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Compression ratio question

2K views 8 replies 9 participants last post by  IanRiordan 
#1 ·
Hi,

I know something, when you want to know the REAL Compression ratio for a motor, you have to know, the combustion chamber volume, the bore, stroke, gasket thickness and the deck of the motor, BUT.....

I have a question....It is better to decked the block at zéro or keep the deck to 15 or 20 thousand....right now, I have 9.5 CR in my bbc, but I want to raise the CR to 10.7 (with aluminum head)...to stay pump gaz.. It is better to know all the spec (and the deck) to reach this CR or to decked the block at zero...I already heard people here in hotrodders talk about the QUENCH , I really dont understand what is the quench and what those people are talking about. Can you explain me...It is better to have 10.7 CR with a deck at 20 thousand ,play with the piston and gasket to reack this CR or shaved my deck to 0 to reack this CR :confused: :confused:

Thanks in avance everyone...I want to put money, but not to be poor after my project.....

Stephane..

I probably change my mind to built a Big inche small block, I already put allot of money in my big block, I will probably just change my head (full ported 049 oval port head) to a set of aluminum one....Does Canfield are a good choice??? (not really expensive..)
 
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#2 ·
Quench is the distance between the top of the piston and the flat portion of the cylinder head, when a tight space is between them, it forces the fuel mixture out at a very high speed into the combustion chamber, it cools the chamber and makes the mixture burn more efficently. Anything over .060, there is no quench, this is why the head gasket thickness, deck height, piston height, piston design all come into play. Look at this article, it will tell you all you need to know:
http://www.chevyhiperformance.com/techarticles/94138/
 
#3 ·
As far as decking the block goes, I would try to allow the pistons to sit as far in the hole as possible while still allowing yourself to get the right quench. I dont think there is anything bad about zero decking a block except that after that if you want to deck it again, the block is used up.

Adam
 
#4 ·
for optimal quench on street engines the piston should be sitting flush with the deck, and maybe a slight amount of "pop- out". Polishede flat tops are about the best piston for a high compression street engine. Also, keep in mind that you will loose a slight amount of power by using aluminum heads- I would just stick to iron heads with a polished chamber and hair lower on the compression.
 
#5 ·
firestone said:
As far as decking the block goes, I would try to allow the pistons to sit as far in the hole as possible while still allowing yourself to get the right quench. I dont think there is anything bad about zero decking a block except that after that if you want to deck it again, the block is used up.

Adam

Luckily today we have head gaskets that are as thick as .100" thick,and there are even options to deal with bigger problems then that.I run the pistons in my bigblock .015" above the deck,and run .060" thick gaskets.That was what it took to square the engine up when I built it.I also have a 400 block in my shop right now that was decked .050".The pistons were .020" in the hole,they will now be .030" out of the hole.Built the engine because I had a standard block with a burnt deck sitting on the shelf and a rotating assembly with forged dished pistons that I pulled from a 406 with freeze damage,and I figued that I could throw it all together for a few hundred bucks in machine work and have an engine to drop into something and beat on for almost no money.I am torn between running monster gaskets,or having the machine shop knock .030" off the pistons,but I am leaning towards the gaskets only because the assembly is already balanced and the pistons are pressed onto the rods,so it could get difficult to play with the pistons.

As for decking,I would get the quench as tight as possible.10.&:1 on a real driver with pump gas is getting kinda high,and the tighter quench will help limit detonation. .035" between the pistons and heads with a typical street engine is the way to go.
 
#7 ·
One thing that is usually overlooked is that dished pistons usually have circular dishes with a rim about 3/8 inch wide all around the top. Regardless of what head you use, a dished piston ruins the quench except for that little rim.

Ideally you should use reverse dome pistons.
 
#8 ·
compression ratios

These are the factors you need to know to find your exact compression ratio.
Displacement of the cylinder, combustion chamber size, head gasket thickness and the bore size of the gasket, valve relief displacement, the relief of the piston from the deck of the block, if its a dished piston, the displacement of the dish, and if its a domed piston the displacement of the dome.
The area left when the piston is at top dead center is called the unswept area. This is what is left for a combustion area.
Example:
350 cubic inch engine = 716.39 CC per cylinder. (This is the swept area).
65 CC cumbustion chamber, Head gasket .040" thickness, 4.1" bore in head gasket.= 2.76 CC head gasket.
0 deck flat top pistons. 5 cc valve reliefs.
Add up all the CC's = 789.15 CC's. Divided by the unswept area (combustion chamber, head gasket, valve reliefs) = 72.76 CC.
789.15 CC divided by 72.76 CC = 10.85:1 compression ratio.
Your unswept area is also known as the quench area.
Some heads have a combustion chamber design which is smaller and makes for higher compression, or often called a quench head.
A good example would be to compare the closed chamber head on a 4 bbl Cleveland engine to the open chamber or domed combustion chamber of a 2 bbl cleveland head.
The closed chamber head being the better design for quench.
As a rule, I dont subscribe to domed pistons as quite often, while making really sexy compression ratio figures, the dome may actually interfer with the propagation or flame travel of your fuel air mixture.
Over the years, I have found, for me anyway, that the flat top piston is about as good as it gets for power output, getting the boosted compression by selection of head design or milling the heads to reduce the combustion chamber size, or if the pistons have a relief to the deck height of the block, then milling the deck to obtain a 0 deck clearance with the piston.
Another anomaly is that running an open chamber head such as the 2 bbl cleveland, and a dished piston, can be extremely powerful, as you actually wind up with a round ball shaped combustion area, allowing for even power dispersion of your burning fuel air mixture. The limitation is the lack of high compression, however, even with this setup, compression ratios of over 10:1 are obtainable with relative ease, by machine work and selection of pistons.
A rule of thumb on aluminum vs steel heads, you can run one complete ratio higher with aluminum than you can with steel, using the same octane gas.
 
#9 ·
If you're running much overlap then poke the piston out of the hole as any 'quench area' will just fill up with exhaust and pollute the incoming charge.
A BBC should run an open chamber head for superior breathing, the domed pistons have a 'fire slot' in line with the spark plug and seem to work opposite to a SBC(small chamber - flat top).
Alloy head engines need more comp to make the SAME power as iron(not steel) head ones as alloy disipates more heat and heat IS power.
 
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