Engine Rebuild- longer rod/shorter piston
07-11-2010, 10:55 AM
#1
Engine Rebuild- longer rod/shorter piston
Im doing a big bhp rebuild on my 200block yb and Ive herd of longer rods and shorter pistons.
Just wondering what peoples experiences/thoughts/ideas are with this set up?
CheeRS fraser
Just wondering what peoples experiences/thoughts/ideas are with this set up?
CheeRS fraser
07-11-2010, 11:13 AM
#4
Engine will rev better if you increase the rod ratio.
Quite expensive to do unless you are already planning on replacing the rods and pistons with a set of Arrow/CPs or whatever, in which case its definately worth doing if you are spending the money out anyway.
No real downside to it, you'll lose a tiny bit of midrange potentially but nothing you'll ever really notice.
Martin goes one step further with a taller block to allow you to increase the stroke and the rod ratio at the same time.
Quite expensive to do unless you are already planning on replacing the rods and pistons with a set of Arrow/CPs or whatever, in which case its definately worth doing if you are spending the money out anyway.
No real downside to it, you'll lose a tiny bit of midrange potentially but nothing you'll ever really notice.
Martin goes one step further with a taller block to allow you to increase the stroke and the rod ratio at the same time.
07-11-2010, 11:31 AM
#5
As chip says it will help it rev a little higher and more freely up there imo,this is what i use in my engine,wrc spec longer rods and short skirt cosworth pistons and if using around a std cr i dont think its lost any midrange,i have 582lb ft at 5.2k!!
Mine is 8.6 cr though...
cheers danny
Mine is 8.6 cr though...
cheers danny
07-11-2010, 12:13 PM
#7
The reason it works is that at high rpm two things lose you torque in a big way:
1) Decreased volumetric efficency of the engine where the induction stroke timebase is decreased, so basically you cant fill the cylinder properly
2) The piston is moving so quickly that its harder for the now weakened (by the drop in VE) flame front to exert much pressure on it near the top of the cycle where it needs to
Longer rods, by allowing the piston to hang around nearer to TDC they can greatly descrease the second negative effect of big rpm, as it gives the flame front more time to exert pressure on the piston.
I suspect their effect on VE (ie the first of the two points I mentioned) may be an increase as well, due to the fact that the piston also lingers at the bottom part of the cyle where maximum volume is acheived, but Ive no data to proove that and there are some downsides too (ie its at the top on the intake stroke for longer too)
1) Decreased volumetric efficency of the engine where the induction stroke timebase is decreased, so basically you cant fill the cylinder properly
2) The piston is moving so quickly that its harder for the now weakened (by the drop in VE) flame front to exert much pressure on it near the top of the cycle where it needs to
Longer rods, by allowing the piston to hang around nearer to TDC they can greatly descrease the second negative effect of big rpm, as it gives the flame front more time to exert pressure on the piston.
I suspect their effect on VE (ie the first of the two points I mentioned) may be an increase as well, due to the fact that the piston also lingers at the bottom part of the cyle where maximum volume is acheived, but Ive no data to proove that and there are some downsides too (ie its at the top on the intake stroke for longer too)
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07-11-2010, 12:35 PM
#9
If you were going to do it at all with something variable, I think crank throw would be a better thing to vary, but its really not realistic to do either anyway, well not in anyway I can think of that would stand up to 500bhp and not increase the reciprocating or rotation mass by a huge amount.
07-11-2010, 02:56 PM
#10
The reason it works is that at high rpm two things lose you torque in a big way:
1) Decreased volumetric efficency of the engine where the induction stroke timebase is decreased, so basically you cant fill the cylinder properly
2) The piston is moving so quickly that its harder for the now weakened (by the drop in VE) flame front to exert much pressure on it near the top of the cycle where it needs to
Longer rods, by allowing the piston to hang around nearer to TDC they can greatly descrease the second negative effect of big rpm, as it gives the flame front more time to exert pressure on the piston.
I suspect their effect on VE (ie the first of the two points I mentioned) may be an increase as well, due to the fact that the piston also lingers at the bottom part of the cyle where maximum volume is acheived, but Ive no data to proove that and there are some downsides too (ie its at the top on the intake stroke for longer too)
1) Decreased volumetric efficency of the engine where the induction stroke timebase is decreased, so basically you cant fill the cylinder properly
2) The piston is moving so quickly that its harder for the now weakened (by the drop in VE) flame front to exert much pressure on it near the top of the cycle where it needs to
Longer rods, by allowing the piston to hang around nearer to TDC they can greatly descrease the second negative effect of big rpm, as it gives the flame front more time to exert pressure on the piston.
