Board logo

Impact of changed Conrod length
Ivan - 13/1/08 at 04:42 PM

I am looking at putting a turbo on my Cobra’s Chevy 350 and as I would have to change the Conrods and pistons and detonation could be a problem I wondered if longer conrods, which cause the piston to dwell for longer at TDC and increase torque, would increase or decrease the risk of detonation.

Anybody know what the science says here?

oadamo - 13/1/08 at 04:48 PM

if your putting a turbo on you want to lower your compression. so you want shorter rods. or skim the pistons if your doing it locost.
adam

Volvorsport - 13/1/08 at 05:02 PM

longer rods , means a shorter piston , less reciprocating weight , the detonation characteristics wont have much to do with longer rods , more to do with intake temps and fuelling/ignition maps .

you can take advantage of the longer rod , since it will accelerate up the bore slower , less piston speed .

longer rods are definitely not a bad thing

garage19 - 13/1/08 at 05:13 PM

quote:
Originally posted by Volvorsport
longer rods , means a shorter piston , less reciprocating weight , the detonation characteristics wont have much to do with longer rods , more to do with intake temps and fuelling/ignition maps .

you can take advantage of the longer rod , since it will accelerate up the bore slower , less piston speed .

longer rods are definitely not a bad thing


Rod ratio does effect how prone an engine is to detonation.

oadamo - 13/1/08 at 05:18 PM

ive tryed all this on my rs it made little diffrence
if you want more torque just turn the boost up.
adam

garage19 - 13/1/08 at 05:24 PM

Adam,
I think you are confusing changing your compression ratio with changing your rod ratio.

garage19 - 13/1/08 at 05:36 PM

quote:
Originally posted by Ivan
I am looking at putting a turbo on my Cobra’s Chevy 350 and as I would have to change the Conrods and pistons and detonation could be a problem I wondered if longer conrods, which cause the piston to dwell for longer at TDC and increase torque, would increase or decrease the risk of detonation.

Anybody know what the science says here?


If anything a longer rod length will increase your chances of det.

oadamo - 13/1/08 at 05:55 PM

the dwell time is decreased with longer rods on a boosted engine its not worth it as the affects are very little if any.
adam

garage19 - 13/1/08 at 06:31 PM

quote:
Originally posted by oadamo
the dwell time is decreased with longer rods on a boosted engine its not worth it as the affects are very little if any.
adam


Nope, longer rods INCREASE the time that the piston spends closer to tdc.

oadamo - 13/1/08 at 06:48 PM

quote:
Originally posted by garage19
quote:
Originally posted by oadamo
the dwell time is decreased with longer rods on a boosted engine its not worth it as the affects are very little if any.
adam


Nope, longer rods INCREASE the time that the piston spends closer to tdc.


dwell time ????????

Ivan - 13/1/08 at 07:59 PM

quote:
Originally posted by oadamo
quote:
Originally posted by garage19
quote:
Originally posted by oadamo
the dwell time is decreased with longer rods on a boosted engine its not worth it as the affects are very little if any.
adam


Nope, longer rods INCREASE the time that the piston spends closer to tdc.


dwell time ????????


Dwell Time: Longer rods leave the piston closer to TDC for more degrees of crankshaft rotation ie from the time the piston moves from say 0,5mm BTDC to say 0,5mm ATDC a short rod will require say 10 Deg of crankshaft rotation and a longer rod say 15 Deg of rotation so the cylinder will create higher pressures acting on a longer lever arm giving more torque which to some extent answers my own question - ie higher combustion pressures give more risk of detonation but against that you can run l;ower compression ratios and retard the ignition more with less loss of power so still wonder if long or short rod is best for turbo motor.

If I am replacing rods the cost is more or less the same regardless of length so might as well get the right ones hence the question - which is best in a turbo motor.

[Edited on 13/1/08 by Ivan]

Volvorsport - 14/1/08 at 11:42 AM

like i said , piston choice (ie CR) , regardless of how many other factors is the most important , pick the longest rod you can (the shortest compression height piston) .

NS Dev - 14/1/08 at 01:01 PM

Interesting one this!!!

I have no idea as to the answer unfortunately.

Volvorsport, what is your rod to stroke ratio on the volvo engine, and how does it compare?

That might put his mind at rest?

Volvorsport - 14/1/08 at 11:31 PM

80 mm stroke , 152 mm rod CC.

the choices
we have are 158/160 mm in length , that gets down to a 34mm compression height or thereabouts IIRC .

I know of nobody who selects shorter rods , except if there a cheap ass and wants to use an already available rod , that sort of matches .

you should do a search on compression height for pistons , its effects on bore wear and general engine performance benefits over a heavy slug with a large compression height and short rod .

that said , if you cant find a decent piston , or they are already pretty good , then longer rods are a mute point if nobody sells them .

JB - 15/1/08 at 04:33 PM

David Vizard has written many articles on con rod lengths.

I am sure some of the articles are on the web.

Smokey Yunick is another guru on this subject.