Head guru question
Sunday, 26-Mar-2000 16:02:36
As I freshen up my mild 1934cc engine, my thoughts are drifting to my stock valve heads. If I up the heads to 40x35.5 and leave everything
else as is, what kind of hp gain can I expect? Also, what will it do ot the torque band? The engine CR is 7.2:1, the cam is a mild grind, the
are 40 dells and the exhaust is 1.5 with a turbo muffler/stinger hybrid. The engine is being detuned a bit in CR for the sake of long distance
cruising. Thanks all!
Vader: Head guru question (26-Mar-2000 16:02:36)
Rick M: Re: Vader...anytime you can improve flow it helps....it depends on what you want the motor to do... (26-Mar-2000 16:46:15)
RM: Re: Vade.....7.2 cr is fine for long distance. Depends on what you are doing (n/t) (26-Mar-2000 16:48:01)
John Connolly: my input... (26-Mar-2000 22:05:31)
Ed: Save your money and port them puppies... (27-Mar-2000 11:27:03)
Vader: The heads are already ported... (n/t) (27-Mar-2000 13:16:33)
I am certainly not a head guru like Steve Tims, Duncan & Nuss, Clyde Berg and others. What I can tell you is that bigger anything is not
always a solution unless you are looking for something specific. Just like motor sizes. Big motors make power at a lower rpm while
smaller motors need to be twisted higher to develop horsepower. I have always found that big valves help small motors breath better and
develop power at a lower rpm. You don't have the cubic inches to help make power so Cam, Carbureation and overall volumetric
efficiency help create the power. I had a 1689 that would run in the 15's with a set of 44 webers in a full weight 67 bug. I have seen
motors your size make a real 130hp and be extremely responsive and driveable with bigger vales. I don't recommend going over 40mm x
37 on the street as you don't leave much room between the valves. Street driving is for longevity for the most part and going bigger is not
really worth it in my book. Yeah, you can build them to run 12's and 13's but you don't see those motors taking many road trips. It all
depends on what you want. In answer to your question. The bigger valves will give you more midrange and top end. A little more
responsiveness but not astronomical at the bottom. This is based on having done it...not just theory. Get other opinions too... to see what
others who have built a lot of combinations have to say.
if you put in big valves and leave the ports alone, you are wasting your money and time. 35.5 X 32mm valves can flow about 40% more
than the existing ports, so THAT is where you should spend your time. Port the heads! If you plan on more than 40% more flow, then you
need bigger valves too. But to add big valves to a stock port head is a joke. Just cause' almost every VW place is selling a stock head with
a big valve doesn't mean it works!
Hmmm, my response did not go through the first time, probably will appear after I respond a second time...
I just ported my stock dual port heads, BIG DIFFERENCE!!! Better power, freer revving, and I still have a stock carb and intake. My carb
went from slightly rich to extremely lean from the increased flow. you WILL notice the difference, especially with your engine setup.