Author Topic: Over-Square vs Under-Square Bore Configurations  (Read 16873 times)

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Offline BigMike

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Over-Square vs Under-Square Bore Configurations
« on: November 05, 2010, 10:34:14 AM »
I am wondering about bore/stroke configurations and their impact on revving.

I was under the assumption that a larger bore with a shorter stroke is advantageous for high revving applications. For instance, the 4A features an 81mm bore and a 77mm stroke.

However, the 2ZZ is the opposite. A longer stroke compared to its bore: 82mm bore and an 85mm stroke.

So which is which? Which engine is over-square and which engine is under-square? What is preferred for higher rpm power delivery? What do F1 engines use?

Thanks
Mike
« Last Edit: November 05, 2010, 10:42:31 AM by BigMike »
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #1 on: November 05, 2010, 10:35:58 AM »
I should add that it can be written as...

Over-Square bore/stroke   versus.    Over-square stroke/bore.

I understand that, I am just wondering which configuration is preferred for high revving applications?

Thanks
Mike
« Last Edit: November 05, 2010, 10:43:35 AM by BigMike »
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #2 on: November 05, 2010, 10:42:19 AM »
I've also heard that a square-engine is good for Torque. For instance, my 3RZ-FE has a square configuration of 95mm X 95mm

HOWEVER, take a look at Honda's high revving K20. It is a square engine, 86mm X 86mm


Now look at the S2000 engine, the F20C, it has the same over-square bore/stroke configuration of the 4A-GE, a larger bore with a short stroke. The F20C is 87mm bore and 84mm stroke. That is a 1.036:1 ratio compared to a 1.052:1 ratio of the 4A. Hmmmmmmm interesting

We all know the S2000 is a high revving engine, no doubt about that, so is it safe to say that a large bore with a short stroke is preferred for high revving? If so, then why is the Toyota 2ZZ the complete opposite?

Mike
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #3 on: November 05, 2010, 10:47:29 AM »
You know, the small bore of the 2ZZ puts a restriction on the maximum valve diameter compared to an opposite stroke/bore configuration of a same-size engine.

The 2ZZ is 82 bore & 85 stroke. If it was the opposite, a 85 bore & 82 stroke, it would still be a 1.8-liter, but it'd have more room for larger valves in the head :idea:

I think this has a lot to do with high revving. If you have a large bore, then you can fit large valves -- something very much needed for high rpm. If you have a small bore, then you have to use small valves which will put a restriction of flow at higher rpm.

I also know there is valve angle theory to consider as well. The Toyota engines always seem to use a wide angle whereas the Honda engines prefer a narrow angle.

Mike
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #4 on: November 05, 2010, 11:06:20 AM »
Valve angle comparisons

Wider-angle Toyota head (4A-GE): 50-degrees


Honda head:



Toyota did produce a narrow-angle 4A however, 37.5-degrees for the 20-valve:


I also read here that the S2000 F20C has a 51-degree angle head. WOW

Also the 2ZZ-GE has a 50-degree valve angle, just like the 4A-GE.
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Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #5 on: November 05, 2010, 12:42:47 PM »
When we were playing with our old school low tech engines this subject came up a LOT! One case is the GM 350 vs. 327. 350 got the 327 off the line hands down and held a lead to about mid track. About mid track the 327 came to life it seemed, and would overtake hte 350 and win by a length. Both engines had the same bore, but the 327 has a shorter stroke. Please don't ask me the numbers, nearly 30 years since I played that game. But! Adding length to the con rod seemed the help the 350 down the road. So when comparing the differences, consider the rod to stroke ratio too. Still, rhe best for revving, I believe, is an oversquare bore/stroke, with a long rod. I would consider using a 7A block with 1mm oversize pistons, 82mm, with a 5AG crank and rods long enough to fill the gap. Be a hell of a monster with the right tuning.
Now, throw in boost! I would stick with the square engine with a short rod for a supercharged engine. Big bore and stroke work best for torque. A short rod/stroke ratio works best in the lower RPM ranges. Since positive displacement superchargers fall off at the higher RPM ranges build the engine to operate there. The boost is linear to peak an drops off sharply. Turbos, and other centrifugal superchargers work at higher RPM ranges typically, and have a narrower boost range. The engine should be built accordingly. Any way you go about building an engine though, more is better! Just depends on what you want to do with it. Bore/stroke, rod/stroke, boost or N/A, NOS. 1/4 mile, oval, road, autoX, street, cruise. The engine I am building for my drive to work and home is a 7AGE with a mild duration cam and lift in the 9mm to 10mm lift. Maybe use the oversize valves too in a small port head. Should maintain much of the rev happy characteristics of the 4AGE but have the torque necessary to climb these hills as it should. Bore/stroke 81.5mm x 85.5mm, oversquare! Rod to stroke is about 1.5 - 1. (4AGE is 81 x77 with a rod/stroke of about 1.6 - 1) I could easily get 140/140 torque/ horse out of this combination with a relatively wide powerband. Now we need to discuss the head to go further on that bit though.

Volumes have been written and argued on this subject.
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Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #6 on: November 05, 2010, 12:44:09 PM »
Supercharge the above and YAHOOOoooo
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #7 on: November 06, 2010, 12:27:10 PM »
So when comparing the differences, consider the rod to stroke ratio too.
What? なに? これはなんですか? The rod is dimensionless, only the stroke of the crank determines the distance the rod will travel.

