What valve springs to use?
11-17-2005, 10:33 AM
#18
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Originally Posted by Geoffrey
You need to remove the heads to replace the guides
11-17-2005, 11:13 AM
#20
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m42racer & RobS, thanks for the inputs & suggestions - its been very good education for me 
Realistically, I'm not trying to put together a valve train that will sustain even 7500 rpm but to have a reliable and tried system that gives me a bit of insurance from valve float @ redline (~6800).
Instead of extracting the last 2% of hp, I'd rather go with reliability. The car is used pretty much on track only, so adding an extra 10 hours of reliability to me is more important than extra 15hp
Realistically, I'm not trying to put together a valve train that will sustain even 7500 rpm but to have a reliable and tried system that gives me a bit of insurance from valve float @ redline (~6800).
Instead of extracting the last 2% of hp, I'd rather go with reliability. The car is used pretty much on track only, so adding an extra 10 hours of reliability to me is more important than extra 15hp
11-17-2005, 11:18 AM
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Hi,
I'm rebuilding the engine and am debating on what valve springs to use.
My plan is to go with titanium retainers but am deciding on stock vs stiffer springs.
My understanding is that stiffer spring will both rob HP and put more load on the cam (hence wear).
So, does anyone know what safe RPM can the stock valve springs sustain?
The car is mainly used on track and I can see me revving regularly to 6700.
Thanks
1. Usually a tia retainer is around 10grms lighter and any weight reduction in the sprung weight will help valve float.
2. Regardless of harmonic conditions ( which are a factor) example being with a racing grind the stock spring will hit the 9th harmonic at 6600 rpm and the closer the harmonic number is to natural frequency the failure rate is inevitable and you need a safety margin.
Anyway you should have some insurance against missed shifts in competition and or your Brother in Law driving your car.
3. As far as the wear and HP losses unless you are going to run 14mm of lift choose a spring designed for Porsche with out inordinate seat pressures and you will not have any issues with either.
Best regards
I'm rebuilding the engine and am debating on what valve springs to use.
My plan is to go with titanium retainers but am deciding on stock vs stiffer springs.
My understanding is that stiffer spring will both rob HP and put more load on the cam (hence wear).
So, does anyone know what safe RPM can the stock valve springs sustain?
The car is mainly used on track and I can see me revving regularly to 6700.
Thanks
1. Usually a tia retainer is around 10grms lighter and any weight reduction in the sprung weight will help valve float.
2. Regardless of harmonic conditions ( which are a factor) example being with a racing grind the stock spring will hit the 9th harmonic at 6600 rpm and the closer the harmonic number is to natural frequency the failure rate is inevitable and you need a safety margin.
Anyway you should have some insurance against missed shifts in competition and or your Brother in Law driving your car.
3. As far as the wear and HP losses unless you are going to run 14mm of lift choose a spring designed for Porsche with out inordinate seat pressures and you will not have any issues with either.
Best regards
11-17-2005, 11:30 AM
#22
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Originally Posted by racing97
Hi,
2. Regardless of harmonic conditions ( which are a factor) example being with a racing grind the stock spring will hit the 9th harmonic at 6600 rpm and the closer the harmonic number is to natural frequency the failure rate is inevitable and you need a safety margin.
2. Regardless of harmonic conditions ( which are a factor) example being with a racing grind the stock spring will hit the 9th harmonic at 6600 rpm and the closer the harmonic number is to natural frequency the failure rate is inevitable and you need a safety margin.
11-17-2005, 01:12 PM
#23
PT, sorry I can't advise directly on your latest question, though I think you're going to want more spring for that cam, even if you observe the stock redline. I used that cam with larger, lighter valves and titanium retainers. The desired seat pressure, according to the man who set up my heads, was 120 lb, if I recall. I believe that the seat pressure of stock springs at stock height is considerably less than that, but I don't recall the number. I don't think I'd characterize the GT2 EVO as a racing cam, per se, but I know of a number of very successful 930-based race cars that are running them on the track with no issues.
