I’d add to this that rod to stroke ratio matters. I don’t care what David Reher says, because he isn’t building OE architecture cylinder head stuff. Every single SBM, SBC and SBf or severely induction limited. So R/S ratio matters. A bunch. And that affects cam timing. A bunch.
Disregarding all the nonsense written about rod length and how it doesn’t matter, if you cam grinder isn’t asking how long your rods are, find another grinder. Why anyone would ask? Why should anyone question the really smart dudes?? Because architecture and geometry matter.
No one with any serious desire to understand the ICE can ignore what happens when you change the length of the rod, the length of the stroke or both and how that affects cam timing.
No one, not even David Reher would argue that a higher R/S ratio (lets say 1.848 for a 340 and 1.530 for a 408) doesn’t change piston speed at certain points in the stroke. It certainly does.
A higher R/S ratio has the piston moving around TDC slower and moving around BDC quicker. I don’t think anyone would argue that but I could be wrong.
That also means that around BDC the higher R/S ratio will move around BDC quicker and the lower R/S ratio will move around BDC slower.
IMO, that means that IVO and IVC should be different for two different R/S ratios. I don’t think that could be argued, but I suppose it can be.
Now here is where the lines get blurred. What happens to piston speed after TDC moving to BDC and from BDC to TDC and what happens if we look at what piston speed is doing at say...half way down the stroke from TDC and half way up the stroke from BDC??? And how does that affect cam timing?
I say that the higher R/S ratio, the later IVO should be, because the piston is in the way all around TDC. Any flow occurring at low lifts with the piston hanging around will run right into the piston. So open it a bit later and then have it take off and open quickly. That was one reason why the old Direct Connection cams were so far ahead making power when they came out. They were a fairly aggressive lobe for the day.
What about IVC??? Well, that’s a big deal too because with a higher R/S ratio the piston is moving around BDC quicker than with a lower R/S ratio and if you screw around and close the intake valve too late you get reversion. And that has its own set of issues.
Now let’s divide the stroke into quarters or close to it. And let’s say from about .200 lift on (I used .200 because depending on where the ICl is you are generally past TDC a bit but it could be .150 lift or so too...it’s kind of arbitrary unless we are using hard numbers) to ICL (let’s use 106 because a 110 LSA is pretty much the de facto LSA for most stuff guys are using today) and then from ICL back to about .200 lift on the closing side and how that piston speed affects cam timing.
The piston with the higher R/S ratio is moving quicker around the ICL than a lower R/S ratio. And that means that what happens at higher lifts is just as important (I can argue that it’s more important than what happens at low lifts) as what’s happening at lower lifts.
The piston is moving faster and creating more space faster with the higher R/S ratio and therefore seat shape and angles is a huge issue or you leave power on the table. And drivability. So IMO, opening the intake valve a bit later, getting it open faster and holding it there as long as you can and then shutting the intake valve quicker all need to be addressed when you are using higher R/S ratios.
This is one of the reasons people say it’s easy to over cam and SBM. That high R/S ratio changes what your cam timing should look like. You need quicker lobes with less seat timing and the same or quicker at .050 and .200 numbers with a higher R/S ratio than you do with a lower R/S ratio. Thankfully, the engineers at Chrysler evidently had a handle on this stuff and gave us the .904 lifter.
That means with the lower R/S ratio you can use a slower lobe, open it sooner and leave it around TDC a little longer than you can with a higher R/S ratio be as the piston is moving through TDC quicker and not lose power. And since the piston is moving around BDC slower with the lower R/S ratio, you can make the IVC closing a bit later and not run into reversion as soon as you would with the higher R/S ratio.
Rod to stroke ratio also affects exhaust timing but it’s essentially the same issues. You can’t give up blowdown with the higher R/S ratio by opening the exhaust valve too late because the piston is moving through BDC quicker.
So what does all this have to do with AJ’s arguement about the 318?
It’s because guys tend to use weenie cams with the 318. They use low lift, slow lobes that generally have way too much seat timing and not nearly enough at .050 timing. And that just kills power, especially at low engine speeds. It makes the engine sluggish and piggy. A lower R/S ratio can cover that up.
Like I said earlier, cam selection, converter selection and gear ratio will kill a high R/S ratio, induction limited combination so fast your head will spin.
A 318 can make the same power as a 340 or 360. It will take about 200 RPM compared to the 340 and 350 or so more RPM to the 360.
You just can’t slap a bunch of junk off the shelf parts that are designed with GM R/S ratios into a SBM MoPar and expect it to run like it should.
