Originally Posted by
Rob_DOHC
exactly sir, so the higher the cylinder filling the faster (and greater volume) the gasses will be exhausted lol so to increase cylinder filling (more gas) you could for example change the cam (so volumetric efficiency is greater at a lower rpm) and port the inlet (clearly you would do the inlet and exhaust). There is more gas in the cylinder to be pushed out in the SAME amount of time, tbh i think volume is much more important than velocity up until a point in turbo spool up. Looking today, i think the ports are big enough to flow enough lol they are pretty massive! even as standard. I agree with crazy cage that improvements can be made however they will not be vast, at least not as vast, or important as the inlet.
Yeah thats right ... the more you can get in the faster the gas velocity is on exit, but I personally think it's not the inlet side thats the bottleneck but the exhaust.
Originally Posted by
Red_bull
I think you got it wrong on this one mate...
The gasses won't be exiting at the same rate, because of a eventually restriction in flow. If the exhaust port is extreme small this will cause a powerdrop ofcourse, but also the pressure in the cylinder on TDC will be higher then with a right ported head.
I think your forgetting the pressure on TDC.. The pressure on the time isn't always atmosferic. If it flows good you will have a under pressure which causes a higher velocity of gasses entering --> Helping your turbo to spool.
Also a under pressure wil be caused by a high exiting velocity of the exhaustgasses. Also high velocity will make sure the gasses have a high energy when hitting the turbo --> Less lag?
Bit different from the story so far, so i'm really curious to your reactions. I'm i right or am i missing somthing
Yes thats also correct, what you are saying is if the exhaust ports are too small the pressure will 'backup' in the head, restricting the flow of fresh gasses in, slowing spool up as the turbo is fighting against that back pressure (boost pressure in actual fact).
This is exactly what happens when you reach the maximum flow rate of the .36 housing, the gasses cannot exhaust any faster and so you cannot increase further the flow of fresh gasses in (no relevance to spool up, but just an example of exhaust restriction), at which point you would need a .48 housing to increase VE and continue.
Originally Posted by
10th_racer
you need the right mixture of everything really, when my cab was mapped (admittedly it was a 1900 cvh not a 1600 so would be less laggy anyway) it had a stage 3 ported polished big valve head, port matched inlet, cvh34 cam, and a t3 with a .48 housing, 360 bearing, but the rest of the turbo spec was unknown,it only did 210bhp with 250lb/ft, it was on full boost before 3000rpm, peak torque was at 3500rpm, but after 5000rpm it wouldnt make anymore power what so ever, jamie said the turbo was restricting it and holding it back, and as said above that was with a .48 t3 and a good spec head and cam which had done 246bhp on a previous car so we knew the head wasnt the problem, but the compressor side of the turbo was completley unknown which we think is what was holding it back.
I agree a balance of everthing is whats needed, my mate with the above spec (1st post) has a turbo and cam for big VE but a standard head, I think that is were the imbalance is so to speak.
Here is my NMS headwork so you can see that Karl Norris thinks is important to remove/open up on a CVH head.
When we spec'd it I asked for a head that would be responsive as well as flow greater than standard but not at the cost of gas velocity to the point of causing extra lag and this is what I got.....
Inlet
Exhaust
Pics of a untouched head would be nice for comparison!
