AFR Head topic

The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Most of us building high horsepower stuff don't give rod ration a second thought.  I know when we were helping with a pro stock team the reason we went to the super short deck was to get a lot shorter pushrod and shorter runners on the intake.  Had nothing to do with rod ratio.  We build a good piston, build a good crank and the rod length is ONLY math to connect the two.



There are more ways to skin a cat than all of us here could imagine.

whatbumper wrote:Most of us building high horsepower stuff don't give rod ration a second thought.  I know when we were helping with a pro stock team the reason we went to the super short deck was to get a lot shorter pushrod and shorter runners on the intake.  Had nothing to do with rod ratio.  We build a good piston, build a good crank and the rod length is ONLY math to connect the two.



There are more ways to skin a cat than all of us here could imagine.

I guarantee you winning Pro Stock engine builders think very carefully about their rod ratio.

The feet per minute the piston travels is based on stroke and rpm. The difference between a 3.85 stroke engine and a 4.500" stroke engine is almost 750 more feet traveled in a minute. This demand sets what needs to be met in the engine.

gt350hr wrote:500 cubic inches can be had in several ways. 4" bore , 5" stroke ( 502ish) 4.470 bore , 4" stroke ( 502ish) and 4.750 x 3.500 ( 500ish).  10,000+rpm Pro Stock engines run the latter combination. Any guess as to why they run a shorter than 9.800 deck height block?? The rod to stroke ratio was TOO much and it hurt torque. Simply shortening the rod would have made the piston too heavy for 10,000 rpm. Big bores unshroud the valves and short strokes let them rev , but 500 inches is still 500 inches , so they should all be the same right? It's just a 500 ci air pump. LOL

      I am aware of the increases in static compression ratios for turbo engines brought on by Q16 and also the increased  use of alcohol , but lots of people still use gasoline and need the lower c/r. There is also a quiet trend toward shorter rods as some feel getting the piston off of TDC faster allows more air and fuel to be introduced before it "backs up". ( High pressure in the port , opening valve and little space for the pressurized air/ fuel to go with a slow moving ( around TDC ) piston. If everybody did the same thing , they would all run the same, but they don't.

This is not true at all. The reason for the shorter and shorter decks over the years has been because the cylinder heads have been getting taller and taller and lowering the decks was the only way to build an intake that would make any power. Or you could say that shorter deck heights have allowed the cylinder heads to get taller and taller with raised ports, but it's always been about the intake manifold. It has nothing to do with the rod ratio. They have always settled on a ratio very close to 1.7-1.73 (or more, as much as 1.8 ) and even today with the bigger bores and shorter strokes they work very hard at maintaining that ratio. PS heads these days flow much better and can stand a shorter rod but they're still in the 1.7 range. Shorter deck also allows for shorter pushrods and better valve train stability.

And shorter pushrods..... Are a benifit too

An engine package design is rife with quid pro quo.

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Scott Foxwell wrote:

whatbumper wrote:Most of us building high horsepower stuff don't give rod ration a second thought.  I know when we were helping with a pro stock team the reason we went to the super short deck was to get a lot shorter pushrod and shorter runners on the intake.  Had nothing to do with rod ratio.  We build a good piston, build a good crank and the rod length is ONLY math to connect the two.



There are more ways to skin a cat than all of us here could imagine.

I guarantee you winning Pro Stock engine builders think very carefully about their rod ratio.

Probably less than is argued in this one thread. And the team we were involved with wins.

whatbumper wrote:

Scott Foxwell wrote:

whatbumper wrote:Most of us building high horsepower stuff don't give rod ration a second thought.  I know when we were helping with a pro stock team the reason we went to the super short deck was to get a lot shorter pushrod and shorter runners on the intake.  Had nothing to do with rod ratio.  We build a good piston, build a good crank and the rod length is ONLY math to connect the two.



There are more ways to skin a cat than all of us here could imagine.

I guarantee you winning Pro Stock engine builders think very carefully about their rod ratio.

Probably less than is argued in this one thread.  And the team we were involved with wins.  

Here's a couple quotes from Nick Ferri: (you do know who Nick Ferri is, don't you?)

“The rod-to-stroke ratio is also based on how much cylinder head you have,” cautions Ferri. “If the cylinder head doesn’t breathe a lot, then you’re going to want a longer rod. A shorter rod comes off TDC much quicker. If the rod comes off too quick, the cylinder head can’t keep up. Obviously, cylinder heads have improved over the years and we’re running a much higher rpm.”
"Ferri says that 15 years ago the cylinder heads weren’t good enough to support short rods, so longer rods were acceptable. But as technology increased to support larger bores and shorter strokes, the rod lengths could be adjusted to support more aggressive rod ratios.
“Years ago I was at 1.8:1,” says Ferri. “Without being specific, we’re definitely in the 1.70s, and I’ve tested in the 1.60s.”"

