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Pump gas depends on what octane, and how you set your total timing. There are a few other things that can be thown in to help or hinder, but generally up to 9.5:1 will run on regular and anything much higher will require you to run premium. After around 10.5:1 you'll probably have to start mixing in some race fuel.
You can add a water/methanol injection system to an engine, and it will allow a little more compression and not have to up the octane level. I've run one on my 427 Camaro with 10:1 CR and it never pings, even running regular gas. My 327 in the Austin is around 9.5:1 and runs well on regular gas, with no pinging.
Where you set your timing and the rearend gear ratio will have some affect on what you can run also. I usually set mine by ear, then drive it and keep advancing until I hear the ping, or get hard starting issues. Then I back it off until it starts well and doesn't ping. After that I can throw a light on it and see where it's sitting, then record that for future reference.
 

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I may be new to this sight but I have been rebuilding engines for over 25 years.1946 austin is correct.I have a mild street 396 small block chevy using a 150 shot of nos.I have the timming set to 31 degrees total.17 initial,31 total all in by 3500 rpm,approx 730 hp at 6900.It runs on premium 93 octane,but I have aluminum heads.Also it depends on how much overlap your cam has.compression and heat cannot build up when both the intake and exhaust are open.I had a 69 chevelle with iron heads and 10:3:5 to 1 compression and running a roller with 42 degree overlap and it also ran on 93,Just my .02. If I can be of help just ask.Thank You
 

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Hope this helps
used in conjunction with...
Simple Theoretical Power Gains
The nearby formula (see Fig. 1) can be used to calculate the theoretical power gains seen from raising the CR and the chart will save you the effort of calculating those gains. This formula does not take into account the inevitable heat losses, so to allow for this the value of "K" is commonly reduced from 1.4 to 1.3. Using this value we find that, changing nothing else but the compression, output pretty much follows the trend dictated by the formula until about 15:1. From there on up, chemical reactions brought about by the high temperatures and pressures generated absorb heat and only deliver it back to the cycle so late in the expansion event as to serve no useful purpose. Because of this, many learned textbooks will tell you that trying to utilize CR past about 14:1 is a fruitless exercise. But this only applies if no other changes are made to the engine. If, as we shall now see, the side benefits of ultra-high compression are taken advantage of, the situation takes a complete about face.


Dynamic Compression
In the real world, we normally find that theoretical increases are not usually seen in practice because of losses which, to simplify already complex theory, we have ignored. For high-performance engines, part of what has been overlooked by the simple thermal efficiency equation works to produce results far better than theorized. In other words, all the figures in the chart (Fig. 2) are on the low side. For instance, a mildly modified 9:1 350 small-block Chevy would make about 380 lb-ft of torque. Based solely on our thermal efficiency formula, raising the compression to 12:1 should bump that figure to 397 lb-ft. In practice, that number is usually exceeded and the bigger the cam involved, the bigger the gain. To understand how more can be had, let's look at the effect the cam has on the situation. At lower rpm we find that the static CR is never realized because our thermal efficiency formula makes the assumption that the intake valve closes exactly at BDC prior to the start of the compression stroke. This does not happen in reality.


At low rpm, port velocity and pressure waves are too weak to produce any cylinder ramming. Couple this to the fact that even a short cam of some 250 degrees of off-the-seat timing will not close the valve till about 50 degrees after BDC. Fig. 3 shows the typical extent of piston motion back up the bore before the intake closes for three cams. Because of the delayed intake closure we find that during the period the piston moves up the bore from BDC until the valve closes, a significant amount of the induced air is, at low rpm, pushed back into the intake manifold. This means the volumetric efficiency (breathing efficiency) and thus the effective displacement of the cylinder is well below 100 percent. In other words, a 100cc cylinder with a static CR of 10:1 may only trap 75cc of air. This means the dynamic CR, at about 8.5:1, has dropped well below the static CR of 10:1. The bigger the cam, the more this effect comes into play.


