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11/1 Compression Ratio for a cruiser?

Discussion in 'The Hokey Ass Message Board' started by 51 Leadsled, Sep 15, 2009.

  1. Ditto. Plus, opening the chamber around the valve will aid flow while reducing compression. I plan on running 11-1 on pump gas. But, I'll be using aluminum heads, large roller cam, and coatings on the intake valve and intake manifold to keep the incoming mixture cooler.
     
  2. Regular gas here is 7 bucks a gallon, but we can also buy 100 octane at the pump here...

    Mopar lied about stock 410hp and 425hp max wedge motors too; with stock pistons, rods and bore on my 426, and the heads cut .090", the compression is just a touch over 11.5 to 1. I am guessing the real number on the stock motor was closer to 10 to 1.

     
  3. M.Edell
    Joined: Jun 5, 2009
    Posts: 4,138

    M.Edell
    Member

    Maybe, but I have forged 11.1 TRW Pop ups in my engine and probably close to 11.5 as well..
     
  4. rockabillyrodder
    Joined: Aug 27, 2006
    Posts: 78

    rockabillyrodder
    Member
    from Colgate WI

    I ran a 350 with .125 dome pistons .040 head gasket with 64cc fuelie heads on pump gas and had nothing but problems. It would never hot start. I had to ask a stranger at a gas station to crank it while I adjusted timing to start it then quickly adjust it to run. Bought a bag of ice and dumped it on the intake after shutting it down at taco bell then waited a half an hour for it to start. It never ran over 190.

    I ran a o/t malibu with a stock 305 I had the heads milled .040 with a .020 head gasket with no other changes I could only run 93 octane after. If I ran 89 it would either start like shit and run good or run good and start like shit. With a better ignition this would have been corrected though. There are systems with a start retard for easier cranking. I think you can do it, it will just be a bitch to work all the bugs out but not for a cruiser. Cruisers should not be tempermental.
     
  5. RAY With
    Joined: Mar 15, 2009
    Posts: 3,133

    RAY With
    Member

    Going that high of compression on a street cruiser is no more than a bag of worms with the gas thats available. It will take premium,aluminum heads back down timing and a ton of octain booster-You thing its worth that? Your car and your pocket book so do what pleases you.
     
  6. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    What happens is the overlap in the cam bleeds off cranking pressure

    Which cam has overlap during the compression stroke?
     
    6-bangertim likes this.
  7. scottybaccus
    Joined: Mar 13, 2006
    Posts: 4,106

    scottybaccus
    Member


    I measured mine!
     
  8. scottybaccus
    Joined: Mar 13, 2006
    Posts: 4,106

    scottybaccus
    Member

    Almost all of them. The intake valve is still open as the compression stroke begins. It results in some of the pressure leaking out the exhaust, then the valve closes and the cylinder builds pressure for real.

    Spend a few hours degreeing a cam and you will see the cam timing on the inside of your eyelids when you sleep.
     
  9. 61falcon
    Joined: Jan 1, 2009
    Posts: 772

    61falcon
    Member

    whats an "rv" cam? a street car doesnt need more that 10.5:1 compresion to make power. cam and head selection is where the power is made. call a cam manufacture tech line and tell the what you have and what you want to get out of it.
     
  10. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Almost all of them. The intake valve is still open as the compression stroke begins. It results in some of the pressure leaking out the exhaust, then the valve closes and the cylinder builds pressure for real.

    Take a step back, and read what you wrote.
     
    6-bangertim likes this.
  11. I think he meant to say that the intake valve hangs open after the piston is on its way up on the compression stroke. This forces some of the air in the cylinder back into the intake manifold under most operating conditions.

     
  12. Littleman
    Joined: Aug 25, 2004
    Posts: 2,614

    Littleman
    Alliance Member
    from OHIO, USA

    Reversion.........had many problems this year w/ bent valves causing reversion.....chased my tail on tune ups because of it.......self inflicted reversion.........Littleman
     
  13. scottybaccus
    Joined: Mar 13, 2006
    Posts: 4,106

    scottybaccus
    Member

    You guys are supposed to read my mind. Comp cams says it better than me.


    Valve Timing Tutorial
    In an effort to simplify what actually happens inside an engine, COMP Cams® invites you to "take a walk" inside a typical engine, just like the one you might have in your car. We will discuss valve events, piston position, overlap and centerlines. Although we can not explain cam design in such a small space, we might be able to clear up some of the most misunderstood terms and make clearer what actually happens as the engine goes through its four-stroke cycle. We will graphically illustrate the relationship between all parts of the engine and try to help you understand how the camshaft affects the power of the engine. Put on your walking shoes, open your eyes and get ready for a good look inside this engine.

