Rebuild a 454 w/781 heads.... or take it to 496? HP/TQ estimates??

[bowtiepower00]

Their are several problems with building a vehicle to use 91, especially a Tow Rig. Octane Creep would probably be the biggest issue, as engines age and carbon builds up, their octane requirements go up several points.

You are going to be using a small camshaft, your cyl pressure is already going to be pretty high, even with lower compression.

If this was a low-load app, then I'd go 10:1 with aluminum, on a app where you are using a large cam, you maybe able to get away with as much as 11:1+, especially with FI, but if anything goes wrong, you're going to have to run 91 mixed with Octane Boost, or try to find 93 or 94 somewhere. With iron, you'll probably want to stay a half point to point lower.

I'd shoot for 87 octane, the power differences, assuming you are able to actually build a motor to take advantage of the higher octane, are going to be minimal. The engine damage that could result is not.

 

[bigblock72]

The 781 castings w/ mild port work (assuming you can do it yourself) with larger valves, new springs and all necessary machine work will cost about half what a set of aftermarket heads will and will most likely get you to your goals.
The other option is a set of Vortec heads, they flow better, they have a smaller more detonation resistant combustion chamber and will give you about 9:1 compression with flat top pistons. Speaking of compression you need to keep it under 9.25:1 or chances are it will eat itself up especially if tow much at all. I've seen motorhomes destroy a 454 at 9:1 even running 90 plus octane fuel. Please stay conservative for a long trouble free life especially if you run the 781s and domed pistons, those huge combustion chambers invite detonation like you wouldn't believe. EFI sure helps here as well, in fact you'll be happier spending the money on a good fuel injection setup (try and get a knock sensor)then on a set of expensive aftermarket heads.
Although the torque and HP peaks seem a little high the torque is still over 400 ft lbs from off idle to almost the shift point. This is a pretty ideal budget tow motor that won't tear itself apart.

 

[Greg72]

Thanks to everyone for the advice and ideas!!

I wish I could say that you've made my decision easier, but in fact now it's even harder!!

Without some hard HP/TQ numbers it's really hard to evaluate the cost/benefit of some of the changes that have been suggested here. The three combos I want to compare most are:

Roller 454/468 with Merlin Heads (Wes!)
Roller 496 with "781" heads with 2.19/188 valves and minimal port work
Roller 454/468 with AFR Heads (BTP00)

Regardless of the combination, I will do the conversion to a roller cam, and I will use the Holley Commander TBI setup and a decent set of headers.

A copy of DeskTop Dyno 2000 mysteriously appeared on my computer over the weekend , so now I'm trying to plug in some data to simulate these combos to see not only the PEAK numbers they generate, but how broad and flat the power curves are as well....

It sounds like I'm going to have to be very careful about compression ratio, and pick a nice conservative number. This engine will HAVE to be able to tow reliably, so I can't sacrifice that for a sexier peak power number.

 

[bowtiepower00]

You're not going to have to be nearly as careful with aluminum heads. The Merlin's are good heads, but spendy for what you get. Look into Canfield or Brodix for aluminum heads in the $1400-1500 range, assembeled. I would check out Mike at CFMPerformance.com, he does some really good headwork, carries lots of brands, and has good prices. He will do headwork and intake porting too. Very knowledgable and affordable. He will definately have some good input on your build. AFR makes great heads, but they too have stiff pricing, especially on the BBC stuff. You can get CNC ported stuff for the price of their as-cast.

I would consider Aluminum heads a must have- unless you plan on putting your money into the bottom end now and adding them later, but it sounds like you don't plan on doing that.

Seeing as how you plan on doing little port work to the 781's, I'd ditch em. They need a bunch of work to even get close to aftermarket stuff.

Keep your intake runner small, I think Merlin makes some 265cc ovals, not sure if anyone else makes runners that small. As far as heads go, flow numbers don't lie, period. You're not going to need much flow above .600 for a FI engine with a smallish camshaft, but I wouldn't be afraid of throwing some 1.8 rockers on there to get the torque flowing.

For what you're doing, you may want to look into getting a custom grind cam. Most of the FI friendly cams are based on 454's, you're going to want a bit more duration with all those cubes, while still retaining decent LSA. I would also go out of your way to find a grind that's easy on your valvetrain.

Play with DD2000. It is fairly accurate, and is great at showing the differences between combinations, cams, etc. Get the best flow numbers you can, enter in all of the perimeters you can, and get some ideas of how the different combos are going to run.

Huge Torque numbers are great, and are going to be there, but this is a gasser- don't be afraid to use those RPM. You're probably going to want to shoot for max power and torque from 2500-5500 and gear your rig accordingly. Who cares if you're pulling grades at 4K RPM while towing? You're going to be making enough power to do it accelerating.

If you're set on the 781's, check out RATFINK on the 454SS board, he does a great job on heads, he can even get the Peanut Ports to flow well. I believe he is in the south, so shipping may be expensive, though. Lewis Racing Engines up in Washington, I believe, does good motor work too, but I'm not sure how much head work he does.