Bore vs stroke

[tmhutch]

In a recent thread the OP was beginning an engine build with the same question many people have:

Originally Posted by THE_KILLER_TOFU

The real question I have is; is a 302 stroker a more efficient way to obtain power than one of the 5.0 blocks on the market. As I recall, strokers offer better TQ down low than the Big Bores of equal displacement. Is this a universal truth? Thoughts?

To which a couple people responded:

Originally Posted by na svt

Strokers will make more tq than a bb.

Originally Posted by Toby

Strokers typically make more torque and power down low since they have the mechanical advantage. Yes a big bore can make more power up top due to revs, but it won't make more power down low when compared to a stroker of the same displacement.

For many years I believed the same thing. This urban legend started in the late 60's when Pontiac and Oldsmobile had to resort to bigger strokes to keep up with the cubic inch increases of Ford, Mopar and Chevrolet. The blocks weren’t large enough to accommodate larger bores like the competition and just like our 2v brethren, physically smaller blocks also limited cylinder head/port size. Small ports and big cubic inches made instant torque monsters and on the streets these things terrorized the competition. But a funny thing happened at the drag strip. The big bore Chevy’s, Mopars and Fords were dominating. Rarely did a giant stroke, small bore Pontiac or Oldsmobile compete at the top of the game and nobody really had an answer. Then along came the internet and with it access to the expertise of professional racers, engine builders and mechanical engineers. They very succinctly set the record straight. Of course this didn’t stop the myth from persisting but for those who cared to look, the truth was out there.

The main players in this age old question are leverage, friction, crankshaft flex, piston acceleration rate (starting and stopping), piston speed VS flame propagation, valve shrouding, combustion chamber shape/size, and rod to stroke ratio.

Leverage: I’ll address leverage first because it seems to be the leading misconception. Many still believe that a longer stroke will give the combustion charge more leverage to turn the crank. And it does in some areas, but it’s less in others. The most important aspect to understand is that both engines of same displacement are pulling in the same amount of air/fuel mixture. This is the source of energy and regardless of the bore or stroke this finite energy source cannot be altered. In the case of the small bore, large stroke combination the crank provides additional leverage but at the same time has to travel a longer path with the same amount of energy. This same amount of energy has to get the piston further down the bore and travel the additional distance back up the other side. It also has to use this energy to push the spent exhaust gasses out (pumping losses). The net result when considering all 360 degrees of the combustion cycle is that the additional power derived from more leverage is used up moving the piston a longer distance.

It might be easier to think of it this way; Two identical mechanics have to loosen giant bolts on a big industrial engine. Fortunately they are able walk 360 degrees around the bolt attachments. Mechanic number one opts for the 3’ long wrench while mechanic two, knowing the bolt will be hard to turn opts for the 4’ long wrench. The race begins and mechanic two is able to move the wrench faster because of the additional leverage, but because the path at the end of the wrench where he is walking is so much longer it takes the same amount of time to make one full turn as the slower moving mechanic with the shorter wrench. In the end the bolts are removed at the same rate even though the ends of the wrenches offer different leverages and are moving at different speeds.


Friction: So you might ask why I’ve said that the big bore, small stroke engine will make more power and torque? The main part of that answer is friction. Frictional losses account for significant power loss in an engine and almost 80% of those losses come from the piston rings. Even at idle a long stroke/small bore combination with its higher piston speed is losing a tiny amount of power to the smaller stroke/large bore combination. By 5,000 RPM the additional piston speed of a 3.80 stroke engine VS a 3.55 stroke actually starts to show on a dyno graph and accounts for maybe 4 or 5 HP/TQ. Of course losses increase as the RPM climbs but the main takeaway is that a larger stroke does not make additional torque over a larger bore of the same displacement, in fact it makes less.


Crankshaft Flex: Another power robbing component of longer strokes is crankshaft flex. All cranks flex (that’s why we need harmonic dampers) and when they do they absorb power rather than transfer it to the flywheel. This is not hugely significant except in larger stroke cranks but it’s an important piece to the puzzle.


Piston Acceleration: It’s not just the additional friction that robs the longer stroke of power but the energy involved in accelerating the piston harder and to a faster speed, and then having to stop it from a higher speed and reverse direction. Once again the liability isn’t huge but much like crankshaft flex it plays a part.


Piston Speed VS Flame Propagation: When RPM’s get really extreme engineers start looking at piston speed VS flame propagation and combustion expansion. Believe it or not today’s metallurgy and advanced technology has enabled us to build engines that are capable of outrunning these components, which move in the neighborhood of 40 meters per second. Piston speeds in Formula 1 and Pro-Stock are constantly working with and around the limitations. Were not likely to approach these engine speeds but the faster the piston moves the less force the ignited fuel can impart on the downward traveling piston. Again, advantage goes to the larger bore.


