Stainless vs cast Manifold Q's. Sorry, not sorry

Morning all !

Should I scrap the stainless that has cracked again & go back to a standard cast Mani ? That is the question ! Or..... do i put my hand in my pocket & get a hand built stainless manifold created ?

I'm running Horshams' stage 2A map with all associated mods so circa 315bhp.

I've tried a couple of stainless mani's over the years and although this one has been on the car for some 8 years, its only done 5000 miles. I'm sick of having to take them off to repair for either a crack or a wound out stud.

It is probably the weekest part on my car from a reliability point of view.

So i suppose I'd like to know how much BHP a standard one can handle?

Have members ported them to aid flow ? And anything else i cant think of as I've just got up !

Thanks in advance,

Ben

400bhp for the cast mani is the figure I've seen thrown about here mate. I could be wrong though.

I thought about having a bracket fixed to the turbo elbow that allows you to use the gearbox's bolt hole that the OEM elbow uses? Is there space for that? That should support the manifold and turbo better, putting less pressure on the manifold. People also say a flexi downpipe helps.

I can't comment on the longevity of the hand made manis, my only experience was with the H-Dev one and it cracked twice. I eventually put the cast iron one back on

The standard one isn't infallible, it can crack as well but is a little more robust than the stainless ones.

I assume you haven't had a support bracket for the turbo to the block with the stainless manifolds? This is a must in my view, you won't see a professionally prep'd race car without one (rallycross for example).

The cast one seems to survive better without, possibly because the thermal expansion coefficient is similar to that of the turbine housing, while 316/304 stainless steel is somewhat higher.

In terms of performance, it's not so much restriction you should be concerned with as torque shape. The longer runners and larger volume of the stainless manifolds mean that they favour top end over low end while the cast manifold volume is geared more towards low-mid speed torque, it just means you would probably want to run a little more boost for the same top end performance with the cast manifold versus a stainless one, if the compressor is efficient enough to do so.

You have to remember with a turbo engine, the volume ahead of the turbine wheel is critical for building boost in the first instance. The more volume there is, the more mass flow is required to build pressure in order to drive the turbine and get the compressor generating boost. The flip side is that turbine pressure is higher at high RPM for the same mass flow so power potential is slightly less.

Is there a flexi section on the exhaust anywhere? I read somewhere that a bracket to support the turbo can help stop manifolds cracking


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As above a flexi and a brace to the gearbox is essential

I replaced the elbow to downpipe joint with a flexi as that seemed more logical than in the Horizontal part of the downpipe like most are

Go cast if you don't care about looks. Doubt you'll notice much if any difference

+1 for bracing the turbo. I made up a small bracket to help take the weight of the turbo as well and havent had an issue in 5 years with a japspeed manifold (they used to be common for cracking). i also run a flexi de-cat. the theory of adding these parts made too much sense to me with the common manifold issues (heating up/cooling down/weight/cracking)

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Originally Posted by norfy

+1 for bracing the turbo. I made up a small bracket to help take the weight of the turbo as well and havent had an issue in 5 years with a japspeed manifold (they used to be common for cracking). i also run a flexi de-cat. the theory of adding these parts made too much sense to me with the common manifold issues (heating up/cooling down/weight/cracking)

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Got any pics of it? Tips for making one?

Mines still on the cast manifold at 370. Can't tell you if it's better or worse than a fabd one.

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Originally Posted by Sean-B

Got any pics of it? Tips for making one?

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afraid i dont seem to have any pictures. It was quite simple, i used a 'L' shaped bar roughly 5-6 inches long, one end was drilled and fixed onto the exhaust housing using one of the original core bolts. The other end was basically bent over roughly 45 degrees and i found a perfectly situated bolt hole on the block that i used.
i made my flexi de-cat but you can buy those online.

Thanks for the replies. Ive gone with another fabricated one. Thicker material & more substantial welds. Done all the brace work suggested - mani to mani to block & elbow to gearbox too. Flex in the front pipe & a reworked centre box.

Few teething problems but fits great and is okay so far !!

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Originally Posted by G20BEN

Thanks for the replies. Ive gone with another fabricated one. Thicker material & more substantial welds. Done all the brace work suggested - mani to mani to block & elbow to gearbox too. Flex in the front pipe & a reworked centre box.

Few teething problems but fits great and is okay so far !!

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Any pictures? Wanna get an idea of brackets etc to see if I should add anything to my setup


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Never seen the elbow to gearbox one

If it's not a properly well made manifold from the likes of 6boost just throw it in the bin - they're garbage and crack constantly.

Stick with the cast manifold.

Even well made manifolds crack without proper support.

There should always be a supporting bracket at the bottom of the turbo mounting it to the block!