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Old 09-30-2009, 06:19 PM
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Default More Stroke vs. More Bore

So I thought I would get some opinions from the experts on here. I'm building a 555 BBC this winter for the Chevelle. I true 555 is a 4.560" bore with a 4.25" stroke. However, I don't like the idea of buying a $2,300 block and immediately boring it .060" over. So I'm leaning towards a 4.500" bore with a 4.375" stroke (I know, it's really a 556). This preserves the bore and the longer stroke should make a little extra torque. If it were your motor, which would you opt for?

Also, I can get the 556 short block internally balanced and assembled with the Dart Big M block, Eagle 4340 crank and H-beam rods (with upgraded bolts), JE pistons and Total Seal rings from a shop in Nevada for $4,750 + shipping. I would rather give my money to someone local if anyone knows who might be able to give me a deal.

Thanks in advance for your opinions/advice.
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Old 09-30-2009, 06:52 PM
stockerbird stockerbird is offline
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Default Re: More Stroke vs. More Bore

Not being an expert I can only think of watching the rod angle with the longer stroke if you will be using the same connecting rods. I think a ratio of 1.75 is ideal but I don't know much about those really big Big Blocks, but it sounds like either way that will be one honkin motor!!!!
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Old 09-30-2009, 07:29 PM
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Default Re: More Stroke vs. More Bore

It all depends on what you are trying to do. The more stroke you throw at it the more torque you will make. With big cubic inch motors, you utilize the TQ more than the horsepower. We build stock 460 ford blocks with 4.5 stroke all the time. That is a 4.420 bore. A lot of the mopar's use 4.685 Ė 4.75 stroke length. You don't have to run the motors as hard.
The bore size comes into play with the heads. We run the bore big enough to clear the valves. The bigger the bore the better the head will work for you. (Gets the valves closer to the center of the bore) However, if you have a small head that cannot keep up with the cubic inch demand it is useless.
A lot of babble going on there, back on track I donít see any problem going 4.5 bore and increasing the stroke. It will hit the tires a little harder. Probably wonít have to run the motor as hard.
In our engine masters competition motors we always try to increase Stroke before Bore. And as far as rod ratio goes, we like to run around 1.5 -1.6. Typically, a 4.25 stroke is going to have a 6.7 rod (1.57). Our 4.5 stroke is going to have a 6.8 rod (1.51). And our 4.75 stroke is going to have a 7.1 rod (1.49). We run our 460 ci small block ford motors with a 4.25 stroke and a 6.125 rod (1.44).
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Old 10-01-2009, 06:10 AM
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Default Re: More Stroke vs. More Bore

Adding to Shane's comments (though he knows a helluvalot more than me, much more real world experience), increased stroke increases piston acceleration which can have a positive effect on intake tract velocities. Additional air mass requirements (due to piston acceleration) at lower RPM means the heads need to flow the required air mass at lower RPM, and the cam needs to have the timing and lift components that will allow low RPM cylinder filling.

To help compensate for valve shrouding, a very good machine shop (such as MPG) might have a few tricks up their sleeve when it comes to optimizing valve placement on the engine. As far as rod ratio, I wouldn't get too hung up on that as the correct ratio for an engine tends to be the result of whatever rod length is necessary to connect the piston to the crank. I think it's better to design around the stroke and deck height than the length of the rod.

The downside to adding stroke is the movement of mass away from the crank centerline which effects both moment of inertia and polar moment of inertia. The effect here is on response time and torsional harmonics. Lightweight parts, precision balancing, and carefull assembly are needed to help alleviate these factors.

IMO the benefits of adding stroke and keeping bore smaller far outweigh any of the downsides as long as the entire combination is optimized for the stroke. I would also recommend a local machine shop so that you can discuss all of your options face to face.

Good luck.
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Old 10-01-2009, 07:22 AM
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Default Re: More Stroke vs. More Bore

If I had to buy pistons I would keep it standard bore and stroke it. I had everything but a block so I bored my new block to fit what I had. I think the new blocks come .010 under (mine did) and need to be bored anyways. Are you going to tub the car? How fast are you trying to go? street drive it? Call Gary DeWerth @ Colorado Custom Cylinder Heads 303-761-9077 he can help you with your decisions. I have talked with him in the past about the same thing and I would use him before I bought one assembled from a shop out of state. He will be near the same price with better parts and better machine work. and you can get it exactly the way you want it.

Steve
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Old 10-02-2009, 01:53 PM
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Waylon Waylon is offline
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Default Re: More Stroke vs. More Bore

Thanks for the advice. You all pretty much confirmed my thoughts. One shop tried to talk me into going with the bigger bore and shorter stroke. I'll definately give Gary a call and see what he can do. Thanks again.
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