Alfa Romeo Forum - View Single Post - Exhaust systems

mave · 22-05-2008

Originally Posted by symonh2000

Turbo cars often have cast manifolds from the factory.

There are a couple of reasons for this.

A cast manifold is stronger and is better able to support the addtional weight of the turbo, especially when hot. Turbo manifolds can glow red hot after hard use.

A cast manifold gives a small amount of back pressure. This back pressure increases the speed of the exhaust gases leaving the manifold, which in turn makes the turbo spool up at lower RPM's.

Fitting a tubular manifold to a turbo will increse top end power whilst increasing the boost threshold, which basically means the turbo won't spool up until higher RPM.

Tha manifold and the actual blades of the turbo are the most restrictive parts on a turbo exhaust system.

Simon.

Hmmm, not sure about that.... you don't want any pressure loss between the exhaust ports and the turbo inlet, as that's just turning pressure into heat and wasting energy. It should be the turbo itself that's the most restrictive part, but using smooth passageways to minimise losses, ie convert pressure to velocity without wasting energy.

22-05-2008	#16 (Post Link)
mave AO Silver Member Join Date: Sep 2003 Location: Bristol Posts: 2,607	Re: Exhaust systems - What you need to Know Originally Posted by symonh2000 Turbo cars often have cast manifolds from the factory. There are a couple of reasons for this. A cast manifold is stronger and is better able to support the addtional weight of the turbo, especially when hot. Turbo manifolds can glow red hot after hard use. A cast manifold gives a small amount of back pressure. This back pressure increases the speed of the exhaust gases leaving the manifold, which in turn makes the turbo spool up at lower RPM's. Fitting a tubular manifold to a turbo will increse top end power whilst increasing the boost threshold, which basically means the turbo won't spool up until higher RPM. Tha manifold and the actual blades of the turbo are the most restrictive parts on a turbo exhaust system. Simon. Hmmm, not sure about that.... you don't want any pressure loss between the exhaust ports and the turbo inlet, as that's just turning pressure into heat and wasting energy. It should be the turbo itself that's the most restrictive part, but using smooth passageways to minimise losses, ie convert pressure to velocity without wasting energy.