stroke vs. rod length

[Flat Screwd]

Ok this ought to get you thinking. In my oppinion the longer the rod the better. It creates less side presure on the piston along with keeping the tangent angle of the rod to crank to a minimum. the longer rod also allows the piston to dwell longer at TDC. to asure a longer burn. The pistom speed is slower through TDC and BDC making the change in direction eazier. So if this is correct than why dont they " manufactures and after market " make it so straight from the factory. For instance, BBC 454 run a 4" stroke with a 6.135 rod. So wound'nt be better to have a 4" stroke 1/4 long rod and just raise the pin hieght. Why arnt people doing this. Also too BBC offers two different deck hieghts. Why arnt the tall decks more popular? It just seem to me that a tall deck with a 3.75 stroke extra long rod would make the highest revven longest revven motor out there.

[Unchained]

Flathead Screws, We were discussing that subject in the GearHead column on Nov. 15
you might want to check that out. Its under "540 questions"
I didn't know about the piston dwelling longer at TDC with a longer rod. I'll have to think about that one.
I went with an aftermarket tall deck block 10.7 " and a 7.25" long rod to give me a rod angle to stroke ratio the same as a 350 chevy which is suppost to be optimum. The pin hole in the piston is so high it is through the oil ring groove and required a special aluminum oil ring support pin retainer button.
The rod length to stroke ratio for a 454 is 1.53
For a 350 chevy it is a much more desirable 1.7

[Flat Screwd]

Hey Unchanged. I'm glad to see theres someone with a sence of hummor. Flathead Screws, I like it. I might be able to do something with that. But on a more serious note I didnt know ther was a ratio 1.5 or 1.75 and so on. Why would 1.7 be optimum?

[CrazyHippy]

It will in fact "dwell" longer at both the top and bottom of the stroke.
I'm not sure if it will be enough to notice though
BJH

[Unchained]

Flat Screwd, I read that in a mag. about the SB chevy rod angle.
Well I learned something new from you guys about the dwell time with the longer rod. I'm glad I have a long rod. http://free.***boat.net/ubb/biggrin.gif Maybe I should have been setting my ign. timing farther advanced.

[Infomaniac]

This is my favorite subject. You should ALWAYS use the longest rod possible. Especially in a boat.
The dwell time does increase. The best benefit is that piston acceleration is greatly reduced. The piston accelerates away from TDC much slower. This gives more time for combustion to push on the piston while the volume in the cylinder is at it's smallest. The crank is still turning during this time.
Great for blower engines due to a very flat torque curve. Serious torque to be found with long rods. Built a killer small block this way. Had to have custom pistons made but was well worth it. Find some dyno specs on Indy engines. They are all rated in HP but the torque produced is astronomical.
Big RPM engines benefit greatly. This is the feature that allows Indy type engines to spin serious RPM's. The crank is going like crazy but the piston speed is kept reasonable. Rod length to stroke ratio is the key. For example: A normal 454 mentioned before with stock rod length spinning at Indy engine RPM's. The piston would out accelerate the flame front during combustion. Could not push very hard on the piston.

[Flat Screwd]

Ok Unchained , dont cut me short. Its all good in the hood, If youn know what I'm sayen.
Why is an optimun an optimun ? For instance a guy running a 1320 Sporster , long stroke , short rod, says you can out run detonition, in outher words, the piston is at top dead center before there is a chance of predetination ?

[Unchained]

Flat Screwd, The optimum rod angle ratio of 1.7 is just something I read in a mag. Maybe it had to do with the dwell thing. Infomaniac probably knows more about optimum rod angles than me. It would be interesting to compare rod lengths and strokes of several different engines.
Are you talking to someone with a Harley about engine design? Those crude things are only one notch above a briggs and stratton. When I saw a Harley engine apart several years ago I couldn't believe the crudeness. One connecting rod is a fork and the other is a spade. The cyl head has the most indirect port layout I ever seen. The one I saw even had a rotating part for a crankcase vent.
I'm gonna hear it now !

[Infomaniac]

Unchained: See if you can get specs on diesel engine rod length to stroke ratio. They are torque monsters. Also Lycoming and Continental horizontally opposed aircraft engines. They cannot turn more than about 3,500 RPM due to prop tip speeds. They have very long rods to make torque. Cannot make HP at low RPM's so torque is the target.
To answer previous questions about regular mfgs not making long rod engines from the factory. My guess is if they produce a torque beast, they would have to use a very strong crank etc. also transmission, rear end etc. would have be beefed up. Building one for big RPM's would be a valve train nightmare for a stock engine.

[Unchained]

Check out a tech article on connecting rod lengths at grapeaperacing.com
Also at www.aros.net/~rbuck/rick/rodstudy.htm (http://www.aros.net/~rbuck/rick/rodstudy.htm)
[This message has been edited by Unchained (edited December 29, 2001).]