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Author Topic:   4" stroke in a stock block?? Max power / rpm ??
MAXX_EFFORT_1
Dirt Newbie

Total posts: 1
posted November 28, 2001 03:18 PM
I'm tyrying to decide between the 3.875 & 4.000 cranks and I just want to get some feedback about using a 4" crank in a stock block. I've heard people say that it can be done and it will live fine and I've heard just the opposite.... Also is there a max rpm / hp number I should be aware with these long stroke crankshafts?

sdhnc29
Dirt Freak

Total posts: 467
posted November 29, 2001 12:10 AM
If your using a stock block and a 4" crank , your in for more trouble than it might be worth . You will end up using block filler , and grinding threw the block by the pan rails for clearance . You will have to use a small base circle cam , and stroker rods .... even then , you will still probably have to grind on the rods for cam clearance . With a stock block , you can't get a long enough rod in there , to get a decent rod stroke ratio ! So even if you do get this combo together , you will have good torque , and it will be horse power limited . I'm sure that there are plenty of people who will tell you that they've done this , but in my opinion , get a raised cam tall deck block ..... or go with a shorter stroke in your stock block !

There is no RPM limitation to the 4" crank it's self . Your RPM limitation comes from what is called "critical piston speed " . Generally , on a 4" stroke engine , you don't want to turn over 7,200 rpm . This is about 200 rpm over critical piston speed ! What ends up happening , is you will pull the pin end out of the piston , due to too many RPM's .

hughes
Dirt Maniac

Total posts: 141
posted November 29, 2001 07:47 AM
hey sdhnc29 you seem very knowledgeable, where did you learn your stuff. Most of mine is from books, net, and the School of Hard-knocks.

dchaught
Dirt Roller

Total posts: 12
posted November 29, 2001 08:08 AM
A 4" stroke will fit, but does require a lot of grinding, as well as the small base circle cam and stroker rods previously mentioned. I do not agree that it will not spin past 7200. Ask most of the late model guys including myself, and the 434 ci combo will run to 7800, and make a ton of power doing it, if balanced well, and used with lightweight parts. The 3.875" stroke is easier and less costly top build, as it does not require nearly as much clearancing in the block. The 420 ci is about the most popular with unlimited late model guys on a budget (Like ourselves) that can't afford a Rocket block, or some derivative thereof.

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