mustangcat

1) Why don't more automakers use 5 valve design?


2) Why don't car engines get forged and have super high redlines more often?
Ex. The Hayabusa motor pumps out 171 HP N/A out of a 1.3 Liter engine. Since it uses less materials to make, and is lighter, wouldn't it be more efficient for small cars that have low torque demands?(Focus, Cavalier, Corolla, etc.)


3) Why is that Turbo's opperate more efficiently than N/A cars at high altitude?

4) Why can't engines idle at extremely low RPM to save gas? 100-200 RPM.

5) Why is that a 200cc Bike engine can have 40 HP. But a 4600cc(4.6L) engine can only get 300 HP?

67mustang302

1) 5 valve heads are much much more expensive, and a good 2 valve head design*cough*AFR*cough* can perform very well for much less money. And a 5 valve head makes the whole engine package more expensive and complicated.

2) Cost an reliability. A small motorcycle engine has much lighter parts and can rev higher. Higher revving engines have requirements that make them more expensive.

3) A turbo uses the wastegate to control intake pressure, at high altitude where the turbo is taking less air mass in, it can simply turn more rpm and compress it until it reaches a set pressure.

4) The power and range most automobile engines operate in results in an engine that's highly innefficient at those low speeds. An engine that could idle at 100-200rpm would become inefficient as rpm increased and total power output would be very low(think of the first engines, that DID idle at 200rpm in some cases, but only made like 15hp).

5) Horsepower is directly related to torque output and engine rpm. Bike engines have higher horsepower for their size due to higher engine rpm, however if you look at mean cylinder pressures then the difference is much less. Horsepower is a really bad comparison to use vs size unless you factor in rpm. Better comparison is torque vs engine size, which gets into mean cylinder pressure.

mustangcat

Hmm! Very interesting, thank you!

67mustang302

It should be noted that turbo or any blown applications still lose power with altitude, but the extra pressure from boost makes the % loss of power not as severe. The turbo doesn't generate a certain % pressure above atmospheric, but a static pressure. Atmospheric pressure drops with altitude but boost pressure doesn't, so it helps to offset the % loss of total induction pressure.

mustangcat

Is that preset boost percentage automatically adapted at higher altitudes with the wastegate?

67mustang302

Basically. If you have a wastegate that's set to open at 10lbs of boost, then it stays shut until the turbo produces 10lbs of boost regardless of altitude. At higher altitude it would just turn more rpm to produce the boost to open the wastegate. A supercharger would require a pulley change. With a turbo at higher altitude, the same boost pressure would actually be a higher % of boost above atmospheric, since the atmospheric is lower but the turbo's boost stays the same.

GreyStang

Good Q's & good A's [8D]

my little red 90

i'm impressed.

67mustang302

There's a reason well set up turbo engines often perform well beyond what most people think they would. Turbo is just plain bad *** when done right.

whiterabit998

my friends golf gti is a 4cyl turbo with the five valves and he get 30 mpg with the a/c on and can smoke a stock gt.. and he break the bank buying it either!

???
they make these huge unreliable gas hogging engines so we can take them in the dealer every month and spend alot to get them fixed!

thats why ford charges a ridiculous 130 bucks just to check the cel