I was initially looking into thermal gaskets for the manifold when I stumbled across the 1/4" thermal manifold spacers. Haven't seen much about those on any of the forums and it doesn't seem many people run them. For the price of a plenum spacer I could pretty much get a 5pc kit with manifold upper and lower collector spacers, intake manifold spacer, and throttle collector spacer. (Something along these lines: http://www.nwpengineering.com/Phenolic_Spacers.html) Motordyne also makes a spacer that runs around $80. It seems like that should do pretty much the same thing as a regular plenum spacer, lengthening the runners, and there would be less chance of running into a warping issue from torquing the plenum.

Why is it that nobody runs these, is there something I'm missing? What are some opinions on this matter?

 

Look under "applications" on the link you provided.

 

According to the Nissan website they are the same part number, part numbers can be seen in attached images. In addition, motordyne offers one as well:

http://www.thenismoshop.com/MOTORDYN...5DE_p_435.html

I did my research. It seems like this may even be better than a Plenum spacer because if a thermal manifold spacer is used it will cut down on heat closer to the source. I'd love to hear from anyone that has this, otherwise maybe I'll give it a shot and write a review when I get some disposable cash. (Poor college kid status ftl)

 

A plenum spacer doesn't make power by cutting down on heat, it does it by increasing the volume of air inside.

Its been 10 years since the G35 came out... if what you are suggesting was true don't you think someone before you would have posted about it by now?

 

From what I understand runner length has a large part to do with why the plenum spacer is installed. While a spacer on the plenum may add some capacity, it seems that the runners can only pull the air so fast since everything is running at a constant pressure. Unless there is forced induction involved the added air capacity shouldn't effect anything the way I understand it. Polishing the lower plenum ports in addition the lengthening the runners could help with smoothing out the airflow but I don't see any real advantage to the spacer on the plenum rather than the manifold because essentially the runners would gain length either way.

I realize this car has been out for almost 14 years and the plenum spacer is the 'go to' mod, but really I was just wondering why. I'm not saying I'm right or you're wrong by any means, you could be 100% correct. I just wondered if anyone had one because, from the way I understand the fluid dynamics, it should prove to be about the same performance wise and it doesn't seem like anybody takes the manifold route.

 

Runner length has nothing to do with why a plenum spacer is installed, at least on a G35/350Z. That theory might apply to other models so limit my response to the VQ35DE found in a G35/350Z. The reason for the spacer is due to the design of the upper plenum. In order to clear the hood on the early 350Z the upper plenum slopes down to the front of the engine. Because the upper plenum is very close to the opening of the lower intake runners, the front two cylinders actually get starved of air. The plenum spacer fixes this "design flaw" and provides better airflow to the front cylinders.

The VQ35DE "Rev-Up" engine has a different lower plenum design and doesn't suffer air starvation that regular (or non-revup) engines have. The rev-up plenum has its own flaws, mainly due to shorter runners and thus loss of torque, but has better flow near the rev-up's higher redline.

The intake spacers in your first post apply to FWD VQ35 engines that have a different intake design that doesn't apply to the RWD engines. (Apples and Oranges)

The manifold spacer from motordyne is really just to provide clearance for aftermarket injectors. In theory the rev-up engine might see some gains from the runner lengthening, but simply swapping on a non-revup lower plenum yield such good gains that a measly 1/4" spacer seems like a waste of time.

 

^ correct. The lower intake manifold spacers are used to allow clearance for taller injectors - that is all. You will theoretically increase midrange torque at the expense of peak power, but it'd be minimal. I have one spare if you need one that I will let go cheap