bore-to-stroke ratio that was involved, it was the weight "break" given to smaller engines. A lot of stroker BOSS 302 engines played in that game, because of the lighter, smaller block and rotating components.

A shorter stroke engine will rev more quickly and potentially higher in RPM than a longer stroke engine. Gaining RPM quickly under load is one of the keys to building a successful drag engine.


:davis:
 
#6 ·
gasxd said:
this is a bit of an obscure question that i've been thinking about for a long time.

For an all out race motor after maximum power out of a small block, what is the optimum stroke?

We all know that increasing the stroke increases capacity of the engine and the torque exerted on the crank by the piston (T = Fr). Both of these factors increase torque. Increasing stroke with a given deck height also increases the rod angle which creates increased sidewall pressure (friction) and increases stress. Increasing stroke also increases acceleration of piston at a given rpm which means more torque is neaded to drive it (F = ma).

What is the general opinion on the optimum stroke, i'm pretty sure you can't squeeze as much power out of a 289 (no matter how high you rev it) as a 351. Do you get to a point where you have too much stroke in a given block? For instance can you get as much all out power out of a 420 as a 351 or 393?

I'm talking about hypothetical race motors where the power band doesn't matter because transmission and diff ratios can be tailored to suit. I know it doesn't have much to do with real life but i'm just wondering...
I wouldn't be so sure of how much power you can squeeze out of a smaller engine...

How does 1029 HP at 9500 RPM sound from a 330.8 CID BOSS 302 (based on a Dart block) with Yates heads? That's with a fairly big solid roller camshaft and 18:1 compression with a Hogan's "sheet metal" inlet and a pair of Ron's T2-Terminators running methanol. Peak torque of 622 at 7200 RPM...we're talking about 3.11 HP per cubic inch. That's fairly damn good. The bore is 4.155" and the stroke is 3.050". The bore to stroke ratio is: 1.3623:1.

Compared to a 393" stroker Cleveland (mine), which makes something like 630 HP at about 7500 RPM...1.60 HP per cubic inch or nearly half the power per cube of the smaller engine...with a bore to stroke ratio of: 1.0468:1.

More importantly is what you mention in your f=ma with regard to piston speed. For a given bore, we can't do much to reduce mass without grinding the skirts down to a point where they don't work very well. But we can easily change piston speed by shortening the stroke. Piston speed is represented by the following: fpm = (stroke*RPM)/6

For the shorter stroke at 9500 RPM we have the pistons moving at: 4829 fpm. The longer stroke engine at 7500 RPM we have pistons moving at: 4813 fpm. The two are nearly identical (16 fpm difference) and both well within the material limits. The BOSS piston is actually heavier due to the considerably larger bore, but still quite safely within limits at 4829...in fact, it can spin to 10,000 RPM if desired. It would be quite a bit of work getting the 393 to 8000.

Assuming that we were more closely comparing apples to apples with the two engines and both had the same compression ratio and fuel type (and induction system!), it is reasonable to suggest that the 393 would make somewhere around 870 HP at 7500 RPM. That makes for a better comparison, but it is still way behind the smaller engine in power.

To give you an idea of the cost differential for the two engines, the 393" stroker cost me about $15K US. The BOSS engine inlet alone cost $3800 USD. The heads bare were another $4800 for the pair...before porting. The Ron's Terminators were another $2900...and we're not talking about the cost of the Dart block, the ultra-light 4340 crank (the cost of the internal balancing alone was $800), titanium rods (another $2400), Ross custom pistons (you don't get an 18:1 Yates piston at 4.155" bore out of a catalog) another $950. The shaft-mounted rocker arms were $2300 and the valve springs were $900 for just the intakes! The titanium valves were $920 for intakes and another $810 for the exhausts.

All of this is to illustrate a point...it is a lot cheaper to go bigger and slower than smaller and faster...and, whether we're talking hypothetical engines or not, practicality will always play a role in every engine built. The "optimum stroke" for any engine is based on your budget!


