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Low Compression Street Engine's & Tuning Techniques Idea's...How Too's on low compression Engines.

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Old 04-17-16, 04:14AM   #1
TA63
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Default Crankshaft prep

Hello, you guys have any tips and tricks for improving the durability of our stock cast cranks in performance motors? I'd like to put together a 400 that will live happily seeing 6k or slightly more on occasion.
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Old 04-17-16, 12:35PM   #2
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Would not worry
Just as long as you have a competent machinist grinding and balancing it.
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Old 04-17-16, 02:50PM   #3
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Would not worry
Just as long as you have a competent machinist grinding and balancing it.
Exactly, and make sure it magged good.. plus take into consideration life history of that crank, how it's been stored was it ever dropped did it see any detonation.
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Old 04-18-16, 11:47PM   #4
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A good radius in the fillets is mandatory. The BBC rod mod is a good thing. Using Icon 428 pistons and shortening the stroke up about .050, the 6.8 rod is just right. Be sure the grinder understands "radii" AND offset-grinding.

The oil chamfer is also a good place to "help". A leading trough on the main and a trailing one on the rod does the trick. No sharp edges.

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Old 04-19-16, 01:30PM   #5
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U are talking a 400 std engine with hp upgrades right ?
What kinda hp ?

I always have the block line honed to middle spec as the Pontiac block has a tendency to move around in the #2 webbing. Make sure u have .015 side to side in the rods. radius the oil holes in the crank. More info can be posted if more info is supplied.
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Old 04-20-16, 01:25AM   #6
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Make sure the thrust bearing surface on the crankshaft is still usable/does not need repair.
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Old 04-20-16, 02:25AM   #7
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U are talking a 400 std engine with hp upgrades right ?
What kinda hp ?

I always have the block line honed to middle spec as the Pontiac block has a tendency to move around in the #2 webbing. Make sure u have .015 side to side in the rods. radius the oil holes in the crank. More info can be posted if more info is supplied.
Eventually if I find the correct block I would build the lower end to handle 450 hp plus with a set of aluminum heads from SD performance, a roller cam in the mid 230's at .050, and 10.3-1 compression. Short term goal would be 350 or so with ported iron heads, crower 221/229 cam, and a compression just below 9-1.
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Old 10-08-17, 08:01PM   #8
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Have my Scat custom crankshaft form they requested I fill out since I'm paying for a forged 4.0 stroke 400 crank with a custom rod journal. The rod journal is being downsized and offset to increase stroke with a -.020 BBC rod journal size . The diameter of the rod journal I spec'd @ 2.179" for a clearance of .0023-.0025 with the rod bearings that have already been calico'd and professionally measured. I had also spec'd an offset of plus .010 for a stroke of 4.020 as I thought thats all that would be available from a 2.20 rod journal and I wanted strong even fillets. My block has been decked and measured to 10.220.

Scat told me after putting down my deposit they only had a raw forging so I asked them if I might be able to get the additional .010 I had wanted for a 4.040 stroke and zero deck height with my 6.900 rod and 1.300ch piston. They said "maybe" and sent me this form. It specifically asks for rod width and journal width I had professionally measured, rods are .994 so desired journal width of 2.008-2.010 for a .020-.022 side clearance was advised. Im using CB-743HN -.020 rod bearings (FM 113M-std mains).

Was curious about thrust dimension they will grind in and wondered if anyone here might have gotten a Scat crank and measured it and know what they typically run and whether that works well or not? I have the opportunity to have them grind that to my spec since its a raw casting. Possibly make other small beneficial changes too? Thanks for any input ... will be sending form shortly.
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Old 10-08-17, 08:12PM   #9
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Keith can I ask what your paying for it. If you don't want to I understand
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Old 10-18-17, 07:37AM   #10
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Thomas, its no special price, the basic forged crank with a single customization ... I would prefer to let them quote their prices, I wouldnt get to specify multiple aspects for a single price if they had an already cut crank ready when I paid most likely. While its more than the price by double than the factory crank I lost my money on, at least Ill be getting a more reliable piece tho heavier, hopefully shortly. Brian at Scat gave me the green on getting 4.040 stroke from the raw casting vs 4.020 so I will be at my target zero deck and 444ci as well

I am ordering my gasket set and rear main from Butler ... the 2 lip Viton and the Best brand overhaul set. Wanted their input on shaft diameter for rear main seal as Brian asked for that too. Having my thrust bearing remeasured as the measurement I paid to have done some time ago was 1.134 and it appears to me the bearing is supposed to be 1.130 then the crank pads should be between 1.1335 and 1.1390 apart for a .0035-.009 gap, at least for a factory crank. Scat normally grinds thrust to the stock spec which according to my numbers would possibly not even fit as well as possibly have insufficient clearance. I did snap-gauge a 400 crank in my basement today and got 1.144 so am not sure I have correct specs.
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Old 10-21-17, 02:59PM   #11
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Remeasured thrust bearing with main torqued into bare block at shop today 1.133 around the clock. I guess the 1.130 specification has its tolerance too, am not sure how variances in thrust bearings would be optimally accommodated for with the crankshaft thrust grind .. thinking I will specify a similar clearance to my main and rod.. .0025, so I will ask Scat to grind thrust to 1.138 for that clearance on either side. Hopefully oil supply will be able to maintain film there, the bearing thrust surface is Calico'd I believe that is supposed to help.
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