PhantomFE3

the highest octane pump gas we have around here is 92, how much compression can i run while using 92 pump gas? i was hoping for 10.x:1 but i really have no idea, any input is appreciated

Air_Adam

10.5:1 or so is generally the safe limit, but it really depends on how big your engine is, what heads/pistons you use, and most importantly, what cam you use.

Parrydise7

iTrader: (1)

Have a 1969 Chevelle with a 10.25 to 1 c.r. that won't run without major pinging on anything other than 100 octane racing gas. Probably has some serious carbon build-up which may drive the c.r. up even further.

Also, I've been told you can run a higher c.r. with aluminum heads, compared to iron, because of the heat rejection characteristics of aluminum.

As Air noted, there are some variables.

F-BIRD'88

Without going into a lot of what if's and so, if you want reliable street power performance on 92 octane, build your motor with just under 10:1 compression.
cast iron heads- carbed motor.
Build the motor with proper piston quench clearance (.040")

9.8:1 is perfered. about 10.2 with aluminum heads

You'll make more usable, reliable power if you don't get crazy with compression and optimize the total engine airflow, ignition timing and AFR for max output as opposed to too much compression ratio requireing retarded timing ,over camming and over rich afr's to keep it out of detonation.

Night rider327

iTrader: (13)

There's alot more too it than a simple #.

I have a iron headed (1965 327 462 heads) 355 10.18:1 compression, 217-219 psi cyl. pressure, and runs pretty as she pleases on 93 octane.

I have to limit total timing to 34*, and all in at 3000 rpm, adjustable vac. advance can


When you look at compression and fuel needs, you need to think more about dynamic compression, not static compression.

static compression (SCR)... Is based on vol. like head chamber cc, head gasket vol, bore, stroke, etc

dynamic compression (DCR).. Is above plus factors in cam size.

A bigger cam will bleed off more cyl. pressure, therefor makes dynamic compression lower, and lets you run more static compression.

To understand how cams and compression works together please read Pat Kelly's write up. Read it all, and you may have to re read it 4 or 5 times to fully understand

http://members.uia.net/pkelley2/DynamicCR.html


Also other factors comes into play... Iron vs. alum heads. Alum heads you can and need to go up close to or 1 full point in SCR.

head chamber shape.. Old style bowl chambers like mine cant take as much compression as the heart or kindy shaped chabers of alot of aftermarket and vortec heads. The better chamber will let you run more compression w/o detonation

Quench height (the height from piston at TDC **deck heiht**, to bottom of heads, counts the thickness of the gaskets plays a BIG role.. You need and want quench height in the .035"-.045" range. Going bigger (thibker head gaskets) will drop compression, but could cause even more detonation, going smaller could cause a piston to kiss a head/valve

Temps.. Hotter intake air, water, and oil temps will cause more detonation. Building a true sealed cold air set up, keeping water temps around 180*, and oil temps around 190* will let you run more compression

Total timing.. Too much will cause any engine to ping, too little with drop power off like a brick

Timing curve.. A curve that comes in too fast will cause detonation

Fueling... Running lean with cause detonation

devwolf

This has been an ongoing question for quite some time, so I'm gonna throw my mechanical engineer's 2 cents here here to help you guys better understand why some engine run on 12:1 on 93 octane where others will knock and kill themselves. Some say aluminium heads can do it, some say the right cam can do it, quench height, etc.. There are all pieces of the bigger puzzle. Its more complex than "polished aluminium heads can do it".

There are 2 different types of compression ratio numbers. Some of you will already know this. There is static (the one most people call out as their ratio) and dynamic (the real world compression ratio while the engine is moving based on cam specs). A big cam will have overlap and big durations, so the valves are open while the piston is moving; more-so than a stock cam. This allows scavenging to take affect within the cylinder, and a good exhaust system helps this process alone. When you are on pump gas - say 91 or 93 octane, the max safe dynamic compression ratio on iron (or chromoly) heads is about 8:1. Slightly below 8:1, say 7.75:1 would be the safer bet, incase it ever ran hot at the track or overheated on the road before you saw the temp gauge getting so high. Heat makes it more prone to detonation. Now, why did I say Iron and not Aluminium? Because aluminium is a poor cunductor of heat, and this is why its said that aluminium heads are easier to cool. This cooler thermal dynamic is why an Aluminium head can run half a point higher on pump gas and remain safe. Theroetically if you can keep Iron heads cool with a high compression cooling system (which is extremely expensive and has a lot of power leaching from the turning force required for that compressor), you could run that extra half point of compression and be ok as long as it stays cool. Some race engines use iron heads with this ratio because they are much more durable (aluminum is brittle, as metals go) but in order to accomplish this befor edetonation, and keep leaching as low as possible, they use ice water in the cooling system. You might have heard of Leroy, the twin turbo Corvette "gocart" from Tampa, Florida. This is why "Cletus" dumps ice in his cooler reservoir on that car (his turbos bring his dynamic compression much higher in that engine build than is safe on pump gas). This is great for 1 or 2 1/4 mile passes, but it heats up and melts the ice off quickly, so its not practical on the street.

