Here’s a few thoughts on the above posts I hope might be helpful to someone.
Original poster wants a target compression ratio for his 306 inch small block with GT40 heads, TFS-1 cam, and a ~250 rwhp goal.
Shouldn’t have to turn that motor much past 5500 rpm to reach the HP goal. The selection of GT-40 heads and TFS-1 cam is a decent start. A little bowl clean-up work on those heads would certainly help but you should get fairly close without it. You WILL need a better intake manifold, throttle body, and mass air meter to create a matched combo with those heads. Cheapest route for a well-matched intake is from a 90’s 5.0L Explorer. The stock, 5.0L HO intake is particularly restrictive.
One full point of compression (example, 9.0 – 10.0 to 1) will net you ~3% HP increase, assuming you can stay out of detonation. It helps, it’s cheap when building anyway, but it doesn’t pay to push the boundary too far on a street car. Too many variables in daily operation come back to bite you and a street engine operates more under low rpm loads where detonation is most likely.
When we measure the ratio of cylinder volume between BDC and TDC we are calculating static compression. In a vacuum (reverse pun), this tells us too little about the engine’s ability to stay out of detonation on a particular octane fuel.
It is the dynamic or “running” compression ratio that matters. The dynamic ratio will be significantly lower than static and is most dependent on cam timing. No actual compression begins building in the cylinder until both valves are shut (disregarding acoustic supercharging, but you get the idea). Dynamic compression is influenced to a lesser degree by rod length, which determines where the piston is in the bore relative to cam timing everywhere except at TDC and BDC.
Bottom line with compression, the longer your valve timing events (and more overlap) the lower the actual or dynamic compression ratio and so the higher static compression you can run. With the TFS-1 cam on pump gas I’d start getting pretty nervous at anything above 10 to 1. You “might” get away with a bit closer to 10.5 to 1, but the potential miseries that can create are not worth the ~1% HP gain in a street car, IMO. Conversely, In racing more is nearly always worth it.
AeroCoupe’s car made 318 rwhp on 11 to 1 compression, which is around 375 flywheel. That’s 1.22 HP/cubic inch, so that motor has to wind up to a higher rpm than a TFS-1 cam motor to hit peak power, therefore requiring a cam with longer timing events and relatively lower dynamic compression. That’s why his higher static compression works.
As you begin to push the boundary on compression, quench clearance becomes quite critical to prevent detonation. As suggested previously, ideally don’t allow more than .040” between the top of the piston and the flat, quench area in the combustion chamber. You can go as tight as .035” but to prevent pistons from kissing the heads you’d better KNOW your deck height of ALL pistons and not have excessive piston rock in any of the bores. For us mere mortal, home engine builders, .038 - .040” is a great target. Importantly, that does NOT mean the piston is .040” down in the bore, but rather a zero-decked flat top piston with a .040” gasket, etc. Don’t forget about the gasket space.
Recommend not going larger than 190LPH on the fuel pump unless changing over to larger fuel lines. Pressure will drop in the small diameter factory full lines at peak rpm anyway, and the increased speed of flow at all times through the return system causes fluid friction that will heat the fuel. 190LPH will easily feed a naturally aspirated, street 306. Factory 5.0L HO pump is only 88LPH.
+1 on not exceeding 85% duty cycle on your injectors. As the pulse width passes that threshold the short closing time does not allow the injector pintle to mechanically operate reliably and consistently, so fueling will become erratic. Also, the extended degrees of crankshaft rotation during the injection cycle means part of the fuel will not be delivered at the optimum time into a high velocity airstream, for best mixing.
Significantly larger than necessary injectors cause other fueling challenges, mostly at idle where the pulse width is so short that the pintle cannot mechanically respond with consistency. If your injector peak duty cycle falls somewhere between 70% - 85% you won’t need to worry about any of that stuff.
Finally, +1 on Ed at Flowtech Induction for a custom cam for any small block Ford. I have two small blocks now, both with Ed cams. Your TFS-1 is a decent choice for your stated goals and current combo, but if you decide to try something else I wouldn’t look anywhere else. Good luck!
. Glad to hear that I won't need a domed piston to make decent compression gains. How much performance did you gain from raising the compression to 11:1?
