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Old 05-06-2009, 07:37 PM   #1
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Default Highest Compression for pump gas

I Bought a 408W stroker motor and the compression is too high on it. It has 12.2.1 comp and I need to lower it but before I do, I was wondering what is a safe number for 91 Octane? The motor has 225cc aluminum AFR heads on it

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Old 05-06-2009, 07:41 PM   #2
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Depends on a whole mess of things. Cam profile, vehicle weight, gearing, head material, cooling system effectiveness etc.

Generally 9.0-9.5:1 is a good range, and with the right cam can still make tons of power safely on pump gas, provided everything else is up to par. Even as high as 10 or 11:1 can be run, but it often requires extraneous measures to be taken that sometimes cost power.
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Old 05-06-2009, 08:12 PM   #3
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I run a 10.5:1 ratio on premium fuel, only had a problem once... I guess I got bad gas, whe I cut it off it dieseled on me for a few seconds. Pissed me off lol. But I didn't change anything, no timing adjustments, etc, and it hasn't done it again. So I blame it on the gas. But generally, with the Aluminum heads you will be fine at 10 or slightly above. Anything above 10.5 pretty much requires race fuel.
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Old 05-06-2009, 09:38 PM   #4
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Aluminum heads dissipate heat faster and generally allow you to run another point of compression than cast iron heads would. Like Tyler said, 10:1-10.5:1 is about the limit for 91 octane with aluminum heads.
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Old 05-06-2009, 09:50 PM   #5
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Pay attention to what 67mustang302 just said: static compression is only part of the equation.

What you're really interested in is dynamic compression, AKA cold cranking pressure. That's a function of static compression, rod length, altitude, intake valves closing point, and a couple other things. And that just gets you close. Some cams may have a fairly low cranking pressure, but will really build cylinder pressure as the rpm's rise. Cylinder pressure is what makes power.

There are a few calculaters on the web that you can find with a google search. RB Racing has a good one. It's a Harley site, but it works the same on all engines.

Generally speaking, for pump gas you want cylinders pressures below 190 or so. If you have a radical cam with a late valve closing point, CCP should be low so you don't over pressure at 5,000 rpm's.
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Old 05-06-2009, 11:28 PM   #6
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Run e85 its got the octane you would need but you need a special carb to use it and you get less mileage then gas.
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Old 05-07-2009, 12:21 AM   #7
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Yeah, what bob said.

Even doing something as simple as changing cam position to a few degrees advanced can be enough to cause an engine to start detonating. Actual cylinder compression or TRUE compression is a function of static compression, combined with cam profile and valve event timing, head and intake flow, as well as exhaust flow etc. Basically your true compression will be your volumetric efficiency X static compression. It's not totally exact, but pretty dang close.

So if you had a 10:1 engine with 80% VE then you have around 8:1 actual compression, which is decent. A 10:1 engine with 90% VE would be 9:1 actual, which would really be pushing the limits of premium pump fuel, and would likely require a bit more fuel or a bit less timing than the engine needs for best power(or both). Let's say you have a 9:1 engine with 95% VE, then you'd have around 8.5:1 actual, which is doable on pump fuel.

Keep in mind that an engine will always make better power with more timing rather than more compression. The purpose of the compression cycle in the engine is to compress the air/fuel charge, which does 2 major things. First, it brings all the air and fuel molecules into close proximity to one another, so when combustion starts it has less distance to travel and with neighboring molecules closer together it can transfer energy to adjacent molecules more rapidly(ie a denser mixture will burn more quickly and completely). The second thing it does is add heat to the air/fuel charge....which is important for 2 reasons itself. 1 it brings the fuel up close to it's ignition temperature so the spark plug can ignite it, and secondly it adds the heat energy necessary to vaporize the fuel(since liquid fuel can't burn). Compression also helps with air/fuel mixing, but that's another matter entirely.

Now, the whole purpose of advancing ignition timing is because fuel takes time to burn, and you ideally want the burn to start at such a time in relation to the piston that you achieve the highest pressure in the cylinder when the piston is at a point where it can transmit best power to the crankshaft. But, as fuel is burning in the early phase of the combustion cycle the piston is still coming up and compressing the mixture(albeit very little, as it's on the last legs of the upward stroke), and also the combustion wave that is traveling outward away from the spark plug is compressing the air/fuel charge ahead of it. If the air/fuel charge that is yet to be burned is too hot as it's being compressed(and heated even more) by the combustion process, it will reach it's flash point and auto-ignite, sending it's own combustion wave out(detonation), when these 2(or sometimes more) waves collide you get the characteristic knocking sound of detonation. It can kill engines quickly.

