Making My Custom Plenum.

[47une]

Hi all,

Well I am going to be making my own plenum and just need some info before I begin. I have all the materials and equipment at my disposal and I make everything else for my cars, so I thought I would have a go at a plenum. I have done a lot of research around the subject but am still coming up with unanswered questions.

Background info first, it is an RB20DET with KKR430 turbo, bigger injectors, Power FC, front mount, fuel system and all other supporting mods. Let me do the next part in CAPS LOCK --- I AM NOT INTERESTED IF THIS DOESN’T MAKE MORE POWER OR IT IS USELESS UNDER 300KW AND ALL THE OTHER COMMENTS YOU WANT TO POST. I am doing this for me and for me only, I will be doing a before and after Dyno run and it will be tuned by a professional tuner, so I am not worried about the tuning side of things. Ok, now onto the questions.

The first question is, general volume of the plenum? It is a 2.0 litre engine, is there any set rules on the complete volume of the plenum itself? This is for a drift car, so a mix of response and top end power is important, not looking to go huge and make it laggy but lots of power and not looking to go small and make it responsive but lack top end.

The next question is the tapering of the plenum, is there any set rules on what the taper of the tank should be? How big should it be at the front and what should it go down to at the rear?

The last one is more of a callout for those who make plenums or who have custom made plenums, I am just looking at getting some dimensions of the tanks, like total length, size tank height at rear of plenum and tank height at front of plenum.

I am only interested in replies from people who have direct experience with making plenums or who work in race fabrication style workshops.

Thank you all in advanced for your useful replies. I will keep you updated with pics and how it is going as I make it.

[1TUF33]

rb25det should be around 12 liters. so im guessing that a rb20 would need 10 liters of internal volume. there is a lot of technical details in plenum design man, flow characteristics change with pressure and runner length and diameter, air temp even can affect flow. for a turbo engine the details are not as important as in a NA motor, as the air is getting forced around in the plenum. with most plenums tho, the taper at the end is more for asthetics than function. i recommend bellmouthing the internals, yo can get 52mm bellmouths from JNT performance in albion park rail for about $20 each. if you like i can email you a program for flow analysis which i stumbed accross web.....

[47une]

I was under the impression that the taper of the plenum was designed for airflow to be even across all cylinders. All I will be doing is making a top section which will bolt onto the bottom section of the standard runners. I have a seen the difference in flow characteristics between bell-mouthed runners and just radiused flat and the radiused flat did the same if not better than the bell-mouthed which can be seen at the bottom of this page - http://www.supraforums.com.au/forum/showth...6172&page=2

Thanks for the input, it was very useful! Keep it coming.

[ISL33P]

Hi Joe,

Just some tips.

- From my own experience, plenum volume is not directly related to engine capacity, but more the port size. Try about half the volume that has been mentioned above.

- Always use bellmouths as these are what you would use to even out the different lengths in runners, probably one of the most important factors in building a plenum.

- The taper is something i wont disclose for some very good reasons, but have a look at all the top mount ones available and you would get the idea.

Ariel

[Mr Jollwah]

12 ltr plenum I thought that 4-5 litres would be plenty on my RB25.

[Sydneykid]

Why are you bothering? Spend ~$200 and buy a Chinese GReddy copy, then take the time to match it all up. If you spend the same amount of time (as you would making one from scratch) working over the GReddy copy, you will get a much better result.

Cheers

Gary

[47une]

Thats actually a really good idea, I could quite easily do that....... Maybe just cutting the stock runners down the bottom after all the injectors and all the rest of it, cutting the GReddy copy one at the runners and matching the two up.....I will definitely look into that. But really, it is not going to take me much time at all to make one from scatch, this sort of fabrication work is very simple. I know what you mean though, they are a tried and tested design whereas mine is going to be a copy...

[Sydneykid]

I think you will find the RB25 GReddy copy will bolt up, you might just have to do some tidying up around the ports (water and air/fuel) on the head and the manifold. It will take the top feed injectors if you make up some alloy collars and you can also use your standard RB20 fuel rail. You will also have to make up a simple bracket for the throttle clable. A little bit of work to make the DIY genes feel satisfied and a better result in the end.

Cheers

Gary

[47une]

THanks for your help there, I will have a shot at making the custom plenum myself first, as I said, it will take very little effort and cost nothing. If it proves to be a good design I will stick with it, if it turns out bad, I will have a shot at the GReddy copy one and do the mods I need to.

