Engine Raising compression on a relatively stock 302

Just order the Pro-M for larger injectors and no time required.

Again, not wanting to get into a pissing match. Just providing detail on what worked on my combo and I was in the shop on the dyno run and saw the wide band going lean above 5500. Swapped to a 2000 GT MAF and 42’s and made more power, wasn’t lean anymore, and it idled very well.

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!

With this combo you should cover your bases for making up to 280 flywheel HP.

At that power level 19lb. injectors will run at an 85% duty cycle if your BSFC (brake specific fuel curve, a measure of engine efficiency) is at .55 or better. Your well matched combo should be better. Staying with the 19lb. injectors will minimize tuning hassles and create no penalty.

There is a difference between theory and practice when it comes to pumps and injectors. My friend owned a foxbody mustang shop (since the 80's) with dyno up until a few years ago. I've literally seen it all.
Fuel pump upgrades for stock blocks should really just be 155lph or 255lph. 190's don't really serve a point, it's more than a HCI setup setup needs and less and a supercharged setup requires.
The stock fuel system is fine to 500rwhp (with the right pump and injectors), no aftermarket fuel line necessary.
19's will get you very close to 300rwhp, though 24's are the right move at that number.
If you go 2 injectors sizes outside the range of what you really need you are going to have issues if it's not tuned.
So if you need 19's and you run 30's, it will probably be a mess at idle.

Even my old Saleen Explorer 5.0 used 19's, that had an engine with factory P heads and an Eaton M90. The setup produced 286rwhp on an automatic trans and that was forced induction. I daily drove it that way for 100,000 miles.

If this setup was mine, i would run the 19's, a matching calibrated meter and a 155lph pump.

I hate to be that guy but wtf why not...

Coupe, built 331 through a T5 and ran out of fuel up top at 470ish fwhp. Fuel system consisted of a stock hanger, 255 lph pump, stock fuel lines, stock fuel rails, Aeromotive FPR, and 42 lb/hr injectors. Car has an A9L with an SCT chip and was tuned on a Mustang Dyno by a guy in McKinney, TX that is known for his skill set on these cars.

He had to raise the fuel pressure up to 52 psi to get enough fuel up top to keep it from leaning out. I drove it this was for about 4 years and it ran great but that chewed on my. Ended up looking into things and went down a deeeep rabbit hole which means I spent money but learned a lot.

I would tell anyone to go out and build a fuel system for a motor making around or less than 450 fwhp but I can tell you that the factory fuel lines in my car were causing a lean condition with my motor combo but if you are not turning past 6,000 rpm I don't think it would come up. Mine was leaning out past 6,000 so the added pressure allowed it to twist up to 7,600 and not lean out.

Pro-M has a really nice video showing how when you get up to the 255 lph pumps when the fuel level gets low (1/4 tank or so I believe) the return line has so much pressure and volume in low demand states that it can literally cavitate the fuel at the pump inlet. A guy on YouTube showed how he redirected the return fuel away from the pump inlet and says it works very well so that would be a good thing to do when replacing the pump.

Anyhow, all I am trying to say is when you are putting together a motor be sure to utilize the calculators for the injector sizing and then take a look at fuel consumption as well. That will help with a lot of decisions with regards to the fuel system. If you end up getting a tune then pay attention to the AFR through the pulls and work with your tuner. They can do a lot but sometimes fixing it with the tune is just a bandaid for a mechanical or electrical (low voltage to the pump) issue that needs to be addressed.