Detonation can be limited by several things, higher octane rated fuel(higher resistance to detonation, but it burns more slowly), lower compression(less compression heat, which if too low will not provide enough heat energy for efficient combustion, and power is lost), more fuel(richer, causing the mixture to absorb more heat energy on compression as the fuel tries to reach it's vaporization point, and it burns more slowly making less cylinder pressure, and power is lost), or less timing(you start combustion later, resulting in the piston already traveling away from the chamber before the combustion process has had a chance to complete effectively, and the expanding volume reduces temperature in the chamber, but less cylinder pressure is created and power is lost). Things like more efficient cooling systems and aluminum heads can help, but they're not a cure for an overly compressed cylinder with not enough fuel octane.

If you build a high compression engine, and the cam and induction match properly, you end up with higher cylinder pressures during compression and more heat in the charge before ignition(relative to a lower compression engine). If you have to reduce timing in order to control detonation because the charge is simply too hot(not enough fuel octane to handle the heat), then you're reducing overall cylinder pressure and power output as a band-aide to control detonation.....the engine makes less power. In an engine with lower compression that doesn't have as much heat in the charge(the octane can handle the heat) then you can run as much lead as is needed to make the highest cylinder pressures....the engine makes the most power.

You have to remember though, that more compression does not always mean more power. The purpose of the compression cycle is not to generate power by having more compression, it's to get the air/fuel charge as hot as is NECESSARY to achieve the most rapid and most complete combustion. Any more compression and heat beyond that is not only unnecessary, but adds unwanted heat that can result in detonation(or in severe cases, pre-ignition). Ideally, compression heats the air/fuel charge so that the fuel is totally vaporized, and close enough to it's ignition temperature that the spark plug can easily ignite it, but still cool enough that the extra compression heat from the combustion process itself won't overheat the fuel ahead of the combustion wave and cause detonation. Again, any more compression beyond what is necessary for ideal combustion is most likely going to cause problems, and it's un-needed.

Most high compression engines on the street that run premium pump fuel(91/93) get away with it by either running less timing, more fuel, or have a cam profile that helps to reduce overall cylinder compression, but that typically results in a cam that makes the engine want to run effectively at higher rpm than you normally see on the street. Cam's aren't supposed to be used as a cure for too much compression. Also, many larger cams NEED more static compression, which is why manufacturers will give a recommended compression range for their cams. Camshafts that cause the engine to operate better at higher rpm generally have a tendency to lower overall cylinder compression at lower rpm, but it's offset at higher rpm by increased VE(to a degree). They need the extra compression to be able to start the engine and get it to simply run right at lower rpm.
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Old 05-07-2009, 09:23 AM   #8
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I have understood the relationship of static compression to dynamic compression for a while, but have never seen a calculator that gives you pressure and a dynamic CR. Great post!

So in your experience, at what pressures or dynamic CR do you need 89octane, 93octane, 100octane, etc. assuming performance timing?
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Old 05-07-2009, 10:54 AM   #9
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Originally Posted by andrewmp6 View Post
Run e85 its got the octane you would need but you need a special carb to use it and you get less mileage then gas.
This I can help with, and if you compare E85 to 105-110 octane race gas, the price is more than enough to justify a bit less mileage. Although you can get some or all of the mileage back if you decide to go dedicated E100/E85. Also on ethanol you will make more power, run much cleaner for emissions, longer intervals between oil changes, no carbon build up thus no appreciable wear in the engine, and it will run cooler to the point of possibly having frost on the intake on a hot day.

If you have an engine with more than 11:1 its worth looking into and modifying a carb is much cheaper than swapping heads or pistons. Some tunes aren't as good as others, but even the poor tunes pick up power. Most vehicles tuned for it see about a 50hp increase by running ethanol over gas, and that is over race gas or pump gas. If you race you will be more consistent because heat doesnt affect performance as much, and up to a certain point more heat in the engine makes substantially more power.

Currently I have a 98 Formula with its LS1 on ethanol, and a 70 GTO burning it in a 455. The only change I made to the GTO was modifying the carb and it broke the transmission. The Cougar in my sig is going ethanol as well, I like the power and ability to keep the engine cool even on the hottest days.

If you can get E85 near you, its a better idea than swapping heads or pistons to lower compression.
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Old 05-07-2009, 12:41 PM   #10
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I Have 10.8:1 static compression and the car runs just fine. I use 93 all the time but on the occasions when I have used 91 I have not had any issues. I do have a lot of overlap in my cam that bleeds off a bit though
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