I suspect their effect on VE (ie the first of the two points I mentioned) may be an increase as well, due to the fact that the piston also lingers at the bottom part of the cyle where maximum volume is acheived, but Ive no data to proove that and there are some downsides too (ie its at the top on the intake stroke for longer too)
Any how because of the effects the longer rod etc has its why i get away with running a larger duration on my cams too but what effect iyo do you think havind a slightly longer stroke crank will have????
Im only going up to an 80mm stroke as i dont want a bigger bore and i want to keep my rods but all im really looking for is to make the turbo spool quicker without changing much else,hence not going 82-84mm stroke...
This way all i have to do is move the pin hieght by 1.52mm higher up in the piston witch i can do with out having to change my rods too...
cheers danny
07-11-2010, 03:41 PM
#11
More stroke is good for taming big cams, but the downside is that you then start undoing some of the good you have done on the rod ratio if you are on the same length rod still, so its a case of which effect will have the greater influence, which is not something easy to theorise accurately about so I wouldnt like to guess TBH mate, little bit of extra capacity is good though of course.
I'd love to play with some different throw cranks and length rods in my saab engine as its got such an awesome rod ratio to begin with that you can afford some compromise to get more capacity (its 78mm crank and 153mm rods so a 1.96 rod ratio, so massively more scope than the YB) and still have an engine that really wants to rev, but sadly I cant afford to keep buying different cranks and rods so will have to just guesstimate the geometry I want and go for it once!
I'd love to play with some different throw cranks and length rods in my saab engine as its got such an awesome rod ratio to begin with that you can afford some compromise to get more capacity (its 78mm crank and 153mm rods so a 1.96 rod ratio, so massively more scope than the YB) and still have an engine that really wants to rev, but sadly I cant afford to keep buying different cranks and rods so will have to just guesstimate the geometry I want and go for it once!
07-11-2010, 05:57 PM
#12
Happily retired
.
07-11-2010, 06:26 PM
#13
More stroke is good for taming big cams, but the downside is that you then start undoing some of the good you have done on the rod ratio if you are on the same length rod still, so its a case of which effect will have the greater influence, which is not something easy to theorise accurately about so I wouldnt like to guess TBH mate, little bit of extra capacity is good though of course.
I'd love to play with some different throw cranks and length rods in my saab engine as its got such an awesome rod ratio to begin with that you can afford some compromise to get more capacity (its 78mm crank and 153mm rods so a 1.96 rod ratio, so massively more scope than the YB) and still have an engine that really wants to rev, but sadly I cant afford to keep buying different cranks and rods so will have to just guesstimate the geometry I want and go for it once!
I'd love to play with some different throw cranks and length rods in my saab engine as its got such an awesome rod ratio to begin with that you can afford some compromise to get more capacity (its 78mm crank and 153mm rods so a 1.96 rod ratio, so massively more scope than the YB) and still have an engine that really wants to rev, but sadly I cant afford to keep buying different cranks and rods so will have to just guesstimate the geometry I want and go for it once!
Im only in effect using 1.5mm more of the bore with what im doing and as iv now gone twin scroll in theory should get the spool around 5-600 rpm lower and with the new crank being twin counter balanced can now let it rev to 9k!!peak power can now be raised..
The last compromise weve had is 607 hp at 6.2k but still holding 599 hp at 8k and soft cutting at 8.4k to save the crank etc as when i had it apart last year was seeing bronzing on the left of the rear mains and bigs,im hoping the new crank will in effect change all of that so over winter will see if it all works...
Them saab engines would be awesome with a bit of playing with!!are you going to strap the gt35 to it???should be fun!!!
cheers danny
07-11-2010, 06:54 PM
#14
Yeah i thought youd say that and thats why i didnt want to go to made on the cranks stroke..
Im only in effect using 1.5mm more of the bore with what im doing and as iv now gone twin scroll in theory should get the spool around 5-600 rpm lower and with the new crank being twin counter balanced can now let it rev to 9k!!peak power can now be raised..
