Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - forged block
Thread: forged block
View Single Post
  #9 (permalink)  
Old 02-28-2012, 06:31 PM
oldbogie oldbogie is offline
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,014
Wiki Edits: 0

Thanks: 4
Thanked 509 Times in 431 Posts
Quote:
Originally Posted by cherrynova
Hey guys ive got an ole 4 bolt main sbc 383 I'm looking at building up just give me a good Max amount of horsepower I should be allowed to have without forgeing everything. Thanx guys
To a big extent how you use the power is as important as how much in terms of engine life. For example you can take a NASCAR Sprint Cup engine that delivers 750/800 horses at 8500/9000 RPM installed and get a reliable 500 plus miles out of it over 3-4 hours of operation. That same engine put in a drag car is good for a few minutes of racing before you'll have to open it up and inspect, if not fix something.

A 4 bolt block adds time between crank and main cap failures, the extra attachment in the center of the block is designed to deal with the unbalanced forces that are trying to bend the crankshaft around the center main. This puts a lot of load on the number 2 and 4 main. Short of adding counterweights opposite these center throws there isn't a lot that can be done to lower these forces. And that doesn't eliminate them, it reduces them but at the expense of adding considerable weight to the crank which then causes a list of other problems.

Rods and pistons are the weakest link. Factory Chevy rods be they forged steel (including the pink) or powder forged are pretty reliable for a hot street engine into the max output range of 400-450 hp. For circle track you might get them to deliver 500 pretty continuously, for a steady diet of drag racing probably anything beyond 350-400 is treading onto thin ice. Getting up above 420-450 especially if drag racing the engine you need to look to better rods with better bolts and designed for full floating pins. I beams work well into the 500-600 horse range, above that you must look to H beams in steel. Titanium can push the I beam further or use other configurations here's a site where the sucking sound you hear is you VISA card being emptied but shows a different beam shape http://www.pauter.com/billet_rods.htm. Aluminum rods tend to stay with the I beam shape but that as much as anything is a fluke of the material and the processes that can be used to make them.

The block will put up with a lot more, most failures of the block occur where the rod went through it. But when you start pushing 500 and more depending on how hard you beat the block it will start showing fatigue cracking usually around the cylinder ends into the head deck and/or into the crankcase. Another common cracking location is the cooling jacket walls that form the sides of the lifter valley. Always remember that for every action there is an opposite and equal reaction. So, for example, the force pushing the piston down the bore is with equal ferocity trying to blow the head off, expand and stretch the cylinder bore, etc.

High power outputs are hard on the pistons, pin and small end of the rod, there's lots of heat and risk to grabbing the pin so you have to change to better pistons, cast especially hypers are about at their limit getting into the 400-425 hp range, you've got to look to forgings. Again there are forging and then there are forgings. these compared to cast pistons range from expensive to ungodly expensive. this is where the rod needs to become a full floater for the pin, so if the pin gets trapped in the piston there's someplace left for movement without tearing the piston apart.

Detonation is your enemy, it not only takes out pistons but rods and cranks as well. At any power level this must be addressed but at high power settings it can destroy an engine before you even hear it. Piston crown and combustion chamber shape are big time players. Flat tops, D dishes and Ricardo (called Fastburn, Vortec, GT40, Magnum and others by various makers) chambered heads let you push the limits upward on compression. Compression needs to be computed in Static and Dynamic ratios. The cam timing events establish what the dynamic will be, generally looking to hit a range from 8 to 9 to 1. The Static has to be adjusted to hit the dynamic desired.

Lubrication in a high performance engine is a huge issue. Here there are "forces" that are in opposition like no other place, you want a lot of lube but not too much. The pump output varies with engine speed, it's easy to have too little at low speeds and way too much at high speeds. Too little starves the parts and bearings will suddenly let go, too much and you're wasting power driving a pump that's on excessive bypass, bypass heats the oil without it performing it's cooling function in bearing clearances reducing its viscosity potentially to the point of film failure. It eats too much power to do this which comes off the crankshaft while serving no useful purpose. The excess oil floods the rings which increases the amount carried into the combustion chamber where it reduces the fuels available octane resulting in a detonation prone engine which we already hammered on. Yet if you must error on oil, it's probably safer to deliver too much than too little. For a wet sump engine that means a lot more attention needs to be paid to wind-age trays, crank scrapers and trap doors in the pan than is usually considered by the average hot rodder.

So in the end there is an old saying, Speed Costs Money! How Fast Can You Afford To Go?

Bogie

Last edited by oldbogie; 02-28-2012 at 06:42 PM.
Reply With Quote