Take that same 393" stroker, add a stouter block and a 400 HP dual stage nitrous kit and suddenly you're running 1000+ HP for not much more than about $20K US. That is some fairly serious grunt.



:davis:
 
#7 ·
this is all true, the point which i was referring to in my 'F = ma' statement is exactly what 'davis' said, although i do appreciate the figures quoted.

It seems the consensus is somewhere between 302 and 351. It is obviously more efficient (in terms of wallet size) to build a stroker but the question concerned a hypothetical engine which was to squeeze the maximum power out of a given block (deck height and bore).

the reason i'm thinking about such an engine is because i was thinking about doing a thorough analysis on it for my thesis (and it is an interesting question).

my personal favoureite was Benny Gatts XA which i think used a 373 (i think) in the early 90's...

Is it a coinidence that Ford, chevy and chrysler decided on roughly the same capacity for all their high performance small blocks, or was it by chance?
 
#8 ·
gasxd said:
this is all true, the point which i was referring to in my 'F = ma' statement is exactly what 'davis' said, although i do appreciate the figures quoted.

It seems the consensus is somewhere between 302 and 351. It is obviously more efficient (in terms of wallet size) to build a stroker but the question concerned a hypothetical engine which was to squeeze the maximum power out of a given block (deck height and bore).

the reason i'm thinking about such an engine is because i was thinking about doing a thorough analysis on it for my thesis (and it is an interesting question).

my personal favoureite was Benny Gatts XA which i think used a 373 (i think) in the early 90's...

Is it a coinidence that Ford, chevy and chrysler decided on roughly the same capacity for all their high performance small blocks, or was it by chance?
The old saying that the cylinder heads make the engine is fairly true. Today's smaller,multi-valve engines are producing much more performance than larger, pushrod V8s of years ago. For example, my 5.4L 3-valve engine in my F150 makes more power and torque than a 400 (6.6L) in a 1978 F150. To some people a 1.2L difference is an entire engine!

The "optimum" stroke for an engine is going to depend on by what metric its performance is measured. If HP per CI is the yardstick, the smallest possible engine with the biggest heads and a radical cam that doesn't even start making power until 7000 RPM is going to win the competition. That is basically a BOSS 302 in a nutshell. If you were to zoom that period into fast forward, you'd have something along the lines of a 4.8L modular engine in the DOHC variety...with a lot of work or even custom fabricated heads, one could see them pushing 10,000 RPM and making well into the 1300s HP.

If torque is the player, then the longer stroke engine that works well at about 5200 RPM is going to be the winner. However, it is not just stroke that makes tor que, it is bore, too. The reason a lot of engines start with a larger overbore is to increase capacity. Since the Cleveland blocks are notoriously not well-suited for large overbores, we're fairly stuck with stroking. Using a Clevor in a Dart block and alloy head combination makes the best sense in a SBF for maximum performance and longevity.

It isn't a coincident that all three major US manufacturers had the same sized engines. Various sanctioning bodies of various automotive racing associations dictated rules that specified displacement maximums or weight penalties for exceeding those maximums. What the racing bodies didn't do to decrease engine capacities, the EPA did...and/or OPEC when the fuel "shortages" of the mid-70s happened in the USA. The "fixation" of engine sizes by racing organizations was an attempt to level the playing field so that everybody had a chance and so that money didn't dictate the game. Rules are stupid when they try to control the game in such a manner, because it turned into a game where the guy with the most spare engines pushed to the maximum won. Six or eight spare Clevelands later and maybe you were taking home the Wally on Sunday.


:davis:
 
#9 ·
This is an excellent topic with lots of excellent info.

A couple years ago a car magazine did an experiment on this exact topic. I'd have to do some weeding through my magazines to find it but I could.

They built two engines of almost exact displacement, within a couple cubes of each other. Same block but one was built with smaller bore and longer stroke and the other with bigger bore and smaller stroke and put to the dyno test.

The result? No appreciable difference in horsepower or torque.

