Bore to Stroke Ratio

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Dan the man

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How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
 
What are your goals for the engine. What kind of driving?
 
Dan the man said:
How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
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Bore/stroke, 1.1-1.2 suppose to be ideal power to friction compromise.

When building power there's the big thing cam heads cr carbs etc... Gets the bulk of hp per $$$$ then comes the littles thing that can add up hp but $$$ per hp gain is small but usually matter in highly competitive racing and or endurance.
Is it needed depends on your bank account or if there's actual major misalignments.
 
Dan the man said:
How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
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If you want to worry about this, get a 383 or 400 and call it a day.
This is small potatos, don't try to reinvent the wheel, if you want to go nuts, focus on heads.

Of course, I could be wrong.
 
Why worry about something you will not change. Deck hights should be equal, but do not have to be perfect. My 383 was .012 from one side to the other. It got decked, just enough to get them equal and clean up both sides. I have never done a main bearing bore alignment. Set the crank in, on oiled bearings, torque the front and rear main caps and check crank with a dial indicator on the middle bearing. If good, install and torque all the main caps and spin the crank. If it spins nice by hand, move on you are good to go.
 
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Square the bore and stroke as close as possible . Example is 4 inch stroke 4 inch bore.

Square deck off of the crank mains .

Do all machine work with two torque plates bolted on not just one per side when honing

True the lifter bores many of these engines are not trued or are worn bad due to being 50 year old high mileage engines.

The most important is balancing. The closer you get to 0 grams the more power you will make up in RPM's. Balancing is the most critical.

Sufficient oil to the rockers and springs. Springs get hot they need oil to cool them.

There are many things to take into consideration when going for performance and durability. these are just some basic items to consider. RING GAP IS VERY IMPORTANT and changes are made depending on the fuel you will be using and power adders such as boost or NOS.

This is info I get from my son. He has done many engines and the line to get in here is very long.
 
Bore to stroke ratio is the relationship between the width of the piston and the distance it travels.
My Mopar Performance book says the best ratio for circle track is 1.5 - 1.75 the best ratio for
drag racing is 1.75 - 1.85
318/340 ratio is 1.85 and the 360 ratio is 1.71 so i don't think you have to worry which is best for your street machine
 
ir3333 said:
Bore to stroke ratio is the relationship between the width of the piston and the distance it travels.
My Mopar Performance book says the best ratio for circle track is 1.5 - 1.75 the best ratio for
drag racing is 1.75 - 1.85
318/340 ratio is 1.85 and the 360 ratio is 1.71 so i don't think you have to worry which is best for your street machine
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Those are rod to stroke ratios and not bore to stroke ratios.
 
tx..just re read the page and you are correct.
The 360 bore to stroke ratio is .89. Says a ratio above .85 is considered a long stroke and makes more torque...best for street engines.Less than .85 is considered short stroke. Short stroke engines make more hp per cubic inch and are best for racing.
That's from the engineers..me i don't know
 
Dan the man said:
How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
Click to expand...

The easiest way to look at bore to stroke relationships is over square, under square and square.

A square engine is when the bore and stroke are the same number, or close to it. An example would be a 4.030 bore and a 4 inch stroke would be a square bore to stroke ratio.

An example of an under square engine is a 3.940 bore (.030 over 318) and a 4 inch stroke. That’s over square because the stroke is longer than the bore is in diameter.

An example of an over square engine is a 4.04 bore (standard 340) and a 3.313 stroke. The bore is bigger in diameter than the stroke is in length.

As a general rule, an over square engine will make more power. An under square engine will be intake valve diameter limited and therefore induction limited and they produce diesel like torque curves but they will always be down on power and RPM compared to an engine with an over square bore to stroke ratio of equal displacement.

A square engine is in between.

All this means is that as the stroke increases in relationship to the bore diameter the engine will be more induction limited. You can’t fit a bigger intake valve in a smaller bore. And a smaller valve means a smaller port. The smaller port won’t keep up with the air demands you might require. That’s why they get diesel like torque curves but they don’t make power and we all know what that means don’t we???

There isn’t a block out there that doesn’t need to be decked. They should be square decked. It’s not hard to do. Look at it this way. If you bought a set of pistons and half of them had the wrist pin .010-.012 lower than the other 4 (or more…I’ve seen blocks that were .020 off from end to end and that much side to side) wouldn’t you be pissed? You’d call that junk right? So would I because that’s what it is. Junk. So is not squaring the decks.

