pontiac

i was getting a hair cut today and i was reading some magazines. highly respected magazine over here in finland said that 4-valve head wont help if engine is supercharged.

for me it said that all 3rd gen'ers shoud have turbo if it's not attached in the factory. no matter is engine 2 or 4-valve construction, benefits will be same. yihaa!!!

just my 2 cents (euro),


-P

89ProchargedROC

i dont know where you read that but that is pure moranic

think about it for a second?

you have 4 VALVES allowing TWICE the amount of AIR into the combustion chamber to make TWICE as much power with all else being equal and setup properly

why do you think those modular mustang motors FLY with a blower put on?

4 valve motor

ATOMonkey

Just a little FYI. Intake valves on 4 valve heads are not as big as intake valves on 2 valve heads, so it's not twice the the flow, but 4 valve heads have been shown to breathe better. I also believe that everything should be turbo charged. Just like *** intended.

91-ZZ4-Z28

You are assuming that the 4 valves are twice the size of the 2 valves. Depending on the displacement of the engine and combustion chamber size the benefit of a 4 valve motor is in the way it delivers the air and fuel not the volume, and under pressure (Turbo/Supercharger) those characteristics change and a single large valve can provide just as well. So it is correct in some ways but not all.

JoBy

A four valve head is actually much better with turbos.

Increased flow is not the main reson that these heads are better.

These heads have the spark plug in the middle, between all the four valves. The combustion will be from the center and out instead of from one side. This will allow a higer compression ratio at the same boost without knocking. It is usually possible to use 2 unit higher compression ratio, or use more boost.

paul_huryk

Like others said, four valves do not allow twice as much air in the chamber. 4 valve heads do allow more air in at the same lift and duration as a two valve head, making it breathe better with a smaller cam.

The problem is the typical 4 valve motor is 4 liters or under (my TL's motor is 3.2L), making it torque deficient. Add in the typical intake tuning for higher RPM hp and you have a motor that begs to be super/turbo charged to move well. The top dog of that type of motor combo was the 1993 & up Supra: 3L motor. 24 valves with twin turbos at 320hp stock (at a high rpm). Of course the motors are powerful with mods, but putting 600hp through a 3 Liter 6 is not my idea of bulletproof; ditto for the race motors. I would rather have a 4 Liter with 25% less boost or 25% less revs to make the power (more TQ too).

The real advantage of a 4 valve motor over a OHV motor is the fact that the valvetrain is light (small lifters, no pushrods...), so it can take a lot of revs before valve float occurs. Adding that to the high flow with a small cam, you have a good high RPM motor. The LT5 Vette had a 405HP DOHC 350 that didn't have to rev much past 5600 to get the power, today's 2 valve 2002 L06 Vette makes the same at 6000RPM, only a slight RPM increase.

Typically the 4 valve motors run a higher compression ratio, but they require premium gas, so you gotta lower it to put forced induction on one.

Or get a rev kit to give some RPMs to your L98:

IrocZS383

I have to agree with the others, 4 valve heads don't offer twice the airflow that a 2 valve head does but they offer more airflow at lower lift. They do allow you to raise the compression ratio because of the centrally located spark plug and they respond very well to supercharging because any kind of forced induction, super/turbocharging or nitrous, will amplify an already good intake head combo.

Go to www.araoengineering.com

They offer 32 valve heads for big and small block chevys that use stock pushrods. They are a little on the expensive side but they were dyno tested on a engine where 2 valve race heads were used and the 4 valve heads were put in their place and the engine made 129 more hp with every other variable being equal.

JoBy

Well said IrocZS383

ATOMonkey

OHC heads also have bigger intake ports(for same displacement) because you don't have push rods going through the head. They breathe better because of that too. OHC also allows for higher cam ramp rates because they don't have to push a lifter up through a bore. OHC's with belt drives will also retard cam timing at high RPM because the belt streatches and as most people know retarded cam timing helps at the high end.

QwkTrip

iTrader: (22)

You need to start reading new magazines. That one obviously is run by morons.

LIL QIK

iTrader: (1)

I think what I would like most about a 4 valver is the fact that I wouldn't have to run near the lift or duration that I do now. It makes for a much more streetable engine and obtaining the same HP or more. Just my 2 cents.