Exhaust Gas Scavenging (turbo engine)

[1NRO]

If I understand correctly the use of a longer than standard connecting rod will reduce the scavenging of exhaust gases, this can't help spoolup time but is it likely to be significant? I'm imagining an increase in the region of 5% for the rod length.

Thanks for all opinions,

Nik

[Guy Croft]

Nik, hi

OK. Please indicate in detail what do you know already about influence of rod length so that I don't have to go thru a load of things you know already.

GC

[1NRO]

Know nothing in practice (we all start somewhere) but many many hours of research have lead me to believe that a highly stressed engine might benefit from a longer rod.

What catches my eye is the lesser average and peak piston velocity and reduced rod angularity.

Peak velocity at a later stage of the down stroke sees more time for the inlet valve to open before maximum flow demand.

Less tensile loading due to the reduced angle.

A suggestion that less timing may be used would have me believe there may be less tendency to detonate (maybe even an increase in CR)

Reduced piston rock (a symptom of the reduced angularity) might suggest a shorter piston skirt (weight).

What worries me is the low rpm performance trade-off and the tightening on valve clearances and just to stress me some more most of the theory/experiences I've read about have been in relation to N/A engines and not charged.

A resounding opinion it seems is that the gains are small but to me as long as I'm edging in the right direction I'll be happy, I'll be using custom rods and pistons whichever direction I take.

Sorry for been a bit sketchy, I'm too worried I'll tie myself in knots.

Nik

[Guy Croft]

Nik, hi

bit tied up of late but thanks for replying. I want to do some calcs on this but for starters:

1. Longer rod reduces acceleration of rod upper section and piston assy and their velocity too. Reducing velocity on a long stroke small bore engine is not generally a good thing, because it can reduce the pressure ratio across the inlet valve and impair cylinder filling.
2. 5% increase in rod length would be about 7mm on a 145mm long 2 liter Fiat type rod, that is say 7mm. The extra weight of rod will increase rod bolt stress and increase the inertia of the rod/piston reciprocating section out of all proportion to lightening the piston.
3. No great adavantage to shorter piston skirt other than weight saving (counteracted by heavier rod) because oiled skirt-bore friction is to a great extent independent of contact area.

I will remark on the other effects of longer rod when I have looked a few dynamic aspects later on.

GC

[1NRO]

No problem Guy, time is always in short supply and I understand.

The increase in time spent near TDC has in my reading been suggested to improve burn rate with an increase in cylinder pressure but less time for exhaust on power stroke with more exhaust to deal with from BDC, this to my mind might well be a problem (pumping loss) but I haven't found anything in referance to this effect on a turbo engine and the valve timing they use.

Nik

[Guy Croft]

Nik, hi,

one thing you need to know is that the Integrale heads 16v and 8v don't have a pumping loss problem in comparative terms; some heads do, but esp when ported half-decently they flow almost as much out of the ex as they do in thru the inlet... You can run 2-3mm higher inlet lift than std with as little as 8mm or so ex lift with standard valve sizes (plenty big enough) and still get massive results, way over 400bhp.

doesn't answer the main issue I now but worth mentioning en-passant...

GC