ChrisMC specifically. You were talking about oversized exhaust valves, why exhaust rather than inlet? usually on NA engines you get more return from oversized inlet valves as the exhaust gases are forced out under pressure as opposed to the inlet valve having to suck air/fuel in. Are you getting both e and i valves increased? Who is doing the subsequent seat/port work? What size are you going for? Anyone got a BVH and have experience of the SVH prior to it?
I shall have experience of both in the next week or so guys!! jc, Im going to a 35mm exhaust valve (33 mm standard). The inlets at 40mm are deemed big enough for most road applications. I have been told that small exhaust valves & poor standard exhaust port flow are the traditional VW methods of capping power output (on older engines). Hence the big exhaust valves help increase engine breathing & a wilder cam can be used to gain power at the top end! If you compare the VW valve sizes to that of say a Ford CVH (XR3i/RST) or Peugeot XU (205 GTi) you can see that the VW valves are small in comparison..... All the work is currently being done by Autotechnik in southampton... Edited by: chrismc
I had a chat with Steve at C&R about big valve heads in an 8 valve as this is a route in which i am very interested. At the time they had a 2Ltr 8 valve with alot of head work and re-progamable injection system and a whacky cam. The upshot was that this car would pink terribly at 5000 revs and above with a stage 3 style head and would just not rev out. It also created a lot of strain on the engine as it would over heat quickly when pushed to hard (like when your just having a bit of fun.....). So how do you know what the right balance of head work, valve size and cam is ? I think there is a lot of trial and error with this and i will look forward to your postings of your results. 8 valves keeping it real
No probs with the TSR brands hatch engine...that's big valve head and 268/272 split duration cam. No overheating and very rev happy
That talk about problems wil no doubt be to do with that individual engine as the conversion is a common and very sucessfull mod. The exhaust valve, as stated is small, and when all other work has been done will become the limiting factor of the performance of the engine. Better exhaust flow is always good which is why the exhaust is usually the first stop in tuning. As to the regard about if its needed then of course it is, although the gasses may be pushed out it is better for them to leave as little exhaust gas residue in the cylinder as possible. The pack C/stage 3 head will also have reprofiled valves and 3 angle valve seats to further the performance of the engine.
''If you compare the VW valve sizes to that of say a Ford CVH (XR3i/RST) or Peugeot XU (205 GTi) you can see that the VW valves are small in comparison..... '' Doesnt. Mean. sh*t. Most people i knwo say they have no gain what so ever from making the exhaust valves bigger on an 8v head... Theres a shed load more to be concerned about that 'How big can we get this valve' anyway. However, from what i've heard, autotechnic are rather good, and alough i doubt they do in house head work, im sure it'll be good. Edited by: madmk4
My mechanic is putting my head back on right now. It has 42.7mm (1.680") and 34.9mm (1.375") Nissan valves in it now, and is ported and has a Schrick 276 cam and spring kit. I did a dyno on the stock car (Mk2 1.8 GTI) before the mods and I will do the same after the new head is on. Should be an interesting comparison. Nothing else has changed _yet_.
I think youll find Valve sizes do actually make a difference mate! Otherwise a TSR Pack C head would have the same size valves as a TSR Pack A head. The 8v exhaust valve is particularly small, hence why there are gains to be had from increasing its size along with the appropriate seat work, especially on increased capacity 8v`s... My car has gone about as far as it can with a stage 1 head & has all the over tuning work done too (cam/chip/exhaust/induction etc) Im not some kid whose decided to "whack" in big exhaust valves for the hell of it! If the people you know havent gained from the Big valves, you need to realise the bigger valves only increase the engines ability to rev due to better breathing, hence a hotter cam can be used to give more top end power... So Valve size DOES mean Sh*t actually Edited by: chrismc
I think what he means is you can not compare the valve size in different engine configurations ie CVH vs VW 1.8. Of course valve size matters in general. Set aui.....You have 42.7mm inlet valves? If that is the case I would be "very" interested to see what effect the shrouding will have on your performance. Also to recognise the gain in your valve increase you are going to have to port the inlet chamber so much that you are in danger of breaking into a water way. As far I as I know the maximum useful inlet on a 1.8 head is 41mm before you get shrouding. You need 25% of valve diameter between the walls of the combustion chamber and valve to avoid shrouding. You can actually lose power with big valves in the wrong head (Mini's and MGB). As far as allowing the engine to breath more by porting and increasing exhaust valve, if you can't breath in quickly you don't have anything to breath out quickly. I haven't found much evidence of increase in power through increased exhaust valves alone. Exhaust and inlet increases together ..yes. Inlet alone ...yes.
