L series big end diameter

[DatZed]

There is a lot of talk about the smaller L20A and L24 big ends being too small for high out put applications. I would like to know if anyone has actually broken a crank on one of these engines during normal conditions.

Given that Nascar have been running the Honda 47mm big ends for years now, I wonder if this predjudice against the smaller big end is just a rumor circulated by people who haven't actually attempted it.

I am getting a set of rods made and need to select a big end diameter, I can't see any reason why I can't reduce the big end by at least 3mm, giving me the opportunity to offset grind the bigends for a little more stroke.

I would be interested to hear of any experiences that you may have had.

[260Coupe]

My old 240z SC logbooked race car was originally fitted with an L24e donk (old Skyline donk)  with the smaller conrod journals, the crank broke at number six after a big rev. This could have been exascerbated by a loose harmonic balancer, lesson learned I went back to a 240z L24 block and crank (bigger dia journals) for the next engine build.........this went on to turn 135 rwkw at 7000rpm on triple 44 Mikunis, still going strong after 6 years.

FYI

Lou Mondello (Roadster Guru) is now offset grinding U20 and H20 cranks and installing Mitsubishi Evo rods, the strokers are now producing around 120 rwkw from 2.2 litres..........not sure about the longevity as done of these engines have seen the race track as yet .

 

Alan

[DatZed]

Thanks for the input Alan, so perhaps there is a weakness in the factory crank...

 

[hunterzed]

I was under the impression that it was more the physical size of the conrods rather than the crank jounal. Thanks for the info. I've been putting together a stroker kit so its definately good to know that info. Cheers.

 

Steve

[dazzed]

yep,, i have over a period of time broken 3 cranks, all in the same spot on the crank, even after a lot of meticulouse prep work, and its not due to just 1 thing , its a factor of many , touque is the main one , which is what we all want , the more you have the higher the chance is of something breaking , dont expect a motor that is highly stressed to last for ever it wont , but you can prolong engine component longevity , cryogenics is now widely used in high output racing engines these days, maybe someone else can explain this type of treatment and process .

[dalee]

'cryogenics'

 

Sounds simple enough, put the motor in the freezer with the chips.

 

[DatZed]

Well I might trial reducing the big end size and width on an l28 before I risk a stroker crank, I've only got three left!

I will be running an SR20 rod, I'll keep you posted with how it goes.

[NZeder]

yep,, i have over a period of time broken 3 cranks, all in the same spot on the crank, even after a lot of meticulouse prep work, and its not due to just 1 thing , its a factor of many , touque is the main one , which is what we all want , the more you have the higher the chance is of something breaking , dont expect a motor that is highly stressed to last for ever it wont , but you can prolong engine component longevity , cryogenics is now widely used in high output racing engines these days, maybe someone else can explain this type of treatment and process .

Ok so questions for you then.

1. Did they all break at number 6? ie snap the back of the crank off

2. What is your flywheel/clutch setup?

3. What is your harmonic balancer setup? ie stock unit or something else

 

Re the question about journal size - I know that in Japan some were using the L20ET rods as these are 135mm long but on the smaller journal/big end. I believe they offset ground the crank to match the rod. At the little end they installed a bush that again was offset to increase the centre to centre length from 135mm to 137mm (or whatever the L14 rod length is I think it is 136.7mm if I recall correctly) and to take out from the 19mm to the more common L6 sizing of 21mm. However by the time you do all this work on these rods and crank to make them work you could have just purchased some h-beam in the sizing your required from the start. Trust me on that I have spent $$ on rods bushing, shot penning, balancing etc when new H-beams were only about $50 more for the set (this was due to some tricky stuff we did with the little ends )

 

[DatZed]

I am going to use stock SR20 rods for my test engine, but get some Argo's made for my stroker once I have worked out if the cranks are going to break or not. Given that the stroke of the SR is much longer, the  mean piston speed is much higher so there's little chance that the rods will break in an L28 with an 80mm stroke. The pins are 22mm, so I will be using a piston with a 22mm pin and an 89mm bore.

I am picking up some SR rods this weekend so I will post some pics of how they look on the L crank given that the big end width is pretty narrow.