Basically with both engines being equal with the bore and strokes being the only difference I would believe it came down to how much fuel was being burned. Since the displacement was the same and therefore the same amount of fuel being burned the power output was the same. This is for readings at all RPM. When one has more torque because of having a longer stroke the other one with the shorter stroke has more force applied to it with the larger piston (more surface area that force is applied to).

I have my own theory about longer rods. I disagree with the common notion that a longer rod with the same stroke will produce more torque. Think of it this away...

If you need to lift a weight you put your fulcrum next to it and then place a lever under the weight and over the fulcrum. Lets say the lever is 5 feet long. If you stand on the end of the lever you will exert your body weight times the length of the lever in force to lift the weight. If I weigh 150 pounds, times 5 feet, I am exerting 750 ft.lbs. of torque on the lever.

Now I lose 5 pounds and place a 5 pound box on top of the lever. Let's say this box it 2 feet by 2 feet. I stand on the box at the end of the lever and am now exerting how much force? Well, I am still at the same distance from the fulcrum but now 2 feet above the end of the lever. Since I now weigh 145 pounds and the box weighs 5 pounds I am still only going to exert 750 ft. lbs. of torque 5 feet from the fulcrum.

The box would be the connecting rod. I would be the piston, and the lever is the throw of the crankshaft (distance from crank bearing journal centerline to connecting rod journal centerline).

If 150 ft. lbs. of force is not enough torque to move the weight should I get a 5 pound 3 foot high by 1 foot wide box to stand on? No, I would need to get a longer lever (bearing journal to con rod journal).

The throw of the crank will need to be changed for a torque increase with the same amount of force applied to it if the force being applied is to remain the same.

As for the optimal bore/stroke ratio to maximize displacement and keep from having side load problems on the piston I don't know.


Steve
 
#12 ·
as the bore x stroke raito goes up, less force is exerted on the cranck journal and more force is transfered to torque

as the bore x stroke ratio geos down, more force is exerted on the journal but less is transfered to torque

this is a very complicated situation, it gets worse when you realise that as the rod length decreases, the piston makes a larger angle with the block. if you use a shorter rod, friction and sidewall pressure increase.

this relationship means that there is an optimum bore and stroke for maximum power with a given deck height, it is very difficult to calculate mathematically but it has probably been determined experimentally...
 
#14 ·
Well this is bloody interesting...I've been studying physics at high school for 4 years now, and am hoping to to Aeronautical / Mechanical Engineering at ADFA next year, but the coffee is messing with my head and i can't really concentrate on all this :p

A few pics would be nice, perhaps someone can make a little flash / java type thing where you could play with different rod lengths and strokes to see what happens??

Another thing to think about, a guy cut the bores out of his EH (186??) and araldyted some more in at such an angle that when the piston is at TDC the rod is at 90deg to the crank, changes the torque curve completely... But it increases friction and sidewall pressure so it doesn't rev much over 4000 - not that you'd have to...
 
#15 ·
davis said:
Steve,

Torque is twisting force...your lever isn't twisting, is it? That would make it kinda hard to stand on whether you weighed 150 or 145# <grin>


:davis:
Sigh....

The fulcrum is the axis of the rotation. The force is applied around this center and travels in a circular motion around it. Yes, it is a twisting motion. Yes, it is torque.

In your engine the crank bearings are the axis of rotation. The the force is applied to the rod journal and travels in a circular motion around the bearing centerline.

Please reference "Physics 101".

"Simple leverage is the same as torque and both are measured in terms of force and distance (distance is the length of the lever, force is the amount of pulling or pushing applied at the end of the lever). "

Here is a link to a page that elaborates on the definition and physics of torque. I hope this helps.

http://www.akotorque.com/usefull.htm


Steve
 
#16 ·
Ohio XB said:
Please reference "Physics 101".

Steve

No problems mate. Please reference "English 101," because punctuation belongs inside of the quotation marks. Next time, accept a <grin> as a joke...and have a laugh with it.

