Time to go Stage II!

[Alky V6]

Now look at all cylinders at it's lowest cylinder pressure, and see if it matches to the peek exhaust pressure event that may not be happening on the same cylinder, given peek pressure differential. Keep in mind that the valve just need to be near closing to be effected.

Allan G.

You're right, Alan. Cylinder pressure at the point where the exhaust valve is seating is lower than intake pressure by as much as 4 psi for a very short moment (in my case), just as the exhaust valve seats and the piston begins to gain speed on the intake stroke. The difference is so comparatively minor that I didn't bother with it in the previous calculations.
So, in the calculations for my case, the 42 I was using for the cylinder side pressure on the exhaust valve should be as much as 4 psi lower.

 

[Alky V6]

Let's try this again.
We'll use the 88 psi peak pressure presented by one cylinder blowing down as another cylinder's exhaust valve is closing.
Assuming a 1.43 area on the backside of the exhaust valve.
Target intake boost level: 42 psi
Exhaust backpressure at 42 psi int boost: 88. Presented by peak blowdown from another cylinder.
Cylinder pressure to intake pressure differential at the moment of the exhaust valve closing: 42 - 4 = 38
Peak blowdown pressure of another exhaust port theoretically acting on the back of the exhaust valve of interest: 1.43 x 88 = 125.84 psi
Pressure acting on the cylinder side of the exhaust valve: 1.43 x 38 = 54.34 psi
Pressure differential across the exhaust valve: 125.84 - 54.34 = 71.5
220 + 71.5 = 291.5 seat pressure. Still, very close to Crower's suggestion of 287 for my particular application.

This is assuming an exhbp to intbp ratio of 1:1. If we were talking about a 2:1 ratio, I'm guessing that number would be added to the peak blowdown pressure value, adding another 60 or so psi to the required spring seat pressure.
291.5 + 60 = 351.5. Still, real close to Chris' suggestion of a minimum of 350 lbs on the seat.

 

[Alky V6]

It's very evident that a lot of different variables can play into all these pressures, making spring selection a very grey area if you don't completely understand your engine and what's going on with it.
It appears knowing your exhbp to intbp ratio is a very critical variable in spring selection.

If you don't know exactly what your exhbp/intbp ratio is, or is going to be, it would appear that going with Chris' suggestion of a minimum of 350 lbs. on the seat would be the safest way to go.

 

[Alky V6]

Looking again at what my true exhbp/intbp ratio was, and might be with the Stage II configuration, the Stage I setup, using the FI91X turbo, measured 22-24 psi exhaust backpressure at a boost level of 28 psi. What is the approximate exhbp/intbp ratio range using those numbers?
22 / 28 = ?
24 / 28 = ?

The Stage II setup will be adding some exhaust energy to the equation, so I don't expect the new exhbp/intbp ratio will be as surprising. But, I do expect it to be no worse than a straight 1:1 ratio. In fact, if it does turn out to be as bad (bad is really not the right word to use here) as 1:1, that would actually be perfect for my entire combination. Max available boost with the Stage II combination with an exhbp/intbp ratio of 1:1 would be between 45-50 psi.
I would still keep the boost level at 41-42, but turbo spool up, and the ramp up of boost would be quicker, with max boost level obtained earlier in the rpm band.

 

[Mike E]

My last car was a 2 valve modular mustang with twin 57mm t3/t4 turbos. It was one of the first turbo 2V modular mustangs and there weren't a lot of aftermarket parts available then. There was realy only one option for valve springs at the time.. When the car was dyno'ed it would make 570hp to the wheels no matter what the boost was at. At 13lbs. of boost it would make power all the way to 6000rpm. At 20lbs. it would fall off like a rock at 5000 rpm. At 25lbs. it would fall off at 4000 rpm. And so on. The more boost we put too it, the quicker it would fall off.

We suspected it was the backpressure hanging the exhaust valve open, but there were no better "drop in" springs available. I was happy with 15lbs and 570 to the tires so I never spent the money to fix the problem. The few other turbo 2V cars were having the same problem and eventually someone came up with a better valve spring to put on the cars to fix the problem. Backpressure was still less than 2:1 when the power started dropping off. Mine made it a little further in hp than the single turbo cars because my backpressure: hp ratio was lower. With the exact came engine combination n/a the car would pull to 7500 rpm all day long.

 

[Alky V6]

My last car was a 2 valve modular mustang with twin 57mm t3/t4 turbos. It was one of the first turbo 2V modular mustangs and there weren't a lot of aftermarket parts available then. There was realy only one option for valve springs at the time.. When the car was dyno'ed it would make 570hp to the wheels no matter what the boost was at. At 13lbs. of boost it would make power all the way to 6000rpm. At 20lbs. it would fall off like a rock at 5000 rpm. At 25lbs. it would fall off at 4000 rpm. And so on. The more boost we put too it, the quicker it would fall off.

