So much crap in these threads. The direct and real answer to your question is,
If you go up one point in compression you will gain about 4% horsepower. So, if you are now making 250 HP you'll go all the way to a whopping 260 HP. You'll have to decide if that is worth it.
If all you do is bump compression one full point, you will see somewhere between 4-5%, depending on actual compression, before changing anything, if you do absolutely nothing else.
And no, if you paid any attention to what I said, none of it is crap.
I've tested gas stations for water content. Doesn't take a lab. Fill a can, pour some in a mason jar, seal it and shake the crap out of it, then watch it. You can see water separate, if the ground tanks are faulty.
I've seen the numbers in MPG increase, running mid grade over min. This is likely due to octane rating increase, allowing a better tune, combined with more consistent results coming from the stations. Water content doesn't change within the same gas station, just use a known, good gas station (test it yourself)
There is a difference between the capabilities of an engine that runs 8:1, against 9:1.
It just takes a minute to think about the change and how it will change the combustion.
Assuming you've done everything you can to the engine, at 8:1. Timing advanced as much as it can be, air to fuel ratios are sound, throttle positions are correct, etc... Now think about how that fuel is going to react under load, under 9:1 ratio, with absolutely no other changes to timing, fuel, etc.
If you've got the engine tuned so that 8:1 is pulling as much out of each cycle as it can get, there is no room for more advance in the combustion. Anything you do to that combustion is going to cause pre-detonation.
Can you tune a car to run on regular, at 9:1? Yes. It requires LESS timing, which will hurt performance and defeat the purpose of moving up in cylinder PSI.
When you change CR, you change how the engine deals with fuel. This changes the entire tune. Higher cylinder pressure will respond on a higher percentage level to all performance tuning.
No all have done is run numbers on a compression program (summit). My 273 had a blown leaking head gasket. So I had to pull the heads anyway. I'm going to re-ring it and bearings also.
Since the heads are off, I'm going to use the thin head gaskets (why not)?
Since my current heads are open chamber with non hardened valve seats. I thought about putting on a set of 302s to raise the compression, and get the hardened seats.
I'm just trying to decide if the money spent on replacing the heads, will make enough of a difference to justify the cost.
Its just a driver (no track usage). more interested in improving low end and fuel mileage.
Do you think this is a wasted effort ?
It's worth it, if not just for the hardened seats. They are induction hardened, so don't look for ductile iron seats on the exhaust. The entire combustion chamber is already there. The lack of pitting on the seats will tell you, on a used set that is in good shape.
Any increase in cylinder psi/ drop in cc is going to help the engine make more power, per cycle, increasing your potential fuel economy, as long as you tune it for better fuel, it will return the favor.
Not to hijack, but was just kicking this around in my head on my .030" over 360 build. See no point in starting another thread to discuss basically the same question.
Wondering if I could get away with a calculated 9.15:1 static, 8:1 dynamic on 87 octane.
By swapping gaskets around I can go as low as 8.75:1 static / 7.63:1 dynamic. {.055"/4.180" FelPro} My combo is a '90 block with a small roller, 35* abdc at the recommended +3, 308 open chambers measured at 72cc's. Piston's a SpeedPro flat top w/two reliefs, .026" down.
Calculated cranking pressure at 9.15:1 is 160 psi at 3000ft, but I didn't play with the IC point to see what retarding the cam would do.
Running an AirGap and fresh air setup, should help lower tract temps a little, but the biggest drawback I have is the factory TBI's curve isn't adjustable, has no knock sensor, requires a 195 t-stat, and the aforementioned high temps in this area.
Motor's going in a 4.10 geared 5500lb 4x4 truck, sucks gas to the tune of about 11mpg, that hurts at the pump on 1000 mile trips. Trying to squeeze all the efficiency I can out of what I have. Hell, even 12mpg would help.
Pretty sure the answer is no to 9:1/87 octane, and a 1/4 point likely won't make much difference one way or the other, but thought I'd get you guys' opinions.
If your dynamic is down at 8:1, 87 being min-rating octane (I would assume) in your area, I think you are asking for it with that vehicle.
I thought I wanted min octane (85 up here), until I started toying with mid-grade and higher fuels and saw results. Even if it's a wash in dollar per mile, you'll find more consistent results at stations. In my experience, less water in the tanks, too.
If your TBI corrects from o2, you might actually see more out of mid-grade than my carbed engine would.
I'd play with some mid-grade on that engine to see what it does and stick with something that will work on 87 with the build.
It would be nice to hear from someone else running TBI. On the Toyota truck with port injection I saw results in, it went from 20-22mpg, by just changing fuels, on 8.7:1 CR.
I have exactly the same question. Cast iron heads, no quench.
Min octane sucks for everything, if you're willing to tune. I promise.
There are guys fetching awesome MPG in big block cars on the HotRod Pro Tour, with O/D, running premium, because they dial their engines and set the car up right.
