Project Foxbody 306

when building any engine, the very first thing you need to do is determine what rpm range you are going to tun the engine in 80% of the time.

for instance if you are building a street engine, then you are saying that you will spend most of the time in the 1000-4500 rpm range. thus you select parts that operate best in that rpm range.

in your case, i would bump that rpm range to 2000-6000 rpm, since you want to build a street car that you can race. you can fudge the rpm range up or down a bit if you wish

start with the cam as everything revolves around that. lots of good grinds that you can use such as the xe274h extreme energy cam from comp cams. there are similar designs that are roller cams that you can use if you choose.

onto the cylinder heads next. since you are looking for mid range torque, you want something with a 180cc intake runner. that may be a bit small for big top end power, but at the strip you are going to win races off the line, through about 3/4 track, so you are not going to need the big end power in this vehicle, since realistically you are not going to run quicker than 11.70 in the 1/4 mile.

next the intake. you want one that will give decent low end power, as well as decent top end power, but really fatten up the mid range. you have a choice of three, in no particular order;

1: the weiand stealth

2: the performer rpm

3: the performer rpm air gap

the first two will fit under most stock hoods with minor clearance issues, the third may have more clearance issues, it all depends on what carb and air cleaner combination you use, along with what carb spacer you use.

First decision:
Do you want to go EFI or carb?

If the car was 86 or later, go EFI. You won't make any more real power with a carb than and EFI. EFI engines have a lot more flexibility in street cars, you can even have multiple tunes with the flip of a switch.

If the car was 67-85, go carb unless you have a real desire for EFI.

Second decision:
Are you will to spend almost as much money on getting the suspension setup to grip properly? All the HP in the world won't help if the car has no or limited traction. Beware of doing some things of dubious value like removing the quad shocks or removing the anti-roll bar on a street driven car. Do not put skinny tires on the front of a street driven car - you loose almost all your braking power. Going fast isn't nearly as important as being able to stop fast when an accident suddenly looms in front of you.




Far too many people put a dab of this and a dollop of that, and then wonder why the car doesn't run worth beans. Then they think off the shelf computer chips will fix their mismatched parts problem. It won't You have to have a plan for what kind of performance you want: Hot street. Street/Strip, Pure strip, Autocross or Road course. Each one requires a different strategy and a different set of components.. Mismatch the components and you’ll have a car that falls flat on its face when you demand performance.

Everyone thinks HP! HP! HP! and thinks that peak HP is what they need. Peak HP is great for a drag strip car when it has the proper gears and tires to get the car up into the high RPM range where it develops that high peak HP near the finish line. On a street car, that strategy will have Honda Accords outrunning you, because you will never get the engine RPMs high enough without running over everything in your path.

Here’ the strategy: Always remember that there are some tradeoffs in any engine combination. Most of us don’t have enough money to “have it all” as if it was possible by some masterful combination of parts and tuning.

The following recommendations are for 5 Speed Manual transmission cars without NO2 or pressurized induction, stock short block.
1.) Hot street: Broad flat torque curve, high velocity airflow in the intake and heads for best throttle response. Gears suitable for reasonable gas mileage and long road trips without excessive engine RPMs. Stand on the gas pedal from a rolling start to squeeze into that gap in traffic in front of you, and it jumps quick and hard to get you there. Max RPM’s are 5200-5500 RPM for best power. Lopey cams may sound cool, but run poorly in a low RPM street environment.
Use stock cam, stock, GT40 or mildly ported stock heads, Cobra or Explorer/GT40 intake, advanced timing, stock 19 lb injectors, stock fuel pump. Use some good 1.6 or 1.7 ratio roller rockers for extra punch. Use a King Cobra clutch, with stock iron or steel billet flywheel. MAF cars can use a 65 MM TB from the Explorer intake manifold and a 70MM MAF from a 94-95 Mustang. Drive train: 3:55 gears with soft tread compound tires. Use some Ford Racing unequal length headers, stock 2 1/4” cat pipe and some mufflers that don’t drone or get too much attention from the law enforcement or neighbors. The stock computer will handle all this with no problems and doesn’t need any help in 90% of the cases. No skinny or grossly undersize tires for the front: remember you still have to stop quickly in traffic. Make sure all the rubber bushings in the front and rear suspensions are in first class shape. Leave the emissions equipment intact and working. Removing or disabling it won’t get you any more HP or performance. Do not convert to carb or remove A/C: either one will reduce the resale value. Carb conversions cannot be titled for street use or get tags in some places. They definitely won’t pass smog inspections.

