Ford Australia are known to test an engine at full power for 24 hours and only reject them for production cars if they don't pass. The 347 stroker engines they fitted to the TE and similar model Falcons only lasted 16hrs but when stripped it was crankshaft failure not rods or pistons, so they were passed and many were built.
Ross:thumbsup:
So a crankshaft failure after 16 hrs means they passed the test!!?? I wonder what they have to do to fail the test!!??Any idea what the failure mode was and what caused it? I know Jac Mac is dying to say it was the rod ratio.......:lipsrsealed: If so, I'd love to know why and how....:idea:.
The new crankshaft has longer throw journals which increase stroke from 3" (standard 302ci) to 3.4" (347ci). Rod length is increased from 5.09" (302) to 5.4" (347), this increase in rod length attracts some critisizm as it makes for a poorer 'Rod Ratio'.
Ross:thumbsup:
Actually Ross, the increase in rod length makes for a "better" (in Jac Macs book!) ratio it's the increase in stroke that results in the "poorer" ratio, but I'll bet the increase in torque from the longer stroke is easily measurable, as will be the much lower revs at which comparable power is developed!! I doubt that the "disadvantages" of the poorer ratio are anywhere near as measurable or even significant.
What is the effect of that "poorer" ratio? I can believe the increased rod angle at certain points of the cycle will lead to greater side thrust on the pistons and bores, but is it measurable or significant bearing in mind the "normal" (say 1.5 to 1.8) range of values we are discussing? Mind you if there's greater side thrust, there has to be an opposite equal reaction at the other end of the rod, ie on the crankshaft, so that should result in more power. Does it? Is it measurable? Or is it offset by higher friction losses at the cylinder walls and/or by increased blow by?
I read that blow by only really begins to come into play at rod ratios less than 1.5.
On the subject of "poor" 1.5 rod ratios I read somewhere recently that the succesful Chevy engines at Le Mans in 2001,2002 and 2004 were 6" rods/4'' stroke. Ratio 1.5! Less than anything contemplated in most of our motors. You don't get much more of an endurance race than Le Mans. Mind you I don't know how many of those engines started the races or what percentage expired or why.
To my mind, rod ratios are more a byproduct of changing the stroke in a given block. More stroke = smaller rod ratio, generally. If you accept the simplified proposition that the power of a motor is limited by the amount of air the heads can flow, then for the same bore size and heads, the revs that it is developed at is inversely proportional to the stroke. Thus a 289 with max power at 8000 rpm will be more or less equivalent to a 347 at just under 6700. Provided gearing is altered to suit, performance should be similar.
Which will be the more reliable engine? Which will be the more durable engine? Which will be the more expensive engine to build reliably? In real life, is one likely to be more powerful than the other?
As far as a race engine engine such as Ross' is concerned, which presumably will get an annual strip, cracktest and rebuild, what are the effective drawbacks of the lower rod ratio? Jac Mac has mentioned 302 and 351 strokers being less reliable when pushed at higher RPM. That's understandable, everything is! That's because they are strokers, not because of the rod ratio IMHO. Higher RPM than what? At what point does that become a significant factor since geared to suit, with 15% more displacement over a similar 302 with the same heads/cam Ross could get by with 15% less revs. Surely more durable? No?
I think from memory the small journal 351 that I have built for my car is running 6.25 rods and 1.5 CH pistons, which with the 3.5 stroke gives a rod ratio of 1.79. Does this mean, given the same quality of internals, that I can safely use heaps more revs than Ross because I've got the better rod ratio!
Off to put my Nomex on! :flameon: :bash:
