I thought I had worked out all the details on my upcoming 347 build, but I started to order the rotating assembly kit and now I'm second guessing myself on which pistons to order. This will be an injected GT40P iron-headed motor for truck duty with a 5.0L HO roller cam. I'm more concerned about pulling power and fuel economy than 1/4 mile times. I can get the pistons with a couple different reverse dome volumes and I had planned on going with 20.1 cc. With the other parts I'm using, that would give me a static compression ratio of 9.030:1. Now I'm wondering if I ought to go with a 14.2 cc piston for 9.6:1 static compression. Most 5.0L truck computers use a knock sensor and can pull some timing out if it starts pinging but I'm using a 5.8L computer and they don't employ a knock sensor. I don't want to leave power and economy on the table by going with the lower compression pistons but I don't want to have to run premium fuel all the time to keep it from pinging either. Any thoughts on whether 9.6:1 will be a little much for 87 or 89 octane pump gas with iron heads? I've driven quite a few 5.0L and 5.8L trucks and never remember any of them pinging, but I think they all run pretty close to 8.5 or 8.7:1. Just not sure how far I can push it. If it matters, I am having the block milled for zero deck clearance and I'll be using .039" head gaskets, so it will have good quench.

 

FWIW the NLOC guys hate the HO cam they say it doesn't produce any low end TQ in the Lightning 5.8, I find that a bit hard to believe since my 5.8 produced such awesome TQ but then I had a different cam in it(Crane 444232) and even though it's close to a HO cam it doesn't take much difference in a grind to make a big difference in power.

As for the target CR ratio I'd suggest you keep it at around 9:1, you can always dial in more ignition advance to optimize TQ output.

 

Since your building the engine you could do a few things that may help it run OK at 9.6:1. Polish the combustion chambers on the heads. Have the pistons tops coated. These will reduce "hot spots" and the engine to ping.

You can do this yourself if you have the right equiptment available.

A blast cabinet with the correct media and an old electric oven.

Tech Line Coatings | Automotive ( Hi-Performance ) Coatings
Here is a link to a company with products that I used when I built a Ford Turbo 2.3L a few years ago. Some of their products are used by the racing industry and the OEM manufacturers. They do work.

 

I appreciate the responses. After giving them some consideration and thinking it over some more, I think I'm going to stick with plan A and shoot for 9:1. I think I could get away with 9.6:1 if it was a cruiser and didn't have to work very hard at low RPM, but I'm really concerned about what it would do towing 10K pounds uphill - even with polished chambers and coated pistons. I've been to the land of broken rings and holed pistons before and I don't really want to go back. If it were getting aluminum heads I'd have gone with 9.6:1 without even thinking about it, but they would be overkill for this build. I just need a spunky workhorse, not a race car. I'll have higher compression than either a stock 5.0L or 5.8L even at just 9:1.

Paul,

Most of what I've read from Lightning guys about the 5.0L HO cam has been pretty positive. It seems like most of them generally feel like it's a good upgrade from the stock Lightning flat tappet cam. I did cruise over to NLOC's site and dig around a little. I found quite a few threads from happy guys with HO cams and several others looking for them over there, too. I did find some posts by two or three guys that absolutely hate the HO cam and gave everybody running one a rough time, but they were obviously mostly concerned with going fast. I've had the thing sitting around here for awhile and this seemed like a good use for it. It's going in 4* advanced and I've got a set of 1.7:1 '93 Cobra roller rockers to use with it, too. I'm trying to burn up some of my orphan spare parts in this build. It might not be optimum, but I think it will be pretty good for what I'm trying to do. Just out of curiosity, if I didn't already have this cam, what would you recommend considering my intended usage and the other pieces? It will still be a couple months before I'll be ready for final assembly and you just never know what I might run across a deal on in the mean time. I have had good luck with and prefer Comp Cams.