Engine blueprinting

[SteMarsden]

I have a good idea whats involved when an engine is blue-printed but want to ask some generic questions (with respect to PBMW in particular).

Im building a new motor for 2010 and have a selection of 3 x M40's to choose the best parts from.

1. Conrods. Do I choose the 4 lightest (less reciprocating mass) or the 4 longest (to increase the CR)?

2. Pistons. Do I choose the 4 lightest (less reciprocating mass) or the 4 'tallest' (to increase the CR)?

3. Building up the pistons and con-rods. Do I match them for weight or for TDC heights?

Any opinions guys?
TIA.
Ste.

[Motorhole]

I wonder if mixing and matching parts in such a way could cause a problem with balance? or will the whole crank/piston assembly be balanced later?

[SteMarsden]

There is no machining allowed in the regs, so Im guessing that balancing could only be done by ensuring that the con-rod/piston assemblies were 'matched'.

[UweM3]

I have a good idea whats involved when an engine is blue-printed but want to ask some generic questions (with respect to PBMW in particular).

Im building a new motor for 2010 and have a selection of 3 x M40's to choose the best parts from.

1. Conrods. Do I choose the 4 lightest (less reciprocating mass) or the 4 longest (to increase the CR)?

2. Pistons. Do I choose the 4 lightest (less reciprocating mass) or the 4 'tallest' (to increase the CR)?

3. Building up the pistons and con-rods. Do I match them for weight or for TDC heights?

Any opinions guys?
TIA.
Ste.

I would try to select the most equal weights in the fist place. After that you may be able to choose the longest etc. But keeping the balance equal would be my first aim. Vibrations are always a killer.

[billgatese30]

I have a good idea whats involved when an engine is blue-printed but want to ask some generic questions (with respect to PBMW in particular).

Im building a new motor for 2010 and have a selection of 3 x M40's to choose the best parts from.

1. Conrods. Do I choose the 4 lightest (less reciprocating mass) or the 4 longest (to increase the CR)?

2. Pistons. Do I choose the 4 lightest (less reciprocating mass) or the 4 'tallest' (to increase the CR)?

3. Building up the pistons and con-rods. Do I match them for weight or for TDC heights?

Any opinions guys?
TIA.
Ste.

I would try to select the most equal weights in the fist place. After that you may be able to choose the longest etc. But keeping the balance equal would be my first aim. Vibrations are always a killer.

I'd agree, having a lighter/better balance would see more benefit than any minor increases in length.

[AlpineAde]

When balancing always choose for closest weight. Equal length might be equal in terms of one measurement (length) but way out in terms of inherent principles of balance. Me, I'd always go for the freer revving motor.

[GeoffBob]

In agreement with all the above, match your parts according to mass. As Uwe said, it's vibrations that are the killer.

If you are looking for additional info on blueprinting I can highly recommend "Engine Blueprinting" by Rick Voegelin. Avaliable online here

[SteMarsden]

Thanks all,

GeoffBob, thats the book that Amazon recommended to me after I searched for that

Can I then assume that a balanced engine that will have less vibration and therefore rev faster will be more beneficial to me?

[GeoffBob]

Can I then assume that a balanced engine that will have less vibration and therefore rev faster will be more beneficial to me?

That's the whole point to balancing an engine, to limit vibrations. You can rev an unbalanced engine as high as you like (well, as high as it will practically go) but it won't last, something will fatique, and often very quickly. An engine that is cammed to rev high for a race application is best proffesionally balanced else it can literally shake itself apart.

FYI, they only engines that are perfectly balanced at all RPM are the 90 degree V8 (and upwards) and horizontally opposed motors (VW beetle, Subaru etc). With all others it's a case of making the best of what you have under the circumstances, hence the invention of the "counter balance shaft", a useful gizmo that does take up a bit of space.

If you want a copy of that book you can order direct from www.cartechbooks.com . They post them from the Virgin Islands, usually take about 3-4 weeks to arrive.

[ShakeyC]

FYI, they only engines that are perfectly balanced at all RPM are the 90 degree V8 (and upwards) and horizontally opposed motors (VW beetle, Subaru etc). With all others it's a case of making the best of what you have under the circumstances, hence the invention of the "counter balance shaft", a useful gizmo that does take up a bit of space.

I disagree here straight 6 and flat boxer 6 has best primary balance, straight 8's have good primary and secondary harmonic balance not V8; The best is straight or V 12 cylinder which has no need for coutner weights or counter balance shafts and offer good primary and secondary balance.

Flat 4 boxer engines are sort of inbetween they need bigger mass flywheel because at lower revs the opposing rods/cylinder are not directly opposed in harmonic balance.

[GeoffBob]

Straight six has perfect secondary balance but worse primary balance (due to the 120’ spacing of the big-ends) than a four cylinder. Converse is true of the four cylinder ”“ has perfect primary by worse secondary.

I don't think my statement differs from yours with regard to the flat-6 in terms of primary and secondary balance.

While your other statements may well apply to an idealised engine motored (that is to say rotated by an external drive) on a test stand (for the purposes of vibration analysis), they fall short of reality due to the rolling couple caused by combustion during actual engine operation.

A further complication is that primary and secondary balance is not the whole story. Torsional vibration is a very real problem in long crank engines (hence the invention of the harmonic damper) that is also not evidenced during motoring tests (due to the absence of combustion).

While I submit that I should not have used the term "perfectly" balanced, I stand by the statement that the 90' V8 provides the opportunity for optimal balancing in terms of minimal primary, secondary and torsional vibrations. Any longer crank (V10, V12 etc) and torsional vibration becomes a problem.

[teamalsoran]

ok i'll be this wil fly in the face of most of the above posts but here goes,

having built and raced numerous engine different engines with varying degrees of balence and subsqwently made changes or swapped con rods flywheels etc, and were talking engines that rev to 8K i can catagorically state the whole balence thing is overated,

personally i would go for parts that give the most compression, more compression gives more power, balence and especially when were talking a few grams wont notice.

if your serious about a blueprinted engine you need to go through the whole engine and build it so its still bmw spec but all the tolerances are stacked up in your favour, so thats longers rods and pistons, shortest block, thinnest head, so skim them down to minimum thickness, you want the valves to have the thickest possible heads (more cr) but the shortest possible stems (gives more valve lift) the crank should be ground/polished to get the bearing journals on there bottom size limit.
ideal if the rules allow it should be rebored to the biggest allowable size, you might only add a handfull of cc but a few more is still a few more and also gives another slight raise in compression.

to build a properly blueprinted top notch engine is often dearer than building a modified one