Valve shrouding: This another drawback of large stroke/small bore combinations. Almost all cylinder heads experience an increase in airflow when opening up the cylinders and moving the wall away from the edge of the valve. This is especially effective for high RPM engines needing to move a lot of air but the gift of the DOHC 4 valve design just keeps on giving because the semi-hemispherical combustion chamber allows the valves to sit at an angle. As the valve opens it moves away from the cylinder wall and towards the center. Even with bigger valves shrouding does not occur like it does in conventional heads. However, there is still airflow gains to be made by opening up the bore.


Combustion Chamber: Is there anything good about the long stroke/small bore combination you might ask? Yes, one thing. It’s good for emissions. The smaller the combustion chamber the better the heat retention and flame travel. This provides a cleaner, more emissions friendly burn. How much power does this translate to? Not much. But at the other end of the spectrum where combustion chambers get very large there are issues with poor combustion efficiency and heat loss. If the chamber gets too big, flame travel becomes sluggish, fuel can drop out of suspension and uneven heating can cause detonation issues. Mechanical engineers have formulas to determine appropriate parameters for combustion chamber size but that is off our radar given the size and efficiency of the DOHC cylinder head.


Rod/Stroke Ratio: Engine builders debate over the merits of long rod/short rod combinations and many like to point out the torque benefits of the lower rod to stroke ratio resulting from increasing the stroke while maintaining the same length connecting rod. People can debate away but the differences are pretty much meaningless within a reasonable R/S range of 1.5:1 and 1.9:1. Outside those parameters interesting things start to happen but today’s efficient combustion chambers and cylinder head ports have laid the discussion to rest so I consider it a non-issue for the purposes of this discussion.


Bottom Line: Strokers make less torque (and HP) than big bore combinations. Not more.


Is there any reason to go with a stroker as opposed to a big bore. If cost is a consideration, yes. Strokers are an economical way to increase the size of the 4.6. Is it as efficient as the big bore? No. But the 3.80 stroke is not very long and wont suffer much power loss to the big bore at moderate RPM’s. It certainly wont produce more torque as many believe but I wouldn’t hesitate to run the combination myself.

[ModularSpeed]

Great Topic!

This is my one comment; there is a certain race team / car progam, who had to choose between a big bore or stoker in their cornering/road course car. They did a lot of testing, on dyno and track. The conclusion was, to use the BB. One of the reasons was because they could not keep the tires planted on corner exits, when accelerating with the stroker.

I look forward to reading this thread! I will stay out and let the experts hammer it out.

[04MACH!DSG]

I see this going in the same direction as the hp vs tq thread. I noticed that thread is no longer there either.

[na svt]

[littlemags]

very cool....
Iam particular to see how my combo turns out since I waiting on dyno results from my 3.80 stroker. Another member on ModularFords did a VERY similar build to mine..(almost exact) MINUS the stroke....

[massacre]

Definitely a great subject, and very informative.

[vertigyn]

Subscribing - interesting read!

[Fosters]

If you're going thru the trouble of building a stroker motor, might as well add 1k for the big bore boss 5.0 motor to the build cost, and get a big bore stroker. Makes no sense to skip on the 5.0 block.

[tmhutch]

Glad you guys are liking the tech stuff. Just for the sake of clarity, I dont think the 3.80 stroke is a big detriment over the big bore at reasonable RPM's. As affordable as it is to add cubic inches via stroke it's kind of a no-brainer. If you HAD to choose between one or the other it would be a different story and I will add that aluminum big bores have a checkered history of block flex and poor head gasket seal. The alternative being the iron 5.0 block and I think some of you did more than make a subtle point of your disdain for the added weight in the TORQUE VS HORSEPOWER thread.

As far as the certain race team and burning rubber, if you can direct me to the testing I may be able to flesh out what is going on there. It may be that class limitations created conditions more favorable to the longer stroke, or maybe they were pushing the limits of the combustion chamber design and it performed poorly at low RPM with the larger bore. It's hard to say without more detail but a radical change like bore/stroke modification doesnt exist in a vacuum. Other relationships are altered when this happens and a highly tuned race car that incorporates exacting relationships for any number of components can easily deliver unintended consequences.

Just to give an idea what I'm talking about, some of the secondary changes that occur from a change in stroke include rod/stroke ratio, piston weight and the relationship between IVO to piston location in the induction stroke. When the induction cycle is tuned perfectly from intake to exhaust and any of the above components are altered, there will be changes in an engines performance. But to the extent that tires are burning rubber from the change indicates room for improvement somewhere else. Whatever the case, it isn't the result of additional leverage.

[THE_KILLER_TOFU]

As the "OP", thanks for this write-up. I had pretty much been sold on the stroker kit being the best way to go. I'll make a little effort to do more research next time around. Any idea how much the FRPP Boss Block weighs over our WAPs?

