crossflow conrod lengths

[Anonymous]

Hi all.


I have been lurking for a while now. I have a XE ute and a spare motor so I have been planning to warm up the spare engine for a while. Part of this will be to up the compression and possible change conrods. I have looked about for the lenghts (centre to centre) for thes standard 250 conrods but cant find it anywhere, and it isn't in my gregorys service manual.

I'm currently running a 350 holley with extractors and a 2.5" straight through exhaust.
So far I have found that the 200 crossflow conrods are 6.27" and the AU six ones are 6.06" (both off this forum I think). I would appreciate if anyone could verify these numbers or even better provide the actual length of the stock conrods.

If anyone could give an indication of what sort of gains I might achieve by changing conrods that would be appreciated.

Thanks guys, keep up the good work.

 

[xctasy]

Yes, you are right.

The longer 6.27 rods in a 250 reduce the side loads significantly. Ford Oz spent a few million on lengthening the rods to 6.06 from 5.88 at the AU launch. Up till then, all OHC used 200/250 rods.

Spend another 1000 bucks for a rebuild of just the recipricating components and cylinder bores.
Grab onto:-

a)new shallow 1.163" tall compression height ACL race pistons,
b)resized post 1970 - 1976 200 Log or 1976 to 1988 3.3 X-flow rods (6.27") then you'll benefit in the following ways:-

1.Rev range will be increased by about 500 rpm, with the right cam and sump to control oil splash and windage.
2.The horspower loss due to friction is much lower.
3.On engines which show a 10% reduction in rod: stroke ratio (improving it from 1.62:1 to 1.8:1) there is a 4 hp per litre increase in peak power, with no loss in torque at the any point.
4.I infer that the 250 six would show 16 hp more if the cam is optimised, and rev to 500 rpm more if durablity mods were made, with no loss in low speed torque.
5.Wear will be reduced.
6.There is a slight reduction in effective compression which reduces engine pre-ignition
7. but a slight increase in residence time (the time the piston is subjected to heating). The result is that both factors cancel out any improvement in detonation resistance.
8. Fuel economy may improve at part throttle, but increase at wide open throttle.

Notes:Some guys have tried doing tests with Chrylser LA V8's (Hot Rod), A-series Minis (David Vizard) and small block Chevs (Kevin Bartletts engineers when doing Formula 5000), but no one to my knowledge has done a simple assessment of the pros and cons without messing about with combustion chambers and bore to stroke ratios.

I've gone as far as I can. Until someone does back to back tests without changing bore to stroke ratios, we'll keep talking maybees.


Further Plans:

I've got acess to:-
a 200 Log 6 with 4.71" rods (1.51:1 ratio)
a 200 Log 6 with 6.27" rods ( 2.01:1 ratio).
Unfortunatly, the main bearing diameter is different, which changes the internal friction.

Someone needs to do a back to back comparison with the same head, carb, exhast, ignition, camshaft and gearing.

I'll go on record with what I've stated. Anyone who has proof can dispute it.

 

[Anonymous]

Brilliant, thanks xecute.

The plan was exactly to spend about $1000 with camshaft timing new acl hypereutectics and rebuild, with balance. I have acess to a good garage, so I'll do the work. So when the new engine goes in it will have a different cam so I wont be able to do a comparison....

But recemty I crashed my motorbike (I'm fairly OK, thanks all for your cocern) so my spare dollars are directed at that, It sure does suck not having a bike in the tasmanian summer.

Cheers,
Chris.