Hmm was reading random stuff about cars (really? were you grim? thats odd, and not like you), and seeing what some manufacturers have been producing over the years, and found that the 412kw engine in the Lambo murci is acutually MORE undersquare than our venerable sixes

as the murci uses a 84.5 x 92.8 (.091 ratio) compared to the i6 having .93 ratio.

It also makes power to 8000 rpm. obviously there are more cylinders, and thus more valve area for every cubic inch capacity, but s**t, it bucks the trend of undersquare engines not loving rpm.

found on http://www.ultimatecarpage.com/car/3629 ... 560-4.html

its a good site for looking up details on just about any newish make and model

The engine itself, would be more expensive than say any entire FPV.

If you throw enough money at engineering you can get it to hold together.

If you stick with a 2:1 rod/stroke ratio then a square engine is ideal for revabillity. Once you change the rod/stroke ratio then undersquare engines can actually be made to be more revable. Also in the days of soft iron blocks and equally soft piston rings, side load would cause a lot of premature bore wear. With todays high nickel content cast iron blocks and chromolly rings this is no such problem. The limiting factor in a high revving naturally aspirated falcon engine is not the design geometry. Its actually the valve train and also the limited flow capacity of the heads.

Id like to hear more engineering theory on R/S ratios. This part is new in my pick-up-parts self education.

a 2:1 ratio, does this mean the rod is twice the length of the throw of the crank?

Am I then correct in assuming that you would need more than a 1:1 ratio for it to work also, since once the throw of the crank exceeds the length of the rod, it is then impossible for the assembaly to rotate?

what effect does, say a 1.5:1 R/S ratio have over a 2:1 RS ratio? What are the pro's and cons? And how would you adapt this to improving the reliabilty at revs using the sohc inline 4.0l block?

Hi Grim... My guess at answering such a question is that say the stroke is 10 inches... And the rod is 10 inches... It's impossible, but let's just say it is possible, just for theory. When the piston is half way up the bore (ie - the crank is at 90 degrees) the angle of the rod is 45 degrees from pin to pin (crank to piston). It then goes up to top dead centre (at is at 0 degrees) and then comes back down (and goes out to -45 degrees)...

This means that every revolution the engine does the rod is wobbling backwards and forwards a helluva lot.

The AU and the EF are examples of things being different. The shorter conrod but longer piston means the actual bore and stroke are the same, but the dynamics of the angles and sideways loads placed on the piston and sides of the bore are different. I seem to remember the AU has the same rod and piston measurements as the BA???

More suitable for higher revs would be a longer rod and a shorter piston (????)


Like XCABBI says though, head flows and valves and cams!

Ask yourself what you want from the engine at high revs. Maximum cylinder fill or revs for the sake of revs (maximum reliability by the use of minimum side load)? If cylinder fill (power) is what you want then it comes down to much more than rode:stroke ratio's. Cam specs and port flows come into it more than anything else. Short rods give you higher peak piston speeds which draw the air in faster on the inlet stroke, but they also can be a hindrance when using high duration cams as the piston could be half way up the bore on the compression stroke by the time the inlet valve closes (in extreme cases). Long rods give you the dwell at top and bottom dead centre and have the piston a lot lower up the bore when the inlet valve closes but the trade off is you sacrifice peak piston speed.

Even though 2:1 is perfect in text books, in reality its far from the truth. Holden 308's are up around the 2:1 mark and formula 1 engines from the 80's turbo era were down to 1.5:1 (worst case with respect to bore wear). 302 windsors are around 1.7IIRC and 302's with the 289 rods were in the 1.8:1 region. Anyone want to work out what the falcon I6's are. IIRC EA-EF had 5.88" rods whilst AU-FG have 6" rods.

Stoke is 99mm (3.89")

So with a 5.88" rod ratio is: 1.51:1
And with 6" rod ratio is: 1.54:1

Well ultimately you want both, reliabilty and top end cylinder fill, or at least as close to both as you can.

So it seems that as far as R/S ratio goes the AU bottom end is the best starting point for a SOHC 6. If you were to take things very very seriously, and custom build a crank shaft, would it be possible to reduce the throw of the crank, and use an even longer rod? for arguments sake? Ive seen them making custom crankshafts on channel 31's Gasolene program for V8's and was wondering if the same could be possible for the i6, so your basically using the block, modified in bore, and modified custom crank. Then, once the bottom end is developed to a point where it has its maximum potential to be friendly at higher than stock rpm, set about designing the top end to go with it. a 6.61in rod would give an RS ratio of 1.7:1, which would seem a good compromise of cylinder filling top end, and bottom end reliability.

