Intake Plenum for a 260Z

[Ron Earp]

I am wanting to make a intake plenum for my 260Z. What I wish to do is make a "log" style plenum that will tie both carbs together. I can then take that log and connect it to a cone style filter out at the front of the engine bay to maybe help pull a bit of cool air in the engine.

Maybe the attached picture will help a bit with what I wish to do.


The space is tight in the engine compartment. I think it is even tighter than a 240Z on the intake side as the carbs seem to be a bit wider than what is on 240Zs.

Anyhow, I can envision a round tube that has two flat areas on it to mount to the carbs. I could drill holes through the tube to mount the bolts into the carbs and the outside holes can be patched with duct tape or whatever. The end of the tube near the firewall will be sealed. The end pointing toward the front of the car will be connected to a cone filter.

My problem is what to make it out of. I don't want to use muffler pipe as it is too heavy and I can't "work" with it well considering my tools (I have no torch to cur metal, welding skillz are so-so). PVC pipe won't bend or crush and is also heavy. Ideally I could see some thin walled aluminum tube, maybe 0.040", 0.060", or something in that range that I could manipulate with common hand tools.

Any ideas or suggestions?

[Rich A]

Ask and you shall receive.
Look at this website.
They make a box for 6 cylinders.
TWM Induction - Air Boxes (805) 967-9478

[Ron Earp]

Rich, that is perfect!!!!!

But $375, that is steep, woooo wheee!

However, it did give me an idea. Maybe I can use flat alumium for the base, then make a thin walled half round cover from aluminum to cover it. A plate can be cut for the back and riveted and sealed there. On the front some type of 2.5" or 3" round tube can be fit to another cover plate.

Nice find, thanks! Anyone got a six cylinder one of those lying in their junk pile they want to sell and send to be by Friday??? Can't hurt to ask....

Spoke too soon - it is too wide. I only have 4.25-4.5" of space from the carb face to the shock tower, this box requires 5.5". Dang.

Hot damn, the four pot one will fit!

[Gregg]

Ron, how about stove and/or AC duct pipe from your local home depot? It's light and simple to work with. Comes round, rectangular.

[Ron Earp]

Gregg, that is a good idea. I had given some thought about stopping in over at Lowe's and checking that option out too. I think that stuff comes in 3" round I know. I've not seen square, but that would be good because I bet they would make a square to round adapter. I measured some more, the damn rear of the carb to master cylinder measurement is only 4" and it'd hit any of the TWM pieces.

[Jac Mac]

Ron, Put the carbs in the plenum as well so they reference the same pressure should you be careful enough to find a spot for the K/N Filter that gets a bit of pressure. Make the lid removable to service carbs as well.

P.S.-- A large rocker cover off a truck or older six cyl can be used as an airbox-flat plate/sheet alloy onto carbs with provision to screw the cover onto the alloy plate. Extend the plate & cover in front of the carbs so large inlet tube can be connected for cold/ram air.

[Doug Clark]

I concur with Jac Mac. Also don't underestimate how much this'll screw up the carb settings. When I built a closed box setup for the TZ250 I used to have it was a hell of a job to get it right with a fully closed box and I was never 100% happy with. That was with a bike though and car drivers are less sensitive to throttle control-reaction than bike riders so it may not be an issue.

Also make the box as big as possible, the bigger the box the slower the air in it and therefore the higher the pressure inside it & the inlet should have the smallest cross sectional area of the whole thing, the idea is to gradually slow the air and bring it up to pressure in the box.

I made mine by making a polystyrene model of the required shape, wrapping it in FRP and then using gasoline to disolve out the polystyrene, ugly but effective. Seal the polystyrene with glad wrap or paint to stop the resin getting into it.

There is math somewhere to work out proper size of the box and the inlet area you should be able to find that on the web, if the inlet is too big it can also cause issues.