[nzdingo]

Thats actually a really good idea, I could quite easily do that....... Maybe just cutting the stock runners down the bottom after all the injectors and all the rest of it, cutting the GReddy copy one at the runners and matching the two up.....I will definitely look into that. But really, it is not going to take me much time at all to make one from scatch, this sort of fabrication work is very simple. I know what you mean though, they are a tried and tested design whereas mine is going to be a copy...

hey mate, i am in same boat as you except my makeover of the engine isn't as drastic as yours. i am also giving making my own plenum chamber a go. so thank you for starting this forum because i didn't even have a clue that there was a set amount of litrage you were supposed to have. i have an rb30 an have made a custom top mount exhaust manifold with turbo and am making the intake aswell because it will make it about 10 times easier and nicer looking when i plumb up the frount mount. and am the same as you don't really care if it makes for power or not.

so cheers mate.

[esky_mad]

no the rb25 greddy copy will not bolt up. the RB20 and RB25 head on the intake side is a different design.

IT WILL NOT BOLT UP. not to mention water lines on the intake manifold are completely different. i have a custom top half welded to the stock runners on my RB20. im also not interested in massive power, just the fact that having a larger high mount turbo requires ugly piping.

goodluck mate.

[47une]

Esky mad,

Who did the custom top half? Did you do it yourself or did a workshop do it? Do you have pictures of it? Would it be possible to get some measurements of it for me? If you could that would be fantastic, save me a lot of time.

Joe.

[Sydneykid]

THanks for your help there, I will have a shot at making the custom plenum myself first, as I said, it will take very little effort and cost nothing. If it proves to be a good design I will stick with it, if it turns out bad, I will have a shot at the GReddy copy one and do the mods I need to.

How will you know if it's a good or bad design? Do you have the equipment to check it? Flow bench? Individual cylinder EGT analysis? Individual cylinder A/F ratios? You could easily end up with one (or more) cylinders lean by 5% and one (or more) cylinders rich by 5%. And you would never know by looking at the mixed exhaust output (EGT or A/F ratio) from all 6 cylinders. That's the problem, you may not find out that its has bad distribution until it's too late.

Cheers

Gary

[ISL33P]

considering the standard plenum has a 12+% varaince from the lowest to highest flowing runner, 5% isnt too bad....oh yeh thats right the standard one is good for 300rwkw lol

[nzdingo]

How will you know if it's a good or bad design? Do you have the equipment to check it? Flow bench? Individual cylinder EGT analysis? Individual cylinder A/F ratios? You could easily end up with one (or more) cylinders lean by 5% and one (or more) cylinders rich by 5%. And you would never know by looking at the mixed exhaust output (EGT or A/F ratio) from all 6 cylinders. That's the problem, you may not find out that its has bad distribution until it's too late.

Cheers

Gary

hey gary,

with you saying that i am now seriously considering not making one lol. but i am in new zealand an am not sure where i can get one of these 200 dollar intake plenums that were mentioned earlier, that is y i was going to make one. i haven't got alot of funds thats y i am making most of the stuff i can. if you could help me track a cheap one down, or even point me in the right direction that would be great,

cheers rory

[47une]

Well when it is copied off a design like the Plazmaman or any of the others, it cant be that far off, and i have been looking for an excuse to put an RB25 in or rebuild my engine anyway!

[47une]

Good call ISL33P. I have read heaps of times how far out the standard ones out, so really, if I design mine close to copying all the aftermarket ones out there, I cannot get it as far out as a standard one!! Thanks again for your help!

[Sydneykid]

considering the standard plenum has a 12+% varaince from the lowest to highest flowing runner, 5% isnt too bad....oh yeh thats right the standard one is good for 300rwkw lol

Why do I feel like we have had this conversation before

I have never ever seen anything remotely close to 12% variance in a standard inlet manifold. Recently at 315 rwkw we got ~3% lean on #6 and ~2% on #5, with 1, 2, 3 and 4 pretty much the same. That's using 3 EGT sensors and 3 A/F ratio (lambda) sensors in the individual primaries from each cylinder of the exhaust manifold. That's the 4th engine now with similar results on the chassi dyno plus 1 on the engine dyno, no fluke or rouge numbers.

Cheers

Gary

[Sydneykid]

hey gary,

with you saying that i am now seriously considering not making one lol. but i am in new zealand an am not sure where i can get one of these 200 dollar intake plenums that were mentioned earlier, that is y i was going to make one. i haven't got alot of funds thats y i am making most of the stuff i can. if you could help me track a cheap one down, or even point me in the right direction that would be great,

cheers rory

Ebay, or try the Group Buy and For Sales threads on here.

Cheers

Gary

[ISL33P]

not a problem. More than happy to help anyone who is a DIY'er.....its the only way to really learn anything

i can show you the flow results if you like

Edited March 3, 2008 by ISL33P