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Do The Rules Talk About Rod Length?


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Posted

I looked through the rules on rebuilding an engine. I am building up a factory block to be my race engine and I am not going higher than a .5 increase in compression and I am less than 1.5% increase in displacement and I want to use forged internals. All of these are mentioned in the rules but it doesnt say if I can change rod length as long as the other perameters dont change. Can I go from a 5.7" to a 6" rod in a factory block with no points?

Posted
Engine/Drivetrain #8
8) De-stroked engine +4

The stroke is only affected by the crankshaft throw, not the connecting rod length. The length of the connecting rod does not affect stroke or displacement, nor does it affect the compression ratio (given identical piston-to-deck heights, piston dome/dish shapes, head gasket thicknesses, and combustion chamber volumes while varying the wrist pin locations).

 

As an extreme example, if you have a crankshaft with a 3" stroke and use a 6" connecting rod, then the piston moves 3". If you change that connecting rod to be 10 feet long while keeping the same crankshaft, then the piston still moves 3".

 

Changing the connecting rod length affects rod ratio, rod angle, piston side loading, and piston dwell at TDC/BDC.

 

In Kubs' case, he will likely be using a stock crankshaft, a longer connecting rod, and a piston with a raised wrist pin location. His displacement increase is likely due to a typical cylinder overbore, and the compression ratio increase could be from having the heads milled, block decked, cylinders overbored, thinner head gasket, or a combination of those. Now, if Kubs is going to a longer connecting rod because he is destroking the engine without moving the wrist pins, that is another situation, but the engine is not being destroked because of the change in connecting rod length. But, with is <1.5% change in displacement, I doubt that he is changing the crank stroke.

 

I'm not assigning a points value to altered connecting rod length here, but I'm saying that I don't believe that Engine/Drivetrain #8 applies in this case.

 

Mark

Posted
Engine/Drivetrain #8
8) De-stroked engine +4

The stroke is only affected by the crankshaft throw, not the connecting rod length. The length of the connecting rod does not affect stroke or displacement, nor does it affect the compression ratio (given identical piston-to-deck heights, piston dome/dish shapes, head gasket thicknesses, and combustion chamber volumes while varying the wrist pin locations).

 

As an extreme example, if you have a crankshaft with a 3" stroke and use a 6" connecting rod, then the piston moves 3". If you change that connecting rod to be 10 feet long while keeping the same crankshaft, then the piston still moves 3".

 

Changing the connecting rod length affects rod ratio, rod angle, piston side loading, and piston dwell at TDC/BDC.

 

In Kubs' case, he will likely be using a stock crankshaft, a longer connecting rod, and a piston with a raised wrist pin location. His displacement increase is likely due to a typical cylinder overbore, and the compression ratio increase could be from having the heads milled, block decked, cylinders overbored, thinner head gasket, or a combination of those. Now, if Kubs is going to a longer connecting rod because he is destroking the engine without moving the wrist pins, that is another situation, but the engine is not being destroked because of the change in connecting rod length. But, with is <1.5% change in displacement, I doubt that he is changing the crank stroke.

 

I'm not assigning a points value to altered connecting rod length here, but I'm saying that I don't believe that Engine/Drivetrain #8 applies in this case.

 

Mark

 

Mark you are correct. I am going 0.030 overbore and keeping the same stroke. The compression increase comes from a thinner gasket only. I just want a longer rod to reduce the side loading of the piston. No destroking or deck clearance changes are being made just moving the wrist pin higher in the piston and using a longer rod like mentioned. So no points?

  • National Staff
Posted

Sounds good. I assumed a change in the crank/stroke since you said that compression was not changing much. The change in the wrist pin location makes sense.

Posted
Sounds good. I assumed a change in the crank/stroke since you said that compression was not changing much. The change in the wrist pin location makes sense.

 

The rod will get heavier with the increased length, but those pistons sure will be lighter also....

 

Not quite sure how I feel about this.....

Posted
The rod will get heavier with the increased length, but those pistons sure will be lighter also....

 

Not quite sure how I feel about this.....

What is your concern, exactly? Perceived power increase (which will be handled by the weight/power ratio limits), perceived durability increase, or something else? It doesn't sound as if he proposes to install an Unobtainium reciprocating assembly, but rather to use a combination of affordable and commonly available SBC parts; Kubs, please correct me if I'm wrong. Also, just because the wrist pin moves up doesn't necessarily mean that the piston will be lighter (depending on manufacturer), but you are correct in your statement that the rod will be heavier.

 

I'm actually hoping that you are local buddies and I just missed the inside joke.

 

Mark

Posted
The rod will get heavier with the increased length, but those pistons sure will be lighter also....

 

Not quite sure how I feel about this.....

What is your concern, exactly? Perceived power increase (which will be handled by the weight/power ratio limits), perceived durability increase, or something else? It doesn't sound as if he proposes to install an Unobtainium reciprocating assembly, but rather to use a combination of affordable and commonly available SBC parts; Kubs, please correct me if I'm wrong. Also, just because the wrist pin moves up doesn't necessarily mean that the piston will be lighter (depending on manufacturer), but you are correct in your statement that the rod will be heavier.

 

I'm actually hoping that you are local buddies and I just missed the inside joke.

