how much compression can i run on a street motor, my heads are going in to get cleaned up and new valvetrain, and i was thinking about getting them shaved as well, they are aluminum vette heads, and i believe i have 9.5 to 1 compression right now, how much more can i go, and still have a streetable motor? also if i do get them shaved, how much power increase am i looking at?

 

I'm running 10.4:1 with aluminum heads. It takes premium tho I beleive there is a formula to calculate how much power a given increase in compression will give. From what I've heard it isn't a huge increase, but it makes a difference. There are a million programs out there that calculate the compression of your cylinder based on gasket thickness, pistons, and chamber volume.

 

If you are going to get them shaved you don't want to take a lot off. I would shoot for maybe 10:1 compression at most. Talk to the people doing your head work. They can tell you how much they can safely mill your heads and still have things line up. Remember your manifold is going to drop some and your distributor will drop some. You won't be gaining that much going to say 10.5:1. Allen

 

I run alittle over 11:1 and haven't had any problems, that is with aluminum heads and fuel injection. I always run premium gasoline, and whenever I get the chance I run some 110 octane, it doesn't need it, but I like the way it smells With aluminum heads and a good tune you can pull off running 11:1 w/out any problems on pump gas.

 

alright i'll talk to them, and see how much they can mill off...but does that hurt anythign if the distributor and intake drops down a bit more than stock? does anybody have maybe any rough guestimates from going from 9.5 to 10, or 9.5 to 10.5?

 

It's not just aluminum but the combustion chamber shape, spark plugs, cam and pistons that determine how much compression you can run. Also running a cooler thermostat allows higher compression.

 

i know on a flat top piston and my fastburn heads that i had cc'd (54cc) that my compression was around 12.5 to 1. i drove this around town without even knowing what compression i had till i took it apart. i then went back with some custom weiscos with a 30cc dish under my fastburn heads so that i could be around 9.0-9.5 to one for using the nitrous the way i like. it really depends on the cc of the head and what it all adds up to , depending on rings and valve releifs on pistons you need to get you heads cc'd after any work that you have done to them so you know what you are looking at.

 

Well, hmmm, though you may think you were at 12.5:1, I doubt it. I seriously doubt it. It has a LOT more to do with where your pistons are decked at . Zero deck, .040 down in the hole. How thick is your head gasket and head cc. Anyway I know all of this for my engine so I do know my TRUE compression ratio and I KNOW that at 10.3:1 I'm on the ragged edge of detonation, based on real time monitoring of the motor while being put through it's paces.

 

well it was at zero deck height with two valve releifs and a 54 cc chamber. we sat at gtp here in houston and calculated it after we changed pistons and it came to about 12.2 to 12.5. this is the place i had the heads cc's with 2.08 /1.65 valves in them, plus the heads had beem cut a couple times from being installed ran and changes so that ate up some of my combustion chamber

 

well i'm not gonna get into running flat top pistons and deckin the block, i just wanna get as high as possible just by shaving the heads...i'm sure the engine builder that i will be using will be able to tell me the compression when all is said and done with...but i'm still wondering if shaving the heads is bad for the intake or the distribotor because they sit down lower?

 

plus considering the only way to keep it from detonating was running 28 degrees of total timing with a 50/50 104 mix( i ran it one time with 114 on 36 degrees and picked up 4 tenths) . i could beleive it . On the je pistons i ordered it called for a 64 cc chamber head to make it 10.5 . but i had a 54 cc chamber with zero deck and everything else the same as the 10.5 so it would figure to be the differance of 10cc and how much compression that would cause. all these measurements was giving 5 cc to the stock thickness gasket which i used and it said that was the total amount to calculate for the gasket on the package from fel-pro. i drove this motor on the street every day with pump gas but like i said, if you are gonna drive something like this on the street with pump gas you need to go to a colder plug( i ran ngk-6) plus pull timing, and when you go to the track to really get down you need to put some race gas in it. to be honest depending on how much you want to get milled with make the differance on what you wanna do. i had 5 thousandths cut three times off of my heads for each time i dissassembled the motor. now i have had friends who picked up 3/4 a point in compression with a 20 or 25 thousandths shave but that seemed kind of extreme. best bet is to get with your machine shop, cc your heads, then figure what you want and what can be done

 

as long as there is not a extreme amount cut off the heads you should have no prob.

 

alright cool...because my heads are going in to get cleaned up and like a said a new valvetrain, and from my understanding, milling heads is pretty easy and pretty cheap, so i figure while they are there i mines well get the compression boosted up a bit because it couldn't hurt...i'm not looking to gain 50 or 100 horse just by milling my heads, if it makes any kind of difference i'll be satisified...

 

I'd say about 13:1 depending on variables ...but, read this

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

from http://www.speedomotive.com/Building%20Tips.htm

"Compression Ratio" as a term sounds very descriptive. However, compression ratio by itself is like torque without RPM or tire diameter without a tread with. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel, engine and air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbo-charging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:1 unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine significantly.

At one time compression pressure above a true 8:1 was considered impractical. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compression up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the high RPM limited cylinder pressure due to the short time available for cylinder filling.

Before continuing with theory, a little practical compression information is in order. If you have a 10:1 engine with a proper .040" assembled quench and then add an extra .040" gasket to give 9.5:1 and .080" quench you will usually experience more ping at the new 9.5:1 ratio than you had at 10:1. Non quench engines are the exception to this rule. Some racers make the effort to convert non-quench engines to quench type engines, as with our Mopar Squish Deck Heads. Compression ratios that work are as follows:

PUMP FUEL

8.5:1- Non-quench head road use standard sedan, without knock sensor.

8.5:1- Quench head engine for tow service, motorhome and truck.

9.0:1- Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.

9.5:1- Same as 9:1 except aluminum head used.

Light vehicle and no towing.

10:1- Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam. A knock sensor retard is recommended with 10:1engines.

RACE GAS

12.5:1- Is the highest compression ratio suggested with unrestricted race gas engines.

ALCOHOL

15.5:1- Is the highest compression ratio suggested for unrestricted alcohol fuel engines.

Satisfactory use of 14:1 - 17:1 compression engines can be made when restrictor plate or small carburetor use is mandated by the race sanctioning. High altitude reduces cylinder pressure so if you only drive at high (above 4500 feet altitude) a 10:1 engine can be substituted for a 9:1 compression engine. General compression rules can be violated but is usually a very special case such as a 600 HP normally aspirated engine in a 1500 lb. street car with a 12:1 compression ratio. The radical cam timing necessary for this level of performance keeps low and medium RPM cylinder pressure fairly low. At high RPM detonation is less of a problem due to chamber turbulence, reduced cylinder fill time, and the fact that you just can't leave the above combination turned on very long without serious non-engine related consequences.