Piston choices

Discussion in '1974 - 1978 Mustang II Talk & Tech' started by jeffnoel, May 18, 2004.

  1. Thanks thats the reassurance I was looking for. I'm running flat tops w/reliefs combined with 58 cc heads and a 272 duration cam I have between 150-160 psi per cyl in Illinois (pretty flat here)
  2. >Poor exhaust scavenging(an intake charge which is polluted from the previous >combustion) drops engine efficiency as well as power. (this is why headers and >performance exhaust system add power AND fuel mileage over a stock exhaust >setup)

    If this were true then you'd see new honda civics with lumpy idles. Most cars on the street would never benefit from exhaust scavenging. A lot of things that increase horsepower you won't find on cars that get high mileage for good reasons... they aren't efficient.

    >If they ground a camshaft specifically to cause just the exhaust side to lose air flow, >the engine's power levels would drop, as well as fuel mileage.

    Indeed, in the example given the guy talked about running an 11:1 engine off pump gas. I'm guessing his engine didn't have the same amount power on the street as it had on the strip running race gas. In order to prevent the engine from detonating due to the low octane, you either have to reduce the fuel in the chamber or lose compression.

    >the engine's power levels would drop, as well as fuel mileage.

    There is no direct correlation between an engine's power and fuel mileage. If there were then everyone would buy a Mustang Cobra and be getting 50 miles to the gallon.

  3. Why would you see Hondas with a lumpy idle?
    A lumpy idle means that the engine is getting a dirty intake charge due to large duration valve overlap. When long valve overlap is occurring, both valves are open at the same time, and the engine exhaust stroke is pushing the spent charge out through both exhaust and intake valves. Only when you begin to rev the motor will the incoming air form a moving column of air pressure, the force of which keeps the exhaust stroke from exiting into the intake tract. What I was referring to, is proper(as close to 100% as possible) exhaust evacuation(scavenging) There ARE mechanical principles which add both power AND fuel economy. A cleaner intake charge is one of them.
    A more efficient exhaust system, like a clean air filter, is better than free horsepower, it's horsepower with a side benefit of mileage.

    Increasing the purity of the intake charge in ANY engine will increase both its power and its fuel mileage. This has nothing to do with changing the cam profile, and everything to do with enhancing the flow characteristics of the exhaust system, starting with the primary tubes of the headers.
    If you improve the exhaust efficiency,(scavenging effect) the engine's idle quality will grow smoother, not lumpier.
    If you'll notice, economy cars of the past had crappy exhaust systems, and got crappy mileage(and power), whereas today's economy cars have computer-designed and specifically tuned headers, with superior exhaust systems than in the old days, and as a result, they make more power per cubic inch, and get far greater fuel mileage than ever before.

    If you decrease the fuel in a combustion chamber, you make the charge leaner, which will generate more combustion heat, and detonate easier than a rich charge.
    What you were talking about doing is decreasing the exhaust efficiency of the motor by decreasing lift/duration of the exhaust valve, and by doing so, you're decreasing the quantity of useable oxygen in the cylinder, which makes the charge effectively richer. (Less oxygen = rich condition)
    A richer dirtier charge will definately decrease detonation, but at the cost of power and efficiency. Just as increasing the carb jetting beyond an ideal air:fuel ratio will decrease both power and fuel mileage.
  4. Blue Thunder, I agree with everything you've stated in your last response. I couldn't understand why you said some of the things previously:

    >Poor exhaust scavenging(an intake charge which is polluted from the previous combustion)

    Since you said this, I assumed you were talking about using exhaust scavenging combined with valve overlap which does pollute the intake charge and give the engine a lumpy idle at low RPMs.

    >There ARE mechanical principles which add both power AND fuel economy.

    I never argued this... only that power and fuel economy are not directly correlated. In other words there are instances where you can increase one and actually decrease the other. Take a HEMI engine... great for power but bad for fuel economy. A quench style combustion chamber is more efficient due to the localization of the charge in relation to the spark plug.

    You are right about a clean intake charge and a good exhaust system. However I'll add that fuel injection, OHC design combined with variable lift and increased computerization have had huge effects on the efficiency of engines in recent years.


  5. The overhead cam/extra valves per cylinder/variable lift all increase the purity of the intake charge at low rpm without giving up airflow at upper rpm. Kind of cheating, in a way, but effective, nonetheless.

    They're trying to design a motor that has no compromises, which isnt easy.
    About 10 years ago, I designed a fully hydraulic actuated valve setup which would be the epitome of efficiency, since you could have short duration and low lift at idle, and the computer would increase both lift and duration of the valve events smoothly and perfectly proportionately with RPM. The motor would stay in its peak torque output throughout the entire rpm range. No compromise, perfect efficiency, perfect power. The problem I ran into was trying to find hydraulic linear actuators quick enough, powerful enough, and compact enough to move the valve springs.
  6. Blue Thunder,

    I got to see a ford 2.3 head that was equiped with hrydrolic valve actuators at a small auto repair shop in Safford AZ. I believe that they were running into the same problems that you mentioned.

  7. What about a setup that opens and closes the valves also-- wouldn't need to worry about spring pressure then.

    maybe even an electronic setup with solenoids.???
  8. It would take a seriously powerful linear actuator to return the valve to/and hold it on the seat at high rpm, with the big airflow. The pressure against the underside of the intake valve is hideous when the motor is revving.
    I thought about electric linear actuators, and they just don't have anything strong enough that will fit on a head, and be powered by anything short of a tow-behind generator. At least not yet.
  9. I'm surprised the coates system is still around. I checked it out about 10 years ago,(I still have the info package they sent me) and it was very costly, so I passed it up, and figured the company wouldnt survive because of cost issues.

    Very cool idea, the rotary valves, for various reasons, but doesnt offer variable valve timing/duration. I wouldnt mind having a set of coates heads, though!