If a point on the circumference of the earth is where you're standing, and the center of the earth is the point of rotation, how much torque are you applying to the lever?


:davis:
 
#18 ·
The more you push your cucic capacity out the more horsepower and torque you should create generally speaking, obviously sidewall pressure will become and issue especially when using a standard 302/351 windsor or cleveland block, so again it depends on your budget and what times your aiming for as to what will be a more optimum stroke for your application as stated the more cubes you have the less you have to rev the engine to create power and torque which means less maintance and greater longjevity. Im in the process of stroking my 351 windsor which was previously stroked to 408 cubes using a standard 351 w block with no grout filling etc... and after looking at the internals and the bores of the block everything looks fine just as good as it did when it was assembled and i had no issues at all but then i only did 4000-5000 kms on that engine but it was at full song for most of it ;). that same engine created 460 rwhp on pump fuel running 11.1 comp with a small solid roller cam ( 580-590 ish fwhp)

My new engine uses a dart block with a 4.125 bore 9.5 deck height (windsor block) but with cleveland mains using my old eagle crankshaft (machined down to 2.75 on the mains) same rods and just new pistons to match the larger bore will give me 434 cubes there abouts. Im going to be running a heap larger roller camshaft but still not extreme and 13.1 comp with methanol (mainly cause theres no cheap/good alternatives to avgas which is now banned) and my parts supplier is saying i should get somewhere between 800-850 HP id guessed a more conservative 750HP but ill find out soon as the old saying goes "theres no substitue for cubes" unless your into hairdryers and id rather leave those to the ricer ****.
 
#19 ·
Ohio XB...an interesting theory, but a longer rod should actually produce more torque. See in your demonstration, the force you were exerting was always perpendicular to the lever- the ideal case. A connecting rod is a different matter. It applies a perpendicular force at only one point during a revolution (that depends on the rod/stroke ratio).
At some angles of the revolution, the longer rod will give a better torque, and other angles the shorter rod will give better torque. The longer rod will give longer dwell, but a higher piston velocity when the piston is halfway down (maximum velocity). the shorter rod will give shorter dwell but a lower maximum piston velocity. So basically there's advantages and disadvantages to both.
Probably the biggest disadvantage to the longer rod is a greater weight- more rod, more block (for same piston).

To the original question- sounds to me like you're a mechanical engineering student like me (or similar). The way i figure it is you want the cylinder to lost the MINIMUM energy to the outside world (heat), so you want the minimum surface area for a given volume. That could (note you have to take into account the surface area of the piston and head) also minimise frictional losses at the rings, further improving efficiency. Not really sure at what point in the revolution you'd want the minimum volume (it's useless at BDC because the piston can do no work there) so you'd have to do an optimization for that one...along with the rod/stroke ratio.
Sorry this was so long...
 
#20 · (Edited)
Please reference "English 101," because punctuation belongs inside of the quotation marks.

Thanks. I can never remember which way that goes.



If a point on the circumference of the earth is where you're standing, and the center of the earth is the point of rotation, how much torque are you applying to the lever?
Once scientists figure out 100% just what gravity really is and what really causes it (Two bodies in space will be attracted to each other and that attraction will be proportional to their mass, but why they are attracted to each other is still not understood) then I will be able to figure out the answer to your question.


XD408, good post.

Madmelon, I agree with the attributes you mention about the advantages from dwell and such as I am aware of these aspects too. My concern is that the piston is over the centerline of the crankshaft and only travels in a perpendicular path to the center of the crankshaft. This is the point from which the force is being applied to the end of the connecting rod. The force acted from the rod to the rod journal on the crank is in a less than a perpendicular direction due to the rotational travel of the crank. Therefore 100% of the force of the piston is received by the con rod journal for only a millisecond while the con rod and rod journal are perfectly perpendicular (90 degrees to each other). The rest of the time the rod is either also providing force in the direction towards the crank or away from the crank which is a load on the lever in a direction other than the direction we are trying to turn it and therefore there is energy lost.

