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Originally Posted by 1fast62
im wanting to buy a set of 115cc aluminum heads and i was told today that when you use alum heads they disapate heat so fast that it will drop your compression ratio... this was a machine shop and he said if i build my motor to 10 to 1 it will drop it a whole point so it will be 9 to 1.. Is this true!!??? because im trying to build a 10 to 1 ... please let me know what you guys think..
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The effect proves to be more in the world of mathematics than it shows on a dyno or with ETs.
It's actually a power loss due to the faster rate of heat transfer moving heat into the cooling system that would otherwise be pushing on the piston. The compression ratio does not change though one could argue the absolute pressures do.
In reality the faster heat transfer of aluminum lets you push the SCR more than high enough to compensate for absolute pressure differences caused by lower combustion chamber temperatures. The advantage is you can get to ratios that an iron head can't get to without encountering detonation. Aluminum also knocks about 50 pounds off the front end. Another thing you can do to counter this effect is to run the engine hotter, aluminum heads will tolerate much more temperature than iron heads, you can safely run an aluminum headed engine at 220 to 230 degrees, even hotter with an oil cooler. Just don't do this with a 400 block or you'll get cracks in the head deck. Another thing that can be done is to have a thermal barrier applied to the combustion chambers. These kinds of tricks totally negate any efficiency differences because of metal type heat transfer rates and at least for a race engine let you push aluminum really hard far beyond what can be done with iron parts. But for a street engine this isn't necessary perhaps not even desirable.
Building to 10 to 1 takes attention to details. The combustion chamber is also formed by the piston crown shape as much as the chamber in the head. Typically the OEM uses a circular dish piston along with heads that have larger combustion chambers than is advertised so that factory compression ratios range from a quarter to half point lower than claimed. This buys them a lot of space against warranty repairs for detonation damage when the engine is young. The circular dish piston is very inefficient at building what's called mechanical octane into the motor. You make up for this deficiency with less power than advertised, higher fuel consumption than advertised, and often find the engine prefers a higher grade fuel than advertised as the requirement. Although modern EFI engines hide the latter need because they employ a detonation sensor circuit that pulls timing advance out when it detects pinging, which is another reducer of all performance parameters from power to mileage. They do this with the driver being none the wiser unless you constantly check quarter mile times and gas mileage.
The solution to maximizing engine performance at all corners of the envelop has to include an effort to arrive at the highest compression ratio the fuel you buy can stand. To this end you need to design as much mechanical octane into the engine as possible while increasing the burn speed and staying under the detonation limit. So let me say that depending upon the cam which we'll get to, the compression ratio of 10 to 1 with an aluminum head should be good, but you need some edge on your side and the way to get that is to improve the squish and quench functions and maximize the burn speed across the chamber. A flat top piston is the best solution with a small chamber head, in the case of the SBC that would be a 58 or 64 cc chamber in preference to 76 or 78 cc chambers. The flat top doesn't interfere with burn speed as a dome does. It also works with the step in the head's chamber opposite the sparkplug side to emphasize squish and quench. If it becomes necessary to use a piston with a dish to keep the overall compression ratio under control for the fuel used, then the compromise piston is one with a D shaped dish on the spark plug/valve pocket side of the head's chamber. This keeps a flat piston surface to close opposite the squish quench deck of the head.
The same parts do two functions favorable to good combustion at different times in the compression and power stroke. Squish is an action resulting from the piston closing toward the head as it comes to Top Dead Center (TDC). This squirts the far side mixture toward the spark plug. This action breaks up any remaining large fuel droplets and mixes them into close contact with the oxygen of the air. The action also packs most of the mixture in front of the spark plug making it more likely to catch fire. All of this reduces miss and late fires, speeds the burn and develops a more complete burn sooner in the power stroke so that more effort is pushing against the piston where the rod has the best mechanical advantage on the crank. This increases power output, reduces fuel burn for that power, and helps get the burn over with before another ignition point can get started in the chamber. The quench function happens as the flame front speeds into the far side of the chamber. Here pressures and temperatures are going up very quickly. At the far side of an open chamber or one with a circular dish piston it becomes likely the unburnt mixture will explode before the flame front gets there. So keeping the closure of the piston to the head a tight clearance of about .040 to .060 inch results in a space that has a lot of surface area to its volume. This pulls out the high heat building on the far side of the chamber ahead of the flame from, preventing it from exploding. The best power and greatest efficiency an engine develops is just under the detonation limit. The longer you can delay that event the more the power and mileage can be had from the fuel used.
