The V8 Owners Forum -

On most factory pistons the gudgeon pin is offset slightly to the left of centre, when viewed from the front in a clockwise rotating engine, this done to reduce piston slap when the engine is cold. This means tdc for the piston occurs slightly before a straight line through the big end and main bearing journals becomes parallel with the centre line of the cylinder in terms of crank rotation. If the same piston is reversed in the cylinder, then tdc occurs after the straight line between the big end and the main becomes parallel with the centre line of the cylinder, which should result in less side thrust on the piston and a mechanical advantage re rod to crank angle during the power stroke. This was recommended by the Chrysler works race team way back, if more power was a priority using standard parts.

The Rover 4.6 has an offset pin, I might just try it on my budget build. Your thoughts would be interesting.

Cheers.

Omission, obviously cam and ignition timing would need to be retarded to suit.

Your description makes it sound like Rover got it wrong?, what was the advantage of doing it 'their' way around?

Chris.

Why change something that clearly works. Hundreds of thousands of vehicles running as built and no issues with the design offset. Main issue is when rebuilding an engine and you forgot the way the pistons etc go and you have to work it out.

Is one of those things - why bother.

"Why bother " ......the same reason halfwitts keep climbin mountains and then jumpin off them
Coz its there , anyhow whats the harm in giving it a whirl ,but I suspect that the gains would be minimal and thusly hard for the lay person to measure.
Funny this should come up as I was just reading that my car engine has said offset , cant find it now but the offset was bugger all ,like a few thou . I wonder what the mopar offset was . If it was 20 thou that could be different . Some pistons you can see the offset .
As thundefchief says there are other things to consider too .

I did some work for a well known car company a couple of years ago, when I investigated their engine CAD models I found that the bore centrelines did not intersect the crank centreline so queried this with them, they assured me it was correct and explained it, didn’t fully understand but sounded similar to what’s described above.
They also made v6 engines with 90 degree crank offsets but ground offset to make them effectively 60, and an 8 cylinder block with the rear 2 rear pots unfilled

Sorry I've not been on for a few days, work got hectic! The reason for the pin offset in factory engines is to reduce piston slap, so the engine is quiet when cold. On the compression stroke, side thrust on the piston is opposite to side thrust on the power stroke. The side thrust transitions from one side of the piston to the other earlier with the factory configuration than it does with the piston reversed, which puts less shock load on the piston/gudgeon pin, a quieter running cold engine ensues, the cost is a slight mechanical disadvantage re rod to crank angle compared to when the offset is reversed. We can't have our posh, expensive new car rattling on a cold start, not good for brand image!

Some performance engines are designed with the centreline of the cylinders offset relative to the centreline of the the crank to take advantage of the same effect as reversing piston pin offset, the Desaxe engine: https://en.wikipedia.org/wiki/Desaxe

Most aftermarket performance pistons have zero pin offset, a halfway house which perhaps says something.

I believe the 4.6 and 4.0 pistons have a 0.6 mm offset, so there should, in theory at least, be a slight potential gain to be had, every little helps .

It sounds reasonable from a theoretical point of view! Whether you would be able to measure a difference is a different question.

Care to estimate some numbers?

Chris.

There has been a debate about this on the interwebs, those of you who are old enough will remember tuning guru David Vizard, here's what he has to say about this question.



Never a power loss reversing the pistons.

Yawn I lasted 2 mins , he's worse than James May at blithering

So he's saying there is a few BHP to be had, single figures.

I guess if you have exhausted all of the easier ways to get power and you needed every last one, yeah, why not. It's an interesting concept.

Chris.

This stuff is interesting, I build on a budget, if I have an engine stripped and I'm re using factory parts, then why not try it? It's a world away from credit card tuning, not that there's anything wrong with that, but it's fun and satisfying to gain advantage using guile rather than money.

I think (but would have to check) that if you just fit the piston / rod assembly to the wrong bank, you will achieve this.

If you do so, please report back to say if the piston slap is noticeable as that seems to be the only disadvantage.

Chris.

Mc Tool wrote: ↑

Sun Aug 08, 2021 3:19 am

Yawn I lasted 2 mins , he's worse than James May at blithering

Not half. Got to laugh that in the first 30 seconds he says he's going to get straight to the point but then spends more than half the rest of the time doing the polar opposite. Hope he's better at tuning engines than he is at presenting.

GDCobra, Viewing that video, you came in half way through the story. There's a chap on Youtube, Uncle Tony's Garage, who is an ex magazine editor who does budget builds, re using some standard parts to the best advantage. There were others who work on current publications, and have have a subscription video channel with lots of industry sponsors, Motor Trend channel and Roadkill who said that old school stuff was now irrelevant, due to increased availability of aftermarket components,(extra expense) which it certainly is not the case if you are re using factory pistons for your build. That's why there was all the initial rambling.

Anyone familiar with vector calculations will see the advantage of reversing the pin offset, less horizontal thrust component, more vertical thrust component on the power stroke, less friction, more torque, the cost is potential piston slap when cold, not a worry to those interested primarily in performance.