Datsun Roadster Forum-311s.org

Ok, started a thread...

I've been talking some about an anti-reversion plate elsewhere on 311s regarding headers and design. The plate "idea" is from a David Vizard YouTube posting on reversion. At about 18 minutes or so into it he uses a U20 or more likely an L-series head to demonstrate how much less flow velocity there is at the bottom of the port (he calls it "terrible"). As the cam overlaps between intake and exhaust a pulse wave can travel back or "revert" into the combustion chamber, especially at lower rpms. This plate would insert a tongue into that area creating a dam that blocks the reversion without interfering with the other areas of flow. Figuring out how tall and what angle is best is where the challenge sits and may need a flow bench and some time (working on that). This anti-reversion method is already adapted on some headers and exhaust systems with offset "cones" or steps in the pipe but that influences the entire flow path in the port or tube. Some head/port designs don't need this (he mentions earlier experiments with other engines) but it was pretty motivating when he went direct to the Datsun head specifically. I also believe I am seeing data on the datalogs I can collect/analyze that suggests reversion is occurring. Another advantage he mentions is that this will allow one to use a higher duration cam (think B or C cams) without losing (would actually gain) low end torque while still keeping the higher rpm advantages. BTW, I've spoken with some well know experts who have said the primary reason why this hasn't already been done for engines like ours is that it is generally against various racing rules that their customers had to adhere to, so they figured out how to address reversion in other ways that usually require significant head mods but that were still within the rules.

I'm not pretending that I know exactly what I'm doing, but I do hope to be able to work with others who can fill in the gaps and get to the point of "before and after" dyno runs to test the concept. I started the thread now just to begin the community input... there's a wealth of knowledge here on 311s and I'm very open to input. I do think this could be an easy mod to add to the engine (like installing a manifold gasket) that could benefit all U20s with more torque and make running a B or C a better option for all.

Thanks for continuing to pursue this Greg. A very good video for the (me) novice. Among the 300+ comments included 3-4 Datsun builders searching for the added low end power. Greg, I know you have a U20 super-street build with EFI so I don't know what type of power-over-stock (you likely have a ton) or power-over existing your looking for or might expect? As you suggest, if this cone/reed/plate gasket-of-sorts was added to a stock (or B cam) U20, any meaningful bump would be enjoyed by many community members. Thanks in advance for opinions from the 311 experts.

Greg, here’s an interesting video that has an anti-reversion chamber …. Interested to see some real world testing.

https://youtu.be/t2L94tgydNo

Thanks all, keep it coming! I'll reach out or comment to any YouTube postings that speak to this. Unfortunately they don't often respond, and many don't do actual tests. Lord knows I've had trouble getting my own car on the rollers! The plan really needs to be to first get my car on the dyno and finally adjust the timing map so it's as good as possible now, then test the mod. I honestly don't recall which size header is on the engine now (1 1/2" or the 1 5/8") and there are pros and cons to both, especially how they may impact torque down low, but the videos I've seen are not black and white regarding this. If I have the smaller header on it now that will be a confounding factor in my continued testing, as I have a new 1 5/8" I'm going to work with for now. Programs like PipeMax have indicted the smaller pipe is actually better, others have some U20/stroked R16 dyno results that suggest at least higher peak HP with the bigger 5/8. Yes, my engine is different (some mods for better or worse?) and I run 40mm TBs and an Isky cam but I know I gained low end torque over the 37mm chokes I had in the 45mm Dellortos, which initially does not make sense (?). My mods probably make reversion even worse, but I gather that the port design is already problematic as it is (as David V shows in the video).

As always, thanks for the encouragement! I sure hope this works...

Played with heavy stock paper for a template so you can see what I'm thinking. Would be all one plate of course... thick enough and proper material to handle the heat but not interfering with manifold installation. Once designed and proven they would be relatively inexpensive to make (I've talked to and visited a local metal fab company that makes a variety of industrial and aircraft parts).