Tophat Intake Plenum - Review/Writeup (RB25 Version)

[240sxed]

Well, I ordered my tophat intake plenum after being ripped off by another guy nico name "shedrifts" "bmxpsycho". He told me it was an rb20 manifold and from the pictures i couldn't quite tell, especially since i had ordered it since before I dropped in the motor.

So after about 2-3 months of waiting, I found out that he would be out of town, and that the flanges would be coming. So...after about another month I finally get my manifold, for the extra wait, I get some money off, and a throttle cable bracket pre-fabbed to fit perfectly with my KA throttle cable (forgot to switch to rb).

The manifold is pretty decent looking, not quite the greddy manifold, but not quite the greddy price either if ya noe what I mean. The only problems I had were: The fittings on the back were tapped too close to the IACV valve which I will have to get one fitting taken out completely plugged up, and tapped on the other side of the plenum. Second thing, they didn't wait for the paint to properly dry after spraying, so it looks like bubble wrap spots which really turns out to look sorta like poka-dot silver.... Third concern is that you need to source your own bolts. The throttle body needs 4 new bolts, slightly shorter with more thread, and you need 4 more to thread on the passenger side into the manifold on the bottom side.

So no biggie, got the bolts, gonna respray it. but having that fitting rewelded is making me weary.

(vspl)(bumper support just trimmed !(poka-dot)[IMG]http://www.speeddreamz.net/modules/gall ... d.jpg[/IMG]

(missing hole is where the fitting/aicv valve don't fit)

(Fitting in my hand)

(flanges/holes on the bottom lookin good)

Modified by 240sxed at 11:27 PM 8/19/2005
Modified by 240sxed at 11:28 PM 8/19/2005

[ehacker01]

They are steel right? the only problem I have with those manifolds is that steel doesn't dissipate heat as well as aluminum. I would think it would heat soak really bad.

[TheOne]

ok ok, some sort of review without pics???? wt fudge!.

[Joe]

seriosly

pics within 24 hours or ban

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(kidding, try to get them tho )

[240sxed]

Haha... i was gonna take pics but i was late for work...and at this moment im at work bored so yeah...

[burnin240sx]

I just wanted to let readers know that this is the first and I mean first RB25DET intake that we have made. and as any first time doing things there are issues to work out. And these will be fixed.

Sorry about the paint the UPS Guy was waiting on us packing up the box and we thought the paint was dry enough to box up.

I know you called today about the fitting but if you have anymore issues don't be afraid to call us. We will gladly take care of any problems.

[rbsileighty]

These manifolds have been out for a while and I've always had some questions about them... if you don't mind taking a min to answer them.

What kind of calculations do you run when you make these plenums? How are you selecting volume sizes between the RB20 and 25? Do you have any dyno numbers on these manifolds? What rev range are they tuned for? Thank you very much for your time Top Hat!

[ehacker01]

Also what are your thoughts on the heat soak of a steel plenum as opposed to aluminum. Have you by any chance used a temp. probe inside of the intake manifold to see the difference of Al. vs. steel?

[240sxed]

I doubt the heat soak will be that significant, I don't think the air stays in the manifold long enough for it to effect the charge air. With the intercooler maybe.

Also what are your thoughts on the heat soak of a steel plenum as opposed to aluminum. Have you by any chance used a temp. probe inside of the intake manifold to see the difference of Al. vs. steel?

[Kansei240sx]

The only question i have is, does it lean out cylinders 5 and 6?

[s13burger]

This was the first one that we made for the 25. It's completely different than the rb20 plenum we produce. Cylinders 5 & 6 won't run lean cause it's forced induction.

[sparksta]

Leaning out is just part of the design with forward facing manifolds.... and just because its forced induction that doesnt mean it wont lean out on cylnders 5 and 6

[Carl H]

agreed its just the inherent design of the motor and manifold that 5 and 6 run lean, i mean hell rb26's run lean on 5 and 6.

[accel junky]

Uh...forced induction has nothing to do with leaning out cyl 5/6.

Its a matter of flow, which is dependant on manifold design.

[narfdanarf]

If that sort of manifold design automatically makes a car lean in the 5th and 6th cylinders, then why wouldn't the stock manifold automatically make the 3rd and 4th cylinders lean.

[krayton]

Curious people. everyone says 5&6 can run lean with that type of design.

well lets hypothetically talk about an NA engine. now the air has to travel farther to that point.

So wouldnt it just run rich? i mean, less air in 5 and 6 with same fuel as 1 & 2, would mean it would be rich.

...not lean

or am i just *** backwards?

[sparksta]

* NOTE * I'm not 100% sure on these concepts but they are my understandings of this condtion * NOTE * The stock inlet is centrally located so by my thinking all the cylnders have the same opportunity to get air. In a foward facing manifld cylnders 5 and 6 are the furthest away from the inlet so when air comes in it is sucked in the closer cylnders first leaving 5 and 6 struggling to get air. Again these arent 100% truths they are just my understanding of what is happening. Anybody that knows for sure feel free to set me straight because i would like to know a definte answer as well.

[krayton]

^well thats how i see it. (if the circumstances are there. thus me using NA)

but less air would just mean rich.

