ATMO Tuning

However in a GTA the Atmo (odd fire) engine always feels smoother and revs better (easier) than the Turbo (even fire).

That will be down to cams, induction and ecu too though. R25 V6's even fire rev lovely...

. An evenfire will sit quite happily at say 600rpm, but an oddfire will shudder a bit at low revs.

I beg to differ (you just KNEW I would )

I've had a high lifft cam odd-fire engine running efi and idling as smooth as silk at 600rpm. This was a complete accident and we were all very surprised. In other words, give it accurate fuel delivery and it's happy.

The TT DeLorean, running spot on at idle generates a wobble that's very visible on the car it's in because the bonnet fit is poor and it's beautifully amplified and judders up and down all the time.

Now, I know of no answers here, only heresay, but I've heard several people talk about the odd-fire PRV being peculiar for its smooth running. The first even fire engine was the Z7U and I reckon this had to have evenly spaced inlet valves opening because otherwise one bank would end up with more air than the other, being forced induction.

It makes sense to me that an engine whose pistons fire at 60 degree intrervals but where the pistons are held at 90 isn't going to balance very well. Again, no proof, just instinct!

The first even fire engine was the Z7U and I reckon this had to have evenly spaced inlet valves opening because otherwise one bank would end up with more air than the other, being forced induction.

It makes do difference about being forced induction or not. Evenfire is evenfire, so there is no need to alter valve timing per bank. An evenfire is two identical 3 cylinder engines with even pulses between the two, unlike the oddfire 'pair'

Were not talking about engine balancing here, were talking about engine in balance from firing order irregularities, nothing to do with V angle.

If you had a 4 cylinder engine that was usually firing every 90 degrees (90-90-90-90), but then designed one (if possible) to fire at 90-40-140-90, then you would feel the definate hiccup. It doesnt matter if you balanced every component in the engine perfectly, it will still have a judder. The V6 though would suffer it more than an inline 4 though, as the pistons are opposed to each other on the V, so it rocks the engine from one side to the other. If the pulses are equal (as in even fire) the rocking will be regular and smooth (almost cancelled out) - but put it in an oddfire setup and you feel the irregularity. By restricting valve output and timing, you can reduce the impact of several firing strokes in the order to smooth it out. Once the revs are up, then engine is moving so fast that this cannot be felt.

The oddfire will feel very smooth at high revs, as it is not mechanically out of balance (like the evenfire - see below). The firing pulses are happening so quick that the oddfiring is no longer a problem and the engine revs smoothly. The F1 V6 Turbos and Lemans cars ran an oddfire crank.

The evenfire engine has a shudder at around 1500rpm to to mechanical imbalance by offsetting the journals. Its basically a crank made offbalance on purpose by splitting the journals.

I've had a high lifft cam odd-fire engine running efi and idling as smooth as silk at 600rpm. This was a complete accident and we were all very surprised. In other words, give it accurate fuel delivery and it's happy.

The TT DeLorean, running spot on at idle generates a wobble that's very visible on the car it's in because the bonnet fit is poor and it's beautifully amplified and judders up and down all the time.

!

Ah, but that could just be good chance and bad luck! Both of those cars are non original and altered - could be down to engine mounts, or the fact that one has big manifolds and turbos and silencers hanging off each side of the engine and the other has the exhaust at the back in the middle.

If you just simply get two GTA's, evenfire and one oddfire, the atmo will shudder at idle and the odd fire will not. Its the best comparison as one type of car has had the two different engines.

So back to the original question: Why the balance shaft on the 3 litre engines?

We'll have to agree to disagree.... I've heard it from several sources that the odd fire engines were inherantly smooth. Why they varied the cam timing by 2 degrees left to right is still open to debate but I'll let you compare a -DeLorean- engine at idle next to a GTA turbo. I think you'll find the DeLorean's fuel injection smoother than a Carbed ATMO.

I've been looking at this thread for a couple of days and wondering if I'm brave enough to join in.....but in the end couldn't resist it and I'm even sober- if clearly deranged.

First of all I should say I am not an expert and what I'm about to say comes from studies I did over 20 years ago.....but physics is physics, even now.

I've also never worked on a PRV.

From what I remeember there are a number of different factors in play when assessing "smoothness".
These are, at least, mechanical balance (simple mechanics of reciprocating pistons in cylinders), the degree of overlap of the power strokes (this is why more cylinders always feel smoother), cam profiles (if the engine won't stay running at low revs, it will seem rough), damping of engine mounts, weight of the bodyshell etc etc etc........

The mechanical balance of the engine is just geometry, and a major weakness in all PRV's.