Sometimes you hear someone say "polishing the heads can allow higher compression, or small quench. This is true but for a couple reasons that people often overlook. When you mirror polish a raw casting or CNC chamber, you are reducing the surface area by removing the little imperfections. This does 2 things. First, the reduced surface area allows less heat to transfer into the head at any given point. Second, the smoother sruface allows less hot spots and "pin points" during a power stroke which reduces the chances of a detonation at high ratios. A smoother head is less likely to provide a place for the gas to ignite ahead of timing. The quernch area size's contribution is still debatable for reasons we arent going to get into here, but generally, smaller quenches tend to be favored. I bevel the sharp edges of the block and head just a tiny bit with high grit paper and a polish. Getting rid of the sharp edge allows a more diffuse thermal gradient in those areas. Do not actually bevel it, just round the sharp edge, so to speak. Though beveling in some other areas can reduce piston skirt wear, but thats another post entirely. A thinner gasket and taller deck (to compensate for the thinner gasket) also helps prevent detonation. It also allows you a saftey cussion should the engine need deck machining after a failure - you can always remove more metal, but you cant put it back like you do in welding, lol.

When you hear 10.5:1 or 12:1, people are talking about static CR. This means, in a perfect world, if the valves opened and closed in 0.0000001 (you get the idea) seconds,at perfect BDC and TDC, based on engine geometry, thats what it would compress at. But this engine does not exist. An instant opand and close would have such steep lobes (basically verticle raise and drop, with a flat section along the duration, on the cam) that it would rip itself apart. So while that number is a good number to know, especially when using forced induction, its not realistic and gives very little info about the engine beyond geometry. In the racing and engine building industry, we focus on the dynamic ratio. The static number is more about marketing and vanity than actual useful information. Dynamic is the more accurate number that takes the cam and valve system into account. It calculates compression based on open and close durations, IVC, etc.. This is the number you need to focus on. You can find many dalculators online to help you determine this number while you decide on parts and machining specs for your build. Remember dynamic means "changing". A dynamic compression will change as RPMs go up. Think of dynamic compression is a measure of the engine at 1RPM. As the RPMs increase, the pressure rises. Though its entirely possible to really mess up and get one that drops with RPM if its badly timed with a completely incorrect cam. It raises because of pressure waves and scavenging with increased piston velocity. Think about a rubber band rebounding. Air pressure has this property too. In atmosphere (under pressure), think of it as an elastic substance that always wants to be at an equal pressure. When a valve opens after TDC, the cylinder has a negative pressure for a split second. Once it opens, the pressure tries to equalize, and since air has mass to it, if the valve is open just past BDC, the velocity will cause it to rebound (so to speak) and suck more into the cylinder than the actual volume would a 1RPM (making sense now?). This is what people mean when they say 'exhaust scavenging', its those pressure waves from a moving mass that help pull air through. Just as exhaust scavenges, so does a cylinder, and the intake manifold design can help the cylinder do its part by properly directing these pressure waves (thats why intakes can give 25hp just from bolting something like an LS6 verion onto an LS1, it's been refined over the years). So, if you have a dynamic CR of, say 7.5:1 at 1RPM, then at 8000RPM, this could be 8:1. It just depends on the engine build.

To be more clear here, the compression ratio itself doesn't nessisarily go up with RPM (thats a way of visualizing its affects, but not really based on math, so to speak), its technically the PSI that goes up from intake inertia. That extra pressure results in what could be seen as a higher compression at rpm, but thats probably a bad name for that process. A ratio is not a correct way of stating inertial PSI increases, and that number is very speciffic to each engine (its chaotic at high RPM), so theres no "golden chart" to show this stuff. Its based on a lot of assumptions. Ultimatly, its the PSI that increases the potential for detonation, not the compression ratio itself. Its the peak PSI at any given engine speed, theres a red zone for detonation based on temp, surface and materials. Dynamic compression simply means the more realistic compression on an engine with no real inertia, with valve timing events added in.