You are limited on rod length to 6.123 unless you want to turn the rod journals down to 2.100 (SBC large journal size). If you do that you can find longer rods than 6.123 plus you have a much better selection of rod bearing available.

Just like rod length, you are pretty much married to what ever bore diameter Chrysler gave you unless you get an aftermarket block. And any more, you don’t have much of a choice in stroke lengths either. You get what Chrysler gave you or you get a 4 inch stroke. You might still find a 3.79 stroke crank around, but it’s a beeotch getting pistons for them IF a you want to buy your pistons off the shelf. If you don’t mind ordering a custom piston then the 3.79 stroke crank is a good option.

As a general rule, I’m not a fan of over square engines. You can make a bunch of torque but what does that get you? You can smoke the tires easier?

But this is all just mental masturbation because for a 12.5 second car you can use a correctly spec’d and built 318 and do it all day long. Or a 340 or a 360. You don’t need a stroker crank. You can run in the 11’s and not square deck the block (BTDT). You should at least deck the block (you wouldn’t rebuild a set of heads and not deck those would you??? I know I won’t do it) so that should be done. Then to get the decks equal isn’t much more work. Either way the block should at least be decked.
 
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This is talked about a lot in MotoGP as some of the bikes are “Screamers” and some are “Big Bang” aka Under bore and over bore. And it changes year to year as motors are being developed.

As Rat said, your limited to the parts you can get off the shelf. My brother builds custom Cosworth racing engines, seeing the costs for custom parts to rebuild these things is more than your willing to spend for a turn key car you’ve not purchased yet.

Clearly your way over thinking these kind of things. It not like your racing and the B/S is going to add anything to engine combo for improved lap times. Also we’re talking about 50-60 year old engines. There are what they are… Engine development is expensive. Just look at the cost of a Steve Morris engine…
 
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Rat Bastid said:
An example of an over square engine is a 3.940 bore (.030 over 318) and a 4 inch stroke. That’s over square because the stroke is longer than the bore is in diameter.

An example of an over square engine is a 4.04 bore (standard 340) and a 3.313 stroke. The bore is bigger in diameter than the stroke is in length.
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YR, you need a small revision in your discription here.
 
Nothing wrong with asking questions just to learn . Thats the difference between builders and buyers ...
 
Scody21 said:
This is talked about a lot in MotoGP as some of the bikes are “Screamers” and some are “Big Bang” aka Under bore and over bore. And it changes year to year as motors are being developed.

As Rat said, your limited to the parts you can get off the shelf. My brother builds custom Cosworth racing engines, seeing the costs for custom parts to rebuild these things is more than your willing to spend for a turn key car you’ve not purchased yet.

Clearly your way over thinking these kind of things. It not like your racing and the B/S is going to add anything to engine combo for improved lap times. Also we’re talking about 50-60 year old engines. There are what they are… Engine development is expensive. Just look at the cost of a Steve Morris engine…
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And in Moto GP you have (at least it used to be…I haven’t followed Moto GP in awhile) twins running up against in line 4’s and they have to compensate with displacement differences.

There are so many factors that go into bore/stroke and rod/stroke ratios that you can get in the weeds real quick.
 
Rat Bastid said:
And in Moto GP you have (at least it used to be…I haven’t followed Moto GP in awhile) twins running up against in line 4’s and they have to compensate with displacement differences.

There are so many factors that go into bore/stroke and rod/stroke ratios that you can get in the weeds real quick.
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V4’s vs inline 4’s with 990CC pushing over 250HP. Aka Rocket Ships…
 
Dan the man said:
How is this calculated? Do sbm's have a good bore to stroke ratio?
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Does it really matter? Like, is it going to change your mind and make you get a different engine.?
You just like a chat don't you.?
 
Bakerlite said:
Does it really matter? Like, is it going to change your mind and make you get a different engine.?
You just like a chat don't you.?
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I don’t what wrong with OP trying to learn all he can, only down side is some get hung up on little details that’s can cause a build to get out of hand.
Got keep perspective that’s all.
 
Rat Bastid said:
The easiest way to look at bore to stroke relationships is over square, under square and square.

A square engine is when the bore and stroke are the same number, or close to it. An example would be a 4.030 bore and a 4 inch stroke would be a square bore to stroke ratio.

An example of an under square engine is a 3.940 bore (.030 over 318) and a 4 inch stroke. That’s over square because the stroke is longer than the bore is in diameter.

An example of an over square engine is a 4.04 bore (standard 340) and a 3.313 stroke. The bore is bigger in diameter than the stroke is in length.

