What differences does it make to the way an engine operates (Its Charcacter??) by having different lower engine dimensions in crank throw, rod length and piston diameter but the same/similar displacement.

Here are a few dimensions for a few popular engines we all know and have an interest in on here.

Cosworth YB.
CT:77mm, RL: 128.5mm, PD: 90.8mm, OC: 1993cc

Zetec E Black Top.
CT: 88mm, RL: 140.8mm, PD: 85.0mm, OC: 1997cc

Zetec E Silver Top.
CT: 88mm, RL: 136.1mm, PD: 85.0mm, OC: 1997cc

Vauxhall XE.
CT: 86mm, RL: 143.1mm, PD: 86.0mm, OC: 1998cc

Mistibushi 4G63.
CT: 88mm, RL: 150.0mm, PD: 85.0mm, OC: 1997cc

Mitsibushi 4G63 Long Rod.
CT: 88mm, RL: 156.0mm, PD: 85.0mm, OC: 1997cc

All above are standard dimension engines bar the 4G63 LR as thats what im currently doing research on for a friend and want to know what difference having the extra 6mm in rod length makes (with the correct pistons to suit. IE location of pin, crown height, skirt length, comp ratio etc) to the engines characteristics.

Hope this will make for an interesting thread.

 

First thing you should do, is right the rod ratio down for each engine, thats a very important figure to look at when considering geometry.


To answer your question though specifically about the evo engine, the longer rod will give it a higher rod ratio
Before: 1.7
After: 1.77


Implications of that:
The piston will linger near to top dead centre for long, as the same amount of movement from the crank at TDC (where its initial movement is essentially horizontal not vertical) will result in a smaller vertical movement of the little end (and hence piston) down the bore.

What this means is that you can work the charge harder at higher revs.

So basically, it will make more power at high RPM

Downside is at lower rpm you can end up not being able to run very much timing, which can lose out on power, especially when running big boost.

 

should make for a good read this

 

Chip whats the calculation to work out the rod ratio?

 

rod length / crank throw




Simple as that!

 

Cool.

Is there an ideal rod ratio for the "perfect" engine or is it all down to application of use required?

IE a Focus WRC engine will have a lower rod ratio as it will never really see high revs, but wants to maximes torque (due to inlet restriction), compared to say a circuit race car turbo engine (Time Attack) which will want a higher rod ratio as it will allways be seeing higher revs as it wont have any mandatory restriction to have to deal with?

 

As you say mate, its about what RPM the engine is doing.

Ironnically the late saab 2.0 turbo engine bottom end has a rod ratio of over 2 and likes about 9Krpm, and comes with a turbo good for about 4K rpm

 

Sounds like Saab to me!

Hmm lovely bit of boost we seem to be making here at IDLE!

Does all this make a difference to response both on and off throttle in a similar effect to lighter flywheel?

 

Bit of an old thread ( I was bored and browsing lol ) and as im working on a long rod engine at the mo, when ford motorsport built the WRC escorts they used a long rod, and subsequently gained approx 40ft/lb of torque in their restricted rev range, so the gains must not all be up high in the revs.

 

It depends what fuel you are using and what CR you are on etc.

Essentially, with the fuel they were on the original rod ratio was probably optimised for about 2Krpm effectively, so even a longer rod that moved it up the rev range only moved it to the midrange.

If that makes sense?


The effect of the longer rod is the piston hanging around longer at TDC to work the fuel harder, this is particuarly useful at highrpm, but its also potentially useful at lower rpm in certain cases.

 

Indeed it does, its all about how much fuel we can burn efficiantly.

Oh, and the bore data is wrong in the OP's first post for the YB.

 

am i reading this wrong?

so a longer rod gives an unrelated, but similar effect to increasing CR?

I.e. allows you to build power (torque)in high and low RPM ranges by increasing cylinder pressures earlier?

(ignition timing and detenation supression dependent of course..)

Hence with the fuels that ford were running in the above example meant they could increase cylinder pressures closer to det, at lower RPM's without having to increase the CR or advancing the timing?

 

MarkK, You are missing my point mate about the fuel. I was saying they changed fuel at the same time.

The problem with having a rod ratio that leaves the piston hanging around for a long time at TDC is that it makes it easier to find detonation (or harder to avoid it) but when the rally cars are running very det resistant fuel it means that the rod ratio that is optimum for them is greatly different than on a normal road fuel at the same rpm.

Ie because of the fuel they were using (both before and after the rod ratio change) the rod ratio change worked well for them at those rpm, but on a more normal fuel it quite probably wouldnt have done so.

 

Yes a longer rod is a similar effect to more CR, but ONLY at top dead centre, the effect is shorter lived than compression.
So the rod will have a big effect at 0 degrees of crank rotation, but at say 15 degrees (where most performance engines tend to make peak cylinder pressures) the effect is less but for an increase in CR its still marked.

 

The engine has to be specced to suit the requirements, its not just a case of throwing a longer rod in otherwise the piston clearance would be a problem


Fuel or fuelling ?

Also having the rod at TDC (or around the cylinder filling point) gives a greater opportunity and timing scale to fill the cylinder at any given revs.

Why would the YB have the shortest rod then ? because its the oldest engine design !
I cant think of any engine that uses rods that short anymore, the crank stoke on the YB is relatively short, hence why a decent specced YB will love to rev, add into that longer rods and you are heading into a territory that also elongates your engines life expectancy.

 

absolutely!!

im not sugesting that you use a longer rod to increase CR, im just pointing out that the effect it has on cylinder pressures throughout the combustion cycle is similar to increasing the CR...

 

only dependant on how you choose to fill the cylinders, valve timing has to be considered to effect dynamic CR.

 

agreed,

what do you mean when you say "how you choose to fill the cylinders" ????

valve timing is irrelevent for the point i was making, i meant an example in which the timing and static CR remained constant...

 

The fuel, ie what petrol they are using.


The "cylinder filling point" is much lower down the stroke, more like 90 degrees, rod length has little effect there, however what does happen is that when it hangs around at TDC it wastes time that the cylinder could be getting filled, but then conversely you see a greater pressure differential when it does start moving, but that entirely depends on the valve timing as to which of the 2 effects is better, so very difficult to make any sort of generalisation about.


Agreed the rod ratio is suitable for an 8v transit van engine, but thats what the geometry was designed for, the YB wasnt designed to be a performance engine, it was a modified version of a very much non performance orientated one.

 

Rod length will still have an effect at any angle as its reducing trying to grind the pistons against the bore !

and correct the cylinder does begin to fill way before TDC, this is where along stroke crank would be better, but the short stroke will then aid on the power stroke by the speed the piston comes back down the bore.

I can't believe im having a techical discussion whist waiting for my child to be born !! pmsl.

 

Yes there is a drop in side loading as a result of the longer rod all the way down, I was saying in terms of cylinder fill / piston speeds, that once you are heading towards 90 degree the effect is very negligable

Agreed what you gain on stroke length increasing or decreasing speeds where you want it, it will have the opposite effect where you dont.


Indeed, you could be out interviewing swedish aupairs instead!

 

p.s

Its the reduced side loadings and there improvement in reliability that no doubt are the reason for the seeming "mismatch" on the saabs 1.96 rod ratio!

 

Congratulations mate

 

Sorted this now Mark