When you (mathmatically) look at the forces created by engines (eg pistons moving up and down) you find that the ideal engine is a straight 6 or a flat 12. V8's feel smooth as they have lots of overlap on the powerstrokes, but they still virate. Straight 6's don't need to vibrate at all- they have natural balance.

All V6's are unbalanced, its just some are more unbalanced than others. The best balanced V6 is a 60 degree one, the best V8 is a 90 degree one. The PRV is a shortened V8 and at 90 degrees will be unbalanced, and no amount of balance shafts will eliminate the mechanical vibration completely.

Whether odd fire or even fire makes it better or worse I'm not sure and having looked at the text books again, I realise the maths is now beyond me. My hunch is that even fire will be better at there are pistons going up and down to balence each other out, but I cannot prove it, and becuse they are going up and down at the "wrong" angle, there will still be some second order vibration.

Why does the 3 litre have a balance shaft? Who knows- maybe the greater piston mass makes the vibration worse, or maybe as the 610 was supposed to be a move upmarket, they tried to mechanically cancel out the vibration.
I'm more puzzled by why the balance shaft is at the top of one bank only-they are usually at the bottom, near the crank, this seems eccentic, or more probably a bodge.
Porsche use two on the 944- one each side of the crank- as straight 4's are unbalanced. V4's are even worse in mechanical balance & they even have uneven power strokes (like the odd fire)- as you can hear when a Subaru goes by.
What conclusion can you reach to all this?
No idea other than a little knowledge is a dangerous (and probably boring) thing, and that all PRV's are unbalanced, relatively rough engines, but they are torquey and light.....?
Anyone fancy a drink?
Lee

We'll have to agree to disagree.... I've heard it from several sources that the odd fire engines were inherantly smooth. Why they varied the cam timing by 2 degrees left to right is still open to debate but I'll let you compare a -DeLorean- engine at idle next to a GTA turbo. I think you'll find the DeLorean's fuel injection smoother than a Carbed ATMO.

Disagree on what? Ive said the oddfire is a smoother running engine in every single one of my posts than the evenfire above idle..and you can't compare two different cars with the same engine, they have different harmonics, engine mounting...we may as well compare a Laguna V6 to an A610, but the Laguna doesnt have the same resonances because the engine is mounted transversly...

..and its pretty obvious that a FI engine would be smoother than a carbed one...but that has nothing to do with V angles and split crankpins...

double post..

*tips hat to Mr Metters*

Proper info, like it

The crankshaft on all PRVs has counterweights on it and I heard from one "expert" that this is the simple reason why when changing out pistons, you must get things re-balanced.

Oh the balance shaft is definitely a bodge

In terms of gauging "smoothness", I'm thinking purely in terms of idle as this is usually when a shake is most apparent. It's clear from DeLorean engines that they all vary, but I know of one car whose engine is so smooth at idle that if you stood your mug of tea on it, there's barely a ripple. Guess that was a good day on the balancing line!

When you (mathmatically) look at the forces created by engines (eg pistons moving up and down) you find that the ideal engine is a straight 6 or a flat 12.

Edumacate me on this - I thought it was the Flat 4 - like the old Beetle engine, a so-called "boxer" engine because the pistons are paired and 180 degrees out of phase meanging they fight each-other like a boxer..... or does that engine oscillate about the centre line, viewed from above?

...and I thunk the Scooby engine was a flat 4, not a V ?

When you (mathmatically) look at the forces created by engines (eg pistons moving up and down) you find that the ideal engine is a straight 6 or a flat 12.

Edumacate me on this - I thought it was the Flat 4 - like the old Beetle engine, a so-called "boxer" engine because the pistons are paired and 180 degrees out of phase meanging they fight each-other like a boxer..... or does that engine oscillate about the centre line, viewed from above?

A boxer 4 is perfectly balanced, but more cylinders the smoother running, so boxer 12 is even better... Straight 6 is almost perfect as cylinder 1 is in the same position as 6, 5 as 2 and so on..

I'm not brave enough to continue- and I've lost the text book anyway.
As I said mechanical balance isn't the only factor - and after all, you can't really change it can you!
Ok Dave I know you have a 4 cylinder GTA, but that's not a flat 4........
Cheers
Lee

I'm not brave enough to continue- and I've lost the text book anyway.
As I said mechanical balance isn't the only factor - and after all, you can't really change it can you!
Ok Dave I know you have a 4 cylinder GTA, but that's not a flat 4........
Cheers
Lee

Yep, and sods law that old 4 pot will probably feel smoother than the odd, even, upside down fire V6's..

What abouta rotary.....?

Or a V5????