Sounds to me like he gives it plenty of thought. I know two other current, winning, NHRA Pro Stock engine builders that I talk to and they both talk about rod length. I'd say if you're not looking at it as a design criteria then you're leaving something on the table...or just getting lucky.

Scott,
        I am well aware of Nick's statement and am in complete agreement. So again I offer this ,   9.800 block , 3.500 stroke , 6.135 rod ( for 1.75 rod to stroke ratio) make the c/h on the piston 1.915!!! That is NOT a 10,000 rpm piston in ANY application unless you know more about pistons than I do. Shorten the block and the C/H gets down to a more reasonable number allowing 10,000 rpm. You can say what you want about heads and manifolds causing the change but look at the obvious!

Scott Foxwell wrote:Sounds to me like he gives it plenty of thought. I know two other current, winning, NHRA Pro Stock engine builders that I talk to and they both talk about rod length. I'd say if you're not looking at it as a design criteria then you're leaving something on the table...or just getting lucky.

Which is funny because this Darin Morgan quote always comes up when the topic of rod length is discussed.

"Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

http://www.superchevy.com/how-to/project-cars/0608ch-race-engine-building-tips/

<sits back in chair to continue watching this interesting conversation>

BigBlockRanger wrote:

Scott Foxwell wrote:Sounds to me like he gives it plenty of thought. I know two other current, winning, NHRA Pro Stock engine builders that I talk to and they both talk about rod length. I'd say if you're not looking at it as a design criteria then you're leaving something on the table...or just getting lucky.

Which is funny because this Darin Morgan quote always comes up when the topic of rod length is discussed.

"Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

http://www.superchevy.com/how-to/project-cars/0608ch-race-engine-building-tips/

<sits back in chair to continue watching this interesting conversation>

I promise you if Darin was designing a competitive Pro Stock engine he would give it more consideration than that. This isn't "opinion", it's just part of how an engine works. For the masses, he's right... they're not building an engine that really needs a lot of consideration to rod length and I'm sure, in context, that's more where his comment was targeted but when you start wanting to build serious power and are looking more in depth and more into details about how and why engines do what they do, rod length is just another piece of the puzzle. It's not a deep dark secret and it's not black magic voodoo. You have to remember; every engine, even at the highest levels, is a series of compromises and you have to pick your battles.

gt350hr wrote:Scott,
        I am well aware of Nick's statement and am in complete agreement. So again I offer this ,   9.800 block , 3.500 stroke , 6.135 rod ( for 1.75 rod to stroke ratio) make the c/h on the piston 1.915!!! That is NOT a 10,000 rpm piston in ANY application unless you know more about pistons than I do. Shorten the block and the C/H gets down to a more reasonable number allowing 10,000 rpm. You can say what you want about heads and manifolds causing the change but look at the obvious!

You got the cart before the horse. No one ever ran a 9.8 deck with a 3.5" stroke.
Whatever man...I'm not guessing on this. It all came together as a result of the combinations. If you want to believe they shortened the decks to get shorter rods, more power to you. I know different and I don't need to argue about it. The main point I was trying to get across is that rod length is definitely a design criteria in upper level engine building.

Scott ,
I'm on your side when it comes to the rod length issue. there is a direct relationship between cylinder head flow capacity , cubic inches of the engine and rod length. 15+ years ago I had a Sprint car engine builder who used the common 3.8 stroke and 6" rod with a Brodix -12 head. He got a set of All Pro 13s which promised more flow and power . On the dyno it proved correct but the car wouldn't come off the corner. I suggested a switch to a 5.85 rod for faster piston speed off of TDC . He replied that that was a step backwards. When he ran out of ideas , he built one with the 5.85 rod and it "woke up". Soon afterwards almost every 410 engine was "backed up" to 5.85 rods and I think it's still the length of choice , I could be wrong. But put a 5.85 under a 12 15* head and it would be "out of breath" by the flag stand!

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

gt350hr wrote:        Scott ,
           I'm on your side when it comes to the rod length issue.  there is a direct relationship between cylinder head flow capacity , cubic inches of the engine and rod length. 15+ years ago I had a Sprint car engine builder who used the common 3.8 stroke and 6" rod with a Brodix -12 head. He got a set of All Pro 13s which promised more flow and power . On the dyno it proved correct but the car wouldn't come off the corner. I suggested a switch to a 5.85 rod for faster piston speed off of TDC .  He replied that that was a step backwards.  When he ran out of ideas , he built one with the 5.85 rod and it "woke up". Soon afterwards almost every 410 engine was "backed up" to 5.85 rods and I think it's still the length of choice , I could be wrong. But put a 5.85 under a 12 15* head and it would be "out of breath" by the flag stand!