Racing 97, it sounds like you have some knowledge of harmonics. I'd be very interested to hear more detail on this, and I suspect others would too. Care to enlighten us a bit on what you tested, how you tested it, and what conclusions you were able to draw from it?
Racing 97, it sounds like you have some knowledge of harmonics. I'd be very interested to hear more detail on this, and I suspect others would too. Care to enlighten us a bit on what you tested, how you tested it, and what conclusions you were able to draw from it?
11-19-2005, 12:58 AM
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There has been some discussion of springs over on the "Bird".
According to John Doherty of Doherty Racing Cams, the stock spring at installed height is about 60 lbs +/-. As reference, his performance springs are 90 lbs and his racing springs are 110 lbs.
According to John Doherty of Doherty Racing Cams, the stock spring at installed height is about 60 lbs +/-. As reference, his performance springs are 90 lbs and his racing springs are 110 lbs.
11-21-2005, 01:00 PM
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[QUOTE=Rob S
Racing 97, it sounds like you have some knowledge of harmonics. I'd be very interested to hear more detail on this, and I suspect others would too. Care to enlighten us a bit on what you tested, how you tested it, and what conclusions you were able to draw from it?[/QUOTE]
Okay I will try and I appologize for the length of the story.
Here is what I have observed about Frequencies and Harmonics and how I tested my assumptions. This was 1977 or so and the after market was not in full bloom.
Needless to say we were trying to get more RPM out of the Porsche 2.5 and 3.0 racing engines and began breaking things and not finishing races. After finding broken spring remnants in the oil sump screens, I was forced to learn how and why, and my conclusions and the method are following. First the Porsche spring design that spurned the development. The spring at that time, and currently for Porsche vehicles up to 1995 is a Progressive rate winding with a round wire and a Constant rate inner with a round wire, I found that there was no difference between a racing department spring and a 1966 911 street spring. The outer spring has coils wound tighter at the bottom than the top, this is a design possibly for a variable frequency rate as the active coils progressively decrease as the compression increases, thusly raising the rate. This coupled with the Constant rate inner, was an attempt to dampen spring surge by having dissimilar natural frequencies, but it did not work with the new racing profiles, the inner spring would reach coil bind and the outer would just flop around, due to the pitch on the top coils having so little proximity to one another at new frequency rate (cam ramps). As the Turbo era was in full regalia no one from Germany had any interest in the Normally Aspirated engines.
So I learned as best I could that if the engine RPM gets high enough that pulses from the opening and closing of the valve become contiguous with the natural frequency of the spring the vibrations grow exponentially and surge occurs. We will define Natural frequency as if you struck a component with a force and its given ability to resonate like a tuning fork, well the cam is the striking force and depending on the mass of the valve train it can hit it with 2000 g’s. As the engine rpm goes up the harmonic number of the spring goes down and the spring approaches its critical frequency. Picture the valve opening at 8000 Rpm on a 280 degree duration cam taking place in .006 seconds to give you an idea of the magnitude. The natural frequency of the spring we would like to see during this process would be 1/10 of frequency excited by the cam profile or a harmonic number not less than the 10th. This would be after we have established the required spring load vs. RPM so that we build a safety margin which is the first order of business. Basically you want the Spring with the highest harmonic number or the highest differential in natural frequency between the cam and the spring design in the rpm range but also we want to have enough spring force to run the maximum rpm that you want to in our case it was a single pattern Schrick cam that we wanted to run a 8200. We have programs for this now but in 1978 I observed this by rigging up a cam tower, cam, rocker, and head assembly up to a 25hp electric motor and triggering a strobe tach. In the case of our stock or Porsche racing spring the outer spring rotated in a right hand direction back and forth and the inner spring to the left and the retainer surged down the valve stem with the valve moving at one speed and the retainer at another until the keepers were exposed and actually separated from the grooves, it just hammered it into what looked ghostly blurry chaotic mess. We would lower the RPM and it would come back to its original position and when we raised it up the same thing would repeat.
This occurred at 6400 RPM and I believe we calculated it to be the 9th harmonic and this RPM was peak Torque in these motors. So before the Optron currently in use to day I had the opportunity to see for myself and test a new spring, and cure our problems. And to this day my ears still ring, you have no idea how loud this was with out the other engine dynamcis to dampen the clatter of the valve event.
