I plan on running pump fuel with no additives. In a 390, with a 268h cam, what compression ratio can I get away with in my truck at higher elevations? I live at 6000 ft. and play at 9000 ft. during the hunting season. A compression ratio north of 9:1 - I am hoping.

I have C6AE-R heads with hardened seats already. Now I am looking to buy the pistons. Anyone with a similar builds have any suggestions?

 

Please define pump fuel. Are you meaning the 87 octane stuff or the 91 stuff or the in between stuff. Hell, I know a place where you can buy several different levels of high octane racing gas from a normal gas pumps and at the local aerodrome I can get 100LL AV gas from a pump.

 

You can run 10 to 1 or a bit better with careful tuning. A useful addition would be a water injection system which is commercially available. Check that forum for more information. It's inexpensive, easy to install and a lot of people running turbos swear by them.

 

I was planning on using 91 octane

 

Post the specs on the cam. I'll compute a dynamic compression ratio, and compare that to what some other people are getting away with. The big question is, do you ever plan on driving it at a lower altitude? If so, then you either need to build it to run pump gas at a lower altitude, or use an octane booster or race gas when you do. Which is it?

When I was laying out the details of my engine I spent a ton of time gathering data on cams and compression ratios at altitude. As a very general rule of thumb, assuming the same fuel, you can get away with 1.5 points of compression more at 6000' than you can at sea level. After running all the calculations I could, I determined that my engine would take 12.5:1 on 91 octane pump gas without having to back my timing down. When I ordered my pistons, Arias got them wrong, and they came out at 12.0:1. With a little testing, I found out I could run 89 octane with no problems. I commonly ran 91 octane at 44 degrees advance at the track with no issues. My engine was very different than yours, but my point is with a little calculation you can estimate pretty close.

-Scouder

 

The only problem I see is if you build the engine for 6000 ft and have to bring it down to sea level for whatever reason , you could be in real trouble with detonation.

 

Just back off the timing and fatten it up at sea level...

 

Yeah, but if he jacks up the compression to the max to make up for the thin air up there and he is already using 91 octane, backing the timing off and fattening it up just won't help. You can't fix to much compression with tuning adjustments.

 

ok guy's I'm at sea level and want to go to the hills ,should I be keeping the compression under 10.5 or?? for a daily driver? the points you bring up here have me wondering about my build i am hopeing to start soon

 

Going up is not an issue as the air pressure drops as you go up. But coming down can cause problems because the air pressure rises and hence the max compression numbers actually rise faster than ambient air pressure by a factor of whatever your compression ratio is.

 

The best thing to do is build the engine for the lowest altitude it will see. Let's be realistic for a moment. Let's say you have a 300hp, 9:1 compression engine at sea level. You add 1.5 points of compression, taking it to 10.5:1. Your net gain from that change will be somewhere around 18hp. Now take your 300hp engine up the hill to 6600 feet, which is where I am. Your net loss is about 70hp, so you are now at 230ish. Add back 1.5 points of compression, which would be about right for a daily driver, and you get back about 14hp. So now you are at 244. Are we really willing to sacrifice driving at lower altitudes for 14hp?

Raising compression, advancing timing, and using a cam with a narrower lobe separation anlge are all good places to start when you want to gain back some of the power lost to altitude, but they aren't going to be that significant. You simply don't have any air pressure, so you have less oxygen, so you have less fuel, so you have less power. Simple and unavoidable. The only way to overcome the losses from altitude is to force air into the engine or add nitrous. Both add oxygen to the cylinder, allowing more fuel to be burned, therefore more power.

All that being said, I built my engine for my altitude because I knew I would never take it down the hill. I have other vehicles for that. After all the work of researching the cam and compression for my engine, and seeing that altitude still wins the battle, I decided I will never build another normally aspirated engine up here. They will all have some kind of forced induction.

-Scouder

 

Is that all the HP gain is going to be going from 9:1 to 10.5 at altitude? I had no idea. I guess I run 9:1 then.

 

ok so same cam and elevation. stock compression.

a F100 4x4 with dual exhausts and glass packs dumped behind the passenger side door. No headers. Given that I have piles o intake parts parts. Is is worth while to add a 4 bbl cast OR edelbrock performer to upgrade from a 2bbl stock on a 360?

The reason I ask is because I am selling parts to build up another truck. Without the headers, how much air does the a daily driver need at elevation?