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Home > Help Needed / General Tech Chat > Turbo manifold sizing


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8174 Posts
Member #: 90
Post Whore

Somewhere around Swindon

I built a turbo manifold previously, i am not happy with it so want to do another.

Last one was 1.5 inch ID but would 1.25 be better or stick to 1.5?


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38 Posts
Member #: 11761


I can't claim to be an expert in any way, but . I've used 1 1/2" on mine as it covers the cylinder head ports and is pretty much the max to fit in flanges without making the tube ends square. The original cast turbo manifold looks pretty small tho, so there might not be a right or wrong, unless someone has tried both...


169 Posts
Member #: 11017
Advanced Member


What size turbo and state of tune? Recommended size for a 1300 engine is between 1ľ and 1Ĺ", however you want maximum velocity prior to the turbo.
Use smaller diameter tubes for widest power band and larger size where top-end power is more important and sluggish throtttle response is acceptable

Edited by Steve220 on 9th Oct, 2021.


3475 Posts
Member #: 655
Post Whore

Northern Ireland

On a friends car, circa 2.4 or thereabouts, it would make close to 1000hp with 1.75" OD header tubes.

It's so easy to get caught up in thinking ports need to be big with a turbo.

Although it does make sense to have smooth flow, transitions etc etc

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will


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301 Posts
Member #: 10128
Senior Member


This is why I love this site, I'd have assumed bigger is better initially, but of course velocity is king.

Turbo Phil

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4586 Posts
Member #: 20
My sister is so fit I won't show anyone her picture

Lake District

You want high pressure upstream to drive the turbine and low pressure after the turbine to create a high pressure differential.
So donít oversize the runners.



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