Head work

[fubar andy]

I was having a discussion today about head work.

Long story short, how much difference (driveability/power) would having some serious head work on the GTiR head compared to standard?

 

[The Doc]

Depends on the level of work done? If the full lot i.e chamber balance,multi angle valve seats, port fill and re shape...the answer is it wll make a huge differance.

 

[PobodY]

What can be changed? - Overall cylinder compression, overall cylinder capacity (although that's fairly minor compared to changing the bore), more efficiency from the valve train?

On my part this is just academic interest because I'm not planning on any head work unless there's a need to take it off (to replace the gasket for example) and then I might see what seemed like the best value work. - I've always thought there was a bit of black magic in head work.

 

[fubar andy]

Depends on the level of work done? If the full lot i.e chamber balance,multi angle valve seats, port fill and re shape...the answer is it wll make a huge differance.

It would be high quality work:

Head scanning
Gas flow checked beforehand
Valve seat cutting (whatever that is?)
Full gas matching flow
Chamber balancing and porting with a CNC machine


I'm not an expert in head work, but I'm reading a lot about it and I'm possibly looking at turning the GTIR engine into a real "race spec" engine, with a full on Motec/Syvec ECU with boost control and all the sensor additions etc etc (yes I'm thinking of keeping it again :hide

I'm trying to gauge how much £1000+ worth of head work would be beneficial to a GTiR head.

As an example: how would a 300bhp @ 1bar standard head would compare to a 1bar £1000 head worked GTiR head? What would be the gains bhp wise, turbo response and overall fuel efficiency?

 

[PobodY]

They get under your skin don't they? - Even when you get pissed off with the whole thing and think about scrapping it... :roll:

I suppose it's like Jim was saying at Silverstone; how many other cars can you get a complete engine rebuild so cheaply?
How many other cars will take some fairly amateur repairs and keep going? - Obviously the professional work lasts a lot longer.

I suspect it's totally worth doing things like re-seating the valves (with recut if require) and balancing the volumes; if you're getting more compression, that's more torque. - It's stopping at the point where you're getting diminishing returns I guess.

Even blueprinting the head is going to give a performance gain over a standard used one.

 

[jay1]

porting and polishing makes a huge difference, you can unleash so much extra power just from that!!

 

[warpspeed]

I ported and port matched the plenum/throttle bodies/head/exhaust manifold on my car and with a 3" system from the turbo back made 274hp@flywheel on 0.7bar, throttle response and driveability is much improved.

On the drive to Silverstone and back I averaged 31.6mpg sitting at 80mph.

 

[fubar andy]

Liking the MPG figures.

Was that a DIY port polish, or done by a pro?

 

[warpspeed]

Done by me, I've been porting heads for years for myself.

 

[The Doc]

39mpg on cruise,5 angle valve seat, ported and polished by my self, combustion chamber cc'd and matched to block, cam timing set correctley and turbo on full boost by 3800rpm ( gt3076 ish billet more like 3576) . Proper head work reaps big benefits especially the exhaust side on a gtir.

 

[PobodY]

Funnily you should say that; Ed was telling me about how the SR20 head hates exhaust back-pressure when we were at Silverstone, so it makes sense that there are huge improvements to be made on the exhaust side.

 

[fubar andy]

5 angle valve seat.

What's that / what does that involve?

 

[The Doc]

The valves are removed and then rather than the seat it sits in having 1 angle on it the cut 5 in a radius so on low valve lift as it just opens it promotes earlier gas movement and increases velocity of the gas earlier, also it un shrouds the valve allowing an improvement on overall maximum gas flow...
Gas velocity is as important as much as overall flow...hence making ports too big does not mean you get more power.

 

[fubar andy]

The valves are removed and then rather than the seat it sits in having 1 angle on it the cut 5 in a radius so on low valve lift as it just opens it promotes earlier gas movement and increases velocity of the gas earlier, also it un shrouds the valve allowing an improvement on overall maximum gas flow...
Gas velocity is as important as much as overall flow...hence making ports too big does not mean you get more power.

So lets say, hypothetically... :hide:

I decided to DIY some mild, but high attention to detail port polishing. I smoothed as much I could see and do with the right equipment, port matched the exhaust sides, cleaned up the throttle bodies (as they are bloody awful) and generally smoothed the head as much as I dared.

Could I send the head away to have this "5 angle valve seat" process by someone else?

At £1k for some hardcore head work is a lot of cash, where I could (possibly) obtain 70-80% of the work by myself and gain some decent MPG and BHP without handing over crazy money that really isn't going to be worth it in the long run and I could spend that on a decent Syvek or Motec ECU?

Comments?

 

[PobodY]

Is the "angle" cutting just a series of successively larger (or smaller) radius cutting tools? - I wouldn't try it myself since I suspect all the skill is in knowing how much to cut for each angle.

 

[johnny gtir]

I am no expert on porting but I thought it was a quite hard process and to get it right the good ones used a flow bench to see the start and end result testing it along the way. As uneven or to little will make it worse and part of it was knowing when to stop so you don't take to much all of which could make it worse than standard. I am sure it was Danny who told me once on the Gtir head it's not a must but if I wanted it done only to use one of a very select few places

 

[Makaveli_se7en]

A bit long, but this is one of better explanations on process of cutting valve seats:

https://www.youtube.com/watch?v=eMNFeToqzJU

Certainly not a DIY step, but if you know what you are doing there is no reason porting can't be don't at home with a die grinder and appropriate dies/grit rolls. I find 40 grit tapered rolls to be awesome for light clean up jobs.