Best regards
Racing 97, it sounds like you have some knowledge of harmonics. I'd be very interested to hear more detail on this, and I suspect others would too. Care to enlighten us a bit on what you tested, how you tested it, and what conclusions you were able to draw from it?[/QUOTE]
Okay I will try and I appologize for the length of the story.
Here is what I have observed about Frequencies and Harmonics and how I tested my assumptions. This was 1977 or so and the after market was not in full bloom.
Needless to say we were trying to get more RPM out of the Porsche 2.5 and 3.0 racing engines and began breaking things and not finishing races. After finding broken spring remnants in the oil sump screens, I was forced to learn how and why, and my conclusions and the method are following. First the Porsche spring design that spurned the development. The spring at that time, and currently for Porsche vehicles up to 1995 is a Progressive rate winding with a round wire and a Constant rate inner with a round wire, I found that there was no difference between a racing department spring and a 1966 911 street spring. The outer spring has coils wound tighter at the bottom than the top, this is a design possibly for a variable frequency rate as the active coils progressively decrease as the compression increases, thusly raising the rate. This coupled with the Constant rate inner, was an attempt to dampen spring surge by having dissimilar natural frequencies, but it did not work with the new racing profiles, the inner spring would reach coil bind and the outer would just flop around, due to the pitch on the top coils having so little proximity to one another at new frequency rate (cam ramps). As the Turbo era was in full regalia no one from Germany had any interest in the Normally Aspirated engines.
So I learned as best I could that if the engine RPM gets high enough that pulses from the opening and closing of the valve become contiguous with the natural frequency of the spring the vibrations grow exponentially and surge occurs. We will define Natural frequency as if you struck a component with a force and its given ability to resonate like a tuning fork, well the cam is the striking force and depending on the mass of the valve train it can hit it with 2000 g’s. As the engine rpm goes up the harmonic number of the spring goes down and the spring approaches its critical frequency. Picture the valve opening at 8000 Rpm on a 280 degree duration cam taking place in .006 seconds to give you an idea of the magnitude. The natural frequency of the spring we would like to see during this process would be 1/10 of frequency excited by the cam profile or a harmonic number not less than the 10th. This would be after we have established the required spring load vs. RPM so that we build a safety margin which is the first order of business. Basically you want the Spring with the highest harmonic number or the highest differential in natural frequency between the cam and the spring design in the rpm range but also we want to have enough spring force to run the maximum rpm that you want to in our case it was a single pattern Schrick cam that we wanted to run a 8200. We have programs for this now but in 1978 I observed this by rigging up a cam tower, cam, rocker, and head assembly up to a 25hp electric motor and triggering a strobe tach. In the case of our stock or Porsche racing spring the outer spring rotated in a right hand direction back and forth and the inner spring to the left and the retainer surged down the valve stem with the valve moving at one speed and the retainer at another until the keepers were exposed and actually separated from the grooves, it just hammered it into what looked ghostly blurry chaotic mess. We would lower the RPM and it would come back to its original position and when we raised it up the same thing would repeat.
This occurred at 6400 RPM and I believe we calculated it to be the 9th harmonic and this RPM was peak Torque in these motors. So before the Optron currently in use to day I had the opportunity to see for myself and test a new spring, and cure our problems. And to this day my ears still ring, you have no idea how loud this was with out the other engine dynamcis to dampen the clatter of the valve event.
Best regards
11-21-2005, 01:23 PM
#26
Racing 97,
Great info. I love this stuff. There is alot more to building these engines than just screwing them together. I have seen for myself, what you are saying. The difference between an engineered Cam and Spring combo and one sold from a Catalog is huge.
Great info. I love this stuff. There is alot more to building these engines than just screwing them together. I have seen for myself, what you are saying. The difference between an engineered Cam and Spring combo and one sold from a Catalog is huge.
11-22-2005, 01:18 AM
#27
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Wow, very cool even just understanding 10%. I need to dig my college physics books out of the attic and bone up. If only my professors had used these kinds of examples in school.......