The .040 to .060 dimension is inclusive of all the dimensions above the piston and below the head. For most engines this is the clearance space between the top of the piston and block's head deck, for a small block Chevy the factory spec is .025 inch. However, this needs to be measured as pistons come with different distances between the pin center and piston crown standard for the SBC is 1.56 inch, but there are many pistons out there that are made for restoring the .025 dimension on blocks that are zero decked for mass rebuilding purposes. So you need to careful about this. Zero deck blocks for racer purposes are done to level and square the deck in old castings and to increase compression ratio. To do this you need to use a standard 1.56 height piston and adjust the clearance with the thickness of the head gasket. The head gasket also contributes to the piston crown to deck clearance, gasket run from .050 inch for the typical composition gasket used with aluminum heads to .015 inch for steel shim gaskets used with iron heads. There is a range between these two points and a great selection of materials. Lets set copper aside now, these are best used with O ringed blocks or heads and is at best the purview of competition engines. Without O rings it can be difficult to keep copper gaskets sealed, so just set them off the table. Aluminum and cast iron parts have different rates of expansion and contraction which causes them to rub back and forth in relation to each other. This results in a condition called Fretting or Brinelling where the rubbing action wears the softer, in this case the aluminum, of the metals. This action leads to cutting grooves into the aluminum which eventually lets compression and coolant leaks to form. To reduce this, multi-layer gaskets are used so that the adjacent gasket rides with the upper and lower parts and a third element in the sandwich takes the rubbing forces in a shearing action. This usually results in a thicker gasket where the top and bottom layers are steel, perhaps stainless, and the mid layer is a composite of some sort. These tend to be on the thicker end of the scale but are less expensive than what comes next. Next is the multilayer steel again often stainless, these are thinner getting down toward something less than .030 inch, but they are real wallet drainers. They function similarly to composition gaskets with a thin face adjacent to the parts and a center section that takes the rubbing action but has no shear loads in it. Functionally they do an excellent job. Some people have done a single layer steel gasket with a rubber coating to keep aluminum heads and cast iron blocks sealed up with a minimum of distance between them. I just haven't been able to bring myself to this, hopefully someone that has will chime in and let us know how long the seal lasts and want if any damage is done over time to the head.
Camshaft as promised discussion has a huge impact on compression ratio selection. We usually talk in terms of the Static Compression Ratio (SCR) which is just the volumes above the piston crown at TDC divided into the sum of those volumes plus the cylinder's volume swept by the piston. Introducing the effects of the camshaft on cylinder volume introduces the concept that no compression can take place till the intake valve is closed on its seat. To that end it is necessary to know when the cam closes that valve in crankshaft degrees which is usually after Bottom Dead Center (BDC). This radian location is calculated using formulas that position the piston stroke in the bore relative to crankshaft rotation. This becomes lost stroke length which when popped into the SCR equation gives what's called the Dynamic Compression Ratio (DCR). One quickly sees their SCR going down when this is done. Generally we're looking for a DCR in the range of the upper 7's to one to lower 8 to 1 for a street engine with iron heads that will burn regular at normal engine temps of 180 to 200 degrees. The mid 8 to 1 range will burn mid grade fuel, the upper 8 to1 to lower 9's will burn premium fuel. Someone suggest that this number times 10 will equal the octane of the fuel needed, and that's pretty darn close. An aluminum head will tolerate from .5 to 1 full ratio higher. Its also been discussed in this column that 36 degrees of spark advance is about optimum with FastBurn Ricardo style heads whether called Fast Burns, Vortecs, E-Streets, GT40, or Magnums at different brands. You will find that if you have to compromise something to stay under the detonation limit that reducing advance will cost more power sooner than will reducing compression ratio. You can also play with mixture ratios, engine operating temps and air inlet temps to get on top of detonation problems, Worst case there are water or water alcohol injection kits that do a fantastic job at suppressing detonation. This is just to point out that 'there's lots of games and trades that can be made in this area. A good compression calculator can be found at
http://www.kb-silvolite.com/calc.php . By the time this is done you'll wish you paid more attention in trig, physics and chemistry class.
Anyway once again I'm on my way to a book while it was suggest you get some to read. There's a lot of data out there much of it good some not that good but some of the better engine builders like David Vizard, John Lingenfelter, a good one for you would be How To Build the Small Block Chevrolet by Larry Atherton and Larry Schreib this gets real detailed for shop work. There are many more.
Bogie
02-14-2011, 09:19 PM
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