[sparksta]

well i suppose that is correct in theory but the afr ratios would be off plus i think not enough air and too much fuel might lend to higher temperature in the combustion chamber which might in turn melt pistons or possibly worse. again just a thought so dont hate me too bad if i'm wrong hahaha

[wawazat8402]

the air is forced into the plenum fast enough by the turbo that it goes directly past the first few cylinders and the majority hits cyl 5 and 6. this means more air is going in cyl 5 and 6 which means it will be LEAN and have HIGHER combustion temps. a RICH mixture leads to COOLER temps. this isnt really a huge problem, if you are that worried about it, pull your injectors and get them flowed. then put the highest 2 flowing injectors in cyl 5 and 6.

[sparksta]

ahhh thanks for clearing that up... hard to think about things in a differnet way after you develop your own theories about it

[krayton]

the air is forced into the plenum fast enough by the turbo that it goes directly past the first few cylinders and the majority hits cyl 5 and 6. this means more air is going in cyl 5 and 6 which means it will be LEAN and have HIGHER combustion temps. a RICH mixture leads to COOLER temps. this isnt really a huge problem, if you are that worried about it, pull your injectors and get them flowed. then put the highest 2 flowing injectors in cyl 5 and 6.

just curious if you have any info to support this.

im also wondering more about flow since not all the cylinders are pulling in air at the same time.

but to clarify your saying that with turbo, air is being pushed in so fast that it skips the first cylinder and goes all to the end? this is what causes those end cylinders to be lean?

[Carl H]

check out SAU or better yet SDU, this has been beat to death millions of times......under boost the cylinders are not 'sucking in' air as air is being forced thru the manifold, however due to the front facing design air will take the path of least resistance so it will keep going into the manifold till it hits something, say the back of the manifold.fluid dynamics ownz j00, idealy its best to keep the stock side facing manfold for fuel distrobution as it was designed to keep flow to the cylinders relatively equal.even my freeking RIPS manifold suffers from cyls 5 and 6 being leaner than the rest, its just the nature of the beast.

[Kansei240sx]

So, what manifolds other than stock provide better air flow and something that doesnt go over the top of the engine, becuase i hate that so much. Are there any manifolds ie: The greddy manifold for the RB25 that has little to no leaning among cylinders? Or is that just something typical with side mounted style throttle bodies. Becuase the GT-r's come with that style of manifold stock. Im just curious

[Carl H]

So, what manifolds other than stock provide better air flow and something that doesnt go over the top of the engine, becuase i hate that so much. Are there any manifolds ie: The greddy manifold for the RB25 that has little to no leaning among cylinders? Or is that just something typical with side mounted style throttle bodies. Becuase the GT-r's come with that style of manifold stock. Im just curious

if you think the greddy manifold doesnt lean out cyl 5 and 6 you are fking kidding your self........NO front facing plenum provides air to all cylinders equaly, its the inherent flaw in the design.rb26's have been known to run lean on 5 and 6, and agian its due to the front facing plenum.there is no way around it, like stated before get your injectors cleaned and ballanced and put the 'richer' injectors in cyls 5 and 6 working your way down the line.

[Kansei240sx]

Got ya, thanks carl.

[narfdanarf]

How is it then that the stock manifold doesn't go lean in the 3rd and 4th cylinders, it's the same damn principle. Actually wouldn't it even be worse cause the turbo is shooting air down the 3rd and 4th runners, hell it's not even flowing past the other runners, just going down the center ones.

[Wulfgang]

You guys are oversimplifying the issue. Even the OEMs cannot accurately determine which cylinders are going to be unbalanced just by guessing or getting out the tape measure or looking at the shape of the plenum. The fact that the OEM manifold has lean spots should tell you that designing a balanced plenum is NOT EASY. Now consider the length of an inline-6 and realize that nobody, not even Nissan, can make a perfect manifold for it.

TopHat, Greddy, NISMO, whoever... none of them can make any claims about the performance of their manifolds just by looking at the shape. To make an accurate claim an experiment must be done either on a flow bench or on a computer (via detailed 3-D CFD simulations). Otherwise it's just a crapshoot. The dozens of intake engineers at Nissan have access to plenty of flowbenches and do numerous experiments using CFD before deciding on a design. Does Greddy have that much engineering power? So what makes you think that you've got it all figured out in the 5 minutes it took you to read this thread?

Also, even if you could design a balanced manifold without a flow bench, you would have no idea how it performed throughout the rev range. A plenum that performs good at 3000 rpm may be a total piece of !@#$ at 6000 rpm. Cylinders 5 & 6 may go lean at 6000 rpm and be totally rich at 3000 rpm. In fact, that's a reasonable expectation considering that friction losses increase with the square of rpm.

Even the F1, CART, and IRL teams have trouble with this. They make HUGE plenums and use individual throttle bodies to reduce the unbalanced effects, but they still have to tune each cylinder separately (using multiple lambda sensors or a flow bench or CFD or more likely all three) to get the best results.

Any discussion of intake plenums is pointless without real world measurements.

Btw, Carl, there is no difference between "sucking" and "blowing". Ask your girlfriend . In all seriousness, the air moves solely based on the pressure differential, and it does not care one bit whether you think it was sucked or blown down the intake piping.