posted on August 10, 2010 06:59
Engine Tech, Building the Nissan VG30DETT part 2
By Mike Kojima
In the last installment of our build, we delved into building the bottom end of our VG30DETT engine for the goal of reliably achieving 550 plus horsepower. In this segment we will look into what was done to our top end including the cylinder heads to support this power level.
See part one about the bottom end here!
Our VG30DETT heads and lower intake manifold were ported and polished by JWT to increase flow without losing velocity. JWT stressed keeping the runner volume as low as possible while removing obvious obstructions in the ports flow path. The valve seats were matched to the port walls and combustion chamber to remove the slight mismatch that the factory does not bother to correct. The ports were also straightened for a less obstructed run into the combustion chambers with an emphasis on tumble upon entry into the combustion chambers. This kind of porting helps keep turbo lag to a minimum and does not sacrifice too much bottom end power to gain top end. The old school style of porting where the ports were simply hogged out to be as big as possible often reduced flow velocity to the point of making a car lose much of its bottom end power. The latest style of porting that most good tuners use emphasizes port flow velocity to maintain good low end power as well as peak flow. As a final touch, the combustion chambers were polished to get rid of any potential hot spots that could cause detonation.
|Smooth porting on the intake port gives a nice line of sight straight to the combustion chamber. The black coating is Swain Flow Coat which makes the port walls slicker.
After replacing the valve guides and valves with new genuine Nissan parts, JWT finished the heads with a multi-angle valve job which includes a 30 degree back cut on the valve face for better low lift flow. Multi angle valve jobs work by unshrouding the valve seating surfaces and making a smoother flow path for the incoming air or the outgoing exhaust. These valve jobs commonly consist of a throat cut, a seating cut and a top cut. Stock factory valve jobs usually are just seating cuts. Multi angle valve jobs create a smooth transition from the port to the cylinder as the intake valve opens. The converse effect is also true on the exhaust side of the engine but the flow is improved as the flow proceeds from the combustion chamber to the exhaust port. These type of valve jobs increase the flow the most at low lifts where the valves are first opening or about to close. This is important because valves spend more dwell time opening and closing than the do at maximum lift. A proper valve job can contribute up to 50% of the total flow gain enjoyed by good headwork.
|You can see the contouring of the valves and the reduced diameter of the intake valve stem. The contouring is less aggressive on the exhaust valve as it has to deal with a lot of heat on a turbo engine. The white stuff on the exhaust valve is thermal barrier coating and the black stuff on the intake valve is flow coat.
Wednesday, August 11, 2010 3:05 AM
Wow man, it's been years since I looked at a VG combustion chamber. There looks like there's so much room for oversized seats and oversized valves compared to today's heads. In today's heads, the valve heads are almost all touching each other with stock sized valves.
Wednesday, August 11, 2010 5:22 AM
Did you see the valve guides? They were cut flush with the port wall. I didn't flow the heads, we were not hogging out huge amounts of material out of the ports and when this engine was built, there were no good alternatives for the exhaust manifolds which where the main cork. Personally on most turbo motors, I don't spend a lot of time or money on head flow.
On most motors I build for my own use, I don't go super radical on power stuff but I am big on durability and reliability.
I used stock valves because they are the most durable and the stock exhaust valves are inconel or something way better than typical aftermarket. I kinda forgot what it was about them. Perhaps Michell or someone else who is a current VG expert can chip in here.
Wednesday, August 11, 2010 6:24 AM
Mike is this the same engine for your 300zx featured as an SCC project car?
Wednesday, August 11, 2010 7:20 AM
Yup, stock exhaust valves are made from inconel. There do exist in the aftermarket 1/2 mil and 1 mil oversize valve options. Some of them also use inconel for their material.
Wednesday, August 11, 2010 8:19 AM
@ Wrecked: Yeah this is the same engine. It's nice to find these articles online as my old SCC issues that feature project 300ZXTT are starting to fall apart as I've paged through them so many times.
Wednesday, August 11, 2010 9:25 AM
Does the 300zx still have the parachute? hahaha
Wednesday, August 11, 2010 10:00 AM
Yes it is the same build but I have had many requests to have some of my old stuff on here so I have reissued some.
In our long term plans, we have another VG build on the horizon.
Wednesday, August 11, 2010 12:27 PM
Great article as usual Mike.
I have few questions:
1. Did you guys bench flow the head before you ported and polished it? If so, how many CFM did it flow? How about after?
2. Is there any reason why you stick to stock valves instead of using some aftermarket (maybe +1 mill) valves?
3. How about cutting down a bit the bronze valve guides to increase flow?
(I've heard that cutting down the valve guides flat with the intake/ exhaust ports helps bumping the CFM quite a bit but at the same time the valves are susceptible to bend under high rpm, so maybe cut down a bit instead, and not all the way down?).
Thursday, August 12, 2010 2:37 AM
Thanks for the explanation Mike.
I asked about the oversize valves because I noticed there is a lot of material between the valves...and still keep it reliable.
Thursday, August 26, 2010 2:04 AM
Mike, the OEM VG30DETT exhaust valves are made from inconel like you mentioned.
Tuesday, December 21, 2010 12:32 AM
I remember that engine build in scc, but never saw it completed. I saw a black sentra ser over at jwt many years ago, and wondered if it was coleman's. I seem to remember him having one that he would talk about in scc.
Monday, May 09, 2011 3:48 PM
Any new updates on the project, Mike?
Wednesday, August 01, 2012 5:10 PM
what happened to this project, it was one of my favorites. thats the same 300 that was in scc right?
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