Still, rhe best for revving, I believe, is an oversquare bore/stroke, with a long rod.
Interesting! That is what the 2ZZ has and the 4A and S2000 engine do not have! Man this is such an interesting thing to think about :yesnod:

Hey, I just looked up the BEAMS 3S-GE and it is a SQUARE engine, like the K20: 86 X 86. Also, check this out, the blacktop version BEAMS, built in 1998, produced 207 HP @ 7600 and 159 TQ @ 6400 rpm, WITH NO VVTL!!! The best K20, the K20Z1 built in 2005, produces 210 HP @ 7800 and 143 TQ @ 7000 rpm WITH VTEC.

WOOOOOOOOOOW DUDE! Toyota has Honda beat!! Look at what they are doing with the same size engine without a super agressive top-end cam profile! Look it only makes 3 less horsepower but it makes 16 more ft-lbs of torque!! I bet you anything the BEAMS will smoke a K20Z1. WOW I didn't realize this and has totally made me more a believer in Toyota engines. TOO BAD WE DON'T GET THESE AWESOME ENGINES STATE-SIDE!!! :maddest:

A short rod/stroke ratio works best in the lower RPM ranges.
Isn't it the other way around? If you have a longer stroke, then the time at which the piston remains at TDC is increased, so more pressure is developed. The only bad side is that the longer stroke will also cause the piston to travel back down the bore with much greater velocity than compared to a short stroke... They should come up with a variable stroke engine. So at lower RPMs you could have a longer or shorter stroke, and as you rev the stroke changes. Wow that would be sweeeeeeeeet
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Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #8 on: November 06, 2010, 01:23:17 PM »
Ok, so a quick bit on rod/stroke ratio. First the shorter rod allows a faster piston acceleration from TDC reducing piston dwell at TDC. A longer rod slows acceleration from TDC and allpws a longer dwell at TDC. From a power standpoint, the combustion pressures are higher because combustion is more complete with the longer dwell. Slower acceleration form TDC keeps pressures high throughout the power stroke/cycle. The crank centerline/rod relationship is different also. a shorter rod reachesa the 90* angle to crank centerline earlier in the crank rotation than a longer rod. This give a few more degrees of crank rotation to produce peak power. Piston speed is faster in the cylinder bore with a short rod than with a long rod. Higher piston speeds increase frictional losses through the rings as well as an increase in heat produced by friction. With higher velocities comes another parasite, moment. Decelerating and accelaerating the piston increases wear on the bearings, increases oil temps and takes a bit of power to get the job done. I could go on all day on this subject, but, in a nutshell. This subject should spark your interest Mike. I read a lot on this subject decades ago. Why do you think I went with the 6" rod in my stroked 22R?
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Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #9 on: November 06, 2010, 01:36:38 PM »
By the way, I said tuning has a lot to do with the way an engine performs, either way. The 22R is an oversquare bore/stroke engine. Getting power out of that beast above 6000 RPM is tricky. The entire rotating assembly is massive as 4 bangers go. I had to lighten the rotating assembly, use a longer rod, higher compression, more rod bearing clearance, high volume oil pump, light weight rods and pistons, balance the thing to the tightest tolerances than the guy at the shop wanted to do, and still couldn't get power above 7200 RPM.  Power dropped off a cliff after 7500 RPM. That engine had around 210 hp at 6800 RPM and about the same in torque at about 5000 RPM. Held power  pretty well to about 7200 RPM and barely had enough to keep itself spinning at 7500 RPM. Almost like it had a rev limiter at 7500 RPM. Physical redline is the term  I believe. That was an expensive engine! You saw the rods I used. Wish I could get a set of thise for the 4AGE! A set of tose would probably weigh around 300 grams! The stock early rods are right at 490 grams. Late rods are like 530 grams.
« Last Edit: November 06, 2010, 01:43:14 PM by Sirdeuce »
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Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #10 on: November 06, 2010, 01:51:44 PM »
There is so much to absorb about those little engineering nightmares we refer to as cars. My favorite desciption of an engine is, a machine that is constantly trying to self destruct! Ther are so many destructive forces going on in that chunk of metal, if you subject a human being to a few seconds of those stresses it would disitegrate. Some of the greatest engineering feats are taken for granted under the hood of a car. DYuuuhh, put gas in the litte hole in the side of the car, get in , turn the key, point it down the road. Ain't nuthun toit.
"I slept with faith and found a corpse in my arms on wakening; I drank and danced all night with doubt and found her a virgin in the morning."

Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #11 on: November 06, 2010, 02:08:11 PM »
Don't get me started on cylinder boe centerline to crank centebore relationship. Combustion chamber? Port design? We could go on all year with this subject and never scratch the surface.
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Offline BryanH

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #12 on: December 10, 2010, 08:40:06 PM »
Didn't read the whole thread but F1 engines have very, very short strokes.  They do rev to 18,000rpm (used to be 20k rpm until they imposed a rev limit).

Starting in 2013 F1 will be using 1.6L 4cyl engines (I kid you not!) with turbocharging and direct injection.  Max revs will be 12,000rpm I believe.

Offline Sirdeuce

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #13 on: December 10, 2010, 09:41:38 PM »
Revive the 4AGE!!!!!
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Offline BigMike

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Re: Over-Square vs Under-Square Bore Configurations
« Reply #14 on: December 11, 2010, 02:07:50 AM »
WOW
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