Thoughts on head design benefits

[VIN18M]

Ok guys, so I've been pondering building a new head for my car - already I know!

The concerns I'm having is the amount of lag or doughiness down low I'm currently experiencing with the setup I have at the moment. Basically my boost gauge starts to show positive pressure at about 3,500rpm, however power doesn't really kick in till 4,000rpm but it does come on extremely hard and I am getting full boost (22-23psi) just on 4,000rpm.

I know the big cams are creating a fair amount of overlap which means it's harder to get positive manifold pressure, however from what I've read online the td05 20g usually wouldn't hit full boost till 5,000rpm on a 4g93t so they are probably helping me spool the bigger turbo up quicker?

The theory I came up with the other day was that if I was to rebuild my Satria GTi head which has smaller ports, with a new valve job, heavy duty springs and run the same big cams with a 0.8mm metal head gasket (currently 1.5mm), this should bring the compression up significantly and help with the lag I'm currently experiencing?

Having spent so much on the car already, I'm really not keen on starting again with a new head, but again I've been driving a car with 10.5:1 compression and 10psi of boost for the past 8 years I'm not sure if I'm just used to the responsiveness having never experienced turbo lag before.

A few people have told me already that everything sounds pretty normal for this size turbo on this size engine but I can't help but think things could be improved?

 

[EVOCPE]

I was certain GTi and GSR ports are the same

Just stick to how the engine is atm, if anything get smaller cams, I wouldn't touch the comp either

Once you start playin with all these you will need to go get another tune

 

[VIN18M]

The exhaust ports are the same, but the inlet ports are narrower on the GTi head - similar to the small port and big port 4g63ts.

I love the sound of the cams and the power surge when they come on with boost, obviously going a bigger turbo will involve more money, another tune, dump pipe and plumbing changes and will probably mean more lag at the end of the day.

Smaller cams might make it friendlier down low but will essentially reduce the overall power the engine is making right?

 

[evo-gsr]

Not necessarily. I would have a chat to your Tuna amd see what they think about reducing the cam size.

 

[GeoKez]

And if you have adjustable cam gears, play around with the exhaust cam timing. It will effect spool times better or worse depending where your at now.

 

[VIN18M]

At the moment I'm running 6 degrees advance on the inlet cam and 6 degrees retarded on the exhaust cam. The idle the tuner has achieved is very smooth considering the size of the cams and I was told everything is where it should be.

Wouldn't advancing/retarding timing require the ECU to be adjusted accordingly?

Do my spool times sound normal given the mods? The car revs out to 7,500rpm no worries at all.

 

[4piknup]

And if you have adjustable cam gears, play around with the exhaust cam timing. It will effect spool times better or worse depending where your at now.

I was looking at kiggly mivec 2g build. He states the intake cam timing has twice the affect as the exhaust on spool. Thats why evo9 has the adjustable intake and not the exhaust.
But the mivec cam moves a heap (like 30º or more. It's in the evo9 fsm) full movement will change the spool point about 1500rpm but may cost you 100 hp up top.

Just chuck in the std cams and try them

 

[GeoKez]

Interesting Jack, got a link?

The correct term I should have used was lobe separation adjustments. But as a 'backyarder', I found a quick tweak and test of a particular cam, helped give me a direction to head. That said, I have not had the luxury of a good tuner, so for me I guess its a case of small steps looking for gains. If a tuner has dialed your cams in, best disregard my comments.

 

[4piknup]

I shall quote mr kiggly.

While the question about spoolup is a very valid question, it is also a tough one to answer. The more I advance the intake cam, the better it spools and the less power it makes up top. With recent setups at fully retarded to a ~130deg centerline it would not spool at all on the converter (we're talking 0.5psi and hanging out at 3500rpm) and fully advanced to a ~85deg centerline it would spool up to the 2-step rpm in about 2.0 seconds. The fully advanced position would sacrifice about 100whp to the retarded position at 9k.I can't do a good A-B for exact spoolup rpm due to the torque converter and automatic setup interacting with the power curve. If I had to field a guess though, I'd probably say a 1000rpm difference at least with my 76mm turbo. I just looked through some logs and where I have normal timing just after launch the converter stalls up to about 5500rpm for somewhere in the 15-20psi range. At 6000rpm and about 25psi the boost ramp-up shape starts looking pretty vertical, which I think is pretty good for a 2.3l with a 76mm turbo and an 87mm turbine wheel in a 1.1AR housing. Wild guess based on this response is if you could lug it with a 5spd it would be full boost around 6k.
Thanks,Kevin

 

[4piknup]

Really think about this before attempting. He is moving 3 teeth on the belt. And the way I read its closer to the pistons.

Danger will robinson, danger...

 

[VIN18M]

When we did the play dough trick on the pistons we had heaps of valve clearance, but again I think Mick has tuned the car perfectly so everything is where it should be.

Good info there Jack, I think after reading that I'd rather just change my driving style to adapt to the new setup/characteristics of the car than lose the top end power.

 

[ENGINR]

I'd put a smaller turbo on it and sacrifice a bit of top end for more low down response.

 

[VIN18M]

I don't think the combination of these cams and a smaller turbo is going to help things.
Surely a 20g isn't considered that big for a 93t anyway? I've seen cars on this forum with big garret turbos hanging off 93ts and talk of gt35rs and the like...

 

[CYBORG-X]

Once cams are dialed in, you Advance your exhaust cam.. you never retard it..
And you advance your inlet cam usually half to start with
Problem with big cams is the overlap, and loss of captive effort untill the airspeed matches the cams.
Play with the cam timing.
Its all about testing...