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Old 12-02-2005, 09:05 PM   #1
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347 - Short Stroke vs. Long Stroke Comparison...

Say I was to purchase one of these Boss 302 blocks set to come out next year....

I'm just curious about what the engine's characteristics would be if I was to go with a 3.25'' stroke, and a 4.125'' bore, versus the more often 3.4'' stroke, 4.030'' bore?

Same cubic inch displacement. With the bigger bore and shorter stroke, would it want to pull harder at higher rpm's, and would it still want to make the same amount of torque?

I'm planning this build for a street car. A mild solid-roller comes to mind, with at least an AFR 205/TFS 'R', and a Edelbrock super victor jr. (EFI).....

Anyhow, which bore/stroke combination would you go with, and why?

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-Anthony-
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Old 12-02-2005, 09:22 PM   #2
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In such an application, I will always take bore over stroke with today's heads that will support plenty of cfm to support the rpm range. Camshaft and valve timing events will different between the two as well despite the same CID.
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Old 12-02-2005, 09:26 PM   #3
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Larger bore helps with unshrouding of the valves

Your pistons end up with better compression height

It will be more Rev-happy, torque will not suffer
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Old 12-02-2005, 09:31 PM   #4
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I would agree on taking bore size over stroke. With the shorter stroke the piston speed will also be lower.
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Old 12-02-2005, 09:34 PM   #5
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The bore to stroke ratio has no primary effect on the torque output of an engine contrary to popular belief. There are secondary effects as have been pointed out. A larger bore unshrounds the valves more and a shorter stroke ALLOWS higher engine rpm for a given piston mass.
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Old 12-02-2005, 10:11 PM   #6
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Pretty interesting points made....

I definately like the idea of slowing down the piston speed at a given rpm, less wear and tear parts I would think....
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Old 12-02-2005, 10:28 PM   #7
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I would do the larger bore also but don't know what the bore limits of a 302 Boss block would be. The Rblock is 4.185.

I would do the TFS R ported head, TFS Hi port Stage 3 head or how about the new Canfield head. a Comp Cam XE282HR or custom cam and the Victor EFI port matched. The super Victor is too large for a 347 stroker. Solid roller's on the street r a pain.
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Old 12-03-2005, 01:01 AM   #8
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IMO go for stroke hands down. Piston speed is power. With a big 4.125" bore you're also using a heavier piston as opposed to 4.03" so keep that in mind. I believe there was a thread on hardcore50 about this a while ago and if I recall, guys were saying go for stroke rather than bore.

Any engine will rev high as long as you have the proper parts to support it.

If you're dead set on goign big bore use a 3.40 stroke crank, 4.125" bore and enjoy your 363" (I think it comes out to a 363)
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Old 12-03-2005, 04:24 AM   #9
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If using no power adder, run a 3.5 stroke crank and a 4.130 bore if the block can take it. The pistons are only .050 shorter than a 347 ( 3.4 stroke) piston and the .005 oversize on the 4.125 spec will allow for exacting fit on piston to cylinder as a honeing is needed. I have a 357 inch stroker 302 in the work and have all the parts execpt for the pistons. I am now going to wait and see when the new Ford block is available what it can take. Otherwise I have a late model roller block I can use. I planned on using some shaved 347 pistons and some Brodix heads.

Take the big stroke and the big bore in one package
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Old 12-03-2005, 11:08 AM   #10
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Quote:
Originally Posted by Grn92LX
IMO go for stroke hands down. Piston speed is power. With a big 4.125" bore you're also using a heavier piston as opposed to 4.03" so keep that in mind. I believe there was a thread on hardcore50 about this a while ago and if I recall, guys were saying go for stroke rather than bore.

Any engine will rev high as long as you have the proper parts to support it.

If you're dead set on goign big bore use a 3.40 stroke crank, 4.125" bore and enjoy your 363" (I think it comes out to a 363)

Don't forget to factor in the weight of the different crankshafts and rods since lengths are not the same between rotating assemblies. As for the Hardcore discussion the conclusion was for the SAME CID engine getting your cubic inches thru bore was better than stroke, which is what the question is for this thread, You are correct that the consensus there is use as much stroke as possible to get as much displacement as possible but not at the expense of bore size.
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Old 12-03-2005, 11:15 AM   #11
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Is the 3.25 kit available in a ong rod version?

Smokey liked the rod ratio for good reason. Every little bit helps. More durable as well.
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Old 12-03-2005, 11:26 AM   #12
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As has been stated before in the past the only rod ratio we should be concerned about is 8 rods to 1 crank. For all of our combos and how we use them, rod ratio should not even come into the discussion. I've had the fortune to develop good relationship with good engine builders none of them talk about rod ratio.
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Old 12-03-2005, 11:38 AM   #13
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....I agree with tached....

On stroke vs. bore I agree with Jack H.

"Piston speed is power." I don't see it that way. Power/torque comes from optimal cylinder filling and achieving max. cylinder pressure at the right time. Piston speed just falls out afterwards -- your parts need to be able to support the speed you end up with.

Keep in mind, the optimum valve events (cam choice) will likely be different depending on which approach you use to get to the displacement target.

If you can afford it I'd select the bigger bore just because it comes with the stouter block -- whatever stroke you decide to use.
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Old 12-03-2005, 01:13 PM   #14
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I read an article somewhere Coast High made a 5.31'' rod, (thereabouts) to keep the wristpin from intersecting the oil ring....

Any idea on where I can get rods of that length? I went with Eagle's internally balanced stroker assembly last time, and everything went great, besides the fact that it consumed a tiny bit of oil.....
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Old 12-03-2005, 01:23 PM   #15
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Cool

Quote:
Originally Posted by tached93gt
Don't forget to factor in the weight of the different crankshafts and rods since lengths are not the same between rotating assemblies. As for the Hardcore discussion the conclusion was for the SAME CID engine getting your cubic inches thru bore was better than stroke, which is what the question is for this thread, You are correct that the consensus there is use as much stroke as possible to get as much displacement as possible but not at the expense of bore size.

You are right Thomas, I forgot about that. I'll have to search for that thread, I coulda sworn the conclusion was stroke is the way to go. Ah well, big bore and big stroke in the same package is the way to go then

You are 110% correct in the best rod ratio for our cars being 8:1. Leave rod ratio for people who read mm&ff magazines

blue93trunk, probe made a 5.315" rod and it uses a 331 piston with a 1.175" CH. I use probe ultralight 5.315" I beam rods in my 347. I'd prefer a 5.4" with a lighter 1.09" CH piston on top of it but my builder called me and told me they were on a crazy long back order for the 5.4's so I used the 5.315" instead. www.coasthigh.com or www.adperformance.com should have the rods you want. I think crower makes a 5.315" too?? If your engine was consuming some oil, something was off with the machining or build.
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Old 12-03-2005, 04:40 PM   #16
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Well, you said you were building a "street car", but then listed huge AFR 205 heads, a solid roller cam and a Victor intake...it sounds like you're trying to build a high-winding N/A combo to me...go with the short stroke/big bore combo. Piston speed doesn't necessarily equal power...too much and you're giving up a lot of power on the top end from additional friction.
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