The ratio of the inlet to exhaust size I think is the important item here. I have some engine design/tuning software somewhere (apparently expensive as h*ll?) and I'm sure at one point I went thru the PB and 2E engine and worked some stuff out. Anyway long and short of it is in both engines stock exhaust valves are way restrictive so I agree wholeheartedly with chrismc about the "capping of power".
My car is basically at the limit of what it will do power wise, using the standard injection & standard valve sizes! Bearing in mind it has a 2L bottom end, like for like it is drawing in a greater charge than an 1800 equivalent on every induction stroke. As a result of all the induction mods/flowing work done so far, & the power achieved, there is no issue with how much charge the head is flowing IN..... A pack C head with 40/35mm valve sizes is ideally required as the standard sized valves do not allow maximum power to be produced any higher than 5000-5500RPM... The increase in power will come with a change of camshaft to utilise the extra breathing of the engine!!
Don't get me wrong, I am not telling you that you are wrong, i am looking for some advice if anything. I can only find a few examples of people who have changed exhaust valves only and got repectable gains. Also there are "issues" wih big valve 8v heads and breaking into waterways when opening up the throat to utilize the new big valve Read this about halfway down http://www.pumaracing.co.uk/gentune.htm I am more inclined to believe this than some one who is selling something. The exhaust valve is the easy one to change as the increase in size is relatively big and you can port the throat without much skill or danger of breakage. I wanted to hear from people who have BVH before I go ahead with any changes
There are three reputable people for doing heads for Dubs: Dave Crystal (I think ) in Oxfordshire supplies TSR, AMD etc - Same heads an thats gospel. Grant Motorpsort in Bristol or Puma Racing.
All I can say is looking at the valve sizes offered on the respective heads..........Stage 1 40/33..Stage 2 40/35 & Stage 3 42/35 tells its own story.... These sizes arent chosen as a marketing ploy. On an 1800 stage 1 is all you need, but for larger capacity engines it is necessary to increase exhaust valve size to maximise the horsepower potential of the engine I too read the Pumaracing website a while ago, but I have since found out that the small exhaust valve is a traditional VW method of capping the power.. I shall have my BVH before long, so I look forward to being able to give you a before & after view of the engine!!
Nice one. I would be interested as I am rebuilding two engine at the moment. One standard 1.8 8v (mild porting and skimming) One 1.9l BVH 8v with 276 or 285 cam. Another thing that I noticed is that the guy at Puma was running a 165bhp 8v engine a while back. Not this post (or forum) but it answers a question that was asked ages ago about top whack possible out of a NA 8v 1800.
Chris, before you decide to preach on to me, perhaps you should read and understand what the person is writing. I was stating that comparing valve sizes from other manufacturers engine to a VW head is total nonsense. A bigger exhaust valve may well give slightly better performance, as i say, the valve size isnt the only factor that matters here. The exhaust valve size isn't as important as the inlet size, because the piston is moving at a silly speed, pushng the gasses out. Infact, the best gain you'll get from the exhaust side is with carefull consideration to the shape of the port and any obstructions there may be there, this doesn't just mean a mirror finish wall.
Ian carvells engine is an 1800 with a pack D head, webbers, cam, exhaust and a proper setup, crica 180bhp IIRC. What relationship does valve size have with CC ?
