Why when everybody ports a set of heads do they go with a 1.60" exhaust Valve? Why doen't anybody use the 1.55" or even the 1.50" exhaust valve? A 1.55" exhaust valve is used in many Race head applications Brodix EB series has heads with 2.18 intake and a 1.50" exhaust valve, A local Nascar engine builder uses them on his SB2.2's and on the new Chevrolet Nascar heads.

Is the exhaust port that bad? Couldn't you use the extra room by running the 1.50" exhaust valve to cram a 2.05" or larger valve in there? I'd rather be putting more intake valve on the head than exhaust side.

I have my opinions, lets hear yours

 

Many ported LT1 heads out there with 1.56 exhaust valves, 2.00 intake.

 

Simply put, it makes more power. On many of our race motors we use 1.625" valves because it makes more power that way.

 

you sure you want the real answer???? It may cause nightmares?

 

:Lay it on me, I'm ready

 

I'd like to know this too.

 

give me a day to dig out and I will get back to you..cliff notes though...it is still done that way because people are used to doing it that way. I will explain later.

 

Excellent posts, Dennis.

I look forward to the fallout if you explain. Don't you get the urge to hang a sign over the door of some engine builder's shops saying:

"We've always done it that way, that's why."


Jon

 

so if it works they stick with it?

I don't quite understand....I thought sometimes bigger valves flowed less depending on a few things

 

I think it depends on how well a certain combination works. Everything has to work together (port design, powerband). I'm sure someone with more knowledge will chime in!

Most of the time people pick the biggest thing they can get because you know what they say. Bigger is better

 

"there is more than one way to skin a cat" is another point that should be made.

 

why would you skin a cat? you don't eat them do you?

 

Smart *** !
But you never know people eat some crazy crap!

 

Ok guys I will try and make this as short and sweet as possible. Some time ago it was found that installing a large exhaust valve increased performance of a vehicle. The old days of the 100/80 rule (for an engine to operate properly, the exhaust port must flow 80% of what the intake does). I am certainly not a historian but even then these laws were being questioned. Within your exhaust port lies an incredible amount of power and almost NONE of the power is from net airflow. Just think about what the requirements are to make an engine produce excellent power. This not withstanding things that COST power.

1. It must receive a quality fuel mixture at the proper AFR and emulsion rate
2. It must compress the mixture efficiently
3. It must ignite the mixture in a timely factor in order to take mechanical advantage over the crankshaft angle.

At this point we have already made our power…it is done, no way possible to make any more additional power…AND THE EXHAUST VALVE HAS NOT EVEN OPENED! First rule, the exhaust valve does not make power…IT COST POWER! Reduction in output power due to mis-designed exhaust components are caused directly by pumping losses within the motor.

Ok…so now we see that the exhaust valve does not make power…but when properly designed it cost as little as possible towards the net output of the motor. So what does an exhaust valve have to do to free as much power as possible?

1. IT MUST HAVE THE SMALLEST AMOUNT OF TURBULENCE POSSIBLE!! We will touch back on this subject and why later.
2. It should act as a one way valve. Forget the old stories of the super capitulating extra terrestrial overlap exhaust system that pulled copious amounts of nuclear powered oxygen atoms into the combustion chamber through the exhaust….. Sorry junior, Carbon and Nitrogen aren’t very good oxidizers.

Wait a minute? There must be more? What about how big the exhaust valve is? Why not install a 2” exhaust valve so we know we have zero pumping loss? If we recall, the intake valve is key to our gross power potential….we also know that the intake valve must be large as the gas that it moves is colder, moves slower, and is heavier by volume than the exhaust gas. Fuel separation and turbulence is also an issue…. The intake valve should be centered in the bore as best as we can… These are all things that the exhaust valve could care less about! So lets get the intake right…and our exhaust will work out for us! We know that the exhaust gas is relatively light by volume, hot as hell, and is moving hella fast across the valve seat. The less turbulent the exhaust seat design and port is, the faster the exhaust gas can move, the faster the less lobe event we need. OOPS…I left the cam out of the bag…oh err…Cat I mean? YES…you can fix the small exhaust valve with the proper camshaft if the port is designed properly!

In simple terms….1. Design the intake tract correctly, put the exhaust wherever there is room left. 2. Design the exhaust port around the room that you have left along with the rules we posted above. 3. Contact an experience cam grinder to get the exhaust out for you….Simple as that!

So back to the original question…why do cylinder head manufacturers use such a large exhaust valve…The only answer I can find is Habitual in nature. Most OEM’s including the General have been far away from this practice for the last 20 or so years.

Sorry for all the spelling and grammatical errors…You must remember…I grind on metal all day!

Dennis

 

Damn dennis thank you so much for all your great contributions to this site.

Some day when I have more money I'm going to have you do something radical w/ you.
-B