I still fail to see how doing this with a longer rod will aid in producing power with the geometry of an engine's function being what it is.

Whew. Might be hard to explain in words. Drawings would be easier.

This is a good discussion.


Steve
 
#21 ·
falcon coupe said:
Forget English Davis, you are starting to sound like an Aussie :)
...and it's too damn bad that we can't get decent Australian beer over here! Somehow, everyone here seems to think that Foster's is all that...just goes to show the power of marketing. Buttwiper beer over here is in the same category...

I'll be heading back to QLD sometime next year...probably at the end of your summer. I'll spend a couple of months at my mate's place in Gold Coast.


:davis:
 
#22 ·
Ohio XB said:
I still fail to see how doing this with a longer rod will aid in producing power with the geometry of an engine's function being what it is.

This is a good discussion.


Steve
The "longer rod" benefit is a myth...at least in terms of the difference between something like a 393" stroker with a 6.000" versus a 6.125" rod, for example.

The basics of connecting rods are the shortest one that does the job will be lighter than a longer one of the same design and material composition and the longer the rod, the more (in crankshaft degrees) the piston will dwell near TDC. In a high RPM engine, this is generally better for materials but worse for "maximum" power.

Here is an image I've used for over a year or so to demonstrate the short rod versus long rod issue:



Those who argue that the angle (force vector) of intercept of the short rod engine increases bore/piston side loading over the longer rod can see in this very exaggerated rod length difference that the angles are not so significantly different. Brought into reality with a comparison between a 5.78" rod and a 6.03" rod, just a quarter-inch difference in length, is a ratio of:

6.03:5.78 or

1.043:1

My image shows a ratio of (big to small) of:

2.185:1.675 or

1.304:1

To compare it to a 5.78" rod (at the ratio indicated in the image), the bigger rod would need to be:

7.537"

...to (approximately) equal the "differences" expressed in my (rather crude) image. This is a ~23% longer rod.

It doesn't take an image where the angles are visually no different (as is the case between a 6.000" and 6.125" rod) to illustrate the point, IMO. Rather, as I've done is exaggerated the point until there is a somewhat visible difference in the angles and said what sized rod it would take to make that big of a difference. It is ludicrous to suggest using a 7.537" rod in a small block Cleveland engine--even if it could somehow be made to fit.

Here is a worthwhile read along the topic of long versus short rods.


:davis:
 
#23 ·
davis said:
...and it's too damn bad that we can't get decent Australian beer over here! Somehow, everyone here seems to think that Foster's is all that...just goes to show the power of marketing. :davis:
Check the side of your next American can of "Fosters".........the small print says "brewed in Canada".

I do like the TV ads over there though! We don't get those ones here.

........I thought it was Budwater?

The biggest crying shame was me trying to get a vegimite sandwich for an appetizer at the Outback Steak House.......didn't have a clue what that was!

And damn, selling Kookaburra Wings would be a criminal offence down here! It's like us tearing up one of your Golden Eagles! ;)
 
#25 ·
Racer said:
Too much physics theory!

It ever works or it doesn't. Rod ratios/lengths have been proven time and time again to be worth virtually nothing.

I think that the myth is related to the commoner as opposed to perhaps someone like a dirt or asphalt track racer whose vehicle is run at peak RPM for extended durations. In that case, I see the possibility of the longer rod giving a bit more longevity in keeping things together to finish the race.

For the rest of us...it is worthless...except that it gets harder to buy "short" rods as the volume market is moving toward longer and longer rods.


:davis:
 
#26 ·
Some prefer long rods, some prefer short, (don’t go there madam!)

I thought this thread was a debate between long stroke vs. fat bore, not a balancing act between rod length and pin height?

Over square vs. under square, (i.e. par = Ford 400, 4” x 4”)

I think we’d all agree that a long stroke motor makes a great low revving, long lasting, quick off the lights, torquey street motor;
while a short stroke, big bore motor makes a better hi revving race motor, but with a short parts life.

Pick the toy that matches your game (and budget).
 
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