Originally Posted by bad boy rs

good read this learnt a thing or 2

Originally Posted by jamieRST

but i am not on about a massively ported,badly worked head.here is some info ive found for you too.Most 2 valve per cylinder heads are capable of being modified for between 10% and 15% power increase without increasing valve size. Some heads, like the CVH, have even more potential and 20% is possible.Properly done with due consideration for port sizes and valve seat profiles, a modified head will increase power throughout the rpm range. Low and mid range torque increase significantly as well as peak power.The exhaust valves are actually far too big as standard. 37mm is a huge exhaust valve for a 1600cc engine. The 2 litre Pinto has 36mm ones, Peugeot 1.9 Gti are 34.5mm, Golf Gti even smaller. Under no circumstances fit larger exhaust valves and any tuning firm that does this can be safely avoided as they have no clue how to modify this engine properly. There are certain things that absolutely must be done to portshapes to get the CVH head to flow to its full potential. The area around the valve guide where the port bends is critical and this must be straightened out and shaped to blend nicely into the valve throat. You MUST remove the valve guides to get at this area so any head which just has a bit of polishing and no real constructive port work will not show much of an improvement. That means most off the shelf heads you are likely to buy. Every head I do has the guides removed first so that the ports can be modified to the correct shape for high flowThere are some very common misconceptions out there about tuning turbo engines. In particular whether improving flow through cylinder head and induction system porting makes any difference to power or whether all you have to do is turn the boost up. The simple answer is that improving flow can be even more important on a turbo engine than a normally aspirated (N/A) one. All engines make power in proportion to the amount of air they can breathe and anything that increases air flow into the cylinders increases power. To generate a pressure difference there has to be a resistance to flow. Try blowing as hard as you can through a straw and then do the same just through your open mouth. You can generate enough pressure through the straw to puff your cheeks out and almost no pressure without the straw. The amount of airflow is much greater without the straw of course. All the pressure rise is telling you is that there is a resistance to flow but it doesn't tell you how much flow is taking place. The same thing applies to a turbo engine. What the boost gauge is telling you is how much pressure it takes to generate a given amount of horsepower through the restriction of the head and induction system that the turbo is pushing against. If you reduce the restriction you'll get more airflow and power at the same boost level or alternatively the same amount of power at a lower boost level.a big valve head with the correct spec,is good imo

Originally Posted by jamieRST

but it doesnt matter if your chasing power figures and cant say theres no point unless youre pushong 250bhp plus.jim hearne had a zvh with a big valve head (inlet) and properly flowed etc running a pretty much standard spec made 226 atw thats 260+ at the fly.sean richardson 220bhp zvh,added a big valve modified head and made 300bhp.fast road/race spec mk4 xr3is have proper modified heads and they dont push 250bhp.

Originally Posted by Karlos G

ZVH's MUST have a big valve heads or they run out of steam by 5500rpm because the their too restrictive!
So non of that has any relavence to out little turbo charges CVH's mate

Read the article I put up and learn about how it works for turbo charged applications......... in case you do not know it was written by one of the most reputable tuners in the country who also happens to be the creator/owner of this very forum

Originally Posted by jamieRST

so who is wrong and who is right then?and why

Originally Posted by Karl

The ability of a turbocharged engine to rev to high rpm's is based primarily on two factors.

1. strength of engine which also encompasses design to be correct for such an application

2. Ability of the engine and turbo to flow sufficent air at these rpm's in order to maintain torque. (this is what makes an engine feel revvy)


The reason the turbocharged ZVH normally suffers with holding high rpm torque is its rated capacity in comparison to the cylinder head flow rate. Essentially this means that the head itself becomes a flow restriction in relation to its capacity and hence engine torque falls quite rapidly with rpm. This can be overcome to some extent with a larger turbo and heavily reworked head which can help increase engine breathing at higher rpm's but this does of course lead to the usual lag, drivability and most importantly cost issues that come about in building an engine capable of using such a large turbo!

hope that helps explain things a little!