Ported and Polished Cylinder Heads
The Basics
Power in an internal combustion engine is created by adding more air and fuel into the cylinders. A good way to do this is to remove the restrictions found inside the cylinder head. I started off with a set of Thrasher Ported and Polished Heads. These are the 1st Generation design, and TEP has since made improvements to increase flow at all lift numbers.
Modifications
Due to a clearance issue TEP had with the available upgraded valve springs and titanium retainers hitting the stock rocker arm, I was forced to change to a different spring. Interestingly, when I questioned Thrasher about this, I was informed that it was an installation error and not a clearance problem. About a month later, another ClubGP member had the same problem. Once confronted with this, their "solution" was to SELL me another set of valve springs and retainers. This incident has taught me to double-and-triple check all clearances, regardless of what any manufacturer tells you.
I ended up calling Zooomer at ZZP and ordered a set of Comp Cams LS1 105 lbs valve springs (P/N 26915) and steel retainers. Zoom suggested these as he felt that they would go well with my HL camshaft, which it appears they have.
After my most recent and unfortunate mishap, I ended up salvaging my heads by upgrading to a set of oversized valves.
I spoke with Paul Jerkatis at PRJ Performance and decided to purchase a set of
1.84" intake and 1.60" exhaust valves. The valves are a one-piece stainless steel design which are far superior to the L67's stock valves,
that are 1.80"/1.52" and have a two-piece design and an inherent weakness at the joint. The Si valves also feature a swirl polish to aid in increasing flow at all lifts.
Generally, one would think that having an exhaust valve that is only 13% smaller than the intake valve would do more harm than good,
with forced induction motors however, you have to do almost as much work to get all that extra air out of the cylinder as you do to put it in.
In addition, increasing the exhaust valve size usually shows an increase in average cruising MPG. This is because the motor has to do less work to get the air out,
and thats equates to greater efficiency.
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While I had the heads off the car, I decided to optimize them for use with my TOG headers. Originally, the exhaust work done to the heads was limited due to the fact that I was still running stock exhaust manifolds. With the addition of headers, more exhaust work was required to get the most flow out of my heads. I had the exhaust ports openned up to just shy of 1 5/8" - the size of the TOG primaries.
The additional exhaust work and oversized valves have allowed me to process more airflow per pulley size, I have dropped approximately 2 psi (down to 7) with the 3.25" thanks to the head work. I have also noticed approximately a 10% increase in gas mileage while driving around town. Now it's time to head to the track to see how she does!