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64pro you just fell into the quicksand. Let me throw you a rope.

The whole rod length controversy started with Smokey Yunick's claim that the longest rod that will fit is best. Bill Jenkins experimentally determined that his engines performed slightly better with a specific rod length but he didn't know why. We've tested rods and found interesting conclusions that support Bill's theory over Smokey's. These days engine testing done with cylinder pressure transducers and precision crank angle encoders to detect things that previously were only noticed only by observation (like it "ran better" but didn't know why.) People that focus only on peak power are missing the big picture of engine design and winning races.

When designing an engine combination the rod length is very low on the decision heirarchy. I have my HP48 programmed with various calculations and all the variables are stored and interlinked. The first thing is cubic inches. It is by far the number one path to power. When normally aspirated, the bigger the better. If the rules stipulate a limit then you have to go to the next rung which may be bore vs stroke to get maximum practical rpm. (Key word is practical) If the bore is also limited by the rules to a specific size (like the Carrera rules) then you're stuck with a stroke length.

Piston acceleration, cylinder head flow and durability all have to be considered before you start ordering custom rods.
Once you have stroke nailed down then you can juggle Piston CH, Deck height and rod lengths. A shorter CH piston is almost always lighter. Too short and the ring pack has to be too close. Then the piston can rock in the bore and doesn't seal properly. Just about the last thing I solve for is rod length. Even then it's more practical and cost effective to use a standard length rod and a custom CH piston then have a custom length rod made and a standard CH piston. It's even more practical to use a standard piston, rod and spend the money saved on cylinder head work.

Bottom line is there isn't one blanket statement you can make about rod length that's true in all cases.
 

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A good thick skull means a well protected brain, less susceptible to passing fancies and TOTW's. I chose 6-1/8" rods for the DD because they fit between the 327 crank and the OTS 350 pistons. As PW has stated, there are lots of other areas to get a bigger bang for the buck, unless you are on an unlimited budget.

There are lots of pistons available for 350 sbc's with 6" rods. Look at Flatlander Racing for a comparison.

http://www.flatlanderracing.com/pistons.html
 

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Mike Goble said:
A good thick skull means a well protected brain, less susceptible to passing fancies and TOTW's. I chose 6-1/8" rods for the DD because they fit between the 327 crank and the OTS 350 pistons. As PW has stated, there are lots of other areas to get a bigger bang for the buck, unless you are on an unlimited budget.
I enjoy learning about all of this...doesn't mean I'm going to build something like this though.



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DriveWFO said:
Ok, I understand now. Can a 6" rod be used on a standard 350 crank without any machining?
Yes, as far as I know it can.



DriveWFO said:
Would the pistons need to have the wrist pin located closer to the top of the piston?

You would only need to do this if you don't want the piston to be .300 above the deck at TDC. :eek: :eek: (this tends to mess up a valve job really fast :D )

So yes you would need either a piston with the CR aprox. .300 higher in the piston OR a block with a taller deck by about .300 so that the piston will not "POP" out of the cylinder at TDC :)
 

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The 6-1/8" rods are about the same price as the other lengths. I guess they sell enough of them to get the cost down.
 

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DriveWFO said:
Is there a benefit to turning the crank journals down and running a 6" rod in a 350?
Look at the illustration below.
You can see that the diameter of the journals won't make an effect on the length of the rod/piston assembly. Even a 3.48" stroke crank with 1.888" Honda journal sizes won't help fit a 6" rod unless you change the piston pin location or CH (Compression Height).

Just to clarify for other novices who may not not know what we are talking about...the picture shows all terms that are being talked about in this thread. The engine block "deck height" ultimately determines how much you can physically fit into an engine. You can't change rod, piston and crank dimensions without effecting something.

The formula is:
1/2 x stroke + rod length + CH + Deck clearance = Block Height
You can solve for any of the variables if you stayed awake in math class.
 

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64PRONOVA said:
My statement was based on the typical switch from 5.7" rods to 6" rods. I don't think anyone on this board is considering using any other lengths for SBC. That's one of the reasons why 6" rods are the craze nowdays.
Hey, I was helping you out!

My example of too short and too long rod lengths was to counter the arguement that rod length "doesn't matter" or "it matters little".

It's like they are proving that stopped clock is extremely accurate twice a day but ignoring it gets progressively less accurate the farther away you are from that time.

The same is true for the "rod length doesn't matter" arguement despite Dragonowski's "infinite rod" example. Even though his trigonometry shows little effect with a super long rod the truth is that if you go too far (in either direction) you will have a significant effect on performance. The differences between the optimum length and slightly less optimum length is not a lot.

