OT:Camshaft correlation between cubic inches ?

[CaptCobrajet]

Everything else constant, with the same cam......200 rpm increase for every .200 change(decrease) in STROKE.  Then throw in the shorter rod.  I say the torque peak will go up by 600 rpm and the power will peak about 800 rpm higher.

[plovett]

So all this is meaningless unless the new engine uses the same cam and induction.  It ain't happening.  Joe is smart enough to get a new cam to match the new combination.  It is still fun to talk about.

JMO,

paulie

[Joe-JDC]

Everything else constant, with the same cam......200 rpm increase for every .200 change(decrease) in STROKE.  Then throw in the shorter rod.  I say the torque peak will go up by 600 rpm and the power will peak about 800 rpm higher.

Ok, but the rod ratio is 1.94 not 1.67(3.75" stroke x 6.25" rod). You can get a 5.7" rod to work with a 302 3.000" stroke for 1.9 rod ratio.  Joe-JDC
So if I want to keep the torque peak and horsepower peak at the same rpm, what do need to do to the camshaft specs?  That is the real question. 

[gt350hr]

  Joe ,
     Maintaining the same rod to stroke ratio on the smaller engine will KILL the bottom and mid range power even MORE. Rod to stroke ratio affects piston speed off of TDC and back onto it. A larger engine has ( simple physics) a harder "pull"  ( vacuum) on the cylinder head than a smaller CI engine. That "can"  create a situation where the head is more than the cubic inches "need". A good example id the Boss 302. In stock form it has a near "ideal" 1.75 rod to stroke ratio. Those who subscribe to the :out of context" quote from Smokey Yunick (rip) that you have to use the longest rod possible in a race engine , or those that think the "dwell"  time at TDC makes huge power increases are dillusional when it comes to that engine. Why? The head is as big as a 396-427-454-502 big block Chevy but is on a 302 ci short block. A short rod to stroke ratio increases the piston speed off of TDC ( but no affect on maximum piston speed) making the head think the engine is bigger ( harder "pull" , vacuum again) as the initial piston speed mimicks that of a larger engine.
   The popularity of 4.250 stroke crankshafts for FEs has led to BIG torque numbers over a large RPM range because the big cubic inches have the head "bordering" on being too small. 50 years ago a 600hp "true 427ci" Tunnel port was a race only , radical engine. Now days a 482ci Tunnel port is perfectly streetable
      I used the "302" Ford X cam "big " for a 302 in a 408ci 351W and it is actually mild enough to use a stock , low stall . C4 converter . In a 302 , I would have to put the trans in neutral at a stop light unless I had a 3,000 or more converter.
     Sorry if I said a bunch of stuff you already knew , no intent to irritate you.
   Randy

[mbrunson427]

Randy, I made a quick graph to try and understand what you were saying here. Becomes pretty clear once you put it into a picture. Figured I'd share in case some other guys find it interesting too. I ran this simulation at 6000 rpm.

[Joe-JDC]

The only place I have experienced rod ratio being a player has always been where the ports were too big to start with for the cubic inches.  Most engines that I have dealt with over the years that made incredible horsepower/cubic inches had the longest rod possible to allow right at a 1.000" pin height.   We have guys that have 400 cfm sbf heads that use a 6.000" rod in the 8.7/9.0/9.2 decks, and they generally run through the lights at 8200-8400 rpms.  Rod length will definitely affect total timing for best results.  The number one reason I use the longest rod is to reduce piston side forces on older thin cylinders.  The longer rods seem to keep the rings square better and the pistons can be lighter making for an overall reduction in balance weight.  JMO.  I have read all the pros and cons on both, and my personal preference is the longer rod whenever economically feasible.  Joe-JDC

[gt350hr]

   In a drag race scenario I agree. I have had situations on engines that turn corners where rod length made some serious differences. When the cubic inches increase and the heads are "moderate" , the long rod slows down the piston speed to allow for more valve opening and less restriction when the piston does get moving , which in turn makes more power.
   The only negative to a super short piston is the lack of skirt material . Despite the lesser loading on the cylinder wall , there simply isn't mush skirt there to handle the load.
   Randy

[plovett]

Short answer might be 8-10 degrees @0.050" for every 50 cubic inches.

JMO,

paulie