With the theory in here, we should be able to almost come up with a complete "guide". Which I hope to use one day after Uni is finished with, and I can take the au off the road to make into a weekend fun car.

Possible, yes. Easy/cheap no.

You'd also loose capacity. shorter throw = shorter stoke.

If you wanted a longer rod, try reducing the piston height (I don't know how much leeway there is in the AU/B series pistons)

Easy for EA or XF owners to just get the short throw crank in there. (3.3 crank for an XF and 3.2 for an EA). There's a guy In Wollongong who has de-stroked his TD cortina down to 3.0 litres and spins it to 8000rpm on a regular basis. After all it is a turbocharged 8 second car.
Apparently the long stroke of the 4.1 was accelerating the pistons too much at the power levels he wanted and the rods would just bend like bananas. He figured that if he increased rpm with a short throw crank he could get a lower piston speed at higher rpm but yet still get the close to the same cylinder fill with the long throw crank.
This is where my spreadsheet comes in handy fiend. Give me 2 weeks and I'll have it modified so you can see the acceleration of each component of the rotating acceleration and the inertia forces through each part.

Sounds good Cabbi. Like Databrother says, using a smaller crank would result in smaller displacement. Depending on the length of the rob (or piston height) you could produce a motor from the same block that displaces less than the 4 liters (let's say 3.5) but has a compression of only 7 to one or something - Perfect for high revving forced induction one would hazard a guess....

If you used longer rods to compensate for the lack of crank stroke, you'd get a fair bit of compression back, but maintain the same 3.5 liter displacement.

Note - 3.5L is just a number pulled from thin air, but by doing the bore / stroke computations in reverse we find that a bore of 92mm (standard) and a stroke of 88 (standard being 99) would result in a inline six 3.51 litre. This smaller stroked engine would rev to 6400 at a similar piston speed to the standard 3.95L revving at 5500.

Interesting?

I'd like to think so.

Now all you have to do is find a large enough chunk of billet 4340 so you can machine one up.

Big grin at you mate! I'm sure there are people on this site capable of doing such. It is beyond my current level of machining expertise and knowledge. However, I wonder if the nice people at Wade cams would be interested?........

so any updates on this, read it all lastnight very intresting and would love to build something up as a slow project

Hi everyone New to website and ford land. Been a old school v8 commodore fan mostly, however dad has given me his AU and i'm sentimental. But anyhow back on subject...

The big difrence i see between the mainstream mentality of the ford six vs other stables is in the RPM/diff ratio department.

The way to be fuel efficient is to make power at low RPM burning as litle fuel as possible close to detonation, which is retarded on the mapping of the engine management.

Generally speaking fast isnt economical, so consider the following:

Programable engine management with high flow injectors, a Bosch Motorsport fuel pump, BP Ultimate or Shell racing pump fuel will need to be used. (higher engine speeds and other factors will place too high a demand on stock stuff, and higher octane will be needed otherwise detonation will occur).

Camshaft choice. You guys know better than me about the common choice cams, but the grinds that make power 4000-7000 RPM are the ones you want.
Yes double valves springs etc is the go but just as important are the fasterners holding your engine together! ARP and half a dozen other companies do cromolly studs to hold your head on and caps down. And then Consider better con rods. Most ppl i know who build motors use H beem rods instead of I beem. I believe ACL DO awsome high silicon lighter than stock pistons with huge ring lands. The bigger ring lands give more support to the rings. And they offer a 30cc dish to lower compression too (more on that latter).
The AU crank is apparently the way to go. Ballancing is not as important in the i6 as is the case in a V configuration, but if you want to reduce engine harmonics and cause vibration and oil leeks get the assemble ballanced.

Head porting is ussually the way to go to make the heads flow. And making the ports match between manifolds and the head. With a non restrictive throttle body.

Doing these will allow the motor to rev and make power upto 68--RPMish and hold together if treated well. Atleast i am hoping it will becasue this is my plan
But please consider not going the asperated method of producing power. Bang for buck, lower your compression ration and turbo charge the engine!

A thick decrompression head gasket can also lower compression ratio.

WIth a lower than standard compression ratio and using Forged pistons with an "Electronic" speed controler.
A custom tuned length exhaust header with a T4 flange. And a GT balll bearing medium font case flow compressor. Plumb the gas with 3" stainless pipe (better thermasl properties) into a decent front mount intercooler (fully sick!) and then plumb that into a sheet metal fabricated plenum.

Feel free to run 14psi low 28psi high boost.

Now this is where you start braking diffs anfd gear box's people!