 

Mark

 

Turbo,

I have no dog in this ........Kubs and I have met and he is a Corvette guy, so I like him. I just have never contemplated this for a TTA car.....I meant nothing malicious by my post...

 

Pistons will more than likey get lighter being .300 shorter, but the reciprocating mass will probably still gain weight depending on how nice of a connecting rod he uses

 

Chances are that if Kubs is going to build a competitive TTA car the ole L98 is going to have to make "some beans" to be @ 8.7:1, but TTD-TTA is alot of points.

 

I'd be taking intake manifold points for sure...but what do I know , I'm a S-U-R kind of guy...so far.

Posted

Gotcha. Even if the rod/piston combo manages to make more power, the weight/power ratio limits will step in and require that the power be dialed back or weight be added. The good thing about the weight/power ratio limits (in addition to the points) is that Greg doesn't have to evaluate every last nut and bolt combination and assess points values to them, as he would probably have had a complete mental collapse by now if that was the case!

 

Mark

Posted
he would probably have had a complete mental collapse by now if that was the case!

Mark

 

there is some debate as to if that has already happened or not

  • Members
Posted
The good thing about the weight/power ratio limits (in addition to the points) is that Greg doesn't have to evaluate every last nut and bolt combination and assess points values to them, as he would probably have had a complete mental collapse by now if that was the case!

 

Mark

 

 

 

When you watch a grown man eat dirt with a spork, is that an indication that he has had a complete mental collapse??

Posted
The rod will get heavier with the increased length, but those pistons sure will be lighter also....

 

Not quite sure how I feel about this.....

What is your concern, exactly? Perceived power increase (which will be handled by the weight/power ratio limits), perceived durability increase, or something else? It doesn't sound as if he proposes to install an Unobtainium reciprocating assembly, but rather to use a combination of affordable and commonly available SBC parts; Kubs, please correct me if I'm wrong. Also, just because the wrist pin moves up doesn't necessarily mean that the piston will be lighter (depending on manufacturer), but you are correct in your statement that the rod will be heavier.

 

I'm actually hoping that you are local buddies and I just missed the inside joke.

 

Mark

 

Turbo,

I have no dog in this ........Kubs and I have met and he is a Corvette guy, so I like him. I just have never contemplated this for a TTA car.....I meant nothing malicious by my post...

 

Pistons will more than likey get lighter being .300 shorter, but the reciprocating mass will probably still gain weight depending on how nice of a connecting rod he uses

 

Chances are that if Kubs is going to build a competitive TTA car the ole L98 is going to have to make "some beans" to be @ 8.7:1, but TTD-TTA is alot of points.

 

I'd be taking intake manifold points for sure...but what do I know , I'm a S-U-R kind of guy...so far.

 

Just to clarify I want to go with a 6" for the reduced side loading of the piston. If there is a performance enhancement by doing so, then so be it.

 

Danny, I see where you are coming from about it being heavier but everything is going to forged and not cast so the rods could possibly be the same weight or less, and the pistons will deffinitely be lighter.

 

I am building the car for TTA and I am trying to take full advantage of the rules without breaking any (which is why I ask questions publicly so I KNOW I am not breaking any). I have to take points for pretty much everything on my suspension for it is a Z51 and points are based off of the base car, so sway bars and shocks are getting better as long as im taking points for them and I am doing del a lum bushings up front and I have heim jointed members in the rear.

 

The engine is pretty much going to be all custom. Lloyd Elliott is porting a set of stock 113 heads and hooking me up with a custom grind cam as well. The intake is being changed to a single plane EFI with an LS throttle body. I have already talked to Greg about all the changes I want to make and their point values and he says I am not violating anything. My goal is about 380-390 RWHP to not have to add any weight to my already heavy street C4 (not ready to take the plates off and turn it into a full W2W car yet).

 

I started in TTD with 14 points and everything added up puts me right at 79 points which is the limit for TTA. If I get one more point its TTS for me. Like I said I am taking full advantage of everything I am capable of doing to the car to make it the best TTA car I can make it. Driving it will be another story!

 

Sorry for the long post but this is where I am coming from with all my questions.

Posted

380-390 hp .... I love the optimism......

 

 

In the end, just drive it like you stole it! That's usually enough.

Posted
The good thing about the weight/power ratio limits (in addition to the points) is that Greg doesn't have to evaluate every last nut and bolt combination and assess points values to them, as he would probably have had a complete mental collapse by now if that was the case!

 

Mark

 

 

 

When you watch a grown man eat dirt with a spork, is that an indication that he has had a complete mental collapse??

 

 

I don't know - I was in the Bay area all last week - seems to be pretty common for the locals late at night.

 

I think there should be a call in comfort number (not the 900 numbers already on my speed dial) for the Nat'l and Regional Directors.

Posted
380-390 hp .... I love the optimism......

 

 

In the end, just drive it like you stole it! That's usually enough.

 

I dont think its too far off. I know even ported L98 heads dont flow the best but it should be enough. I am also not keeping the TPI and going with a better flowing single plane EFI.

 

And I do drive it like its stolen!

Posted
watch that adjusted horsepower:weight limit stuff too

 

I checked into that too. Now I just need the funds to finish the project.

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