I've been lurking this forum for about 2-3 years now and have finally found a question that I can't seem to find an answer to, so I broke down and created a membership.

So I've got a 1974 Highboy with a really tired 390 FE engine, I have plans in the future for a ZF-5 transmission swap so rebuilding my FE engine does not seem practical if I'm just going to swap it to a BBF bell housing anyways in a few months. I have a complete 460 and a complete 400 m that I can use, but I'm having a hard time deciding which one to build. Sure the 460 makes more hp/torque in stock form, but I think I can make very similar numbers with a 400 and still maintain decent (at least, better than a 460) fuel milage. I know my 460 would produce around 210-250 horsepower and upper 400's torque with a rebuild, intake and exhaust... all at the hands of 8 miles per gallon.

So I have everything to build a 400 m with a .030 overbore (406?) and edelbrock performer intake and Holley 600 cfm 4 barrel with headers to true duals with 40-series flowmasters, realistically what kind of power/torque could I expect out of this much alone? And how about adding a mild cam and bumping up CR to 9-9.5:1 by decking the heads? Also, what type of fuel milage is everyone else seeing with their 400's, I have heard these M blocks actually do relatively well compared to 390's and 460's.

 

Go ahead with the 400 build, especially as you have the parts already.

What pistons do you have ?

A mild cam package would be in order, as well as block milling to get a zero deck.

(Head milling is only required if the heads are warped.)

You could easily achieve a +/- 400hp and +/-500 torque with the right parts selection. (And still be a street engine.)

With a manual tranny I wouldn't be surprised to see an average of 14mpg. (Depends on how you drive !)

(The 460 would achieve way more than 210-250HP. )

 

a 460 will net very strong numbers for a truck engine, but a bigger plant heavier, larger parts, larger heavier. A 460 doesn't fit real well in that chassy. It can be "shoehorned" to fit. The 400 factory brackets and all work very well, A big block will naturally build more torque, due to the longer intake runners, more air to charge the cylinder, more power. heavier rotating assy. better lugging capability with a manual. A 400 with some compression and a cam will run very well, they are a square engine. Lots of piston velocity and good midrange capabilities. Not as healthy as a 460, but no slouch. A good 400 vs. a good running 460, in the same truck, infront of the same trans, for me, burn the exact same amount of gas. checked on the same roads with the same gps. 400 is more responsive to timing adjustments. probably due to a higher piston velocity. The deal is that the 460 had headers and an aluminum intake and the 400 has maifolds and 2bbl iron intake. I bet I could shave another mile or better out of the 400 with a set of headers and intake. That would tell me to lean to the 400. The 460 had no measurable change from headers or manifolds on the exhaust side. 460 is a way over square platform, 385 stroke and 4.36 bore, 400 is 4x4. A 460 has the capabilities of nearly unmeasurable potential on account of the cylinder being able to accommodate very large valve configurations, leading to a substantial increase in power development. The 400 is limited to what you can shove in there with a 2" valve, and the significant shrouding going on by the smaller radius cylinder wall. The 400 seems in my opinion to start to fall off at about 5000 rpm, which makes perfect sense because the combustion efficiency likely is not there for long enough duration.
Fundamentally the idea is like trying to feed a garden hose with a mcdonalds straw, just doesn't happen. With advanced head castings, revised valve angles, and increased combustion chamber efficiency the 400 has a new lease on life. That makes it potentially a great platform.
Moral of the story, in a pump gas build for street use, there is likely nothing holding back the 400. I would say in equal territory, with adequate tuning, a 10:1 400 on pump gas or a 10:1 460 on pump gas, both modded equally, as far as cam/trans/timing/exhaust, I would say the 460 would lose 1-2 mpg and make measurably more torque, but the 400 likely would cool easier, run plenty hard, and be a better fit in the chassy.

 

So in the grand scheme, power and mileage are directly related on how much you want to spend. I lf you already have parts for 400 then rock em! I'm on my 6th build so I guess I like wasting money lol but I would expect a mild build to make 14-16 mpg and still be fun. IMO the bracket master II from Lunati would be a good cam for you between 9.5-10:1 as it will build good power through 6k and with a stick you can even degree it a bit to tweak where you want your power to roll in. What exactly do you plan on doing with your build?

 

My pistons are the ones that come in the Federal Mogul rebuild kit, cast, dished pistons. 14 miles per gallon sounds a hell of a lot better than the 8-9 I am getting with my 1973 Super Camper with a 460. I would definitely like to do a mild cam in the future, especially after reading all these comments, but right now I really just need to get a reliable engine in the truck as soon as possible. Thanks for the quick response!

 

Thanks for the response and tons of awesome information. As far as the 400 falling off past 5000 rpm, this engine will be built with reliability and fuel milage in mind first, and overall power second. With that being said, I don't plan on revving past 3500-4000 rpm to often (unless I see a Chevy up ahead of me). I am pretty sure I'll be going the 400M route on this truck.

 

Without .020 coming off heads or more you'll be about 8.5-.8.75:1 with open chamber and dish. Recommend a comp #268h as it's good power and torque 1500-5500 w added compression. Ran one myself

 

Like I said in a previous response to FMJ, 14-16 would be right what I'm aiming for so that news sounds great. I'll definitely look into the camshaft you recommended, I've never put a cam in any of my trucks in the past but my entire family seems to really lean towards Lunati for any make/model. As far as my end goal for the truck, I would like to make it a week day work truck and weekend toy up on the mountain, it is going to be my daily driver when it is finished as long as I can get above 12 or 13 mpg. I am currently rebuilding a '79 Dana 60 kingpin for the front end, 4.10's and a Trac-Lok limited slip, so all of this engine stuff will have to come after I get disk brakes and a '79 steering box under the pick up.

 

https://youtu.be/FERWWwSfVPM

This was comp #268h a few years ago. Went Lunati about a year later (much more duration w negligabne lift increase)

 

I have a Summit K-5200 camshaft that has pretty good specs, similiar to the Comp Cams 268H but a little more exhuast flow. Plus its much cheaper and leaves you a few more dollars for upgrades.
https://www.summitracing.com/parts/sum-k5200