In the case of the Duece engine the performance gain of 5.9" optimum length rods and 5.7" was small (about 3 hp) but the piston weight saving was even more important benefit considering we were near the durability limits of the vintage block.

Mike's use of 6.125" rods are probably past optimum length for that stroke but on the right side of the curve. Example: Say 5.7" makes 450hp and 5.9" makes 453 and 6.125" makes 451hp all at 6,000 rpm.

Keep in mind that with smaller, and lighter pistons, better rods and crank and 4 bolt block, the engine can now tolerate a higher rpm. This more than makes up for the difference. Now with the extra rev range a slightly bigger cam can be used and the 6.125" rod engine makes 460+ at 6,800 rpm.

So did the long rod make all that power? Not exactly.

Consider this:
The first 331" engine made about 405hp at the crank with 5.94" rods and a 2 barrel. With shorter 5.7" rods it could make 500+hp using the same piston CH. How? Easy! Use a 3.75" stroke.
 

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"You forgot to add the other sentence. The shorter rod will have a tendency to promote detonation since the velocity of the piston as it approaches TDC is more radical. With a longer rod the velocity is reduced thus lowering the threshold for detonation to some extent."

the other school of thought is that with a shorter rod the piston accelerates away from tdc quicker, not letting the pressure build as high thus reducing detonation.
shawn
 

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IMO, no. You'd don't want it too be excessive but there's a lot of latitude.
Piston speed is what you look at. Figure out the cubic inches you'll need to get power you want. Then figure bore and stroke to keep piston speed within part durabilty limits. Bigger bores allow more breathing room but longer strokes increase piston speed for any displacement. It's all about juggling the numbers. Pro's and cons for everything. Rod length, Pin height can only vary so much before they become impractical.

I think a good efficiency compromise combination is a 4.125" bore and a 3.625" stroke, but that's doesn't necessarilly make the most power overall.

For example a Honda that makes 125hp per liter has very efficient combination of bore vs. stroke. It can rev to 9,000 rpm.

However, a less efficient 400 inch Chevy will kick it's butt.
 

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I have not seen much on bore/stroke combos, but I am very interested. I've been thinking more and more about a 377. (similar to mad's) I do like the idea of being different, but i have experienced the awesome power of a well built, transmission cracking, pulls your arms to your chest, Brodix Headed 406 in an s-10.

Do the destroked engines really wrap up faster? can/do they develop the same torque that a 406 or a . . stroked 427 sb can produce?
From the physics point of view it makes sense, but I also understand that sometimes paper and reality are two very different things.
 

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This all goes back to the David and Goliath legend.
In the movies it's the unarmed kungfu master kicking butt on a hoard of gun toting bad guys.
In hotrodding it's the myth of 302 Z/28 beating big blocks. HotRod had a dream engine give away. The engine that won the dream was a dual quad, tunnel ram, high rpm 302 with modern heads. Made only 406hp.

The real science of engines that if all the other variables making power are the same then the larger displacement engine will make more power.
The other variables don't increase power the way that displacement does.

I've found that unless you are restricted by a cubic inch rule, the easiest and most cost effective way to make normally aspirated power is to increase displacement. Even power addders like nitrous and boost increase the amount of oxygen which effectively adds displacement.

Smaller engines can be made to be very efficient but it costs money. In the case of the Honda example, it can cost $10,000 to make the same power as a bone stock 350...with a 2 barrel!

I'm all for diversity and being different.
Considering small blocks are the same size on the outside, unless the have x-ray vision nobody can really tell you are being "different".
 

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Bore Vs Stroke ratio questions (merged)

Ok, I have seen a few articles on bore vs stroke, and the dyno sheets say cubes make torque or horsepower not longer strokes. If everything is the same, heads, cam, intake, etc. 301 cubes with a short stroke and large bore say 3.00 x 4.00 will make nearly the same hp & torque as a 305 with long stroke and small bore 3.48 x 3.736.

In a performance engine, a larger bore would be an advantage due to more room for larger valves and less shrouding by the cylinder wall. Shorter stroke would make lower piston speed at a given rpm and may allow more rpm.

But, my question is about economy, is there any advantage to small bore long stroke at lower engine speeds? I am thinking 1800 - 2300 rpm in a daily driver. GM spent a bunch of money on the 305 when they could have just made 302's with a 3.00" stroke and a 350 block. Does the longer stroke, higher piston speed change the B.S.F.C. on the dyno? All the info I have seen doesn't include the small details like how much fuel per HP of the different combo's.

I am just thinking out loud, but I know some of you guys really understand and have experience with this sort of tech stuff.

Thanks for any input, KenDog
 

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GM did make 302s. Rare to find anymore but they was made. I wonder if a aftermarket company would reproduce them. Those were some rappy strong engines stock ! :awkward:
 
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