matth76 1 Posted January 17, 2006 Report Share Posted January 17, 2006 HiI was looking at Stealth's website and they seem to specialise in rebuilding and also uprating engine cylinder heads - one way is to "gas flow" it. How much does this normally cost and are the power / torque gains worth it? Does it improve low down torque as opposed to top end power?I assume a 'big valve' head is different to a gas flowed head? And what is a ported head? What are the costs and power gains for each?Thanks for any info.Cheers Matt Link to post Share on other sites
VR6Pete 1,455 Posted January 17, 2006 Report Share Posted January 17, 2006 I've got a Schrick Big Valve head from stealth on mine.... you wont get many gaines from a standard engine, but couple that with some cams and a dastek, then it comes into its own! Also, an ideal time to get your clutch and chains done. Link to post Share on other sites
antera309 3 Posted January 17, 2006 Report Share Posted January 17, 2006 This is the way I understand it so anyone feel free to correct me if I'm wrong...A "gas flowed" head has the inlet & exhaust tracts (the passage between the valves and the ports on the side of the head) widened and polished. The widening allows the head to flow more gas per second and the polishing reduces air resistance due to friction against the sides of the tract. The polishing also removes any imperfections that may cause air turbulence."Porting" is a widening of the ports on the side of the head itself to match the internal diameter of the inlet and exhaust manifolds. This eliminates the "step" between the head and the manifolds, which is a further cause of air turbulence.On a well-flowed head, the valves themselves become a bottleneck. This is especially true of engines like the VR6 which have only 2 valves per cylinder (rather than the 4 valves per cylinder arrangement which is more common these days). Increasing the diameter of the valve stems and fitting appropriate larger valves reduces the bottleneck and so liberates more BHP. This is quite an involved job, so you usually find that big valve gasflowed heads are quite a bit more expensive than normal gasflowed ones.When gasflowing a VR, you should also get the throttle body enlarged, as this is one of the most restrictive parts of the engine.You should also consider a set of performance cams, which hold the valves open for longer and increases the effectiveness of the head still further.An ECU remap will be necessary after fitting that lot, but once done you should see over 200BHP on an otherwise standard VR.The benefits of a big valve gasflowed head are even greater if you are running (or plan to run) a supercharger.Hope this helps!! Link to post Share on other sites
matth76 1 Posted January 18, 2006 Author Report Share Posted January 18, 2006 Cheers. Thanks for your help. Lots of useful info. Link to post Share on other sites
FishWick 21 Posted January 19, 2006 Report Share Posted January 19, 2006 Increasing the diameter of the valve stems and fitting appropriate larger valves reduces the bottleneck and so liberates more BHP. I think you meant DECREASE valve stem diameter With doing that you need to make them from a stronger metal too usually.There are many arguments for and against benchworking a head but I'd always leave it as the last modification, after first confirming that the engine's current state of tune can't be taken any further.Just be careful not to go too far or you'll slow the gas speed down too much and lose bottom end. Link to post Share on other sites
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