Best replacement pistons for stock '69 engine
If the Ica(Intake Closing Angle) on your new cam is later than on the stock cam, then the Compression ratio needs to keep up, else the take-off power with the same gears and TC will go soft. How soft depends on how far from stock that you stray.
A higher than stock Compression ratio does not automatically put you into a higher-octane fuel. It's all in the combo. Increased cylinder pressure raises power everywhere in the RPM band. Whereas a 4bbl does nothing until the rpm gets up high enough to open it. With hi-way gears and a lo-stall, this can take a really long time.
You said TTI exhaust, but did not say Headers. Headers always add their power to the whole combo sortof as a percentage, and the percentage will vary from zero to maybe 10 or 12. So say your engine makes 150hp, then at 10% that would be 15 hp. but if your engine made say 250 hp, then 10% is 25hp.
If you pay 1000 smackers for your exhaust, then at 15 hp, that would be 1000/15=66.67 dollars per hp. But at 25 it drops to 40 dollars per hp. But here's the deal, this type of power increase occurs at or near the power peak, say 4200 on the 15 hp, or say 25 at 4700 on the other. But at a stall of say 1800, the power increase could be ZERO. So what this means is that when you take off from a dead stop, neither a 4bbl, nor headers, at a stall of 1800 may be any different from stock.
With a given engine (318 in this case) at zero mph there are only two things that will make a difference, namely cylinder pressure and increased stall. By far, stall gets the biggest gains.
As soon as the car gets moving, TM(Torque Multiplication) comes into play, so now gear ratios get the next bigger gains; not in horsepower, but in acceleration over time, which feels and acts like an increase in power.
What I'm saying is, for you;
So, just a daily driver engine with a little more pep courtesy a 4 barrel intake and carb, plus TTI exhaust.
A lil more pep is a easy as say a 2800 rpm stall and a bigger number rear gear. The 2800 will let your engine start off at zero mph with a boatload more power, and the gears will allow the engine to zip thru the rpms quicker, and it will do all this with or without a 4bbl or headers. Both of those would be icing on the cake.
Finally, if you stay with a stock-spec cam, you will sorta have to stick to the stockish Compression ratio. But if you go to a bigger cam, and I would if spending all this money anyway, then as mentioned, the Compression ratio has to keep up, else low-rpm power may suffer.
The stock 318 cam is so small (240/248/112) that you can easily jump it up two or perhaps three sizes before getting into fuel-economy issues.......if the Compression pressure is well co-ordinated.
Here is the big picture among cams; first the stocker, then progressively larger theoretical ones, installed at split overlap, and arranged as
intake/compression/power/exhaust/LSA/Ica/o-lap,and, on the end I tacked on the compression ratio to run a DCR of 7.99/ 8.01@600ft elevation, and the resulting pressure/VP (see*1).
240/130/122/248/112/50/
20/9.10/160/133VP
248/128/120/256/110/52/
32/9.22/160/131VP
252/127/123/256/108/53/
38/9.29/160/131VP
256/125/120/262/108/55/
43/9.39/160/129VP
>The numbers I want you to see are the Effective overlaps that grow from 20 to 43. These numbers only matter if you have headers. Overlap is the time, in degrees, during which both the intake and the exhaust valve are open, with the piston dwelling at TDC. At this time, the header puts a yank on the plenum that gets the fuel/air charge moving towards the chamber much much earlier than the falling piston can accomplish. And so this makes the power come on sooner and stronger, than it ever could with log-manifolds.
>I varied the LSA (112 to 108), to achieve very little Compression Ratio compensation to achieve the target 8.0Dcr/160 psi targets. But look, we still have a variation from 9.10 to 9.39, to achieve that target.
>I kept the power-extraction purposely long; 120 to 123, so you can get the same fuel economy in steady state operation as the 318 cam does.
>By comparing the intake durations (from 240 to 256), we can see about 2.5 cam sizes. Each cam size ( about 6 or 7 degrees) moves the power up about 200 rpm, so from bottom to top about 500 rpm; if the 240 peaks at say 4200, then the 256 might peak at 4700. I'll guess about 25/30 hp difference, meh maybe a tad more.
>Finally, watch the VP number; it is shrinking from 133 to 129. I tried to keep the shrinkage to a minimum because VP is what you feel on the gas pedal as performance. The bigger the better. The only ways to make this number bigger are; more cylinder pressure, or, a bigger engine . The only ways to get more cylinder pressure from a given engine size are; more compression ratio, or an earlier closing intake (which usually means a smaller cam).
*1, Read about VP here;
V/P Index Calculation
