VVT RPM limits and questions
#1
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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
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
#2
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
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
#5
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.
#9
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.
#11
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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.
#12
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.
#13
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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.
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.
#14
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.
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.
#16
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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.
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.
#17
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 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.
#18
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,
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,
Last edited by Ken Hill; 02-15-2018 at 07:00 PM.
#19
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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.
#20
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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.