2.9 Race Heads

Speedster56 · 12-08-2020, 11:29 AM

I have a 2.9 race engine that currently has a set of stock 2.7 twin plug heads cut for 2.9 P&C. I have available a set of ported twin plug heads with 41mm Intake and Exhaust, with standard valves of 46mm(I) and 40mm(E). I am hoping someone can advise me on how these heads may perform in the motor compared to the standard size. Engine has RSR spec cams, 46 PMO's, 12.5 compression, etc,
I recognize that intake and exhaust will need to be matched.
Thanks for any comments

stownsen914 · 12-08-2020, 05:20 PM

What is the intended use of your engine? If it's track/race and you don't need to worry about low end performance, it will certainly make better power in the mid and upper ranges with ported heads, especially with RSR cams and 46mm carbs. Even more so if your stock heads are 2.7 heads which have the smaller, non-S 32mm ports (2.7S heads are 35mm).

Speedster56 · 12-09-2020, 09:15 AM

Thanks for your comments Stow. Just to be clear, it is a full race car to HSR spec and only used for that purpose. I run it to about 7300 rpm through a close ratio gear box on 17" Hoosier R7's.
The ports are opened to 41mm on both sides but there is a step in the exhaust port as shown in the photo. Since I didn't do the mods on these heads, I wondered if there is a technical reason for this step?
Thanks for any comments

stownsen914 · 12-10-2020, 05:25 AM

These are 2.7 heads, right? It looks like they've been machined for an insert that may be missing in this pic (referring to what looks like a step). The inside of the port looks a little funny. Can you take some additional pics showing the port a little closer from different angles?

Speedster56 · 12-10-2020, 11:46 AM

Below are additional photos of the head. The step in the exhaust port is half circumference only. I also noticed the small step on the edge of the combustion chamber which I don't recall on the stock 2.7 heads, and I don't have a stock one handy to compare. The surface of the exhaust port is slightly rougher than the intake side. The valves have a lot of racing hours on them.
Again, thanks for any comments.

stownsen914 · 12-11-2020, 02:21 AM

The step in the exhaust port could mean that it was meant to accommodate an insert of some kind, or perhaps the headers had an interlocking portion that protruded into the head? Judging from the orientation of the step, it looks like the port is offset toward the roof of the port. I have heard of people doing this to improve flow. The 41mm measurement is taken at the outermost portion of the port?

The step in the combustion chamber leads me to wonder if these were used with non-standard cylinders or a different style cylinder head gasket.

jpnovak · 12-15-2020, 06:15 AM

The step is likely anti-reversion step. It is located on the short side of the port to ensure the radius stays as long as possible.

It is also highly likely that the port is machined concentric to the exhaust studs to match a larger exhaust header. There is just more material on the short side of the port and it was not blended the way the long side was.

The modern equivalent to this is a "D" shaped exhaust port.

Yes, this can also be done on the intake. Effectiveness depends on overall shape of the rest of the port.

BURN-BROS · 07:38 AM

That's a later Thermal reactor head. The step is the register for the reactors. Presumably, the person doing the port work didn't want to take the port floor all the way down and make the area closest to the seat sharp.

The step was to get proper deck height while getting the piston higher into the combustion chamber to increase compression. Example; Running 0 deck at the cylinder, yet retaining 1mm at the head...