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I supported Toymods
Location:
Brisbane, QLD
Registered:
May 2002
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Increasing Conrod Length
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Mon, 09 February 2004 02:52
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Hi guys,
What are the practical implications of doing this? What are some practical methods of doing this with or without crankshaft modification? Pros and cons? Is increasing the block height with a thicker head gasket applicaable in this case?
Is it cost effective to get a new crank made, based on an existing stock or an aftermarket design? What materials are recommended for the 600HP - 1000HP range? Rough idea on cost (for a 6 cylinder) in terms of fabrication and materials?
Please oppine!
Cheers
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I supported Toymods
Location:
melbourne
Registered:
June 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 03:39
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why do you want to do this? no offence, but you seem to be confused as to what you want.
what you have here is what we in the engineering business like to call "a solution looking for a problem"
as far as i know:
The effect of increasing the length of a conrod is the reduction of inertial loading due to the conrod having to move through less of an angle as the crankshaft turns.
If you think about it, the crankshaft should not need modification at all, unless you want to change the stroke of the engine. it may need modification if you wanted to SHORTEN the con rods, due to them being closer to the crankshaft at BDC.
i am almost certain you would have to space the block with a spacer plate, as using head gaskets wouldnt give you the required height, and would be "as dodgy as all f*ck"
i would not reccomend changing conrod length unless you have a really, really good reason for it.
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Location:
1st street on the right
Registered:
November 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 03:47
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You could lengthen, then either deck the piston or change pin height. Maintains comp ratio and chamber vol either way.
But why? And what engine, perhaps someone knows the solution if we had more info.
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I supported Toymods
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I renounced punctuation
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May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 06:38
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Nishad, you would increase rod length by getting new ones made. To use the same block, you just get new pistons made with reduced compression height. You could extract a further mm or 2 by using smaller gudgeon pins - TRD has some lovely 20mm ones rather than the usual 22mm. You would use a crankshaft of standard dimension.
The main reason why one might use longer rods is to increase the rpm at which critical average piston speed is reached (roughly 4000ft/sec). The other effect is that it will increase the dwell time of the piston at TDC - this gives the burning charge more time to act on the piston.
With regard to getting a custom crank made, it is possible, but it will certainly set you back a pretty penny. If this is for a 2JZ, then I think you're probably wasting your time (money) getting it done - the standard crank is quite robust. As an example the JUN stroker kit for the 2JZGTE is about $14000 AUD.
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I supported Toymods
Location:
Brisbane, QLD
Registered:
May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 09:20
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Hi guys,
Firstly thank you very much for the quick response(s) 
ShinyBlueSteel:
Well, the question was an exercise in extracting information from a larger knowledgebase (which is you guys) for the purpose of my personal education to a greater or lesser extent. Oh yeah and then there is my addiction to "Solutions looking for Problems" Good to see that the engineering community hasn't completely lost their sense of humour! (Turns out real estate agents and accountants already have! Sad sad world we live in)
Like Dave explained, the really really good reason(s) for increasing the length of the conrods are;
Due to the reduction in the angle the conrod moves through the cylinder:
- you can increase the RPM at which the "critical piston speed" is reached (thus we achieve a higher reving motor).
- due to resulting reduction in the intertial loading and the pressure on the cylider walls, the motor will tend to increase revs faster (needless to say, frictional losses are reduced).
Due to the change in the velocity curve of the moving piston, the piston "spends more time" closer to TDC, i.e. increases the dwell time around TDC thus giving the burning charge more time to act on the piston during the powerstroke. However, the negative effect of this is the consequent reduction in dwell time around the BDC reducing the low RPM breathing ability of the motor.
I believe you are wrong when it comes to saying the height of the crank has no effect on the conrod length. With everything else being the same, if you reduce the crank height by a x mm, then the stroke is reduced by a factor of x mm, true. However, the length of the conrod will have to be increased by a factor of x/2 mm to ensure that the compression volume to remain the same (because their is a x/2 mm reduction in radius around the axis of the crackshaft).
As for the headgasket are there any disadvantages in this scanario to using the thickest, "robust enough" headgasket (say which has been used in high power super/turbocharged applications to reduce the compression ratio)? Every fraction of a millimeter counts.
Once again, thanks for the adive mate. It's highly appreciated.
Dave:
As always thank you so very much for the quite insightful advice.
I would definitely go TRD gudgeon pins and custom pistons with reduced compression height.