[tmhutch]

There's nothing wrong with the stroker kit. It's one of the best bang for the buck changes we can make with our cars. Under 7000 RPM you wont see more than 5'ish HP/TQ difference.

The Boss block is about 80 pounds heavier, about the same as the pre-NVH Navigator block. Small penalty to pay for the extra cubic inches in my opinion. I do understand the reluctance to pay the extra $1000, though.

[massacre]

Originally Posted by tmhutch

I will add that aluminum big bores have a checkered history of block flex and poor head gasket seal.

Aren't some people getting around the block flex by filling the block? I thought I had seen something about that somewhere....

Also, I was wondering what role cam timing would play in this whole thing. IOW, would you guys set the cam timing up differently for a BB motor VS a stroker?

[na svt]

Originally Posted by tmhutch

Under 7000 RPM you wont see more than 5'ish HP/TQ difference.

Has this been confirmed thru extensive dyno testing?

Posting your references would add greatly to this thread.

Originally Posted by tmhutch

I will add that aluminum big bores have a checkered history of block flex and poor head gasket seal.

Poor head gasket seal is very rare, if not non-existent when dry sleeves are used.

[Fosters]

Originally Posted by massacre

I was always taught that on a basic level, a bigger bore/shorter stroke results in an engine that has higher RPM capacity, whereas a smaller bore and longer stroke results in more low-end but that doesn't rev as high. Now obviously this isn't true in all cases, there are many variables that need to be considered, but I thought that the weight of the longer rods and bigger crank throws plus the rod angle was some of the reason for this. I could be way off though.

That, and piston speed. If I remember right, the 2000 cobra R 5.4 at 7000 rpm reaches formula 1 piston speeds, because of the long stroke. If we had a bigger bore to begin with - like the 4.0" bore on the pushrod 302 - then the stroke would be shorter and wouldn't be approaching the more crazy piston speeds.

[na svt]

Originally Posted by Fosters

That, and piston speed. If I remember right, the 2000 cobra R 5.4 at 7000 rpm reaches formula 1 piston speeds, because of the long stroke.

It's about 6fps slower than a 20,000rpm formula 1 engine, but then again the 2.4 liter F1 stroke is only about 1.57". The 5.4 stroke is 4.165".

Originally Posted by massacre

Aren't some people getting around the block flex by filling the block? I thought I had seen something about that somewhere...

Block filling is a common practice for race engines when allowed. However, it's my understanding that even Mihovetz's doesn't fill his block and its safe to say that he pushes them to their limits.

Originally Posted by massacre

I was always taught that on a basic level, a bigger bore/shorter stroke results in an engine that has higher RPM capacity, whereas a smaller bore and longer stroke results in more low-end but that doesn't rev as high. Now obviously this isn't true in all cases, there are many variables that need to be considered, but I thought that the weight of the longer rods and bigger crank throws plus the rod angle was some of the reason for this. I could be way off though.?

A smaller bore has no benefit when it comes to hp or tq; larger is always better. Strokers can rev high but not as efficiently as shorter stroke combos, Al P and Modular Performance spun their 5.4s and 358s to well over 8000rpm. There are guys spinning 4.25" stroke, 496 big block chevys to 8000rpm.

Stroker rods are usually shorter in order to get the wrist pin out of the ringland.

[massacre]

Originally Posted by na svt

A smaller bore has no benefit when it comes to hp or tq; larger is always better. Strokers can rev high but not as efficiently as shorter stroke combos, Al P and Modular Performance spun their 5.4s and 358s to well over 8000rpm. There are guys spinning 4.25" stroke, 496 big block chevys to 8000rpm.

Stroker rods are usually shorter in order to get the wrist pin out of the ringland.

Ah, thanks guys for your perspective. Interesting about the stroker rods being shorter, too with the pin lowered, I didn't think of it right away but it makes perfect sense.

Obviously there is some "sweet spot" when it comes to a compromise between bore/stroke, is it just a "square"(meaning that bore & stroke are the same) motor that is the sweet spot? Or does it depend on application, like maybe more of one or the other is better in a certain application?

[Fosters]

Originally Posted by na svt

It's about 600fps slower than a 20,000rpm formula 1 engine, but then again the 2.4 liter F1 stroke is only about 1.57". The 5.4 stroke is 4.165".

4.165 * 2 * 7000 = 58310 inches per minute / 12 = 4859 ft per min / 60 = 80.98 feet per second.

1.57 * 2 * 20000 = 62800 inches per minute / 12 = 5233 ft per min / 60 = 87 feet per second

1.57 * 2 * 18000 (they're limited to 18k now) = 56520 inches per minute / 12 = 4710 ft per minute / 60 = 78.5 feet per second.

Where the hell did you get 600 fps? That's bullet speed (ok, maybe a slower 45)... 5.4's don't run on gun powder, they wouldn't get 26.5 mpg

80.98 fps = 61.35mph...

