Low-mount versus top-mount

[paradox]

Title says it all I think, anyone have thoughts on this? Surprisingly, I couldn't find a lot of intelligent info about this.

Reason I ask is that I am taking out my top-mounted IHI RX5 turbos and replacing them with (probably) 2860-5s. Given that I have to get new dumps anyway (the IHI have a weird super small 3-bolt exhaust side) and that the car is running MAP, I'm not super attached to anything currently bolted up to the intake/exhaust sides of the turbos and am willing to dump my current manifold if there's a good reason.

I don't notice a lot of TT RB26s running top-mounts, so I imagine there's a reason?

Cheers,
-P.

[mcfly]

Not many cars are built to make 850whp.

those top mount rx5's are some major turbos and therefore you dont often see top mount setups. Big top mounts were a huge thing in Japan back before the newer generation of large singles.

Those 2860's will not bolt on, you will need new everything on the turbo side of the engine

[paradox]

I think you have them mixed up with RX6s, which are huge. RX5s are actually pretty small (350hp small, but still).

SO how workable are manifolds? The inlet side doesn't have the same pattern as the -5s, was I a little "naive" (to be nice) in thinking I could have them reflanged? And perhaps rework the exit "area" of the manifold to a T25 shape (current exhaust shape is round but dimensions are close-ish)?

(Yes, I'm a newb)

That being said, I'd still like to hear some thoughts on the pros/cons of low vs top. Is it only a clearance issue that makes you go to top-mount? What do you lose in the process?

[j-ran]

I think top mount manifolds are designed to allow more clearance for a bigger turbo, I don't see what else you would accomplish with a top mount... maybe more clearance down below for runner design so you could accomplish equal length runners on big single manifolds etc...

[mcfly]

Ah, my bad. For some reason though rx5's are sticking with me as big. I'll track something down. Maybe they were hybrids.

Either way, what type of flange do the top mount manifolds have and what are your power goals.

[paradox]

Flange looks like a DSM TD05/T25, I haven't measured it but I don't know of other square flanges with round "ports". I suppose I could use something from these guys: http://www.bmcrace.com/index.php?cPath=60_62 ?

Power goals are 500-550whp, but the car spends 90% of it's time on the track so fast spool + mid-range is really what I'm after.

[inhumane]

well IMO the difference is clearance, if you can fit your turbo low the go for it but if its a big turbo your going with then youll have to go top mount

[Skym]

Low mount = better lateral, etc weight balance, as turbo sits lower in car (same reason why GTS-R has a low mount exhaust manifold with a fairly big single turbo). Alot of people call a stock GTS exhaust manifold, turbo a low mount setup, but it's a mid mount setup.

An example of a low mount exhaust manifold, fairly big single turbo (similar to GTS-R exhaust manifold) on a GTR would be the R.I.P.S built Stealth Bomber R34GTR -

http://www.youtube.com/watch?v=v9EGlO632Xc

Only GTR I've seen with that low mount setup.

Here's a stock GTS-R engine bay and can see a similar type of low mount exhaust manifold, turbo setup (at 0:59) -

YouTube- 日産　スカイライン 2000 GTSーR　�エンスー�� 近畿・関西】

Top mount = bad weight balance but gain clearance between engine, strut tower for bigger size single or twin turbo's.

Factory run short runners for a reason (more power, torque at lower revs, better response).

Longer extractors on exhaust manifold = less response from pressing pedal to turbo spinning (longer distance for exhaust gas to travel).

With longer runners, gain mid to topend hp due to extracting more gas from cylinders (no exhaust gas bottling up and reentering cylinders) thus get a cleaner burn inside cylinders and more hp at mid to high rpm's. But you lose bottomend power, torque.

Stock or upgraded stock RB26 exhaust manifolds are the way to go for racetrack / road use.

[paradox]

Thanks a lot for the info Skym, that's really interesting. Guess it's back to low-mount for me.

What are usually considered to be good upgraded low-mount manifolds? I see Allan74 had a Nismo/N1 manifold for sale earlier this week and he seems to do his homework very thoroughly, so I assume that's a good choice. Anything else that's been proven? (again, with spool/midrange in mind)

[Kras]

The stock rb26 manis can be ported out a lot. If money isn't a big deal then you can get tomei extreme manifolds. I'm not sure how much N1 manis are...

[mcfly]

Or you just buy Allan74's manifold and turbos.

Nismo rs580's, stainless dump pipes, n1 cast manifolds. 2000usd I believe

Very good idea there, then all you need is some factory oil and water lines which are easy to get.

[paradox]

Or you just buy Allan74's manifold and turbos.

Nismo rs580's, stainless dump pipes, n1 cast manifolds. 2000usd I believe

Very good idea there, then all you need is some factory oil and water lines which are easy to get.

Believe me I already had the same idea, but they don't appear in his sales thread anymore. :( I PM'ed him to make sure, but I assume they're sold.

[Allan74]

Not sold yet......just considering offers and thinking about replacement parts ;)