Knife Edged Crank

[tommy gun 6]

Hi guys and gals thinking of getting some hi comp pistons for my 205 gti6 so the bottom end will pretty much be rebuilt, thing is i was thinking about getting the crank knife edged.

Does anyone know the pros and cons of doing this and also how much and who to trust for a good job.

thanks

[rallyeash 155]

doubt you will see any benefit. just get a polish done on the journels and fit a billet flywheel.

[Tom Fenton 1,542]

If you were building a race engine and leaving nothing and I mean nothing untouched, then I'd say yes. However for a fast road/track day engine I would agree that it probably isn't worth the effort.

[tommy gun 6]

ave got a billet fly wheel and getting a pully.

bottom end will be full race spec, i only want to do it if there will be gains?

[Miles 331 1 Cars]

What do you think a Knife edge'd crank does?

But if your going for a race spec engine, then it's forged pistons, Steel rods and a steel crank not a standard one

[markw 1]

i think it would be worth doing, as well as "cutting" through the oil cleanly, instead of "smashing" into it, there are benifits of lightening the crank, you may gain 50-100g i suppose but you could also save about 3 kg by swapping to an 8v crank, plus you gain some extra CC too, (i think it fits?)

 

and a standard Mi16 flywheel,clutch cover and plate weigh 12 kg, and the billet steel flywheel 71/4" ali clutch cover and plate weigh 6.9 kg

on my car, thats an 8 kg saving overall , and as the old saying goes by saving 1lb on the flywheel, is like taking 10lb out the car

[tomcolinjones 26]

save about 3 kg by swapping to an 8v crank, plus you gain some extra CC too,

 

does this increace the stroke to gain CC? if so the piston has to travel further, therefore it takes more energy to turn the engine over, and to get it to rev at high RPM? but would increace torque. but i guess you wouldnt be shaving weight of internals to increace the torque, more for a free revving low rotational mass engine? ... am i correct? i'm no engine expert but this makes sense in my twisted head

I guess the extra mass saved from fitting a lighter crank and flywheel / clutch setup would overcome this anyway. ill shut up now

[markw 1]

tom,

 

absolutley correct, you would increase the stroke by 2mm and it would slightly alter the piston speed and the ability of it to rev quicker, but saving 3kg on the rotating mass

of the crank would far outweigh this anyway AFAIK, also the ignited flame acts on the piston top for longer as it has to push the piston down for the longer stroke, increasing torque, but then again i know F**k all either, i`m a builder. lol

Edited April 4, 2011 by markw

[tommy gun 6]

so am i better off using the 8 valve crank? i will be using 88mm pistons so will the 8v crank increase the stroke there for incresing the cylinder capasity or am i in the dark.

[Miles 331 1 Cars]

The crank doesn't sit in oil so never cut's threw it, 8v cranks are never a good idea in high revving engine's either but I won;t go into why on here as it's all my personal experience.

 

Flywheel as said is a good idea, fairly cheap too when compared to a lightened, balanced and faced item

[rallyeash 155]

use an 88mm diesel steel crank, im running one in my RS block

[Batfink 201]

use an 88mm diesel steel crank, im running one in my RS block

 

I have one for sale worked in colins race car without issue..

[welshpug 1,661]

I have two

[tommy gun 6]

been told today that with the 88mm crank the pistons will lift up past the top of the block meanig you have to use a thicker head gasket to stop the pistons from hitting the valves, is this true?

[rallyeash 155]

yes and no,

 

when running the 88mm crank you need to loose 2mm from the piston/rod height, i had steel rods and forge pistons done for mine. although were not cheap at £1200.

 

a thicker head gasket isnt the way to do it

[welshpug 1,661]

Its not 2mm you need to lose, its 1mm

 

specifically with the RFY engine (S16, ZX 16v, 2.0 MI16, Xantia VSX) the pistons are flat topped and sit about a mm below deck, so a 1mm increase in the protrusion height would bring it just past deck, into the gasket space I think.

 

There's been a few builds like this, though you'd be advised to do a dry build and check clearances, i;'d imagine cutting the valve relief's deeper would be a must if you wanted to uprate the cams, but the compression is nice and high!

[tommy gun 6]

welll ive got wossner high comp pistons, would these not already have deep pockets?

[petert 588]

gti6.................................... thing is i was thinking about getting the crank knife edged.

 

 

I thought all XU10 cast cranks were knife edged!

[tommy gun 6]

right i think i will be useing a 88mm steel crank know, that engine could i find one from?

[bobdylan_55 1]

think the 1.9 td had 83mm bore and 88mm stroke

Edited April 28, 2011 by bobdylan_55

[2052NV 24]

Have you thought about just putting in a sump splash guard/windage tray. then there will be no oil mist for the crank to worry about and alot cheaper than knife edging. And i assume your whole bottom end is balanced yes?

 

WIN WIN (if you haven't got one already )