So back to the question. Can you run 12:1 static compression on 93 octane with iron heads? The short answer is yes, you can. But the cam is very important here. If you crank the CR that high up, you need a big cam. And a big cam usually means a high power curve in the mid and top end, but pretty bad at sub 2000RPM ranges. A big cam ensures a low dynamic compression, and dynamic compression is what makes a knock, not the static pressure. Again, 8:1 or even 8.23:1 dynamic is ok on 93 octane with an engine a proper temp with iron heads. And 8.5:1, or slightly higher, dynamic is ok on cooler running aluminium heads. At a real world dynamic CR of 9.5:1 or 10.5:1, you have a big problem with anything but race fuel, even at idle. Engineers tend to laugh a little in their heads when someone says their LS1 is running pump gas at a stock 10.5:1 ration. The engine isnt actually running at 10.5:1, that just something auto advertisements started doing and now we cant go back because people will see "8.5 dynamic" and think "what a garbage engine".

My daily driver is 12:1 static, but its dynamic with my rather oversized cam is 7.52:1. Its got trick Flow chromoly heads with mirror polishing, and -3cc flat top pistons. I run it on 93 octane, and it can even do 91 octane on a cold day. And no matter how hard I beat on it, it doesnt have any cylinder knock.Theoretically I could even mill my deck down even farther, to 12.25 or 12.5 compression, and still be "ok". Dont let the static pressure number scare you, it means almost nothing in the real world. Now, keep this in mind when you go for a 12:1 engine... To do this, you need big cams with high IVCs, lifts, overlaps and durations to keep the dyanmic pressure at or below 8:1 or 8.5:1. When you do this, you are almost always making an engine thats an absolute dog at low RPMs, which can be annoying on the street, and makes for pretty bad gas miliage down there because the effciency is so low as low RPM. Its great for a drag strip engine. But for the street, you are usually better off going 11:1 or 10.5:1 in static, and getting a smaller cam to make for the 8:1 or 8.5:1 dynamic, because that smaller cam gives you a wider power band and acts a lot better on the road where you are starting and stopping all the time. Its better at a stop light becuae you dont have to launch at 3500RPM just to get her moving (which can make for better traction by the way, and spinning tires = lost E.T.).

So there you have it. A 12:1 static compression car is not some 'unicorn' engine, where only a few exist. Most supercars these days are doing 13:1 and some even higher than that. Its all about how you build her and what parts you put in. This does not mean you can go to the garage this weekend, shave the deck height down to 0.015'', and drive off without being killing it. You still need to do some head polishing work and get a big cam (remember you sacrifice streetability when going with a very big cam), and make sure you have proper gaskets. It also helps to have flat or hemi pistons without valve millings in the top, if you are going slightly above the "safe" dynamic ratios mentioned above, because these are just more sharp edges and hot spots that can cause detonations.

If you are one of the people who say 12:1 is suicide on pump gas. I urge you to please, please, do your homework before telling people that. Make sure you understand what you are talking about and not just repeating things you hear. Thats how these urban myths start, because people refuse to do their own studying. Now you know the in's and out's of why its not just possible, but common, to run these ratios. Especially in motor sports. Please teach others next time you hear someone say its not possible, because chances are, nobody has taught them about dynamic compressions. Help us get rid of this internet / urban legend once and for all. The more we understand engines, the more horsepower we can make.

*Disclaimer* - Even with dynamic compression being more the more meaningful and accurate number, there are still many, many more factors here like inerital filling (the pressure wave, 'scavenging' points discussed above). Dynamic compressions really means nothing without those numbers and other things like efficiency curves per RPM. Theres a lot more math involved in this. Which is why i say 8:1 is a 'safe' spot for 93 octane on iron heads. You can go higher but its dangerous unless you know your graphs and charts for that engine.


*Decent static and dynamic compression calculator - http://www.gofastmath.com/Compressio...tio-Calculator

*How to calculate IVC (intake valve closure point) - https://www.corvetteforum.com/forums...lculation.html

*Horsepower estimation calculator (incase you wanna see how compression ratios can affect it) - http://www.gofastmath.com/Horsepower...wer-Calculator

*How to calculate cam overlap (if you want more rompity-romp in your idle and are looking for a cam - these tend to be big cams, and work best under mid to high RPM, so prepare to have bad idle and low RPMs) - http://members.uia.net/pkelley2/Overlap.html

*A video breakdown of what was discussed here, touching a little deeper on it with things like air temperature (density) affecting it, as well as visual representations with a white board-

You Tube

Orr89RocZ

iTrader: (20)

Diggin up alot of old ones eh lol