10 or more ftlb of extra torque for free.
Discussion
Had the exhaust manifolds off, as you do and noticed that there was a considerable difference between the size of the ports on the head and the opening in the manifolds. Was rather annoyed that I had not noticed when refitting the engine.
My heads had been to V8d and initially I made the assumption that the ports had been opened out by them, however measurements from several other standard 5l cars suggested they were close to the original size. I set about opening up the manifolds to match the heads and then the gaskets to match both. Not an entirely straight forward process as each port has to be bespoke.
After much grinding, test fitting and checking with a boroscope the match is now quite good. A quick run on the RR revealed slightly less torque up to 2500, as before at 3000 and then at least 10ftlb more right up to 5000 at 5500 and above it exactly matched before so the peak power is about the same. At 4250 it reached a peak of closer to 18ft lb gain.
Quite why TVR shipped them with such a poor match is difficult to figure. Worth considering next time you have the manifolds off.
My heads had been to V8d and initially I made the assumption that the ports had been opened out by them, however measurements from several other standard 5l cars suggested they were close to the original size. I set about opening up the manifolds to match the heads and then the gaskets to match both. Not an entirely straight forward process as each port has to be bespoke.
After much grinding, test fitting and checking with a boroscope the match is now quite good. A quick run on the RR revealed slightly less torque up to 2500, as before at 3000 and then at least 10ftlb more right up to 5000 at 5500 and above it exactly matched before so the peak power is about the same. At 4250 it reached a peak of closer to 18ft lb gain.
Quite why TVR shipped them with such a poor match is difficult to figure. Worth considering next time you have the manifolds off.
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