We suspected it was the backpressure hanging the exhaust valve open, but there were no better "drop in" springs available. I was happy with 15lbs and 570 to the tires so I never spent the money to fix the problem. The few other turbo 2V cars were having the same problem and eventually someone came up with a better valve spring to put on the cars to fix the problem. Backpressure was still less than 2:1 when the power started dropping off. Mine made it a little further in hp than the single turbo cars because my backpressure: hp ratio was lower. With the exact came engine combination n/a the car would pull to 7500 rpm all day long.

Good stuff, Mike. Thanks for sharing that.

 

[Alky V6]

Does being able to use lighter valve springs eat up less horsepower? Assuming, of course, that you are using enough spring to keep the valvetrain under control throughout the entire planned rpm range.
You can answer that for yourselves. I already know the answer.

 

[turbobitt]

I would suspect that you will start seeing some back pressure. I would guess at least 10 PSI or more at the target boost levels you are talking about. I think mass flow is going to increase significantly over the stage I.

Allan G.

 

[Alky V6]

A good article covering the latest valve spring technology.
http://www.hotrod.com/techarticles/..._need_to_know_about_valvesprings/viewall.html
Some enlightening information about running the springs closer to coil bind than what is normally recommended is included in this article.

 

[Alky V6]

I would suspect that you will start seeing some back pressure. I would guess at least 10 PSI or more at the target boost levels you are talking about. I think mass flow is going to increase significantly over the stage I.

Allan G.

I agree with you. I don't know if the increase will be as high as 10 psi, but it should increase. Instead of having 34.5 psi to 42 psi (.82:1 ratio), I would hope it will be a 42 to 42 ratio. The sim is showing that a 1:1 ratio would provide the best peak power out of the combination.

 

[Alky V6]

In the sim, I'm using a turbine nozzle diameter spec of 2.4". This is the same turbine nozzle size I came up with on the Stage I configuration to get exhbp to intbp pressures to match real world measurements.
Using the same turbine nozzle size with the Stage II configuration really didn't change the exhbp to intbp ratio a whole bunch in the latest sim calcs for the Stage II setup, even though mass flow and power did increase. Maybe because the turbine housing on this monster is so big. I don't know.
Inorder for me to force the exhbp to intbp ratio to 1:1 in the sim for the Stage II setup, I had to lower the turbine nozzle size from 2.4" to 2.0". That's a very large turbine nozzle size change, and because the change is so big to force a 1:1 ratio, I really don't see the exhbp to intbp ratio changing a whole bunch with the Stage II real world versus what I had with the Stage I real world. I do hope there is a change, because as the ratio moves closer to 1:1, the engine will make more power, although the increase would be minor. Nonetheless, I just don't see that the change in the exhbp/intbp ratio is going to be real big.

 

[norbs]

Ok so what pressures are you going to run?

 

[Alky V6]

Ok so what pressures are you going to run?

Exactly which pressures are you referring too?

 

[norbs]

Exactly which pressures are you referring too?

Sorry ,valve spring pressures.

 

[ijames]

Sorry to be a few pages behind - Don, there is a lot of good valve spring info in the engine and advanced forums at http://www.speedtalk.com. How close to run to coil bind, with some spintron data to back it up. Almost all na, not turbo, though. Great resource for engine building tech.

 

[Alky V6]

Sorry ,valve spring pressures.

Not exactly sure yet, but I'm eye balling the Crower spring installed at 2.050". That would give me 312 seat and 706 over the nose.
Coil bind height is 1.190"
2.050 installed height - .700 lift = 1.350 spring height at full lift.
1.350 spring height at full lift - 1.190 coil bind height = .160" clearance
At .700 lift, the spring will be .160" away from coil bind.
To install at 2.050 will require a spring seat (.065" thick), a standard position retainer, and a .035 shim. From what I understand, the Ti retainer that Crower has for that spring is a +100. That's going to screw me up. I already have std valve locks.
Basic spring height that's available: 2.150" from spring seat on head to top of lock groove on valve stem.

 

[Mike E]

Could be a good opportunity for an experiment. Run them with the highest installed height you can get. If they are too low, you'll know it. Then, shim them up. And do it soon, because I need to order valve springs before long.

 

[Alky V6]

Could be a good opportunity for an experiment. Run them with the highest installed height you can get. If they are too low, you'll know it. Then, shim them up. And do it soon, because I need to order valve springs before long.

lol

 

[norbs]

How many minutes will the valve guides/seats last around town is the question with those pressures?. Not a street set-up I take it?

 

[turbobitt]

How many minutes will the valve guides/seats last around town is the question with those pressures?. Not a street set-up I take it?

With the HIPPO oiling you may be surprised.
Allan G.