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#1
12-21-2009, 07:46 PM
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4.25" stroker verses 4.21" stroke
Besides 0.04", what is the difference between a 4.25" stroke and a 4.21" stroke crank? Any real advantages of one over the other?
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#2
12-21-2009, 08:14 PM
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Six of one, half a dozen of the other.
A smaller rod journal on the 4.25, good. A higher ring pack vs. 4.21, not as good. Piston side load? Not as bad on 6.8 rod/4.25. See 1`st line. 6.625 rod lighter than 6.8. See 1`st line. |
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#3
12-21-2009, 08:15 PM
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Depends. Often the 4.25 will have the BBC sized rod journals so you can run 6.8" rods, and the 4.21 would tend to have the Pontiac sized journals so you can run stock length 6.625" rods. Of course you can go both ways with that, so its good to know what rods are intended to be used, and what the main journal diameter is.
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#4
12-21-2009, 08:35 PM
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New motor, blank canvas.
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#5
12-21-2009, 11:55 PM
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then go 4.5 stroke
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#6
12-22-2009, 09:21 AM
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Too basic a question. All is dependent on your driving and use needs.
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#7
12-22-2009, 10:00 AM
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Agreed, too many variables, and plenty of opinions and theory as to why one way is better than another, etc.
I can tell you for certain, NOTHING shows up on the dyno with near identical 455 steet engine combo's the only difference being 4.21/6.625" rods vs 4.25/6.800" rods.....Cliff
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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#8
12-22-2009, 10:21 AM
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Bore = horsepower
Stroke = torque The right combo = Nice ET's. |
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#9
12-31-2009, 10:37 AM
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Seems as if the 4.25/6.8 kits became popular when the cost of the stock pontiac rods were pretty expensive. now that pricing has come down, I wonder if the 4.21/6.625 kits will become as popular. I would assume that the shorter rod = lighter rotating weight
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#10
12-31-2009, 10:58 AM
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4.25 was a scam to make you buy more of their chinese stuff..LOL Oh you gotta use this 6.8 rod .. oh now you need custom pistons...mmmm BS I say..
Not enough potential profit to just sell and 3" main pont rod 4.21 spec crank with a factory bobweight. cheap guys like me would use cast stock or cheap 5140 steel 6.625 rods and good ole heavy trw/speed pro forged stock 455 pistons 
__________________
63 Catalina coupe 467 cid budget drag car 11's 1/4 , 7.20 1/8th pump gas n/a 66 Star Chief Executive 57k mile 69 Le Mans 2 dr HT 350 85k mile 15 sec 1/4 69 Firebird 400 Burgandy/Black 70 Olds Rallye 350 F85 4 speed 3.91's 70 Olds Cutlass Cruiser Red Wagon 350 101k miles 15 sec 1/4 12 sec w 455 74 Cheyenne Super C10 LWB Gen 6 454 w ZZ502 cam 3.07gear 13.1 1/4, 8.3 1/8 2020 RAM 1500 SLT 4x4 5.7 A8 Hemi 2007 Hummer H3 3.7 liter turd 2019 Chevy Spark petrol car 38 mpg |
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#11
12-31-2009, 11:21 AM
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Actually it was found out that if you had a fubar'd crank with screwed up rod journals but would go .050 that the aftermarket BBC style rods could be used. They are 2.200 compared to 2.250 of the Pontiac and also are .010 thinner which makes it nice for side clearance and higher rpms. I have even used the .500 over rods (6.635) in combos with a std stroke Pontiac 4.210 crank and std pin height Pontiac pistons.
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#12
12-31-2009, 07:01 PM
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P-Dude, I understood that. However, being a NON-Business, I've never had to go that route.
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#13
12-31-2009, 07:51 PM
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Quote:
 If you tore up your crank and needed rods and pistons for a new build anyway. You woulda saved money back in the day. That and the smaller oil load area is easier on parts, less heat and quicker rpms. Same deal before they came out with a 3" 4.210 crank to use in a 400 block. Sometimes ya gotta think outside the box. 
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#14
12-31-2009, 08:18 PM
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How about 4.25 vs the 4.375 stroke? I understand the bigger the stroke the more torque. At what point is it too much stress on a 4 bolt main block? Is 4.5 stroke living on the edge?
Thanks |
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#15
12-31-2009, 10:13 PM
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I would have gone the 4.25/3.0/6.8 route if I didn't have a good 455 block and crank. But there seems to be plenty of guys here with 455-based builds running 11's or better- how bad could the 4.21/3.25/6.625 combo be if all of these guys are happy with it?
Geno |
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