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In my previous post, I mentioned that I spoke with an Engineer at Jim Wolf Technologies (JWT) however, it's not advisable to post someone's name on the internet without their permission.
The JWT Engineer explained to me about the cooling passage-way above the combustion chamber "triangle" (between the spark-plug boss and the exhaust valves), causing Ethylene Gycol based anti-freeze to "Flash Over", which causes detonation, and in extreme cases thermal runaway.
The rest of the info (from my previous post) came from studying a sectioned (cut-away) VG30 Head, actual testing, experimentation, more testing, observation and experience.
This is the 4th set of VG30's I've done and I'm mildly impressed with how well the heads flow bone stock. The Nissan Engineers did a very good job at designing these heads (20+ years ago). They designed this head, with the valves very close to the cylinder wall on purpose. The combination of valve location and the port design, work very well together.
But of course, any design can be improved upon, and has significant improvements, look at the VQ (for example).Over the last couple of days, I decided to experiment, to prove (at least to myself) about Oversize valves, and the Cylinder wall shrouding issue.
While running these tests, the collected data show trends that going with O/S valves actually hurts the flow (and performance) UNLESS, you overbore the cylinders. WHY?
The outer 1/3 of the valve is shrouded by the cylinder wall (past Convergant Lift). Going to O/S valves creates additional area that is shrouded by the chamber and the cylinder wall. Additionally, the O/S valve is closer to the free edges of the chamber. This additional amount needs to be included into the calculation as well. The Nominal size valve is just far enough away from the free edges (by design), and are not shrouded by them.
Do the simple math, (compare nomial valve diameter x Pi with O/S valve dia x Pi) the numbers don't lie! To make O/S valves work, you will have to go to 89mm bores (or larger) to get the valves enough away from the cylinder wall. To have a reliable combination, Sonic Block testing, and a whole stash of blocks would be required to do this, because the VG30 suffers allot of core shift.
The combustion chambers would then need to be opened up, thus unshrouding the valves.
From again studying the Sectioned Head & block, there appears that there is enough room to "wet Sleeve" the VG30 block (as an alternative to having a stash of VG30 Blocks). In my test today, I used a 4.00" bore simulator (instead of the 3.5") and ran flow tests on the LH head.
I try to simulate as close as practical the "real world" that the heads are flowing in, thus the 3.50" bore adapter used in all of the previous tests. All of the CFM's and velocites jumped substantially (as expected /predicted)!
However, I was keenly interested in what the flow numbers (cfm AND Velocities) were doing AFTER the convergant lift zone. (Nissan designed the heads to "fall off" after the convergant lift zone due to the shrouding issue). This is where it got interesting! I've never seen numbers that big before on my bench! (both cfm and velocities).
Unfortunately, the heads really started flowing the biggest numbers at valve lifts that are, for this engine, prohibitive. After 12mm lift the Lifter oil gallery is completely open, causing oil starvation of the cam bearings.
The intake ports (with stock sized valves) max flowed at 265 cfm, (at 14mm lift) which equates to over 1000 hp! Obviously, to take advantage of the larger bore, O/S valves AND the accompaning Port and Chamber work would need to be accomplished to optimize the combination. I probably am not far off by predicting that going to a 4.00" bore with the additional required modifications, would likely result in an engine capable of over 1200 hp! (food for thought)
For a Turbo Application, maintaining port velocity is essential to a well performing, well matched combination. The data shows that O/S valves will work very nicely in an over-bored engine. The VG30 Heads "Stand UP & Shout" when the cylinder wall is far enough away to prevent valve shrouding, or the chambers modified to do the same with Nominal sized valves.
Hmmm, how about a Twin Turbo, DOHC, 4 Valve 3/4 scale SBC????? Wet sleeve a VG30DETT (4.00" bore), and that is exactly what you would have, but capable of over 1200 hp!
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