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Old 11-10-2010, 07:56 PM   #1
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Big bore/short stroke vs smaller bore/long stroke

I am wondering the pros/cons of both scenarios and hoping to gain a better understanding of engines in general. Please correct me if I'm wrong, but from my understanding, a bigger bore and shorter stroke is going to yield better motor breathing and more HP up top, as well as an ability to spin the motor faster. A smaller bore and longer stroke (stroker motor for example) is going to give you more torque down low due to the longer rods and torque/angle on the crank shaft, but the motor is going to have problems spinning faster and also might have less reliability down the road with the more awkward angles of the engine internals. I would also like to know which motor takes boost better and why, and which motor will like an N/A setup better.
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Old 11-10-2010, 08:56 PM   #2
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nobody huh?
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Old 11-11-2010, 01:50 AM   #3
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You are correct, generally a shorter stroke will be more rev happy, longer ones like the lower RPMs. I would think the shorter stroke would be better for turbo to spin it up faster, but that is just an assumption. Longer stroke engines are great for NA. Both really can use forced induction though.
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Old 11-11-2010, 03:08 AM   #4
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Small bore is great for boost!
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Old 11-11-2010, 03:44 AM   #5
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Quote:
Originally Posted by nate1235...View Post
I am wondering the pros/cons of both scenarios and hoping to gain a better understanding of engines in general. Please correct me if I'm wrong, but from my understanding, a bigger bore and shorter stroke is going to yield better motor breathing and more HP up top, as well as an ability to spin the motor faster. A smaller bore and longer stroke (stroker motor for example) is going to give you more torque down low due to the longer rods and torque/angle on the crank shaft, but the motor is going to have problems spinning faster and also might have less reliability down the road with the more awkward angles of the engine internals. I would also like to know which motor takes boost better and why, and which motor will like an N/A setup better.

If a big bore/short stroke engine and a small bore/long stroke engine were designed by professionals, there would be no reliability issues due to proper angles used in the design, not some fabbed up aftermarket stroker set-up sold just for cube increase from a stock block.

Other than that you covered most of the basics.

Both love boost, it just enhances each engines characteristics.
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Old 11-11-2010, 03:52 AM   #6
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Originally Posted by nate1235...View Post
nobody huh?

There's volumes written on it in ME textbooks. You're not going to get the knowledge you seek in 30 words or less on teh interwebz
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Old 11-11-2010, 05:11 AM   #7
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Big bore/big stroke. LS3+4" crank. 4.030" bore, and 4" of throw. Big breathing up top, deshrouded valves, and more torque from the longer rods (which is all hp is anyway).

Our engines are designed to rev. Having a longer stroke won't hamper that.
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Valve shrouding on a stock motor is fine though, because ultimately you're just going to blow it up anyways, and then bore out the motor. Voila, valve unshrouded.

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Old 11-11-2010, 05:14 AM   #8
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^^^^

Go as big as you can, just be mindful of super high revs, and short piston skirts.
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Old 11-11-2010, 05:42 AM   #9
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rod length is the same typically in the LS builds, whether it is 4" stroke or stock
The difference is taken out of the piston (compression height) moving the pin up into the oil ring pack. This can cause some issues with reliability and oil consumption but in general seems to work ok.
For N/A this is usually seen as a good thing because the pistons are lighter.
If you want to pound a good bit of boost, I prefer a strong piston...thicker top section by keeping the top ring groove spaced down from the top, and a pin that is not in the oil ring pack. For moderate boost (<10psi) it's not as critical obviously as many stock engines survive quite a long time (mine included).
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Old 11-11-2010, 06:36 AM   #10
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Originally Posted by mistermike...View Post
There's volumes written on it in ME textbooks. You're not going to get the knowledge you seek in 30 words or less on teh interwebz

Yup - and EMC results made a fool out of a lot of internet and magazine experts.

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Old 11-11-2010, 07:08 AM   #11
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There's volumes written on it in ME textbooks. You're not going to get the knowledge you seek in 30 words or less on teh interwebz

Even when you do get the information you are seeking (on this forum) ......it will be buried under incomplete, inaccurate and false information.
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Old 11-11-2010, 10:20 AM   #12
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When its time I am going with a sleeved block, bored as far as it will go and stock stroke crank. Thats gonna be a lot of fun.
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Old 11-11-2010, 10:24 AM   #13
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Re-sleeved blocks are about as much fun as a sticky doorknob.
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Old 11-11-2010, 10:29 AM   #14
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Quote:
If a big bore/short stroke engine and a small bore/long stroke engine were designed by professionals, there would be no reliability issues due to proper angles used in the design, not some fabbed up aftermarket stroker set-up sold just for cube increase from a stock block.

