Its a tough question to answer.  The big reason is, what's a mild cam?  What heads are on the engine?

I will say this, an EFI 5.0 was a torque monster in its own right.  Stock they made like 225HP and 300 lbs/ft.  Peak torque was at 3,200, but from having driven them extensively I'd bet the torque curve before that was about like that of a Diesel; flat.  Keep in mind, those numbers don't seem that impressive, but those are real numbers; net HP.  These aren't the optimistic gross numbers used in the '60s and '70s. 

The big question is how much power does what you have put out.  In general gross HP was about 15% optimistic vs. net, though it can be worse.  A stock 4 barrel 351 was about 300HP and 380 lbs/ft.  Knock that down by 15% and you're looking more like 255/323.  Not too far off a stock 5.0.  Now, your 351 may be turned up a bit, so if we say you added maybe 20HP with some upgrades might be more in the 275/350 range.

What I can say is that a cam swap and some other tweaks to a 5.0 can get you 50HP relatively easily.  I think you'll still be down on torque comparatively, but not that bad, especially with shorty headers.  If you've got the coin for decent heads you can make that 5.0 a real screamer. 

Of course, you could do all that to the 351 too, provided its not a total loss.  I'd start by seeing exactly what's up with it and what it would cost to repair.  You can always add EFI to the 351.  5.0 and 351 use the same firing order, though the EFI manifold for the 351 can be expensive. 

I would also check out the 5.0 to make sure it seems healthy before committing to going that route. Haven’t done a compression test or anything on it yet. For now I’m just thinking out loud.

The 5.0, if its EFI will have E7TE heads most likely, unless its an '86 model year engine.  The E7TEs are probably a bit better than the stock heads on the 351, particularly at low lift.  If the cam idles fine at 600-700RPM its got to be pretty close to a stock grind, likely ground for low end torque more so than top end power.  A stock 5.0 EFI cam is also likely better, if for no other reason than that its a roller, so it will have better area under the curve.

I think the only real difference will be the additional 49 cubic inches the 351 has on the 302.  I'd guess its worth about 20-25lbs/ft.  Not insignificant by any means.  Seat of the pants that's going to feel like something, but its probably one of those things where if you ran the car down a track with both engines the actual 1/4 mile time would probably be pretty close, maybe 0.20 seconds difference. 

Thank you all for your input.  I know there's no definitive answer.  Really just asking from a big picture perspective of those that have some experience with both engines...not trying to get into too many gritty details.  This was my first and only car build so far.  I bought it as a roller in '99 and the 351w is all I've ever known. The car is strictly a driver so I'm not concerned with track times...just fun factor.  The 351 is a '70 model block with '69 or '70 model heads on it for those wondering.

First order of business when I'm ready to get back to work on it will be to pull the 351 and tear it down for inspection to see if the damage extends beyond the crank and bearings.  In theory, I suppose the filter should have prevented any metal from being circulated anywhere else but that may be wishful thinking.

My experience with my 66 coupe,

The first time I got it completed and on the road, I had a rebuilt 5.0 roller cam motor (1990) with a Ford E-cam, stock 5.0 heads, an aluminium dual plane manifold, Autolite 4100 carb (480 cfm), long tube headers.

The second time, I had a 351w, roller cam conversion, aluminum heads, 10-1 compression, dual plane manifold, 750 cfm carb, long tube headers, lots of cam.

The third time I had a stroked 351w (393), roller cam, AFR heads, 10.8-1 compression, dual plane manifold, 750 cfm carb with mech secondaries, long tube headers, and even more cam.

All the motor configurations were with a T5 5-speed, and a 8" rear with 3.80 gear and locker. other than the upper control arm drop, and bigger sway bar and all of the suspension parts new. 5-mid leaf rear springs, and cal-trac bars, all the suspension was stock design. The coupe had 15" wheels with 205 front and 245 rear. I never abused the T5 and with only a 245 street tire, it held up fine.

With each motor, the car was less fun to drive, largely due to compression and cam. With a 245 rear street tire and 3.80 gear, the 5.0 would blow the tires off anytime I chose to do so. The 5.0 would rev to 6000 rpm very fast and was a blast to drive around. It always seemed well balanced for the rest of the car. With each motor change I gained power and torque, but none of it was usable with the rest of the configuration I had.

My point is, unless you plan to do extensive suspension, steering, and brake upgrades (changes), a well built 5.0 (306) is a great match for a first generation coupe. If I had both and had to choose between a 5.0 roller motor or a 351 flat tappet, I would choose the roller motor, and there are a lot of cams out there better than the Ford E-cam, just what I chose back in the day.

Thanks, kardad. That’s very helpful first-hand information. A little background on the rest of my car: stock style rebuilt suspension with poly bushings, Shelby drop, 620 coils, stock height 4 leafs, stock rebuilt steering (manual with ps box), T5z, MS cable clutch conversion, MS power brake conversion, SSBC stock style disc brakes, 9” with 3:50 gear, 17” wheels. Nothing radical but all systems have been freshened up.

The 351 is all I’ve ever had in the car. I’m far from an expert mechanic but I try to learn and do the best I can.  There’s a few things I’ve never liked about the 351.  It’s had a hot start issue since day 1 that I never could fully resolve. Tried a multitude of timing settings, float adjustments, etc that helped but never totally fixed it. My best defense was a high torque starter and cracking open the hood if I new I’d be starting it again soon.  I never felt like the engine was radical enough to require a high torque starter. I always wondered if part of it was simply an oversized engine in an undersized bay.  Also, intake and header choices are very limited to fit the 351 in the early cars. None of this bothered me enough to want to swap engines but this relatively major issue with the crank has me considering my options.

TKOPerformance wrote:

Try wrapping the starter in a reflective blanket.  The starter is probably heat soaking from the headers causing the resistance to skyrocket. 

I had a heat soak problem with my 351/long tube headers.  I got to the place where I could change out the starter in about 15 min with a hot car.  Solution was a reflective blanket and shorty headers that were farther away from the starter than the long tubes.

Oh, in addition - I think the 351 LOOKS better than the SBF in the engine compartment. I like the esthetics of that big iron.  And it still handles good enough with that extra weight.
 

Last edited by lowercasesteve (1/15/2020 5:48 PM)

Counterpoint on the GT40s.  A good upgrade for a stock engine.  BUT, if you want to make more power later they will quickly become a limiting factor.  They don't tolerate much valve lift stock (P heads are even worse due to the exhaust valve rotators).  They force you to use pedestal rockers, which are both weak and expensive.  A lot of the ones scored in junk yard turn out to be cracked.  They're iron, so they limit your compression to about 1 point less than you could safely run with aluminum heads on the same fuel.  Aluminum heads also weight about 50lbs less, improving front/rear weight bias and shedding weight (good for handling and acceleration). 

Sure, they can be ported, etc., but by the time you start to add up the costs of all the work you could buy a better set of heads for the same cost, something like Edelbrocks.

There's some truth to the GT40 heads on a 5.0 makes 300HP.  But, that's an EFI engine with the right matching parts.  Does it work?  Yes, the '94-'95 Cobras were rated t 300HP, BUT they had 24lbs injectors, a different MAF, a different intake, a different computer, a different TB, etc.  On a carb engine you will make less torque and therefore less HP.  Though 275HP-280HP is likely realistic.