VVT RPM limits and questions
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 |
Originally Posted by nitrodann
(Post 1451098)
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 |
Fairly sure the issue was a harmonic of sorts generated in that RPM range. Not sure if different cams/springs/etc really address that.
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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+ |
Originally Posted by andyfloyd
(Post 1451142)
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+ 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 asshole i am, do not currently have subs, but i have yet to spit a lashcap. |
Originally Posted by andyfloyd
(Post 1451122)
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.
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Is there such a thing as too much valve spring or is it just good insurance vs float?
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Thanks everyone,
Much appreciated. |
Originally Posted by phocup
(Post 1451205)
Is there such a thing as too much valve spring or is it just good insurance vs float?
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. |
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? |
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.
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Originally Posted by add lightness
(Post 1452419)
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? |
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. |
Originally Posted by TNTUBA
(Post 1452514)
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. |
Originally Posted by TNTUBA
(Post 1452514)
I'm not going to get into specifics, but i am running considerably more than the ST Heavy doubles.
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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. |
Originally Posted by TNTUBA
(Post 1452610)
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. |
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, |
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.
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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.
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