VVT + Supertech pistons = interference engine
 

by nearly 1 mm on the intake
exhaust is OK


These are with 84 mm, 8.6:1 Supertechs

Yipe. With stock valves?

so my big domed 10:1 pistons are fine but supertechs are not? wtf?

Oh god! And I loved the Miata engine for not exploding when a timing belt fails.

Yep. I seriously doubt there will be any issues with 47* of crank degrees advance on the intake cam though. That's only 24* on the cam. IOW if max lift is at around 90* ATDC at max retard, 24* of cam advance won't make the valves hit the piston at TDC. This is usually a problem with V8's because they have huge valves (2 valves per cylinder) and need huge cam duration to make power.

Those big domes have deep notches. The supertech notches are effectively about 1.3 mm shallower. When I say "effectively", it means the bottom of the cutout reaches the valves 1.3 mm closer.

Well only pukes who don't change their belts break em.

Jason, have you taken a measurement on the 8.6:1 Supertech that someone like Keith at FM could compare to their 9.0:1 Weiscos in order to check clearance? I know one of their recent redesigns allowed them to maintain non-inteference status even with +1 valves.

Timing belts can fail before their recommended change interval. This was a welcome relief after having to worry about whether or not the PO had done a timing belt job on my POS Eclipse years ago.

This thread is relevant to me. Rawr. I don't want to derail it.

Cutout is 3.3 mm below piston deck.
The deck is 20.4 mm above the top of the piston pin.

Jason, out of curiosity what is the deck clearance with those pistons?

They are 0.5mm lower than the stock 01 pistons, which are pretty close to flush with the deck.

Sav, going by piston heights to figure out if you have enough clearance is fraught with peril... the only way to be safe is measure it.

If you'd like the procedure we use at the shop to check this, give me a call. It will be a little more complicated with a moveable intake cam but not tremendously so.

Jason, can you please give more details about how you arrived at this conclusion?

I measured took my head and turned the cams so that it was at full open on both intake and exhaust in one cylinder then turned it over. I placed the piston over the valves so it was resting on the relief cuts. Exhaust side was fine. I eyeballed the distance between the piston's deck and the cylinder surface and placed my caliper beside it, eyeballing an equivalent gap. It was around 1.5 mm. Given that the compressed head gasket is 0.8mm, there's about 0.7 mm of interference. If you do the math using the piston deck height measurements and the cutout depths I measured, it makes sense, assuming that the stock pistons have about 0.5 mm of clearance.

I hope to have a better measurement when I put the head on the block and check clearance with VVT.

Do you plan on measuring the range between full retard and full advance with VVT with a timing belt on to see if you would ever actually make any contact? I understand that the pistons might make the engine interference now, but I don't honestly see that as a concern. Practically every motor in existence now is. I'm more interested in if the pistons create an interference in the range that cams are actually adjustable.

Yes that is my concern but I doubt it is a problem because I doubt VVT can make the intake cam get within 1.5 mm of full lift at TDC. However, better safe than sorry.

Anyone have a procedure handy? -- I'm wondering how to do it while minimizing the hassle of getting the head and belt on and off .... I suppose I use an old head gasket right?

Does anyone have a page of the 01 cam belt / tooth alignment procedure handy? The VVT intake cam gear doesn't have a convenient mark and the mark I made got erased.

When I used to put together B series motors before I bought the miata I would clay them without a hg, testing each degree change on the adjustable cam gears. Just add the HG to the measured clearance. Unless you have mega lift cams you would normally never get total contact necessitating the need for a HG on check. If you start at the extreme retard and adv and work your way to the middle you will likely see the clearance increasing. You can pretty much stop at the point where you have adequate clearance if you see an increasing trend toward 0deg. This minimizes the amount of times you need to do it. Usually 3 on each side was enough for me to find the limit.

With a stock VVT cam I doubt that there will be an issue. With a custom cam with higher lift you would definitely need to clay it case by case.

I have the FM Wiseco 9:1 spec sheets for both the old and new designs. If I am am reading the specs right, the new design has the intake valve pockets .175" (4.45mm) deep.

From the spec sheet for the intake valve pockets:
Depth from TE = .175 VERT

I am assuming TE means top edge.

--Ferdi

Mostly trying to get an idea for what the limitation is as far as valve lift goes.

Jason, do you have a '99 head laying around that you can take the same measurement on? If the '99 head is still non-interference with 8.6:1 Weiscos then IMO it's safe to assume that full advance on the VVT won't cause an issue (if more cam retard = tighter clearances as neo said).