[Jim c]

Made some air boxes and manifolds for MGB s with single webber.
The trumpets came within about 25mm of the lid.
The top end was fine with or without lid.
Bottom end lost about 5HP with the lid on until about the mid range then it was ok again.
The solution was put 60mm holes directly over the trumpets in the lid,covered the holes with K&N filter.
What appeared to be happening was the wave from the intake at low rpm was traveling out and hitting the air box lid and sending it back early.
The holes allowed it to pass and solved the problem.
So Ron what I am suggesting is if you do have a room issue the box ends up flatish in shape,check it with and without the lid on at the dyno to check if you get the same problem.
Any one that has made air boxs would say that $375 is not that bad as it can be very time consuming.

all the best

Jim

[Gregg]

Originally Posted by jac mac

Ron, Put the carbs in the plenum as well so they reference the same pressure should you be careful enough to find a spot for the K/N Filter that gets a bit of pressure. Make the lid removable to service carbs as well.

P.S.-- A large rocker cover off a truck or older six cyl can be used as an airbox-flat plate/sheet alloy onto carbs with provision to screw the cover onto the alloy plate. Extend the plate & cover in front of the carbs so large inlet tube can be connected for cold/ram air.

WOW. Not only a wealth of knowledge, but an ARTIST as well! Jac, can I commission you for a project??

[Ron Earp]

Thanks for all the replies fellows. Jac Mac, I'll give that a go. I also like that idea about making a foam core and laying one up, but I don't have that kind of time. I got to have the car on the trailer and ready to go in 7 days and there is quite a bit of other stuff to do.

I like this sort of thing about this forum, useful responses. If someone asked this question on some other car forums 1/2 the responses would be debating if the airbox would be useful or not, 1/4 would be about come chick's ta tas, and maybe 1/8 of the responses would be useful.

Ron

[Gary Kadrmas]

Ron,

Have you looked at the Group 44 Datsuns air induction plumbing?

It seems to me they used an air box, and plumbed the ductwork from the carbs back to the leading edge of the windshield using the existing chamber where the heater/defroster box is located. This is the same tech fabrication that the Boss 302 Mustangs used in Trans-Am Racing.

I may be thinking of a local Datsun 240Z racer rather than Group 44......but I do remember the box/chamber set up, and he said it really helped with midrange RPMs.

Regardless, I hope you get something in place to give you a slight edge in HP, and good luck with your next upcoming race!!!!

[Ron Earp]

I think you are thinking of Group 44, but not Datsuns. My race buddy is a TR8 racer and Group 44 man, in fact, his car is number 44. They were famous for racing TR8s, TR6s, Jaguars, and other "unracable" British stuff. The TR8s they had back in the day had that exact airbox that you mention. Pulling air from the high pressure point at the base of the windshield.

Thanks for the wishes, I'll have some video to watch if it all goes okay.

R

[Jac Mac]

Originally Posted by Gregg

WOW. Not only a wealth of knowledge, but an ARTIST as well! Jac, can I commission you for a project??

Commission away, you know the address, do I look for a large brown envelope?, My tech drawing teacher from 1963/65 would be mortified if he saw that!

Ron, the base of the W/screen is the ideal place to pick up some pressure air particularly at higher speed & if you ever get into a drafting situations, while in front of the radiator is good, the drafting configuration will kill the airflow at that point, just when you need any power gain.

Jac Mac

[Gregg]

Originally Posted by jac mac

Commission away, you know the address, do I look for a large brown envelope?, My tech drawing teacher from 1963/65 would be mortified if he saw that!

Ron, the base of the W/screen is the ideal place to pick up some pressure air particularly at higher speed & if you ever get into a drafting situations, while in front of the radiator is good, the drafting configuration will kill the airflow at that point, just when you need any power gain.

Jac Mac

Is non sequential bills, no larger than a $20.00 acceptable?

[Jac Mac]

Originally Posted by Gregg

Is non sequential bills, no larger than a $20.00 acceptable?

Great, I can see you have done this before-what ya want me to draw, a REAL transaxle?

[Gregg]

Originally Posted by jac mac

Great, I can see you have done this before-what ya want me to draw, a REAL transaxle?

Nah, I friend of mine should still have a detailed set of prints for a REAL trans. This is personal.


I have photo's of a certain manufacturer with his favorite Ewe. I was thinking of a detailed life size photo. I'm torn between a birthday present and blackmail. I'll let you decide how to use the photo. NO water colors for obvious reasons.

[Jac Mac]

Damn, I will have to send your commission back, I am no good at animated drawings, knowing how involved you like to get in these things I suppose it would have been a three-way deal as well!
Think I will stick to car parts.