@273
I think I get what you are saying; Not saying I understand your thinking completely, but at one time, I had those thought as well. But me, I wasn't smart enough at thre time, to pursue it; probably still not,lol. But here's my thinking;
Let's talk a streeter at WOT;
Peak VE, and peak torque walk hand in hand.
In an LA318, with the stock cam, peak torque is around 2400rpm.
If you want more power at 2400, you have to trap more air.
Compression ratio won't "pull" more air in.
You can compress that ingested volume to kingdom come; it will never increase your VE. Well actually, depending on how you compute VE, yes it is possible; but that is a numeric increase not a real-world increase. That is how science and math can be used to manipulate the world around us; but I digress...
All you can do is make it easier for the atmosphere to soldier it's way into that cylinder before the intake closes. With the Stock cam, what are your options for increased induction at 2400rpm? I mean besides supercharging, and besides a CID change.Well;
#1 is a free-flowing exhaust,to let out the spent gasses, so the falling piston on the Intake stroke immediately following, doesn't have to deal with them anymore. Forget backpressure; that is your engines worst nightmare. and
#2 is a free-breathing induction system,obviously. IMO the bigger the carb the better until you hit low-rpm response driveability issues. and
#3 is a bigger intake valve,but not so big as to induce shrouding, and
#4 is better ports, and
#5 which should be #1, is the often overlooked, increased air density. Sucking hot underhood air is a sure recipe for strangling an engine's power.and
#6 which should be #2, I reasoned, was the ring seal.
If your rings are leaking, the piston cannot properly evacuate the chamber, making it less inviting for the atmosphere to dive in there. And IMO this is the part about high-compression that nobody talks about. This is why I chose a very small chamber volume and Plasma-Moly file-fit rings. I was searching for zero% Leakdown, in an effort to "pull in" as much cold-air as possible at peak VE and below. I already knew my 367 had plenty of high-rpm power, now I wanted more bottom-end with the same cam.
Every time I hear a 318 fan say; " I'm not changing the pistons", I cringe; knowing that 40 year old bores are tapered for sure, and probably oval, and sloppy loose on the pistons and she has ring gaps you could drive a bus thru.. But the first thing they want to know is what cam to put in it.
Even if you put brand new rings in deglazed holes, you will still not get a decent cylinder evacuation. On the induction stroke, everything just slowed right down, and on the power-stroke,all your imagined power is going straight past the rings.
#7 is crankcase pressure; If the pistons and rings have to work against 4psi underneath of them, that is lost power to the crankshaft.
But lets talk about a bigger cam;
so with no other changes, that means more rpm, and that means a "bigger" cam has to move the operating window up higher, sliding the VE window to a higher rpm as well. So the VE slides to a new higher rpm, and swells to a greater number. If you put a 340 cam in there, the VE might slide to 3200rpm. And might increase the VE by 5%. I'm just throwing numbers out there. But to get that extra 5% at 3200, the engine might sacrifice that same 5% at 2400.
To get the 5% back at 2400, your only option is to figure out a way to trap the original volume of air back in there. How are you gonna do that?
IDK either.
IMO, this starts with ring-seal and fresh-cold air.
Now; you can compensate for the lost VE, with say; stall,Scr,and rear gear; but you can't get the 5% back, at least I don't think you can, at least not with the same type of cam.
Now lets talk cruising;
If you are cruising at 65mph,and say that requires 300cc of air in an 800cc chamber; just saying, that is 37.5% VE.
If your engine is a low-compression model, it might take 40% primary throttle to get that 300CC.
If your engine is a hi-compression model, it might take 35% to get that.
If a very hi-compression model, maybe 30%.
This is where Hi-compression pays huge dividends. You are making the same power on less throttle opening. This allows you to install a smaller rear gear, and that is where your better fuel economy comes in. As long as you stay on the primaries, lol.
Of course this filters down at all Part-Throttle rpms as well.
Now lets talk WOT power only, at like racing or something;
If you are targeting a certain amount of power,it is easy to compute your fueling requirement. which will dictate your air requirement, which will dictate your rpm requirement, which will dictate your camshaft requirement. If you can't get the air requirement out of the chosen cubic-inch displacement, at a reasonable rpm, then you back up the bus and get a bigger engine.
Was VE mentioned? Was Scr mentioned? Did I forget,lol?
So the point I guess I'm trying to make is this;
What VE are you talking about? If at WOT, as is usually the case, you are blasting thru the peak just once on your way to where the power is. So the absolute peak value is rather un-important. Higher VE through-out the upper rpm is hepfull but Higher compression may make more power by virtue of the higher expansion ratio. But VE will be little affected by the higher Scr.
But if your peak VE is at 2400rpm, then you might want to put your cruise rpm close to that. If it's at 3200, well you might find that uncomfortable on long trips.
So choosing a perfect blend of cruise-rpm and camshaft, with a small engine, in a streeter, when you are limited to non-overdrive applications, gets tough,
My solution was a small cam, tight ring gaps, plenty of pressure, cold air, full-length 3" duals, and lots and lots of gears, seven of them at one time,lol..
I think I understand your thinking, but for a streeter, I chose pressure over absolute VE.
Besides, you can fudge the absolute VE, by changing carb sizes, and messing with the exhaust. And you can move the VE with intake-sizing and cam-timing.
Or you can just start with a bigger engine .......... like Rusty mentioned. Then who cares,lol.
