Our first order of business was to dial in our SS3 three bolt product which runs a close second in sales only to our SS3 VVT. Our colleagues at Cam Motion supplied us with a selection of grinds featuring our GPI/GPX proprietary lobes ranging from 229 to 235 degrees’ duration on the intake side. The exhaust varied from 245 to 252 degrees’ duration. The lobe separation ranged from 111.5 to 116 degrees and all net valve lifts were in the mid 6s. We labeled these T-1 thru T-5 as shown on the charts.

The ported version looked quite a bit better, this time we were disappointed with the positive result as the porting is very labor intensive and it’s probably going to be requested a lot going forward. We didn’t find significantly more peak power, but we got it everywhere and the midrange torque increase was worth the effort. The only negative is after the porting the manifold it is structurally impaired for any future forced induction applications.

The first head we tested, which we will refer to as brand “Y’, was a nice looking 280+ cc runner GM casting using OEM LS3 valves. It has a CNC chamber measuring a tad over 71CC. We figured the midrange would be a little weak on a 378 cid engine due to the port volume. We find that when people start racing flow benches sometimes the end result isn’t as expected.

This head performs well on the bench and may well look better on a larger engine but didn’t do the LS3 much justice. It picked up about 10 HP at peak while losing 20 lb ft of torque thru the mid range. Now granted it was lower on compression by 1 cc than the stock head but we failed to find that equated to 20 lb ft of torque. Because we were dealing with varying chambers and compression ratios we bumped the timing in both directions up to 2 degrees to find the happy place. We also made sure the coolant and oil temperatures were the same within a few degrees. Again, this was to learn and improve rather than generate propaganda. If however at the end of the day we sell a few heads and cams I can live with that too!

The next head we tested we will refer to as brand X. It has a runner volume in the 270s and had already been milled to 66cc. We will never know how it might have fared against the brand Y on a level field, as we didn’t have an expediant way to find out. All the same it gave us our first real glimpse during our testing to see how compression sensitive our mule would be. We figured we could test our 268cc ported and chambered head at 70cc against our non-chambered version and the stock head. Then start milling to compare with brand X. Below are the results as compared to stock, of our GPI chambered and non-chambered.

What we learned here is that our heads on equal compression picked up an honest 20hp non-chambered and 25hp chambered without giving up any appreciable midrange, thus the beauty of a smaller intake runner which can maintain air speed on a smaller displacement engine such as a LS3. We also observed that the chamber work was paying off. We noted that the chambered head did not require more or less spark advance than the non-chambered, not what I expected. Below are the results of the GPI 268 chambered heads compared to the brand X at the same compression ratio.

The best power opportunity so far seems to be with our 268 cc runner with chamber work and some additional squeeze. This graph illustrates the gain over stock at 66cc. We netted 34 HP over the stock head while improving torque above 4200 RPMs without appreciable loss below.

Now for the cathedral vs Rectangular port show down. We were fully prepared for this bad dude Trick Flow to do some damage and it didn’t disappoint. We have ran these things on some forced induction and nitrous builds that did the work. In our minds however, after the dust settles our good ol LS3 factory casting with a little GPI magic and an OEM intake is a great choice for the average LS3 owner! Our next step based on this testing will be to work out a solution to achieve improved compression on the SS3 equipped LS3 and L99 without fly-cutting valve reliefs or perhaps do a better job taking the mystery and tension out of the fly- cutting process.