As a general rule, an over square engine will make more power. An under square engine will be intake valve diameter limited and therefore induction limited and they produce diesel like torque curves but they will always be down on power and RPM compared to an engine with an over square bore to stroke ratio of equal displacement.

A square engine is in between.

All this means is that as the stroke increases in relationship to the bore diameter the engine will be more induction limited. You can’t fit a bigger intake valve in a smaller bore. And a smaller valve means a smaller port. The smaller port won’t keep up with the air demands you might require. That’s why they get diesel like torque curves but they don’t make power and we all know what that means don’t we???

There isn’t a block out there that doesn’t need to be decked. They should be square decked. It’s not hard to do. Look at it this way. If you bought a set of pistons and half of them had the wrist pin .010-.012 lower than the other 4 (or more…I’ve seen blocks that were .020 off from end to end and that much side to side) wouldn’t you be pissed? You’d call that junk right? So would I because that’s what it is. Junk. So is not squaring the decks.

You are limited on rod length to 6.123 unless you want to turn the rod journals down to 2.100 (SBC large journal size). If you do that you can find longer rods than 6.123 plus you have a much better selection of rod bearing available.

Just like rod length, you are pretty much married to what ever bore diameter Chrysler gave you unless you get an aftermarket block. And any more, you don’t have much of a choice in stroke lengths either. You get what Chrysler gave you or you get a 4 inch stroke. You might still find a 3.79 stroke crank around, but it’s a beeotch getting pistons for them IF a you want to buy your pistons off the shelf. If you don’t mind ordering a custom piston then the 3.79 stroke crank is a good option.

As a general rule, I’m not a fan of over square engines. You can make a bunch of torque but what does that get you? You can smoke the tires easier?

But this is all just mental masturbation because for a 12.5 second car you can use a correctly spec’d and built 318 and do it all day long. Or a 340 or a 360. You don’t need a stroker crank. You can run in the 11’s and not square deck the block (BTDT). You should at least deck the block (you wouldn’t rebuild a set of heads and not deck those would you??? I know I won’t do it) so that should be done. Then to get the decks equal isn’t much more work. Either way the block should at least be decked.
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I never heard of anyone running a surface cut on the deck of a block without square decking it off of the crank tunnel. That would be a waist of time and money.
 
Oldmanmopar said:
I never heard of anyone running a surface cut on the deck of a block without square decking it off of the crank tunnel. That would be a waist of time and money.
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I have..
A lot of reco places just square up the deck to the cutter and take the same amount off each side.
 
Dan the man said:
How is this calculated? Do sbm's have a good bore to stroke ratio? I know that they have a awesome rod to stroke ratio. Machine shop suggested having the main bearing bore alignment checked and corrected if needed and then measure the block deck height and have the block decked and squared. Also he suggested having the longest rods fitted to the shortest ch pistons. Is this procedure worth it or a big waste of time and money?
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I'll try this from a simple man point of view.....

Any B/S ratio can work. You just need to match a particular engine combo with what you want out of it. Close to 1 to 1 makes a great torque engine. Big bore/sm stroke make for a winder.

So with the mains, as long as the engine is in decent condition and you have the original main caps, a hone should be all that you need for a street/bracket engine.

As far as squaring the decks to the crank centerline, that's one thing i would consider $ well spent.

Now to the long rods vs ch. Long rods can help with intake flow on restricted heads with the right cam as it will increase piston speed away from TDC/BDC and that normally helps in the mid/upper range. Again, with the right cam timing. But you can get to the extreme with that, and when you get to the oil rings crossing the pins it's only a good move for a race only engine.
 
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To the OP a lot of the info you will come across is more for racing 7000-9000 rpm engines not that we can't use some of the info. Bore size is the limiting factor when it comes to NA power cause that limits valve size which limits air flow but unless your building something crazy it not gonna effect most of us. Eg. 273 would be the smallest bore most of us would deal with and most of us should be able to get 400 hp or so and good builder who knows but 500 hp at least should be doable. With 318 or bigger bore the strength of the block be a more limiting factor. As for longer stroke the less rpm you'll need to turn to make the same power but over a certain ratio friction will be limiting factor but for most of us who generally wants to make peak power in the 5000-6500 rpm range it a fair trade off.

I think of cam and heads as the amount of power to be built and displacement is at what rpm's it will happen, obviously there's bunch of variables, more efficient the build will need less rpm than a poorly built engine.
 
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