Compression Ratio
For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want a improvement over stock.
Pressure is heat is power. The more pressure your engine has as long as it doesn't detonate, the more power it can make. The problem is that in stock configuration the smog teens barely make 135 psi. as soon as you install a cam with a later-closing intake, the pressure falls , and it falls fast. .
For a streeter, with a 2.76 gear, and a 2000 stall, that teener will never wake up until mid to late 30s mph
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber.
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The street engine doesn't care about it's Static Compression Ratio. It only cares about peak pressure and when it occurs, and how much load is on the crank. The engine cannot begin to make pressure until the stinking intake valve is closed and not leaking. The pressure is determined by something called the dynamic compression ratio, which is what is left of your Scr after the intake closes.
IIRC the cam in the stocker is 240/248/112, which in at 108 the Ica will be 48*
the Wallace says that at 500ft elevation, this is likely to produce
133 psi @ 7.8 Compression ratio, which, as you know, is abysmal.
Now say with no other changes, you install a 262/110 cam. This will change the Ica to 57* and the Wallace says that the pressure will drop to
124psi. So then, your bottom end has gone from bad to piss-poor. This translates as a huge performance loss below about 3000/3500. But yeah, if you wait until ~4000, the cam will eventually wake the engine up. The problem is that with 2.73 gears and stock TC, you'll be going 43mph!, and no the 4bbl will not make this up.... because the 2bbl is good to at least ~3000 all by itself. So it don't matter how you time the secondaries, they will not be contributing power over what the primaries could do on their own, until after ~3000.
Now, on 87 gas, that teener could easily run the pressure up to 150/155psi. To get there with the stock cam would take an Scr of about 8.75. To get there with the 262, would require the Scr to be 9.2
Ok, then imagine the bottom end power loss going from 133psi to 124. Now imagine going from 124 to 153.
The Wallace includes a calculation to help you understand this, called the P/V index The higher it is, the more fun the bottom end will be. Conversely, the lower it is the more of a lazy-dog the engine will be.
the Stock 318 has a
P/V of 111.... and you may know what that feels like...... lol
Now still at 7.8Scr and with the 262 cam, the P/V index drops to 97, but
pumping the Scr up to 9.2, the P/V now climbs to 120.
What do the numbers mean?
Well from 111 to 97 is a loss of 13%, and from
97 to 120 is a gain of 23.7%
So these are numbers your butt-dyno is gonna really feel. But recall that these numbers are ALL ABOUT low-rpm power. If you install a 3500TC, none of this is gonna matter., NONE of it.
What increasing compression ratio does not tell you, is how the engine reacts to changes in throttle openings.
The stock 318 is a lazy engine. When you are cruising along at say 2400 rpm and you floor it, chit takes a while to get moving. Why is that? Because at 7.8Scr, she has a total combustion chamber volume of 96cc, and when the piston starts "sucking" on the carb, that big chamber is like a big rubber band, it takes a long time to get the air moving. But at 9.2Scr, the chambers is about 16.5cc smaller which is 17%, which is a lot! And so the air is a lot more willing to get moving. This makes for snappier throttle response that you can really relate to.
>New thought; Suppose your stock teener makes the same 300ftlbs at 2000 as at 2800. At 2000 this is 114hp. But at 2800 this is 160hp. That is an increase of 40%. There is NOTHING that you can do to your stock 318 short of maybe supercharging it, that can generate this size of power increase. So then this should be your very FIRST "bolt-on". Couple that with a 4bbl for another say 10ftlbs at 2800, and that would be 165hp
> New thought; those 300ftlbs, by the time they come out to the rear axles, in 2.45 first gear with say 3.23s, to be fair, will get you 2374 ftlbs to share between the two tires. Now slap some 4.10s on there, and this becomes 3013 . Now stall it up to 2800 and add the 4bbl and it looks like 3113 ftlbs. But hang on, this does not reflect the 40% horsepower increase in the stall rpm. Now the last time I tried to presemt this power increase, in an understandable context, I got thoroughly slapped for it. So I ain't doing it again. But obviously the extra power is gonna do something at the rear wheels that the modest 100 ftlbs does not speak to.
> so in conclusion: as I said;
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber. Leave the spare tire at home. With the spare not there, you might as well leave the jack home too. Since no one will passenger with me, I took out the back seat too (72 stinking pounds of of folding boat-anchor).