HKS VCam Step Pro + Carillo Pistons - 9.5:1 - Has anyone does this before?

[The Mafia]

Hi Everyone

I'm in the process of building my RB28 engine and I have basically got everything here ready to be assembled. Currently have an extensively modified head (Shimless buckets, 1mm oversize valves, ported, clearanced, springs, Kelford 272, 10.25mm lift cams. 

Pistons are CP Custom 9.5:1
Crank is a 2.8 Stroker. 

I want to fit a HKS VCam Step pro with the 264 / 10mm lift. 

I'm aware that the HKS VCam kit needs the HKS pistons that have a larger valve relief to obtain 50 degrees advance. 

I've spoken to someone that has done the CP + VCam Step Pro and they said they got 32 degrees of advance before there would be clearance issues. 

So, my question is pretty simple - Has anyone does this combination before and can confirm this? I would be pretty happy with 30 degrees advance because that is still a decent gain.

We would be installing a stopper once we know how far it can be advanced. 

Thanks in advance for any information.

[Piggaz]

Can’t just put larger valve reliefs in the slugs? So many factors come into this and really can’t be answered without the donk being assembled and measured.

- head been decked?

- block been decked?

- rod to stroke ratio…

- gasket thickness?

- depth of relief

- valve size

Measure it up and work backwards from there. There isn’t ANY other way to know.

[The Mafia]

I would be disturbing the structure of the piston by doing this. 

The HKS VCam was a late choice. I know, bad idea but It's apparently been done. Just can't run the full amount of advance which I'm okay with. 

[joshuaho96]

On 10/11/2021 at 8:13 PM, The Mafia said:

I would be disturbing the structure of the piston by doing this. 

The HKS VCam was a late choice. I know, bad idea but It's apparently been done. Just can't run the full amount of advance which I'm okay with. 

You could always ask CP how much valve relief you can mill into the pistons before it's a problem. I know some tuners out there mill valve relief out of the stock pistons to get the full 50 degrees of cam advance on an otherwise stock engine.

[The Mafia]

I've been thinking about this. If a Step1 kit can go onto a stock engine and move to 30 degrees advanced, I think a Step Pro should be able to do relatively the same because the CP pistons will almost be the same valve recess area and piston to quench area. 

I should be able to get about 30 degrees out of the 50 degree kit which will be still a significant upgrade. 

The engine builder is going to measure everything up for me and see how much advance I will have. 

[Piggaz]

Except step 1 is 8.6 mm of lift and pro is 10.0mm or whatever it is, Plus the different ramp rates and durations of the cams.

So many variables. Can’t compare a factory engine to your built one as so much can change.

Step 1 moves 90-120

Step 2 moves 75-125

Pro moves 80 - 130

Another variable.

[The Mafia]

Yeah we thought about that too. Sorry I should have mentioned that in my post. Once we have built the bottom end and have dummied up the head we will have a better indication on how much piston to valve clearance we have. 

I'll make sure I post the results so there are answers. It's always good to know this stuff. 

[The Mafia]

Nearly there. Should have some measurements in the next week or two. Going to use a 0.8mm head gasket too. Will have about 9.7:1 compression. 

[The Mafia]

okay, so I had issues with sourcing a 0.8mm Apexi head gasket and purchased the Nismo 0.9mm gasket. 

Careful measurement with the engine dummy assembled showed that with these particular pistons, and Kelford 272, 10.15mm lift cams, at 35 degrees advance I have 2.15mm clearance from the piston at the closest point of the valve lift. 

The pro cams have slightly less lift at 10mm.

A pro kit will work fine locked out at 35 degrees or less. 35 degrees is still a massive gain to be had. 

[RB335]

I did a Step II with 9:1 comp pistons and there was a good amount of clearance, the go was to mock it up and advance the cam 50 degrees one way see if engine is still non interferance and then 50 degrees the other way and repeat on the stand I was not going to risk guesswork at all

Yes now I have Vcam Pro going for 10:1 comp pistons and the go was to get custom pistons made with a dome shape that doesn't interfere where the valves normally travel to

Edited November 13, 2021 by RB335