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bore vs. stroke on a SBM

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bore vs. stroke on a SBM

Postby DavidDean » Sun Jan 29, 2012 2:18 pm

I dont post much over here, but I read plenty. I'm building a 454" SBM 4.125 bore and 4.250 stroke. This is a methanol fueled drag motor. Its based on a tall filled X block. It will have indy 360-1 cnc heads 245 cc int port volume valve size is 2.150 int 1.60 exh. finished by Brian at IMM engines an indy intake and an SV-1. Compressions' about 15:1. Im planning on keeping the rpms below about 7000. My dyno program figures a cam of about 270* LSA of around 114* and lift of about .750. My question is am I trying to put to much stroke in it or can this combo produce 750 hp keeping the rpm below the 7K limit? Thanks!!
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Re: bore vs. stroke on a SBM

Postby Racer7088 » Sun Jan 29, 2012 2:57 pm

I don't know anything about the crank in question but if its built right you dshould have no issues. Just make sure you have enough cylinder length to support the piston properly at BDC. Can't remember offhand how long the Mopar SB is but it's probably long as the deck height is pretty tall compared to an SBC.
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Re: bore vs. stroke on a SBM

Postby DavidDean » Sun Jan 29, 2012 3:06 pm

The deck hite on my block is 9.59. I'm thinking the cylinder length is about 6"
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Re: bore vs. stroke on a SBM

Postby Keith Morganstein » Sun Jan 29, 2012 3:17 pm

I've done the 408 CU/In 4" stroke deal a few times and it worked well. Since you're working with Brian at IMM, I would just default to him. (This is his specialty).
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Re: bore vs. stroke on a SBM

Postby Racer7088 » Sun Jan 29, 2012 3:27 pm

DavidDean wrote:The deck hite on my block is 9.59. I'm thinking the cylinder length is about 6"


You should have no real problem there I can see. 6 inch Bore length - 4.250 stroke = 1.750 bore left for piston.

With most ring stacks you should have over 1.000 of skirt still in cylinder but again I don't really know what the length is.
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Re: bore vs. stroke on a SBM

Postby rustbucket79 » Sun Jan 29, 2012 4:32 pm

We did a 408 late last year and I was impressed at how nicely the 4" arm fit into a 340 block without a lick of clearancing, stock rod length, and the compression height was very nice. I can't see a .125 shorter compression height piston being an issue. The 4" forged crank internal balanced w/o mallory, I'm not sure a 4.250 will work out that way since there's likely to be some cam cutting of the counterweights to fit the package together. Good luck!
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Re: bore vs. stroke on a SBM

Postby ou812 » Sun Jan 29, 2012 4:49 pm

DavidDean wrote:I dont post much over here, but I read plenty. I'm building a 454" SBM 4.125 bore and 4.250 stroke. This is a methanol fueled drag motor. Its based on a tall filled X block. It will have indy 360-1 cnc heads 245 cc int port volume valve size is 2.150 int 1.60 exh. finished by Brian at IMM engines an indy intake and an SV-1. Compressions' about 15:1. Im planning on keeping the rpms below about 7000. My dyno program figures a cam of about 270* LSA of around 114* and lift of about .750. My question is am I trying to put to much stroke in it or can this combo produce 750 hp keeping the rpm below the 7K limit? Thanks!!

Actually your doing the right thing by increasing the stroke to keep the HP peak down and HP up. The cam will need to be different though...maybe 274-276@.050 on a 113lca straight up. .750 need to be at the retainer so you'll need closer to .800 gross on the intake.
I'm working on a 420 cid sb mopar right now, on race gas that should make 725 using a 4.125 crank and 4.060 bore and Indy -2 heads. Will be 14.3:1 and our Indy 4500 converted intake. I expect max HP will be at 7000rpm.
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Re: bore vs. stroke on a SBM

Postby DavidDean » Sun Jan 29, 2012 5:16 pm

Thanks Brian I'm still piecing the bottom end together but I'll be in touch when I'm ready for a cam
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Re: bore vs. stroke on a SBM

Postby andyf » Sun Jan 29, 2012 9:16 pm

I did a 427 inch SBM for Mopar Muscle. That engine required a fair amount of work to get the parts to fit. Oil pan clearance, windage tray issues, oil pump pickup clearance, etc. The 4.250 stroke will require even more fitting time. It can be made to work, but you'll need to spend some time on it or else pay someone to make it all fit.

http://www.moparmusclemagazine.com/tech ... index.html
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Re: bore vs. stroke on a SBM

Postby DavidDean » Sun Jan 29, 2012 9:59 pm

Thanks Andy, I've read your article very informative. I'll be using a BME rotating assembly it uses a SB Chevy rod. I'm thinking the amount of fitting other than notching for the rod will be minimal.I won't be using a windage tray and I believe it clears the oil pickup. Thanks again!
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Re: bore vs. stroke on a SBM

Postby DUNNUCKRACING » Mon Jan 30, 2012 1:40 pm

DavidDean wrote:Thanks Andy, I've read your article very informative. I'll be using a BME rotating assembly it uses a SB Chevy rod. I'm thinking the amount of fitting other than notching for the rod will be minimal.I won't be using a windage tray and I believe it clears the oil pickup. Thanks again!


Actually, David is using a BPE assembly,not to be confused with BME. BPE stands for Bloomer Performance Engineering. He sells small block Mopar parts exclusively. I just got off the phone with Rod Bloomer,the owner.
I use one of his 4.125 stroke cranks in a 360 block with a .030 over bore. This makes for a 421" small block. I make a touch over 600 hp with mine using Edelbrock heads and a flat tappet camshaft. This is in my personal bracket racer. It took very little clearancing to work with a stock block.
I know he has mocked up a 4.250 crank in a 340 block and the clearancing was minimal.
Aftermarket blocks like AndyF used are thicker castings and may take a little more work.
I am just posting this to try and clear up any confusion,hoping it might help someone.
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Re: bore vs. stroke on a SBM

Postby DavidDean » Mon Jan 30, 2012 6:21 pm

My mistake BPE for sure. Rods stuff looks good.
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