Abe which measurments are you talking about?

 

deez. No nutz necessary.

 

I hate to necro a thread like this but I think it's worth clarifying since so much good info is here already...

Does this "Supertechs + VVT = Interference engine" thing apply to other compression ratios for the supertechs or only the 8.6:1 pistons specifically mentioned in this thread?

 

My engine with untouched 11:1 ST pistons, stock VVT head & cam, .020" shave on block, have an interference of about .008" at worst case; valves full open, piston at TDC. I have been running it like this for a couple of years now. If it goes, I get to build a better one!

 

My setup have contact (counting with the stretch margin at 8krpm) if the cam intake cam advance goes beyond 24 crank degrees.
VVT hickups (overshooting the advance too much) will bend valves...
So it's not only the 8.6:1 with stock valves and stock cam

 

Pulling up this old thread. I just built my VVT motor and valves are hitting the pistons at TDC. Of course it (hopefully) will not be an issue during operation. But this last note from NiklasFalk about overshooting the advance too much scares me. Is this even possible?

 

Holy ****. Why didn't I see this before?

I have 8.6:1 and +1mm O/S valves and never knew to check.

 

Is this even an issue with an unmodified head? I feel like Sav and Emilio have both posted builds with stock vvt heads and 8.6 supertechs where no issue was mentioned.

Psyber, are worried because of the o/s valves?

 

I'll have to dig through my records about specs on the head after resurfacing. It does have OS valves though.


The new head has cams, sub, and OS valves and I have a pretty strong feeling it'll interfere so whenever I manage to rebuild the bottom end, I have to figure out this black magic of clay and cam angle.

 

Ugh - not making me feel good when Psyber is worried too. Here is what I have:

block re-surfaced a couple times (unknown). Supertech 9:1 pistons sit -.001 below deck.
Head re-surfaced a couple of times, and then I went ahead and did some porting and the bowl is at 52mL. So calculations say I'm now at 8.922:1 compression ratio (not that it is relevant).

I have stock valves, manley rods.

So the question now is: How would I be able to manipulate the VVT actuator to manually rotate the assembly and check for clearance? OR, does anyone have timing info that would say "no problemo dude, stop worrying'.

 

Albert soothed my paranoia by offering great insight. He stated full advance is equivalent to 3 teeth on the cam gear. You can index it and carefully proceed with clay. He also mentioned if it interferes, you can run a tooth retarded and have a fixed offset on the vvt table effectively shifting the vvt range

 

Thanks for the info man. Sounds like my cover is coming back off tonight.

 

Another question for the experts: If I rotate the assembly through its range, and the valve does not interfere with the piston, does that mean I'm good? I would expect that there is a reason why people clay the piston - is it because you need a minimum gap between the piston and valve to make sure the valve doesn't bounce (or thermally expand) and hit the piston while in use?

 

You need a lot more room (~2mm) that just not hitting at rest, and more on the exhaust side. Unfortunately, you're in something of a hard place with an assembled motor and unknown P-V clearance.

 

Just to tidy this thread up - my journey (and findings)

1. Piston to Valve clearance, while important, isn't nearly as important as "quench or squish". The gap between the top of the piston and the head surface. My block has been ground down 10mils and I'm using a different piston from stock (9:1 supertech). So I have basically a 0mil delta between top of piston and block surface. So I have a head-gasket-only quench distance. At about 40mils, that is definitely the dominant concern. I've seen lots of info stating you should be greater than 35mils to prevent the piston contacting the head.

2. I have tons of room between the piston and valves. I clayed the pistons and found there was ~0.20" on the intake side and nearly 0.30" on the exhaust side. I'm using stock valves, and the valve seats were reground a couple of times. Crazy amounts of room. Really I'd say Miatas only have a risk if you have oversized valve diameters and/or larger cams.

3. It is in fact an interference fit, but as long as you don't plan on breaking your timing belt, it is a non-issue.

4. I am going to clay it again with the VVT advanced/retarded. I'll get back after this weekend.

Thanks everyone

 

Hey there guys,

Do we know if the same thing applies for the normal(non FM) wisecos 8.x cr?
I couldn't find measurements of the valve reliefs for the wisecos

Any input will be very helpful as I am about to tune it and I really dont want to have bent valves by advancing the vvt
As the engine is built by the previous owner unfortunately I don't have any input if the checked that before final assembly.

 

@Mudlfap any findings after advancing the VVT?