    We begin with the piston all the way at the top with both valves closed. Just a few degrees ago the spark plug fired and the explosion and the expansion of the gasses is forcing the piston towards the bottom of the cylinder. This is the event that actually pushes the crankshaft around to create the power and is referred to as the "power stroke" (figure 1). Each "stroke" lasts one half crankshaft revolution or 180 crankshaft degrees. Since the camshaft turns at half of the speed of the crank, the power stroke only sees one fourth of a turn of the cam, or 90 camshaft degrees.

    As we move closer to the bottom of the cylinder, a little before the piston reaches the bottom, the exhaust valve begins to open. By this time most of the charge has been burned and the cylinder pressure will begin to push this burnt mixture out into the exhaust port. After the piston passes the true bottom or Bottom Dead Center, it begins to rise back to the top. Now we have begun the exhaust stroke, another 180° in the cycle (figure 2). This forces the remainder of the mixture out of the chamber to make room for a fresh, clean charge of air-fuel mixture. While the piston is moving toward the top of the cylinder, the exhaust valve quickly opens, goes through maximum lift and begins to close.

    Now something quite unique begins to take place. Just before the piston reaches the top, the intake valve begins to open and the exhaust valve is not yet fully closed. This doesn't sound right, does it? Let's try to figure out what is happening.

    The exhaust stroke of the piston has pushed out just about all of the spent charge and as the piston approaches the top and the intake valve begins to open slowly, there begins a siphon or "scavenge" effect in the chamber. The rush of the gases out into the exhaust port will draw in the start of the intake charge. This is how the engine flushes out all of the used charge. Even some of the new gases escape into the exhaust. Once the piston passes through Top Dead Center and starts back down, the intake charge is being pulled in quickly so the exhaust valve must close at precisely the right point after the top to keep any burnt gas from reentering. This area around Top Dead Center with both valves open is referred to as "overlap". This is one of the most critical moments in the running cycle, and all points must be positioned correctly with the Top Dead Center of the piston. We'll look at this much more closely later.

    We have now passed through overlap. The exhaust valve has closed just after the piston started down and the intake valve is opening very quickly. This is called the intake stroke (figure 3), where the engine "breathes" and fills itself with another charge of fresh air/fuel mixture. The intake valve reaches its maximum lift at some defined point (usually about 106 degrees) after top dead center. This is called the intake centerline, which refers to where the cam has been installed in the engine in relation to the crankshaft. This is commonly called "degreeing". We will talk about this later also.

    The piston again goes all the way to the bottom and as it starts up, the intake valve is rushing towards the seat. The closing point of the intake valve will determine where the cylinder actually begins to build pressure, as we are now into the compression stroke (figure 4). When the mixture has all been taken in and the valves are both closed, the piston begins to compress the mixture. This is where the engine can really build some power. Then, just prior to the top, the spark plug fires and we are ready to start all over again.

    The engine cycle we have just observed is typical of all four- stroke engines. There are several things we have not discussed, such as lift, duration, opening and closing points, overlap, intake centerline and lobe separation angle. If you will refer to the valve timing diagram when we discuss these terms it might make things a lot easier to understand.

    Most cams are rated by duration at some defined lift point. As slow as the valve opens and closes at the very beginning and end of its cycle, it would be impossible to find exactly where it begins to move. In the case illustrated, the rated duration is at .006" tappet lift. In our plot, we use valve lift so we must multiply by the rocker arm ratio to find this lift. For example, .006" x 1.5 =.009". Instead of the original .006" tappet lift, we now use .009" valve lift. These opening and closing points are circled so that you can see them. If you count the number of degrees between these points you will arrive at the advertised duration, in this case 270 degrees of crank- shaft rotation. In this illustration this is the same for both the intake and the exhaust lobes, thus making this a single pattern cam. Some cam manufacturers rate their cams at .050" lift. If we again multiply this by the rocker arm ratio, we get .075". we can mark the diagram and read the duration at .050" lift. This cam shows around 224 degrees, standard for this 270H cam. The lift is very simple to determine. You can simply read it from the axis going up. This is the lift at the valve as we said earlier. Sometimes you will hear lift referred to as "lobe lift". This means the lift at the lobe or the valve lift divided by the rocker arm ratio. In this case, it would be .470" divided by 1.5 or .313" lobe lift. The lift is simply a straightforward measurement of the rise of the valve or lifter

    We touched on opening and closing points a little earlier, but now we want to consider them even further. We talked about when these points occur, and how they are measured. As you can see in figure 1, the valve begins to move very slowly then picks up speed as it approaches the top. It does the same closing, coming down quickly then slowing to a gentle stop. It's kind of like driving your car. If you were to go from 0 to 60 mph in a fraction of a second and stop instantly, you can imagine what that would do to the car, not to mention the driver. It would be much too severe for any valve train to endure. You would bend pushrods, wear out cams, break springs and rockers, and lose all dynamic design. The cam would not run to the desired RPM level as you would have all these parts running into each other. As the valve approaches the seat, you also have to slow it down to keep the valve train from making any loud noises. If you slam the valve down onto the seat, you can expect some severe noise and a lot of worn and broken parts. So it is easy to see that you can only accelerate the valve a certain amount before you get into trouble. This is some-thing COMP Cams® has learned over the years-how far you can safely push this point.

