Conrod length

[Stanpartmanpartwolf]

The amount of possible alteration to 2.5L rod length is minute; compression height is already at a minimum & the actual effects of change are minimal, even when going from 6.25" to 7" in a 4-pot TR race engine, for example.

The Maxspeeding rods are copied from a 1990s Carrillo- same bore fouling issues.

Older Arrow 5/16"UNF rods, that don't foul, break instead.

Current 1/4"UNF Carrillo rods fit.

In essence, rod length is effectively inalterable in the 2.5 engine & there is absolutely zero point in tangling underwear over it.

The physics at issue here, & so on and on, are totally irrelevant, just keep everything light, in line & balanced.

[Nick Jones]

Andreas is saying it IS alterable (by 2.5mm) - though obviously the piston pin to deck height has to change (reduce) to accommodate it.

 

Nick

[Ragtag]

Andreas is saying it IS alterable (by 2.5mm) - though obviously the piston pin to deck height has to change (reduce) to accommodate it.

 

Nick

 

I think that is the point Stan is making. Surely the key measurement is from the top of the piston, not the length of the rod. I always assumed that Andreas' eccentric bush was in order to utilise OE VW pistons which presumably have a different piston top to gudgeon pin measurement. Clearly the fulcrum point is changed marginally but I have no idea what benefit that might accrue.

[Peter Cobbold]

A longer rod with the same stroke and with piston modified to keep crown height kept constant will have an effect on the time the piston 'dwells' either side of TDC. As Andreas says. But the effect on torque is miniscule for the percent change described above.

The second figure shows the piston position for rods of 100 and 300mm lengths either side of TDC (TDC is -180 and 180deg). The difference for a 300% length change is not that great,

http://www.motoiq.com/MagazineArticles/ID/1996/Does-Length-Matter.aspx

So for a 5.75 to 5.85 inch longer TR rod - 2% change - the effect on piston position is negligible. Bring the gap between those two curves together by 300/2 and they will be separated by maybe the ink's width. ( BTW that 2% does not mean 2% more torque !)

More likely in my view, Andreas' positive findings will be due to the more modern VW piston design - skirt ** and that eccentric gudgeon pin - producing less friction losses.

 

Peter

 

**

https://www.researchgate.net/figure/261171062_fig3_Figure-4-Piston-friction-behavior-with-engine-speed

 

Piston friction rises markedly wth rpm , more than other friction losses:

http://www.brickleyengine.com/blog/wp-content/uploads/2011/03/Current-SI-Engine-Friction.jpg

Edited November 2, 2016 by Peter Cobbold

[TriumphV8]

To be honest we "home tuners" will have problems to proof such a difference.

If such an engine was built I will not strip it, build another with short rod and skim

the block instead and measure again.

 

-It is similar to the exhaust manifold- We evaluate and compare and than buy a

manifold, fit it and thats it........

 

Anyway, the rod/stroke ratio can be pushed from 1.53 to over 1.6.

It is not that there are no shorter pistons availiable.

Compression height from pistons can be found less than 1" and TR6 has 29mm.

It is more that it will boost the costs to have a piston and rod custom made.

 

Normally 1.8 would be a ratio that is acceptable!

We are far away from a sound ratio in the TR6.

In my opinion it is the more healthy to do a modification

when the existing situation is quite a lot away from normal.

 

Did you know that FORD did a long rod homologation for the BDA?

And did you know that it was time limited and gave such a big advantage

that tuners continued to use it after homologation run out?

German Zakspeed was one of the victims catched to do so.

All that when its real nonsense? I believe not!

 

Here is a nice discussion I found for the FORD although these are

also only opinions and not test results

 

http://www.turbosport.co.uk/showthread.php?t=149145

 

Anyway if I have to choose to make excentric small ends or skim the block

to make the VW pistons fit I would definitely choose the excentric way.

...... and cut a little bit from the crown of the VW piston to get the rod

another little bit longer :-)

 

As Peter says visually there is not that difference in the diagram

but as there are so many things affected (burning phase of mixture,

friction/acceleration of piston, suction of mixture into engine), so

it may have a noticeable difference in summary and up to now

nobody claimed disadvantages with that.

[GT6M]

could doo it the other way,

 

2 litre crank, small pistionee, and a long rod

a lot better revver t,start of wid, should be even better wid a long rod.

 

I was lookin at this 25 odd years ago, but did,nt go this route

as costs wer alott moer than a 2.5

 

M

[Peter Cobbold]

This conventional tuning seems to me to be a lot of hard work, and expense to match.

Why is it so popular? Supercharging is much easier. Fit a blower to pump the mixture past the valves : no messing around winning a little gain here, another there.

Supercharging is no good for racing because of 'the rules' that penalise it. But it is ideal for developing stonking torque at low rpm in an otherwise standard road TR. And smooth running too. Its a shame that there are no specialists in UK fitting blowers to classics, although I'm sure the machining skills are around. If Moss really have killed off the TR6 Eaton kit there's an opportunity going begging.

Peter

Edited November 3, 2016 by Peter Cobbold

[Mk1PI]

could doo it the other way,

 

2 litre crank, small pistionee, and a long rod

a lot better revver t,start of wid, should be even better wid a long rod.

 

I was lookin at this 25 odd years ago, but did,nt go this route

as costs wer alott moer than a 2.5

 

M

That's what my 2100 runs Markus - 77mm bore and uses a 2.5 style piston with the 2000 crank - so 9.5mm longer rods - custom steel from Pauter.

 

It makes a bit more horsepower than a Dolomite sprint engine in a TR7 with proper extractors and a mild cam - a least the main straight at mallala racetrack in South Australia said so during the TSOA national supersprint.... it would pull about 5 cars lengths out on the TR7 Sprint - both flat out.

[Andy Moltu]

The 2L revs better because of its shorter throw than the rod length. And whilst there may be benefits to a longer rod, the 2L engines used a longer rod so they could use the same block.

 

Building screaming, high revving engines wasn't Triumph's agenda with the six cylinder TR or saloons at the time.

[Mk1PI]

The 2L revs better because of its shorter throw than the rod length. And whilst there may be benefits to a longer rod, the 2L engines used a longer rod so they could use the same block.

 

Building screaming, high revving engines wasn't Triumph's agenda with the six cylinder TR or saloons at the time.

Having scratched the itch (desire to properly build a high revving road 2 litre) and succeeded with a rally cammed EFI monster I can categorically say it is pointless but sounds amazing!