Interesting. I thought I remember reading an article that talked about stroker motors and the more the extreme the stroke gets, the harder it is on the engine internals because of the odd torque angles and parts slightly warping especially at high rpms.
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Old 11-11-2010, 10:33 AM   #15
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Simply put boost is about PSI. The larger the bore the more SI. Long stroke pulls more air and fuel in but if it is forced in big bore is king.
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Old 11-11-2010, 10:48 AM   #16
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I also thought it was interesting to read that immediately after the gas/air combusts on the power stroke is when the explosion gives the most energy and transfers it to the piston. Shortly thereafter the amount of energy pretty much falls on its face and there isn't much more benefit from making the stroke longer (as far as drawing energy from the air-fuel explosion). Seems to me that you would want to have this ideal event happen over and over again as quickly as possible in order to draw as much energy as you could from combustion. big bore/small stroke would win there.
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Old 11-11-2010, 11:00 AM   #17
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Cliffs:

In mechanical terms, the definition of torque is the force acting on an object that causes that object to rotate. In an internal combustion engine, the pressure produced by expanding gases acts through the pistons and connecting rods to push against the crankshaft, producing torque. The mechanical leverage is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC).


So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.


The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.

Here's the link if anyone is interested in reading the full article. It was very informative IMO. http://www.rehermorrison.com/blog/?p=220
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Old 11-11-2010, 01:39 PM   #18
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I like sticky doorknobs.
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Old 11-11-2010, 01:53 PM   #19
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I like sticky doorknobs.



Srsly, though. When you weight the cost of doing sleeves properly and the bad results some folks get with them, unless road racing, I'd go iron in a heartbeat.
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Old 11-11-2010, 01:59 PM   #20
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RHS aluminum block and 502ci.






no sticky door knob for me, thanks.
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Old 11-11-2010, 02:23 PM   #21
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If I do sleeve it, it will go to Erik.
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Old 11-11-2010, 03:02 PM   #22
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ERL Performance is another highly touted shop you might consider. Erik has spoken very highly of them and if I'm not mistaken has done a "joint project or two" with them
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Old 11-11-2010, 03:05 PM   #23
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I like my MID Darton Sleeved sticky doorknob from Callies
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Old 11-11-2010, 04:21 PM   #24
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RHS aluminum block and 502ci

yumm...


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Old 11-12-2010, 04:04 PM   #25
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This thread has so much going on with it that I am afraid to touch it with a stick...

Are you asking this just to learn, or are you trying to set a race motor up for a specific hp or performance goal?
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Old 11-13-2010, 08:21 AM   #26
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RHS aluminum block and 502ci.






no sticky door knob for me, thanks.

I have to say that my favorite part about the RHS block is the fact that you no longer have to pull the heads to get to the lifters.
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Old 11-13-2010, 08:40 PM   #27
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Quote:
Are you asking this just to learn, or are you trying to set a race motor up for a specific hp or performance goal?

Trying to figure out a build I'd like to do. I don't think I would like to build my 5.7 up at all, but rather upgrade my drivetrain for now and start planning my new motor. I think I'd like to go iron block since the GTO isn't much of a handler anyway and no matter what I end up doing, I want to throw some boost at it with a blower. I'm thinking with an iron block around the 6.0 L range with a big bore and smaller stroke with a mean super charger is going to be the best route. It'll be very nice power down low, but not too much to where it's impossible to hook up and my first 3 gears are useless, but when my rpms get higher, the power really kicks in. I'm mostly trying to get a better understanding of motors as far as stroke vs bore, but those are my goals, I think.
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Old 11-14-2010, 06:54 AM   #28
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iron block bored .060, stock crank....good pistons like I described above...that will meet your goals without spending large $$$

that would be 4.060 bore, 3.622 stroke, 6.125 (commonly) rod. A shorter rod will give you more meat on the piston if you have the cash as it won't be off the shelf anymore
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Old 11-14-2010, 07:01 AM   #29
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long stroke w/ small bore- more torque & is something goes wrong you have room to bore the block again
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Old 11-14-2010, 07:37 PM   #30
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Quote:
iron block bored .060, stock crank....good pistons like I described above...that will meet your goals without spending large $$$

that would be 4.060 bore, 3.622 stroke, 6.125 (commonly) rod. A shorter rod will give you more meat on the piston if you have the cash as it won't be off the shelf anymore

Which iron block?
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