Crankshaft Offset grind

Ranger Mike

Grinding the crankshaft is a relatively straight-forward process. The crankshaft nose is placed into the head stock chuck and the crankshaft flange (in some cases the crankshaft seal surface) is secured in the tail stock chuck. At which point the main journals can be ground. The rod journals are ground after the machine is set to the proper stroke and after the counter weights are set. After the crankshaft is ground, the oil holes are chamfered and deburred. Then, the crankshaft is polished with a fine grit belt to achieve a satisfactory micro finish.
The above process can be used to add more stroke or remove crankshaft stroke. This has been done for years. The 12 o'clock and 6 o'clock positions are offset ground.

My question is - Is there a benefit to offset grinding the 3 o'clock or 9 o'clock position?
Would there be an advantage regarding timing?

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brewnog

Not sure I follow. Grinding the pins at 12 and 6 o'clock add/remove stroke; I assume 12 o'clock is pointing back towards TDC? So grinding at 3 or 9 o'clock would change the TDC position with respect to crank angle, and could upset the balance of the engine (as well as buggering up valve timing). So no, no inherent advantage that I see. What were you thinking?

mender

Any deviation from the correct indexing will cause problems. If you feel the need to change something to optimize each cylinder, I'd start with adjusting the individual spark timing; after that, custom grind a cam with different lobes for each cylinder.

As brewnog said, what are you after?

Ranger Mike

lets just say I am looking for optimum performance with in the limits of a restricted class. Engine limited to stock lift cam shaft, displacement, valve size, compression ration can be changed, stock size piston but weight is open as is any piston rings, ignition must be by engine actuated timing intake and card must be stock appearance, exhaust is open, after market connecting rod is ok. stock size crank
am looking for an advantage...
I am having trouble seeing effect of the 3 oclock offset grind on the crank??
Would this modification make the engine think it had a longer con rod and make more power?

brewnog

No, it would just alter the TDC position with respect to crank angle. If you did it by the same amount to all cylinders, all you might see is a change in compression ratio.

xxChrisxx

How would altering the TDC position wrt crank angle alter compression ratio if the stroke is the same? Wouldn't it only reduce CR if there was a change? This is all beyond what i've ever done before.

Is this for your open wheel car Mike?

Ranger Mike

Far be it for me to look for an advantage in a highly restricted racing class...

what ? me take liberties with the Written Word of the sanctioning organization..I am shocked!!

mender

Offsetting the piston pin in the piston will change the piston motion in a way that can mimic a longer or shorter rod after TDC. But first you need to decide what the engine will respond to.

http://victorylibrary.com/mopar/rod-tech-c.htm

Your rules appear to allow a change in rod length, true? If not, can you use a "stock" rod length from a related engine family (Ford 2.3 rod in a 2.0 for example)?

As usual, there are ways to improve performance with things that are hard to measure and aren't specifically noted in most rules. Is that cheating?

Ranger Mike

rules say you can use after market con rod but must be stock length..we already did the piston pin offset thing...lightened flywheel, use 4 inch diameter clutch /pplate package
cold air scoop. CD ignition, thin piston ring package ( we used zero gap rings and too much drag...sucked HP) we run redline oil at $10 per quart..dry sump oiling system...electric fuel pump and water pump to gain a few HP...it would be a lot simpler to bore it .125 over and go long con rod but that is obvious cheating..

mender

Is it OHC or pushrod? Sometimes you can change the motion ratio without altering the cam lobe profile and total lift.

Is porting allowed? Valve stem size? Spring and retainer weight? How much vacuum in the crankcase? Oil scraper for the crank? Min quench and max compression? Angle milled the head and manifolds? Offset the head on the block to unshroud the intakes?

Is this an open forum?

Ranger Mike

overhead cam , porting allowed, valve size must be stock, cam lift is measured on the head at race course and on top of cam follower,
dont know about crankcase vacuum..
there is a combustion chamber min cc regulation as is head gasket spec
dry sump has crank scraper
open forum? i guess??

mender

Just wondering if you want everyone to know what you're up to!

I'm getting the idea that you're running in FC, right?

Ranger Mike

No not FC...nice try though...

mender

Well, what then? Hard to give tips without knowing which engine you're using!

brewnog

Yes, I meant that the stroke would be changed.

greasemoneky

the exact rules from the sanctioning body (pdf, ulr) could be very important.. the rules say what you can't do so that leaves a bunch you can do, and interpretation of the rules.
because "In racing there are the ones who win and then the ones who follow the rules!"

the operating range and power envelope or curve your looking for would help.


I've off-set ground to change stroke, (class displacement limit, and get a better R:S ratio) and made the rod journal alot smaller in the process to use smaller rod bearings.

there are little things add up to huge gains over all,
- valve face size spec ok but what about... valve seat angle? margin width? stem dia?
- radial gas porting, thermal barrier, friction coating

track side cam check.. only on cyl. 1? (a wise man won a few national championships in the 60's b/c the rules only stated that the cam was checked on cyl.1, which he made it in spec, the rest of the cam however was fair game. it was within rules, it wasn't cheating, until they revised the rules.)


need the engine specs and rules!
please! i haven't got any spec engines im working on now, it'll give me something to think about in the bathroom...errr i mean the study...

awdgsx

Just a few tricks.. you can grind cams with a very aggressive ramp rate and actually throw the valves open further than the actual cam lift. Take a look at nascar pistons. If any weight is allowed I would have the smallest area of skirt I could and still hold the piston square. If there is no restriction on compression height you can ofset grind the crank at 12 oclock run a smaller rod bearing and increase the piston height to keep the same compression ratio.