Dimensions of optimum motor
I beleive tat long rod motor may be less sensitive to non optimal cam selection because if there is more piston dwell, then the time spent traversing between TDC and BDC is less. Then it has to be moving (and pulling the charge in with it) between these points faster
Originally Posted by J-Rod
The other thing with a lot of stroke is piston speed. As stroke goes up, piston speed, and the forces acting on the rod and piston go WAY up. For instance, in an old school SBC when you go between a stock 350 with a 3.48" stroke, and a 400 crank 3.750" stroke (i.e. you build a 383). The forces on the rods and the rod bolts @ 5000 rpm on that 383 are about the same as a 350 @ 7000 RPM. Can you see that this is a lot to put a stock rod and bolt up against...
82.5 ft/s = Honda S2000 (3.3" @ 9000 rpm)
79.5 ft/s = BMW E36 M3 (3.58" @ 8000 rpm)
77.7 ft/s = Stroked ls1/ls6 (4" @ 7000 rpm)
76.5 ft/s = CobraR 5.4L DOHC (4.17" @ 6600 rpm)
73.4 ft/s = Ferrari 360 Modena (3.11" @ 8500 rpm)
72.2 ft/s = Saleen S7 (4" @ 6500 rpm)
72.2 ft/s = Lamborghini Murcielago (3.42" @ 7600 rpm)
70.4 ft/s = stock ls6/ls1 (3.62" @ 7000 rpm)
69.9 ft/s = Lamborghini Diablo 6.0 (3.31" @ 7600 rpm)
67.3 ft/s = Honda F4i SportBike (1.67" @ 14,500 rpm)
65.8 ft/s = Ferrari Enzo (2.96" @ 8000 rpm)
65.3 ft/s = stock ls6/ls1 (3.62" @ 6500 rpm)
65.0 ft/s = Porsche [996 based] Ruf RGT (3" @ 7800 rpm)
The piston-speed debate also exists in Formula1 and right after the E46 M3 came out picked up a little steam. During this time there were alleged numbers to what BMW has tested with and what they actually decided on. As you can see, after some number crunching and comparisons, there actually is a sweet spot at least for OEM. After the fact it was clear there was one thing for sure and many things not so certain, that one thing being, that with the pace and timing of R&D the new M5 is gonna one serious piece of *** in the machine world lol. Anyway back to regularly scheduled reliable automobiles as alas I do not have a couple 100 Large I can toss out to either of those two utimate driving machines or even the Porsche GT which unfortunately I have not calulated yet. Thanks again Jrod for excellent tech and ls1 information
TECH Enthusiast
Joined: Feb 2004
Posts: 743
Likes: 0
From: Foley, Alabama-southern Alabama
The time between tdc and bdc is the same with long or short rods,but with the longer rod the pistons stays or dwells near the top part(near tdc) of the stroke longer then it accelerates (quicker then a short rod)toward the bottom of the stroke then it stays or dwells longer near the bottom(near bdc)portion of the stroke then quickly accelerates(quicker then a short rod) back to the top.Usually a longer rod likes a wider lda in the cam because of the piston motion,also heads with good low lift flow #'s as the piston is moving faster at mid-stroke or as intake is still increasing lift.But not always! 
TECH Senior Member
Joined: Nov 2001
Posts: 9,246
Likes: 3
From: Troy, AL
"Would your decision have been any different if you could've turned that 461 to 9,000RPM+ without fear of grenading your motor?
If you don't mind saying, how much do your cylinder heads flow?"
If I thought that the 461 could make better power than the 434 above 8000 RPM and that it could do it with some degree of reliability (side loading and piston speed being huge concerns with a 4.25" stroke at 9000 RPM), then yes, my decision would have been different. However, I see that even the 434 doesn't like to pull hard all the way to 9000 RPM so I don't believe that to be the case.
The heads flow close to 400 CFM (C5R heads.)
If you don't mind saying, how much do your cylinder heads flow?"
If I thought that the 461 could make better power than the 434 above 8000 RPM and that it could do it with some degree of reliability (side loading and piston speed being huge concerns with a 4.25" stroke at 9000 RPM), then yes, my decision would have been different. However, I see that even the 434 doesn't like to pull hard all the way to 9000 RPM so I don't believe that to be the case.
The heads flow close to 400 CFM (C5R heads.)
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Originally Posted by romanss
So with a 3.65" stroke I will end up with 5171 fpm (feet per minutre?)
and with a 3.4" stroke 4817
So what is a good speed?
So these formulas, do you have them memorized or recorded somewhere?
I am impressed.
Thanks,
Roman
and with a 3.4" stroke 4817
So what is a good speed?
So these formulas, do you have them memorized or recorded somewhere?
I am impressed.
Thanks,
Roman
