nitrodann

I remember many references to VVT cams failing when the engine RPM limit is increased in stock cylinder heads.


Questions:
Is it the cams, or is the issue just that the head doesnt have enough spring?

If cams, do we know why?

What mods do the guys running big rpm have to help?

Thanks,
Dann

andyfloyd

I believe the valves float it needs stiffer springs. The lobe shape ramps up pretty steep and it causes this I believe. I think the BP4w cams ramp rate is not as steep and isnt as hard on the springs in this regard.

albumleaf

iTrader: (7)

Fairly sure the issue was a harmonic of sorts generated in that RPM range. Not sure if different cams/springs/etc really address that.

andyfloyd

From Emilio : The NB2 "VVT" stock intake cam has a very abrupt ramp which allows the valves to bounce violently past about 7800rpm.

He reccomends Supertech heavy double springs for 8000rpm+

chicksdigmiatas

iTrader: (11)

It is a float issue. You need more spring. Andrew suggested 7400 revlimit somewhere on stock springs. I am running vs855 springs and have a revlimit of 7600. Not sure what the upper limit of those springs would be on a vvt head. I was running low mileage na valve springs and a hard limit of 7500 with no issues at auto x or the dragstrip. I cant vouch for equal results for you.

The nb springs are lighter than na springs. So i ran the heavier na ones to 7500, and my ish guesstimate was somewhat correct. It is a mix between harmonics and steep ramp from the vvt cam. I have a hard time believing the vvt actuator is perfectly balanced. So my answer is yes? And the current solution is heavier springs.

I, like the ******* i am, do not currently have subs, but i have yet to spit a lashcap.

Savington

iTrader: (31)

+1. It's a valve float issue. VVT cams have over 1mm more lift than BP4W cams and less duration, so the ramps are much more aggressive. More valve spring cures the issue.

phocup

Is there such a thing as too much valve spring or is it just good insurance vs float?

nitrodann

Thanks everyone,

Much appreciated.

chicksdigmiatas

iTrader: (11)

Yeah, there is. Try turning heavy valve springs over vs light ones. Power loss... You want the lightest ones you can get. Probably lightened for fuel efficiency and emissions in mazda engineer world on the nb. Extra strain on the timing belt I suppose too. You know to meet those California mandated timing belt change periods.

Speaking of periods, maybe someone should infuse tampons with horse tranquiizer and rufenol, with something that makes women happy. Gotta go. The eye of mordor is upon me.

add lightness

What's the limit for the Supertech singles on the VVT cam?
In the same vein, where would you guys put the RPM ceiling for the stock pistons for track use? Can they make it to 8k or are they going to fail soon after the factory rods?

sixshooter

iTrader: (12)

Rods will stretch before the pistons fail. Our engines are not very fond of 8k because of harmonics in addition to the valve spring issues. They are not smoothly balanced like Hondas. Think Massey Ferguson.

Leafy

iTrader: (1)

How long do you want it to last? On my motor as built from trackspeed the intake valve lash went from right at the loose limit to all 1-3 thou past the tight limit (fsm numbers) in 1 auto-x season with an 8500rpm redline. From valve float either stretching the valve stems, flexing the head of the valve like an wind blown umbrella or mushrooming the valve seat. I relashed it and add the middle springs to make it from heavy singles to heavy doubles and ran another season with the same redline but I never re-checked the lash before I sold the motor. It still had good hot compression so it probably didnt go much tighter than the fsm numbers if it did degrade again. The new owner of the motor hasnt told me he's blown it up yet but I did tell him if it was me and I was going to run it another year above liek 7500 rpm I'd build the head. So basically at 8500 rpm heavy singles are good for about 2-3 race hours before needing to be re-lashed.

TNTUBA

iTrader: (1)

In my experience, you are WAY better off to have too much spring in a high boost, high RPM motor than not enough. Every time I went up in spring pressure, the car made more power and the valves lasted longer.

The current set I pulled out of the head had two seasons of use on them and looked hardly used. There was a lot of trial and error and a lot of re-engineering other people parts to make them last, but at this point, I am calling the issue fixed.

Spring pressure alone wont fix it......but it does more than most anything else you can do.

I'm not going to get into specifics, but i am running considerably more than the ST Heavy doubles.

patsmx5

iTrader: (16)

Eric, I'm clearly not going to be racing against you any time soon, well, ever. Don't think my auto car would do very good at autocross... But I would like to know where to get some stiffer springs than the heavy doubles, it's something I've been wanting to upgrade for a while. Good to hear you saw gains with stiffer springs. Can you tell me where to get better springs? Or roughly how much stiffer you have run without issue?

Savington

iTrader: (31)

To be fair, you're also running a lot more cam than a factory VVT setup, right?

TNTUBA

iTrader: (1)

That is a fair point.

I am running considerably more lift than a VVT cam and rougly the same durriation on the intake side.

if you were to ever see the cams in person (I think Leafy has and I know CodingParadox has) you would litterally laugh out loud and call me an idiot.

Leafy

iTrader: (1)

I've only ever seen them sitting on a tool box under an ez-up in the middle of an air field. And yes, those cams should have come with a not off the shelf valve spring solution.

Ken Hill

I'm sharing my experience with VVT intake valves and springs on my car, a 2002 with a GT2560 at 15psi. I had the engine rebuilt 7K miles ago with Supertech stainless steel valves and 56# single coil springs on a stock VVT intake cam. I just removed the head because of poor leak down results on cylinder #1. My engine builder found one of the valves in cylinder #1 had beaten itself so much, all the 0.008 lash was used up and the valve was held open. All the other valves are beaten down to about 0.003 lash.

My car is a street car only. No autocross, no tack time. Just a fast turbo car for fun on country roads. I rarely run the engine past 6500RPM. At 15psi the engine makes 270ftlbs at 3500RPM so i ride the torque curve from 3500 to 6000RPM. I'm surprised Supertech stainless steel valves failed after such a short time.

My engine builder is suggesting inconel valves and adding a second spring to my 56# springs to take the total stiffness to 84#. After reading this thread, this seems like a good solution,but I'm open to suggestions from others with more/different experiences.

Thanks,

Savington

iTrader: (31)

This is not an uncommon problem with ST valves. ST has made multiple revisions to their valves to make the seats more robust, but for a while they were very succeptible to failure if your valve seat widths were too aggressive. Inconel would do the trick as well, but it's worth a call to Martin at ST to talk about their updated SS valve.

TNTUBA

iTrader: (1)

One thing you need to remember when setting your valve stem height and spring pressure is that the Over Sized ST valves are .6mm longer and that length is under the keeper. That effectively reduces your seat pressure by roughly 8 lbs with the ST doubles.