347 Intake Manifold

To me, Weekend Cruiser means that you want decent low RPM performance and an engine that pulls hard from 1500 RPM To redline. If you had a different plan in mind, you need to spell that out.

The 425 HP is doable but you will experience a loss of streetable driving performance. Horsepower in the 325 - 350 HP range at the rear wheels is a more reasonable figure for street driven cars.

With that in mind, you need a manifold with longer runners and a cam to match. Mismatch the cam and the intake and you will be disappointed in the car's performance. The short runners of a carb style intake are tuned for high RPM usage and tend to have less low RPM torque for street driving. That means you have wall climber gears (4.30 to 4.86) to get it drivable on the street at low speeds, and those same gears make highway driving miserable.

My comments are taken from "High Performance Strategies for 5.0 Fox Mustangs" do a search to read the whole tech note.

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.

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.

My 347 put down 375rwhp and 398rwtq. I have the Holley Systemax II and a fairly mild cam. So the Holley will do what you need.....

If you have the hood clearance, the trick flow box R is the ticket. I've got a ported victor 5.0 intake on mine. It's not a bolt on a go intake, but makes good power if you're willing to put in the work required to make it work. You will need to measure to see how it sits on the heads, and it'll probably need to be milled to fit. You also have to rig up a pcv system or use a vented breather tank. My set up pulls hard to near 7,000 and makes plenty of power all around.

400+ hp on a 347 is easy to do.

What head and cam are you running? Transmission? Rear gear ratio? Stock hood? 425 flywheel horsepower out of a 347 is easily done with the correct parts. The box R and Holley systemax are good intakes(the Systemax is on the small side for a heavy breather though). The Box R isn't going to fit under a stock hood.

If you're building around existing parts(heads in this case-which 185cc are on the small side for a 347) I recommend the SysteMax. You'll still be able to reach your 425 target.

It's not only the size of the runners that matter. Cam timing,exhaust sizing,compression ratio,etc etc all make a difference. I'm taking a wild guess but sounds like you are running pro-comp or equivalent heads? For a street cruiser the 185's will be fine. I just read into your earlier post and assumed it was a street/strip build and you wanted to make good hp/tq. The 185's with a systemax intake and a good cam will make a very healthy street combo. If you want to error on the more aggressive side use the box R and a 210cc head with the right cam and you could see closer to 450-500hp. With what I think you want to do, I recommend the more conservative approach. Just a fwiw the topend parts will just be limited if you're planning on getting more aggressive down the road(for a n/a combo). As far as inj go...a 30#inj will support 425hp but I'd go a little bigger for a safety margin. You don't want to be limited on fuel supply...including the fuel pump(255 lph).
Edit: grab some 42's if you can find them.

I have a 331, not a 347, but I make about the HP numbers you are shooting for. AFR 185s, Holley Systemax intake, custom cam ,shorty headers, 4.10's, about 10.7:1 compression. No slouch on the low end.

Remember, these intakes are rated at 302 displacement. If they advertise 2000-6500 rpms, expect the power band to be a little lower.

Joe

davevt347 said:

Thanks,I live in vt so the car will only see a few months of driving. Probably only see 2-3000 miles over the summer.even if I don't get the big numbers with heads I have,I'm sure whatever it puts out will be good with the proper cam and intake.i still have quite a bit of money to put into the motor so I'm stuck to work with what I have.still need cam,lifters,push rods, head studs,intake, injectors,throttle body,maf to match.plus accessories.i am taking in everything that you all have said. Just wish I had a garage for the winter to have her ready for summer!!I looked on the for sale forum not much on there.if you know of good used parts sites that would definitely help out.thanks!!!

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I prefer this forum for chatting, but corral.net probably has the best mustang parts sales section.

As a matter of fact, my intake, rockers, lifters, headers, maf, timing chain, shift light, upper control arms, who knows what else, all came from their classified section.
Joe

X2 on the corral I've bought and sold a lot of things over there. There is a lot of movement in their classifieds. Good luck on your project!

Might want to consider 3.55 rear instead of 3.73. 400+ hp foxbody with 5spd, 3.55 is plenty steep. Mine has 3.73 and in hindsight, 3.55 is what I wish I had done.

Gear the car for the power band your combo is designed for. I had 3.73's and when I rebuilt my 8.8" I went 4.30.

Good advice @Grn92LX . I went from 3.73 to 4.10 eventually. Not a DD though.

Joe