    Looking a bit further at the timing points, the first one we see on the diagram is the exhaust opening point. We have all noticed the different sounds of performance cams, with the distinct lopes or rough idle. This occurs when the exhaust valve opens earlier and lets the sound of combustion go out into the exhaust pipes. It may actually still be burning a little when it passes out of the engine, so this can be a very pronounced sound.

    The next point on the graph is the intake opening. This begins the overlap phase, which is very critical to vacuum, throttle response, emissions and especially, gas mileage. The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine.

    The last point in the cycle is the intake closing. This occurs slightly after Bottom Dead Center, and the quicker it closes, the more cylinder pressure the engine will develop. You have to be very careful, however, to make sure that you hold the valve open long enough to properly fill the chamber, but close it soon enough to yield maxi mum cylinder pressure. This is a very tricky point in the cycle of the camshaft.

    <TABLE border=0 cellSpacing=0 cellPadding=5 width=600 align=center height=1092><TBODY><TR><TD height=3 vAlign=top align=middle> </TD></TR><TR><TD height=408 vAlign=top align=middle><TABLE border=1 cellSpacing=0 borderColor=#333333 cellPadding=3 width=564 bgColor=#333333><TBODY><TR><TD width=554 align=middle>[FONT=Verdana, Arial, Helvetica, sans-serif]EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)[/FONT]
    </TD></TR><TR><TD bgColor=#ffffff><TABLE border=0 cellSpacing=0 cellPadding=5 width=585 align=center><TBODY><TR><TD bgColor=#efefef width=279 noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Tighten [SIZE=-1](smaller LSA number)[/SIZE][/FONT]</TD><TD bgColor=#efefef width=287 noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Widen [SIZE=-1](larger LSA number)[/SIZE][/FONT]</TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Moves Torque to Lower RPM[/FONT]</TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Raise Torque to Higher RPM[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Increases Maximum Torque[/FONT]</TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Reduces Maximum Torque[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Narrow Power band[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Broadens Power Band[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Builds Higher Cylinder Pressure[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Reduce Maximum Cylinder Pressure[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Increase Chance of Engine Knock[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Decrease Chance of Engine Knock[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Increase Cranking Compression[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Decrease Cranking Compression[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Increase Effective Compression[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Decrease Effective Compression[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Idle Vacuum is Reduced[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Idle Vacuum is Increased[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Idle Quality Suffers[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Idle Quality Improves[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Open Valve-Overlap Increases[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Open Valve-Overlap Decreases[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Closed Valve-Overlap Increases[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Closed Valve-Overlap Decreases[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Natural EGR Effect Increases[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Natural EGR Effect is Reduced[/FONT]
    </TD></TR><TR><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Decreases Piston-to-Valve Clearance[/FONT]
    </TD><TD noWrap>[FONT=Verdana, Arial, Helvetica, sans-serif]Increases Piston-to-Valve Clearance[/FONT]
    </TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>
     
  14. Kevin Beck
    Joined: Jul 13, 2019
    Posts: 2

    Kevin Beck

     
  15. Kevin Beck
    Joined: Jul 13, 2019
    Posts: 2

    Kevin Beck

    My 2004 vw toureg with the 4.2 L V8 has 11 two 1 compression ratio in the owners manual it say to use 98 octane
     
  16. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 5,232

    Bandit Billy
    ALLIANCE MEMBER

  17. Don't adjust CR with gasket thickness. You want the piston to head clearance to be at .040" for best detonation control, especially if you really have 11:1. Much more than .040" and you get into trouble. Measure what you have and make your gasket selection for clearance rather than CR. Your motor will be happier.

    Clearance can differ due to whether the cylinder deck has been re-machined or not. From the factory a 350 will have about .025" deck height above the piston at TDC. We call that "25 in the hole". That leaves you with .015" for gasket thickness for the correct squish.

    They offer a steel "shim" gasket at .020" thick and I have gotten away with that on a stock deck height, but it's better to lower CR other ways rather then gasket thickness. You can do it with more dish in the piston top or unshroud the valves a little.

    You can also play with more cam duration to effectively lower dynamic CR, but I don't think that is the direction you're wanting to go. That's a whole 'nother story. ;)
     

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