[Toby]

Wow. I agree with NASVT. Correct me if I'm wrong but, I get the feeling youve played with the older 302, 351 pushrod engines before going modular.

[na svt]

Originally Posted by Fosters

Where the hell did you get 600 fps? That's bullet speed (ok, maybe a slower 45)... 5.4's don't run on gun powder, they wouldn't get 26.5 mpg

6fps, sorry about that, I've been thinking in "hundreds" all day.

[Stopsign32v]

Why wouldn't you go big bore/stroker combo? I don't understand going one way or the other over both unless there is something I don't know.

[massacre]

Originally Posted by Toby

Wow. I agree with NASVT. Correct me if I'm wrong but, I get the feeling youve played with the older 302, 351 pushrod engines before going modular.

If you are referring to me, you are correct on this. When I started working on engines, there were no modulars lol.

And it's not about agreeing/disagreeing, I was just throwing out some ideas for the purpose of discussion.

I was just wondering if there might be an application where you want the bore but not necessarily the stroke?

[tmhutch]

Originally Posted by massacre

Aren't some people getting around the block flex by filling the block? I thought I had seen something about that somewhere....

That works great for a race engine but a street car even with partial fill and acceptable coolant temps will cook the oil from the solid part of the block transfering too much heat to the pan.

Originally Posted by massacre

Also, I was wondering what role cam timing would play in this whole thing. IOW, would you guys set the cam timing up differently for a BB motor VS a stroker?

A stroker (vs the big bore) will put more demand on the cylinder head, and it will change how the piston dwells at TDC and BDC (assuming the piston pin isn’t moved). Both of these require changes to cam timing to achieve perfection but these are considerations reserved for big money competitors who are striving for every last ounce of horsepower. If you are interested in cam timing read my “sticky” at the top of the forum. Warning, it’s very technical and not for the faint of heart. That being said, picking a cam is pretty easy because the manufactures have figured out all the hard stuff. All we have to do is put it in there and degree it.

Originally Posted by massacre

Obviously there is some "sweet spot" when it comes to a compromise between bore/stroke, is it just a "square"(meaning that bore & stroke are the same) motor that is the sweet spot? Or does it depend on application, like maybe more of one or the other is better in a certain application?

Nah, there really isn’t any sweet spot. You’ll see OEM’s trending toward square engines but that’s a combination of efficient combustion characteristics (smaller bore) for emissions and the engineered operating range of the engine not pushing piston speeds into inefficient territory. Rule number one is cubic inches. The more the better until stroke gets so big piston speed is limiting RPM. If you’re wondering where that is consider Pro-Stock engines are turning close to 9000 RPM… with a 5.75” stroke. As is true with all levels of professional racing that information has absolutely no application to us mere mortals but it does throw a little perspective into the mix.

Originally Posted by Stopsign32v

Why wouldn't you go big bore/stroker combo? I don't understand going one way or the other over both unless there is something I don't know.

No reason I am aware of other than more important places to spend the extra grand.

Originally Posted by na svt

Has this been confirmed thru extensive dyno testing?

That a big bore wont make more power? Please clarify.

Originally Posted by na svt

Posting your references would add greatly to this thread.

I wish I could hook you guys up with a book or something that puts all this information in one convenient reference but I’m not aware of any such resource. What I write here is the result of 30+ years experience and paying attention to people a lot smarter than myself. If you have any questions, please be specific and I’ll do my best to answer them.

Originally Posted by na svt

Poor head gasket seal is very rare, if not non-existent when dry sleeves are used.

A big bore without the stroker crank will do better. I’ve seen enough guys have problems with the dry sleeves/stroker/aluminum block to be leery of the combination. The most recent was a Tymenski built engine.

[na svt]

Originally Posted by tmhutch

That a big bore wont make more power? Please clarify.

no, this

Originally Posted by tmhutch

Under 7000 RPM you wont see more than 5'ish HP/TQ difference.

Originally Posted by tmhutch

A big bore without the stroker crank will do better. I’ve seen enough guys have problems with the dry sleeves/stroker/aluminum block to be leery of the combination. The most recent was a Tymenski built engine.

That wasn't a head gasket sealing issue.

[JeromeMach1]

I plan to add a stroker crank to my BB motor. Will this reduce the reliability of the head gasket seal? How much of a HP and TQ increase should I see?

Thanks,
Jerome

[na svt]

Originally Posted by JeromeMach1

I plan to add a stroker crank to my BB motor. Will this reduce the reliability of the head gasket seal? How much of a HP and TQ increase should I see?

Thanks,
Jerome

With your cams the power should go up another 20-25rw give or take a couple. As for the reliability of the head gasket seal, it won't change. The problem that he's referring to was not an issue with gasket sealing.

More cubes, more fun.