I'm not thinking JUN stroker kit at all. I'm definitely not looking forward to increasing the stroke I was thinking for the worst case scenario, in case I had to reduce the stroke a little in favour of increasing the rod length, I would need a new crank.
I guess increasing the height of the block is unfortunately out of the question 
As for conrods/pistons do you know anyone who does good custom conrods/pistons? I need the engine to be able to deal with upto 600HP of power (not that I want it for any conceivable reason, just want to build a nicely over-engineered motor that revs to around 9K+ RPM).
Cheers
Nishad
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Location:
nth ringwood, Victoria
Registered:
August 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 09:42
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argo in morpothfor the rods and sps in melb do forge pistons with any pin height you wont
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Location:
Lost in the K hole
Registered:
May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 10:00
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ey bro!
| Quote: |
- you can increase the RPM at which the "critical piston speed" is reached (thus we achieve a higher reving motor).
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from thinking about this for about 2 minutes, im thinking the peak velocity of the piston should not be related to rod length as youre suggesting...
peak velocity will occur when the crank angle is 90deg BTDC and 90deg BBDC. a shorter rod will be less than perpendicular to both the crank and piston at this point, and thus the speed of the crank journal wont be 100% transferred to the piston. the longer the rod becomes, however, the closer to perpendicular the rod orientation will be to both the crank and piston, and thus, the velocity of the piston will actaully increase -> approaching the speed of the crank itself.
ie, what im saying is: the longer the rod - the higher the peak piston velocity (approaching the crank velocity - which is governed by stroke length).
i think the main advantage of the long crank is the fact that it travels through less of an arc per crank revolution, and thus your dealing with an accelerating system of lower inertia. it would also put less side forces into the piston, thus reducing friction and wear on the skirts (in fact you could install pistons with shorter skirts - further reducing weight and friction in the reciprocating assembly)
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As for the headgasket are there any disadvantages in this scanario to using the thickest, "robust enough" headgasket (say which has been used in high power super/turbocharged applications to reduce the compression ratio)?
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YES!! thicker head gaskets will open up your squish clearance between piston dome and head at TDC which will dramatically increase the likelyhood of detonation at high load. instead, you should aim for normal hg thickness with a TIGHT squish clearance, and concentrate on combustion chamber relieving and piston dome contours to get your totla combustion chamber volume up
cheers mate!
hope youre doing well!!
ed
[Updated on: Mon, 09 February 2004 10:06]
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Location:
sydney
Registered:
March 2003
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Re: Increasing Conrod Length
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Mon, 09 February 2004 10:11
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why would u want to make the piston go higher out of the block
u really dont want the top of the piston(the outside part) to go higher than block top deck height
so i dont know y u would fit a headgasket to get more clearance so the piston doesnt hit the head
most common is longer stroke on the crank (means crank needs to be modified)
and then you get a shorter conrod
this is how you get stroker engines
if u want over 600 hp and have the engine be reliable
i would suggest a billet steel crankshaft made up from a brand new hunk of steel
i know that an fj20t with about 750 hp built properly with all good stuff and correct procedures with no cutting coners will cost you around 15000 dollars
then dont forget the other 10000 dollars u need to make the rest of the car handle the 750 hp
just remember this when spending big on the engine
it is only 1 piece in a big jigsaw puzzle and the weakest link will always break first
so my answer is fukin expensive
but hey when u want over 600 hp nothin is cheap
so maybe u can go rob a bank first hehehe
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 10:14
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| sideshow wrote on Mon, 09 February 2004 18:11 |
if u want over 600 hp and have the engine be reliable
i would suggest a billet steel crankshaft made up from a brand new hunk of steel
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In the case of the 2JZ, I'd be making sure it was in good condition, but in no way do I see a need for a billet replacement to be sourced. The engine is very strong from the factory.
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Location:
nth ringwood, Victoria
Registered:
August 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 11:00
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sideshow by increasing the rod length you also move the pin height further up the piston and can change it from 22 to 20 to give you 2mm in length then you go from there moving rings further up and making them 1.1 mm also and oil control 2mm
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Location:
sydney
Registered:
March 2003
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Re: Increasing Conrod Length
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Mon, 09 February 2004 11:51
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movin piston higher means machining top of piston
so i dont know y u would go and fit thicker headgasket incase piston comes up higher
i have heard u can run into probs if piston comes too hi out of the block
if engine is goin to rev over 8000 or 9000
then a billet is best way
i know its expensive but if it was a race car or similar then u would want it to be reliable
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 12:03
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Supra generally doesn't have to rev that high to make power like an RB26. It has more torque due to its capacity.
