ugluk- just off to work at the mo, will digest all that info when I get ther,

cheers!

The high Compression DOES increase power not just efficiency in certain areas, more squish more bang its not about tuning the engine at rev ranges its just about the more you compress the gass the bigger the bang you get, however the more risk of damage to the engine through detonation.

it's not a question of squish, i agree that more squish is better in any engine because it homogonizes the fuel mixture better, it's more to do with ignition timing a Volutetric efficiency, FACT: lower compression ratio and more advanced ignition timing makes more power on the dyno.... but to be honest running on normal unleaded fuel i would use the higher compression engine anyway

Shorter rods on a long throw crank DOES increase swept volume, with shorter rods on longer throw crank the TDC position remains the same the crank is throwing it further up but the shorter rod makes up the difference so the TDC position is the same now rotate the crank 180 degrees the longer crank throw is pulling the piston further down the cylinder and the shorter rods are making it even further down the cylinder - bigger difference between TDC and BDC - more swept volume. Shorter rods are going to be lighter as well. Although I guess manufacturers would know best which is the bigges risk between moving the rings closer together and shortening the skirt or shortening the rods and giving them greater angles to deal with.

if the length of the rods are changed it makes no differnce to the swept volume, with a short rod the piston starts and finishes lower in the bore, and with a long rod it starts higher and finshes higher, but so long as the stroke length of the crank hasn't changed, the swept volume/cc wont change....

i agree that a shorter rod has less mass which reduces reciprocationg inertia inside the engine, but... if you have a shorter rod, you need to inrease the compression height of the piston to retain the same compression ratio etc, effectively this means you need a longer piston, and a longer piston weighs more and causes more friction in the bore hence it will wear out more quickly.... so it's a trade off.

ITs the short rod long crank throw combination that makes the difference not just the length of rod.

At TDC the long throw of the crank makes up for the short length of the rod meaning compression height is the same. Say you have a 4"crank "throw" and 6" rod at TDC the piston is 10" from the centreline of the crank.

at bdc the long throw of the crank is taking it further down the shorter rod means its even further down again. Its the combination of longer throw crank and shorter rods which does increase volume. With the same 4" crank throw and 6" rod at BDC the the piston is 2" fromt he centreline of the crank.

(yes thats one hell of a longstroke but figures are just to help visualise)

If you lengthen the throw of the crank to 5" and shorten the rod to 5" TDC is still 10" from crank centreline, however at BDC it is exactly at crank centreline (ouch that would hurt) therefore youve gained 2" in travel by lengthening the crank throw and shortening the rod. You could achieve the same thing by using the same length rod and move the gudeon pin 1" further up the piston.

my heads exploding at trying to understand all these numbers:D

i think we might be confusion each other with all these different engine codes etc

but this is how i see it, if for example you have a 4" crank throw(half of the stroke) the difference between the highest and lowest point(tdc and bdc) is 8".... it doesn't matter how long the conrod is because it still only travels(if clearance is available) 8 inches in total from top to bottom. fitting a shorter conrod will give you more space above the piston at tdc hence lowering the compression ratio.... this has no effect on the cubic capacity because the bore and stroke measurments are still the same

further to this.. if you increase the length of the stroke it will increase the cc

Just trying to put together an illustration of what Im talking about works better if you can see it.

Right here you go crankrod

Sorry the animation isnt well done and the crank shaft doesnt "rotate" as such but it shows TDC and BDC points with a "stock" crank and rod and with a longer throw and shorter rods to get the same TDC but a lower BDC to increase swept volume. Hopefully you can see that between tdc and bdc the crank throw and rod lengths remain the same with both stock and long throw short rod combos tdc is at the same point but bdc changes. the short rod isnt necesary to increase the capacity - a long throw crank alone will do that - but if you ahve a long throw crank with stock rods and pistons the TDC position will be higher by however much you lengthen the throw, however with shorter rods the top height remains the same and lower height becomes even lower.

Obviously the crank throw \ rod length shown here would not work in real life but it shows how altering the ratio has an impact on volume.

ugluk cool anim.

So I'm thinking that maybe either longer throw crank and stock rods or stock rods and crank, and to hell with the 1.4 idea?

Long throw crank with stock fabia rods and pistons.

Long throw crank with felicia rods and pistons would result in a piston smacking into a cylinder head at high speed.

You have options to go for both long stroke crank and bigger bore pistons and liners I believe they can take these engines out to 1500 reliably or 1600 with overly thin cylinder walls.

just added another bit to the anim to show long throw crank with stock length rods and pistons. crankrod2

Long throw crank with stock fabia rods and pistons.

Long throw crank with felicia rods and pistons would result in a piston smacking into a cylinder head at high speed.

You have options to go for both long stroke crank and bigger bore pistons and liners I believe they can take these engines out to 1500 reliably or 1600 with overly thin cylinder walls.

do I want to go that big though? would it not make the engine less revvy-ie less square?

if its gonna be done I still want cheap road tax so I can drive it to the circuits:D

i see where the confusion was now... i thought you were referring to a stock throw with any given length of rod....

i dont deny that increasing the stroke also increases capacity

no no the throw is where the increase comes from the shorter rod is to allow it to work in the stock block, to keep the TDC point the same.

no no the throw is where the increase comes from the shorter rod is to allow it to work in the stock block, to keep the TDC point the same.

I take it that the timing would need adjustment?

yeah but regardless of how log the rods are, the tdc point would still be the same

well look at the second anim if the stock rods are used with a longer throw crank the piston raises higher at TDC. the only way other than shorter rods to keep the same TDC height is to take the extra height out of the top of the pistons. Extra throw means a piston travels further up and down if its going further up its gonna hit the head unless you shorten something.

:iagree:

i meant in terms of the timing marks

take it you lot had a snow-day today?

take it you lot had a snow-day today?

no snow in glasgow- just a boring day at work!, and haey it may even happen:D

just a boring day at work!,

i'll let you of.

ugluk, teflon, whats your excuse???

i'll let you of.

ugluk, teflon, whats your excuse???

Ta-muchly i'm sure!

Think the boys just enjoyed a post about stuff that they can get their teeth into, as opposed to locked phones:thumbup:

i work in professional theatre nowadays!! i'm in lieu time till the end of next week

Think the boys just enjoyed a post about stuff that they can get their teeth into, as opposed to locked phones:thumbup:

:iagree:

makes a change to have a debate with some people who know what they are talking about rather than all the usuall "will this k&n give me a another 50 bhp" threads:rolleyes:

:iagree:

makes a change to have a debate with some people who know what they are talking about rather than all the usuall "will this k&n give me a another 50 bhp" threads:rolleyes:

one does ones best! I dont claim to be an engine builder, or even particularly knowledgeable about the process,

however I do enjoy "proper" tuning- i'e cams, head work, crank swops and things like TB's rather than chipping and all that jazz.

dont get me wrong, I'll prob be doin a MS install on the Felicia if this goes ahead:thumbup:

i'm still wrestling with the emerald m3dk engine managenment system... i've made the loom it all nearly ready to go in, just waiting on some more parts from germany

i'm still wrestling with the emerald m3dk engine managenment system... i've made the loom it all nearly ready to go in, just waiting on some more parts from germany

whats the emerald like to work with? most of the BMW boys I know (doin all sorts of tasty installs!) use MegaSquirt. think its nearly as flexible and cheaper?