Street/strip: A little more slope to the torque curve with a gently sloping peak. Use slightly larger port volumes on intake and heads for more peak HP. Uses 3.55 or 3.73 gears to get the RPM’s up into a higher range quicker. Be prepared to sacrifice some low RPM throttle response in exchange for high RPM power. This by necessity will be a Mass Air or Mass Air conversion on 86-88 5.0 Mustangs, since stock speed density will not run well with the changes in engine airflow. Don’t get too crazy on any one engine part since you still have to drive the car on the street, and a mismatch can make street driving miserable.
Use stock or mild aftermarket cam, Ported GT 40, or 165-180 CC port volume aftermarket aluminum heads. Use a Trick Flow, Edelbrock Performer or equal intake manifold. Take a 73 MM aftermarket MAF calibrated for 24 lb injectors, and 24 lb injectors, 155 LPH fuel pump, Kirban adjustable fuel pressure regulator. Be prepared to shell out some $$$ for a custom burned chip using data gathered from a dyno run. Mass market chips will not get the job done. Use some Ford Racing unequal length headers, aftermarket 2 1/2” cat pipe and some mufflers that don’t drone or get too much attention from the law enforcement or neighbors. Drivetrain: expect the stock T5 to fail, so save your money for a super duty 5 speed trans. Tremec 3550, TKO 500 & TKO 600 are the best choices. Different gears in a stock T5 case work for some, but there is only so much power you can pass through a T5 in race mode before it breaks, even with stronger gears. Next are the Chassis mods: full length subframe connectors, different springs, different shocks, aftermarket lower and upper control arms with rubber or urethane bushings. Buy all the parts from someplace like Maximum Motorsports, Griggs or Steeda as a kit so that you know that all the parts fit and don’t argue with each other. Carry spare tires and wheels for the drag strip: skinnys for the front and drag radials for the rear. No skinnys for street driving! Over 85% of the breaking power is generated by the front tires, so skinnys won’t do the job in a panic stop situation. Disconnect the front anti-roll bar at the strip; reconnect it before you drive home. Leave the emissions equipment intact and working. Removing or disabling it won’t get you any more HP or performance. Do not convert to carb or remove A/C: either one will reduce the resale value. Carb conversions cannot be titled for street use or get tags in some places. They definitely won’t pass smog inspections.

Strip only: High RPM, High flow heads (185-215 CC port volume), wild cam, high flow intake manifold, 70 MM or larger TB, 80 MM or lager MAF, strip everything out of the car that doesn’t make it go faster. Carbs are OK if that’s what you want, but remember that as the temp/humidity/ barometric pressure/altitude changes, you have to re-jet and readjust the carb. EFI eliminates most of that with its built in compensation or you can tune of the fly with a high end Motes or Tweecer system combined with a wide band air/fuel ratio meter. Use custom headers, dumps and minimal mufflers. How fast you can go on 5 liters is a function of the skill level of the driver/mechanic and the size of your wallet.

TRAILER the car to the race track since it won’t be legal to drive it on the street. Drag slicks in the rear, skinnys up front, use 3.73 or bigger gears (4.xx) in the rear axle. Since you won’t be driving on long trips, the big gears with work with the high RPM power curve to get the best results. Drivetrain: TKO 500 & TKO 600 are the best choices. Different gears in a stock T5 case work for some, but there is only so much power you can pass through a T5 in race mode before it breaks, even with stronger gears. Next are the Chassis mods: full length subframe connectors, different springs, different shocks, aftermarket lower and upper control arms with rubber or urethane bushings. Buy all the parts from someplace like Maximum Motorsports, Griggs or Steeda as a kit so that you know that all the parts fit and don’t argue with each other. Remove the front sway bar, put an airbag in the rear spring of the side that spins the tire the most. Plan on a roll cage if you are truly serious about going fast: most strips will require it once you get to a certain ET range.

Autocross is a combination of Hot street engine and street strip chassis prep. The engine must accelerate quickly from low RPM and needs a broad, flat torque curve. Next are the Chassis mods: full length subframe connectors, different springs, different shocks, aftermarket lower and upper control arms with rubber or urethane bushings. Buy all the parts from someplace like Maximum Motorsports, Griggs or Steeda as a kit so that you know that all the parts fit and don’t argue with each other. Most of the time you’ll never hit third gear, so some 3.73 or bigger gears (4.xx) may help a lot. You’ll have to spend some more money on brakes since it kills brakes quickly. Rear disks, larger rotors up front, stainless steel brake lines, different brake pads. A 87-88 T Bird Turbo Coupe or SN 95 rear axle will be your best bet. Autocross will severely strain 1st & 2nd gears, so your T5 may take a premature dump. Save your money for a super duty 5 speed trans. Tremec 3550, TKO 500 & TKO 600 are the best choices.

All out road race is the most difficult of all: an engine that will run at high rpm hour after hour and never fail, yet pull hard out of the hairpin turns that will require a lot of torque at lower RPMs. In my opinion, guys that can successfully build a winning road race engine are the cream of the crop. Top this off with a chassis built for strip only duty, but with changes to the settings of springs, tires, roll bars brakes and shocks. It’s a whole other world of racing.
You’ll have to spend lots more money on brakes since it kills brakes quickly. Rear disks, larger rotors up front, stainless steel brake lines, different brake pads. Air ducting to cool the brake rotors will be a must. The brake rotors of cars on a high speed road course glow red after several hard laps of racing. Drivetrain: TKO 500 & TKO 600, and T56 close ratio are the best transmission choices.

What is done to engine and drivetrain now?
What is your budget?
What are your drivability expectations?
Are you willing to buy new cam/exhaust once you go turbo?
Getting the most out of a 306, then talking about GT40 heads do not really go hand in hand.
If you already have GT40 heads, are you trying to get max effort out of them?

They totally go together, but you said you want to squeeze as much out of the 306 as possible. By 306 I assume you are talking about the shortblock, and are looking at a heads/cam/intake setup? If that is the case, and if your budget allows the options of aftermarket heads, then there is much more power to be made while maintaining street car function.
If you are looking to get as much out of a 306 with GT40 heads, it is a different ball game.
So, which is it?
Also need answers to the previous questions.
Working over a set of GT40's (port/bowls/valves), going solid roller with rpm is a spendy adventure, more spendy then going aftermarket head and running stock lifters and keeping rpm in touch.

Need more info.