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I supported Toymods
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I renounced punctuation
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May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 12:06
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| humble wrote on Mon, 09 February 2004 20:20 |
I'm not thinking JUN stroker kit at all. I'm definitely not looking forward to increasing the stroke I was thinking for the worst case scenario, in case I had to reduce the stroke a little in favour of increasing the rod length, I would need a new crank.
As for conrods/pistons do you know anyone who does good custom conrods/pistons? I need the engine to be able to deal with upto 600HP of power (not that I want it for any conceivable reason, just want to build a nicely over-engineered motor that revs to around 9K+ RPM).
Cheers
Nishad
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Nishad, I was merely using the JUN stroker kit as an example of cost. They supply a billet crank (amongst other things) as part of their kit. If you really had your heart set on a billet crank, I'd suggest talking to Crankshaft Rebuilders here in Melbourne. I'm sure Shuggy could do something for you.
You wouldn't need to reduce the stroke. You could safely move the gudgeon pin up the piston quite a few mm, and by using the TRD gudgeon pins (20mm), you cleverly gain another 1mm. Of course, all this has to be done in consideration with how much boost you plan to run. If you are going to this sort of expense, I'd strongly recommend chatting to an experienced engine builder.
With regard to rods, as Glenn suggested, Argo Engineering in Newcastle or Ridgecrest here in Melbourne would be helpful. With regard to pistons, SPS here in Dandenong are good too.
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Location:
Melbourne
Registered:
May 2002
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Re: Increasing Conrod Length
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Mon, 09 February 2004 12:07
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You crazy short ass white guy, what ya doing! like you need more power 
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Location:
Madrid - Spain
Registered:
August 2002
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Re: Increasing Conrod Length
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Tue, 10 February 2004 02:30
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I can't see how making the rods longer can be beneficial for reducing inertia. While the angle might be slightly less the mass has increased substantially.???
Yes there may be some negligible change in rotational inertia but you still have more mass to accelerate vertically and horizontally.
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Registered:
June 2002
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Re: Increasing Conrod Length
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Wed, 11 February 2004 23:53
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Correct.
By lengthening the conrod, you will be decreasing the peak piston acceleration. This, in turn, reduces the average piston speed. I am led to believe the current industry standard for maximum average piston speed is around 4000 ft/sec. Of course, by having new longer rods made, the materials used and the forging process will increase their strength and that limit can be upped.
By using a longer rod and decreasing the compression height of the piston, this allows the use of a shorter skirt piston. This will invariably be lighter and also less resistive forces on the bore. That said, if you were going to this expense, you'd probably be considering a moly coating on the skirts as well.
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Location:
Madrid - Spain
Registered:
August 2002
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Re: Increasing Conrod Length
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Thu, 12 February 2004 00:44
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Sounds like a lot of work in the name of reducing piston accelerations.
The average piston speed is zero, so I assume you are talking about the peak speed which would be inversely proportional to the cosine of the conrod angle. Or by average speed are you referring to the average speed for 1 stroke not 1 revolution. Hoy would average pistion speed be a design factor?? I would have thought peak speed would be a bigger consideration.
What would be the effective order of magnitude change that you are expecting for piston acceleration and consequently combustion chamber pressures?
The material changes could bring some good benefits though.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: Increasing Conrod Length
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Thu, 12 February 2004 00:56
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speed is non-directional, so it's speed curve will look sort of like a sine wave with troughs at 0.
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Location:
Redbank Plains,near Ipswi...
Registered:
May 2002
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Re: Increasing Conrod Length
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Thu, 12 February 2004 06:39
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i have done a lot of long rod conversions on race engines. the usual reason is to get smaller pin to crown height pistons. with the gudgeon higher up in the piston , you get less piston flutter and hence better ring sealing. the arguments about longer dwell time at TDC are mostly theoretical. only a few engines benefit from this, typically those with shit-house combustion design to start with. most performance engines already have optimied rod ratios, eg stroke to rod length. longer rods are relatively simple--pay money. at the cheap end, steal them out of a honda (beautiful rods typically), at the dear end, pay Argo, Carillo etc. They start at $250 each. FYI, all 4AG rods are standard at 122mm .
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Location:
qld
Registered:
December 2003
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Re: Increasing Conrod Length
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Thu, 12 February 2004 10:57
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i am guessing at this point we are talking about a toyota 1jz or 2jz. have you considered the possibility of the 1j crank in the 2j block?
i will admit alot of what has been said is over my head, but that would give you room for your desired longer rods.
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