The last compromise weve had is 607 hp at 6.2k but still holding 599 hp at 8k and soft cutting at 8.4k to save the crank etc as when i had it apart last year was seeing bronzing on the left of the rear mains and bigs,im hoping the new crank will in effect change all of that so over winter will see if it all works...
Them saab engines would be awesome with a bit of playing with!!are you going to strap the gt35 to it???should be fun!!!
cheers danny
Im only in effect using 1.5mm more of the bore with what im doing and as iv now gone twin scroll in theory should get the spool around 5-600 rpm lower and with the new crank being twin counter balanced can now let it rev to 9k!!peak power can now be raised..
The last compromise weve had is 607 hp at 6.2k but still holding 599 hp at 8k and soft cutting at 8.4k to save the crank etc as when i had it apart last year was seeing bronzing on the left of the rear mains and bigs,im hoping the new crank will in effect change all of that so over winter will see if it all works...
Them saab engines would be awesome with a bit of playing with!!are you going to strap the gt35 to it???should be fun!!!
cheers danny
If so I think you can make that soo much better
If you remember my old mans engine the last one, it peaked at 8k 613bhp
608 at 8500 and dropped under 600 at 9k
Full boost was just around 4 k in car, this was a 2.0 on STD crank and lower cr than yours, pump fuel etc,
I would look at cams as something isn't quite right to produce power so early as I know your engine is well specced there must be soo much more in it !?
07-11-2010, 07:07 PM
#15
Danny did it peak at 6.2k revs ?
If so I think you can make that soo much better
If you remember my old mans engine the last one, it peaked at 8k 613bhp
608 at 8500 and dropped under 600 at 9k
Full boost was just around 4 k in car, this was a 2.0 on STD crank and lower cr than yours, pump fuel etc,
I would look at cams as something isn't quite right to produce power so early as I know your engine is well specced there must be soo much more in it !?
If so I think you can make that soo much better
If you remember my old mans engine the last one, it peaked at 8k 613bhp
608 at 8500 and dropped under 600 at 9k
Full boost was just around 4 k in car, this was a 2.0 on STD crank and lower cr than yours, pump fuel etc,
I would look at cams as something isn't quite right to produce power so early as I know your engine is well specced there must be soo much more in it !?
502 hp at 5k,607 at 6.2 and still 599 at 8k with only 6.5 degree ignition on v power..
all at 2 bar peak and 1.9 at 8k so im well happy with my cams mate as i can do what i want with them..
The new engine im hoping for 700 hp at around the 8k mark and untill the rev cut with circa 600 lbft ...
The above engine as it is now is only 2.0 ltr still, new one will be just under 2.1..
cheers danny
07-11-2010, 07:13 PM
#16
jimbo you only had 453lb ft mate,iv got 582lb ft!!!!its the best compromise through out..
502 hp at 5k,607 at 6.2 and still 599 at 8k with only 6.5 degree ignition on v power..
all at 2 bar peak and 1.9 at 8k so im well happy with my cams mate as i can do what i want with them..
The new engine im hoping for 700 hp at around the 8k mark and untill the rev cut with circa 600 lbft ...
The above engine as it is now is only 2.0 ltr still, new one will be just under 2.1..
cheers danny
502 hp at 5k,607 at 6.2 and still 599 at 8k with only 6.5 degree ignition on v power..
all at 2 bar peak and 1.9 at 8k so im well happy with my cams mate as i can do what i want with them..
The new engine im hoping for 700 hp at around the 8k mark and untill the rev cut with circa 600 lbft ...
The above engine as it is now is only 2.0 ltr still, new one will be just under 2.1..
cheers danny
Your turbo is bloody huge compared to the one used too !
Ps not knocking yours just saying as you mentioned 9k revs
07-11-2010, 07:38 PM
#18
Yes it was low on torque, but the barrel throttles pulled this down, and obviously the cams, for a 2wd car I thought a better spread of torque and power would be better due to grip ?
Your turbo is bloody huge compared to the one used too !
Ps not knocking yours just saying as you mentioned 9k revs
Your turbo is bloody huge compared to the one used too !
Ps not knocking yours just saying as you mentioned 9k revs
anyway iv got over 400 lb ft from 4.4k and rising and still over 450 lb ft at 8k so it has got a big spread of power and torque!!like iv said i made a nice compromise between the two and will make it even better again...
Your right though there is more in there just didnt want to push it at the top with only v power and std crank...
cheers danny
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