An example here will show just how much influence the delayed intake closure has on the dynamic CR. Let us take three different duration cams, all having a 108-degree Lobe Centerline Angle (LCA) and all timed-in at 4 degrees advanced. Along with this let us say our static CR measures out at 12:1. With a 250-degree duration cam, the dynamic CR will be in the mid- to low-11s. For a cam of some 275-degrees duration, the dynamic CR will drop to around the mid 10s. Because of the piston/con rod crank geometry, the piston tends to move much more slowly around BDC. This works in our favor for shorter cams, but the piston quickly moves out of this sweet spot, so once we get much past about 280 degrees we had better have a decent dynamic CR. To give you an idea to what extent this occurs, we find that with our example a 300-degree race cam used with a static CR of 12:1 has a dynamic CR of only about 8.3:1. This snippet of info should bring home the importance of having sufficient CR for a big cam. If it doesn't, then maybe the dyno test results in Fig. 4 will. These are some tests I did with the 2-liter Ford Pinto series of cams I designed for Kent Cams in England some years ago. I realize that very few of you drive Pintos, but the two-liter version of this engine, because of its geometry, reacts about the same as a typical small-block Chevy, so the results do directly apply. From these results we see that with a 9:1 CR, a 260-degree cam produced (the gray curves of Fig. 4) some decent results from low rpm on up. As expected, it started to drop torque by the time 5,000 rpm was being approached, and power peaked out at just shy of 140 hp. This cam was then substituted for a 285-degree cam. On the same 9:1 CR (blue curves of Fig. 4), this bigger cam dropped 38 lb-ft of torque at 1,750 rpm. That amounts to a 32 percent reduction. The extra duration did not start to pay off until 3,750 rpm. From there on up the bigger cam paid off by delivering an increase in peak torque of 4 lb-ft and almost 26 hp. At this point the head was milled to bump the CR to almost 12:1. The results of this move are shown by the green curves in Fig. 4. As you can see this increase in compression recouped almost all the low-speed torque that was lost. On top of this the big cam/high compression combo produced an increase of 15 lb-ft and 33 hp. Stepping that result up to a 350-inch engine, the numbers look more like 40-plus lb-ft extra and 95 hp. So are these numbers realistic? Sure they are. I have seen well over 100hp increase from a 355-inch small-block Chevy with 25 degrees more cam duration, 100 thousandths more lift, and 2 points more compression.


The big increases seen with a combo of more compression and cam are easier to understand when we go back to the basics. If you check the numbers in the chart (Fig. 3) you will see that the biggest gains from a compression increase happen when moving up from a low compression to a higher one. Going from 8:1 to 10:1 is worth a theoretical 3.7 percent, whereas raising the compression the same two points from 11:1 to 13:1 is only worth 2.5 percent. This means the bigger the cam, the more responsive it is to an increase in CR, especially in the lower rpm range.


Read more: http://www.popularhotrodding.com/en..._compression_ratio/viewall.html#ixzz2DAMqyCZX
 

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Here is an explination that may be a little simpler. First, think of the four strokes of the engine. Intake(as the piston drops with the intake valve open) compression or the power stroke(as the piston rises and compresses fresh fuel and air with both valves closed) combustion(Spark plug goes off...boom, gas expands and forces the piston donw on the power stroke) and the exhaust(piston rises and exhast valve opens to let out the burnt gasses).

Now look specifically at the intake event. As I was sayin, the intake valve is open as the piston is dropping. Because the piston is dropping, pressure drops, so fresh fuel and air enter through the open intake valve to equalize the pressure in that cylinder. But, the valve does not close as soon as the piston gets to the bottom of the stroke(bottom dead center). The piston will reach the bottom of the stroke, change direction and start to rise(the compression stroke), but the intake valve is still open as the piston rises. Not the whole time, but depending on the cam choice, the valve could be open for a fairly long time, or just a short time. At this time, you cant actually really compress much cause the valve is still open with the piston rising. You cant start compressing anything to any great degree until the valve closes.