That makes sense. As cylinder heads get better they can support a little shorter rod. Shorter rod pulls a little harder on the port (there's that pesky piston speed thing again...) , fills the cylinder a little quicker. Great for quick accelerating engines in lighter cars with some gear. Longer tracks, slower acceleration, spending more time at higher rpm, you're not going to need as much cyl head and a longer rod is more benefit. We build two long rod BB Chev combinations, a 509 and a 532, both use 4" stroke and a 6.535 rod. (The typical combination would use a 6.385 rod). It puts the piston about .010 out of the hole on a 9.8" block but with small cylinder heads and a good cam these combinations make 800hp on pump gas, over 700#/' tq (for the 532) and love to rev.
It's interesting to compare OEM designs and look at things like rod lengths/ratios. For instance, compare a 454 BB Chev to a 460 BB Ford and look at their strokes and rod ratios and compare cylinder heads and induction, then compare the performance personalities of the engines. There is a reason for everything they did...nothing is just arbitrary. Two completely different schools of thought and design for two very similar engines.

Scott Foxwell wrote:

You got the cart before the horse. No one ever ran a 9.8 deck with a 3.5" stroke.

EVER ...?
It was quite a while ago, (I realize that most of these people were not even alive back then), but, the Can-Am aluminum 430 big block Chevrolets had a 9.800" deck height and a 3.5" stroke, (actually 3.470" stroke), before they went to running 494 cubic inch engines.
The rods in those 430's were +.400" at 6.535" long.

Bill Jenkins, Wally Booth and some others ran those 4.44" bore Reynolds aluminum 430's in some match race Pro Stockers, also.

dfree383 wrote:

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

Does it take more energy to walk 5250 feet then it does 4491 ft? And the reason for this analogy is I'm not that smart so I have had to learn this stuff in terms I can relate too so in by no means am I belittling you with this question.

rmcomprandy wrote:

Scott Foxwell wrote:

You got the cart before the horse. No one ever ran a 9.8 deck with a 3.5" stroke.

EVER ...?
It was quite a while ago, (I realize that most of these people were not even alive back then), but, the Can-Am aluminum 430 big block Chevrolets had a 9.800" deck height and a 3.5" stroke, (actually 3.470" stroke), before they went to running 494 cubic inch engines.
The rods in those 430's were +.400" at 6.535" long.

Bill Jenkins, Wally Booth and some others ran those 4.44" bore Reynolds aluminum 430's in some match race Pro Stockers, also.

OK, I stand corrected but they sure didn't do it with a 6.135 rod which was the comment I was responding to, but you're right.

The 494 was the engine that set the world on fire. The Cam Am 430 was a "yawner" even in a 1900lb Can Am car with Crower injection. Of course the heads flowed about 350 cfm , not   500 ( or so) like a "modern" head does. I have it on good authority that those Chevy's had 4" cranks ( at least) in the match race engines. I'm sure Wally would admit it now.
    I asked the late great Smokey Yunick at a past PRI show about his oft quoted rod length statement and he said it was taken out of context and allowed "it's all relative".

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

Does it take more energy to walk 5250 feet then it does 4491 ft?  And the reason for this analogy is I'm not that smart so I have had to learn this stuff in terms I can relate too so in by no means am I belittling you with this question.

How much faster is a 6.7 rod the a 6.8 on a 632" ford?

dfree383 wrote:

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

Does it take more energy to walk 5250 feet then it does 4491 ft?  And the reason for this analogy is I'm not that smart so I have had to learn this stuff in terms I can relate too so in by no means am I belittling you with this question.

How much faster is a 6.7 rod the a 6.8 on a 632" ford?

We are not talking rod length sir, we are talking piston speed. Stroke and rpm determine the fpm a piston will travel in an engine. The rods just "hanging" out.

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

Does it take more energy to walk 5250 feet then it does 4491 ft?  And the reason for this analogy is I'm not that smart so I have had to learn this stuff in terms I can relate too so in by no means am I belittling you with this question.

How much faster is a 6.7 rod the a 6.8 on a 632" ford?

We are not talking rod length sir, we are talking piston speed.  Stroke and rpm determine the fpm a piston will travel in an engine.   The rods just "hanging" out.

I prefer that my rods don't 'hang out'....they may 'go along for the ride' if they wish.

Lem Evans wrote:

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:

dfree383 wrote:

Straubtech wrote:The more piston speed you put in an engine the more demand is put on both induction and exhaust.

Can you please explain your comment?

Does it take more energy to walk 5250 feet then it does 4491 ft?  And the reason for this analogy is I'm not that smart so I have had to learn this stuff in terms I can relate too so in by no means am I belittling you with this question.

How much faster is a 6.7 rod the a 6.8 on a 632" ford?

We are not talking rod length sir, we are talking piston speed.  Stroke and rpm determine the fpm a piston will travel in an engine.   The rods just "hanging" out.

I prefer that my rods don't 'hang out'....they may 'go along for the ride' if they wish.

LOL...poor choice of words...