So say you build two motors, and both are 10:1 compression. Lets say one motor has an intake valve closing angle of 60 degrees(after bottom dead center). That means that the intake valve wont close and you cant compress anything for 60 degrees worth of rotation after bottom dead center. Once it closes, the piston will be partially up in the bore, so the mixture actually gets compressed at a rate less than 10:1. In order for the mixture to actually be compressed 10 times to one, the valve would have to close right at bottom dead center, but it doesnt. Lets say your other 10:1 engine has a different cam with an intake valve closing angle of 74 degrees. Thats a bigger number than 60, and it means that it will take even longer for the valve to shut, and once it does, you be able to compress even less than with a 60 degree closing angle, even though the static compression is the same in both motors. That means that even though both motors, have 10:1 static compression, because of the different camshafts, each engine will actually produce different cylinder pressures while running. The one with the bigger camshaft with the 74 degree angle will produce less cylinder pressure. That make sense so far? All in all, there really isnt an exact number for static compression ratio. You could build a 9:1 engine and have tons of detonation problems on premium fuel cause of the cam choice, or you could build an 11:1 motor and be able to run happily on 91 octane.

Anyways, this whole idea that you cant compress anything til the valve closes might sound bad, but its not all bad. At high engine rpm's the air/fuel is traveling so fast and has so much momentum, that even after the piston starts to rise(pressure isnt dropping any more) the air can continue to force itself into the cylinder with its own momentum. This allows for better breathing at high rpms. If you shut the valve too early, you may not allow as much fuel and air in as you could, so you wont produce as much high rpm power. This is just one of the reason that big cams are better for higher rpm, and small cams are better for low rpm. Small cams close the valve quicker and build better cylinder pressure and torque down at low rpm, but the dont allow the motor to breath as well at higher rpm. Bigger cams do just the opposite. Hope that helped some.
 

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Discussion Starter · #10 ·
Thanks for all the info ndimagg! I have a pretty good understanding of cams. If i am correct, the tighter the lobe separation= the more overlap you have which in turn will decrease cyl pressure and also lower compression to an extent. Please correct me if im wrong.
Here is a very useful link.Hope it helps.There is a formula for figuring dynamic compression which is more important than static compression.
http://www.wallaceracing.com/dynamic-cr.php

I had a 383 that had 11.1 static comp. & it would not run on 93 had to use 97.
 

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Casey , I like the comparison of the 9:1 and 10:1 with different cams, and valve timing, that ndimagg posted. I remember back in the 70's I just had to have that big cam in my big block Dart and felt the loss of power. On the hi-way it had good power when I was up in the rpm's but street light to street light it was a bit of a dog.
 

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Very good info ndimagg, especially the cam info and the aluminum head info. Aluminum is more forgiving, and will allow higher compression without the pinging. I've always liked high lift cams with as short a duration as possible. read years ago in "How to Hotrod a Big Block Chevy" that they suggestd this setup for street BBC, and I went that route on my 427 build. I'm running a 284/284 duration with .528" lift, and 112 LSA, with cast iron oval port heads. The cam is a bit lopey, but has excellent street manners without suffering in power. And as I mentioned it runs excellent on regular with my methanol injection system.
 

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Discussion Starter · #13 ·
Casey , I like the comparison of the 9:1 and 10:1 with different cams, and valve timing, that ndimagg posted. I remember back in the 70's I just had to have that big cam in my big block Dart and felt the loss of power. On the hi-way it had good power when I was up in the rpm's but street light to street light it was a bit of a dog.
Yeah Steve it was good info for sure. The heads i have are 64cc and are supposed to yield 9.19:1 comp. According to RHS they can be shaved .052 inches which should raise comp to 10.8:1. Of course my cam will change that a bit but I talked thouroughly with Jerry Cantrell from Schneider cams about my combo b4 deciding on the best cam. Its plenty large for the street, but with the gears and 4 speed it'll work:D
 
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