.660 BTR dual spring pressures at .525 lift?
07-12-2017, 08:11 AM
#1
.660 BTR dual spring pressures at .525 lift? Anyone happen to have this information? These are 155 closed and 400 open.
Curious what kind of open pressure I’m seeing at only .525 lift. Have a pretty moderate cam in and I’m thinking I may need to shim these. Or if you have a spring tester compress a set to 1.280 or so and let me know what you have.
I have a very gradual loss in power at 5800ish and by 6k it falls on it’s face. It’s nice and smooth though. No breaking up. Plugs and AFR look great. It’s a 219/228 @ 112 .525 lift cam. Twin 72/68 turbo setup. Back pressure is 1:1. Feels great to 5800, then it feels like NA power.
Curious what kind of open pressure I’m seeing at only .525 lift. Have a pretty moderate cam in and I’m thinking I may need to shim these. Or if you have a spring tester compress a set to 1.280 or so and let me know what you have.
I have a very gradual loss in power at 5800ish and by 6k it falls on it’s face. It’s nice and smooth though. No breaking up. Plugs and AFR look great. It’s a 219/228 @ 112 .525 lift cam. Twin 72/68 turbo setup. Back pressure is 1:1. Feels great to 5800, then it feels like NA power.
07-13-2017, 07:37 AM
#4
Spoke to Howards though and they said 330# should be plenty. I spoke with BTR and they tested one of their new springs and came up with about 360# at my installed height so I should be fine spring wise. Others are saying you shouldn’t run a spring more than .060 from its rated lift because the spring may surge or have other odd harmonics issues. So I’m still on the fence. May try a few other things before going back to springs.
Duty cycle is around 90%, Fuel pressure is not falling off, AFR is steady, and plugs look great. This is a twin setup, 72/68's on a 5.3. Back pressure is 1:1. So It shouldn’t be stressing the springs any more than an NA setup I wouldn’t think.
.024 gap now. I’ve tried 4lbs of boost with no water/meth and gotten the same result. May try .019 next and see if it helps any. I’m stumped.
Thanks!
Duty cycle is around 90%, Fuel pressure is not falling off, AFR is steady, and plugs look great. This is a twin setup, 72/68's on a 5.3. Back pressure is 1:1. So It shouldn’t be stressing the springs any more than an NA setup I wouldn’t think.
.024 gap now. I’ve tried 4lbs of boost with no water/meth and gotten the same result. May try .019 next and see if it helps any. I’m stumped.
Thanks!
07-14-2017, 12:09 AM
#5
Back pressure has nothing to do with it. You still have to overcome psi X valve surface area off the seat to move the valve. With a 2.02 valve that's ~3.2 sq inches fighting whatever your PSI is at any given time. At 20 psi that's 64# of effective spring pressure lost because the pressure in the cylinder is pushing backwards against the closed valve..
07-14-2017, 05:37 AM
#6
Not sure how anyone can answer without knowing the installed height. You need to measure, then use the spring stiffness to get both the seat spring force and the open spring force. If too low, you can shim to increase the seat/open force values and also even up the spring forces for all valves, i.e., get a consistent spring seat force value.
07-14-2017, 08:26 AM
#7
So exh back pressure wouldn’t be additional pressure acting on the exh valve?
I think it’s more complicated than your description The pressures on the valve at different events vary a ton.
If your back pressure is say 2.8:1 (as mine was at 25lbs) Then by your math my 300lb valve springs would have floated the valves like crazy. But they didn’t. Worked great to 6800 or so on my old 5.3.
The installed height isn’t going to vary enough to matter IMO. I can check it, but we are talking a factory unmodified head. Even if it’s 1.70 to 1.85. The springs should produce more than enough pressure to control a .525 lift cam. This cam is crazy mild… It’s not like I’m trying to iron out a valve train on a race engine. This is similar to a stock cam. These cams work great with stock ls6 springs and should easily REV to 6800.
I appreciate all the input and I will pop off a spring and test it for peace of mind. I’ll also order some shims to get it within .060 to bind height… Like every other time though, I really don’t think the valve train is the issue.
I think it’s more complicated than your description The pressures on the valve at different events vary a ton.
If your back pressure is say 2.8:1 (as mine was at 25lbs) Then by your math my 300lb valve springs would have floated the valves like crazy. But they didn’t. Worked great to 6800 or so on my old 5.3.
The installed height isn’t going to vary enough to matter IMO. I can check it, but we are talking a factory unmodified head. Even if it’s 1.70 to 1.85. The springs should produce more than enough pressure to control a .525 lift cam. This cam is crazy mild… It’s not like I’m trying to iron out a valve train on a race engine. This is similar to a stock cam. These cams work great with stock ls6 springs and should easily REV to 6800.
I appreciate all the input and I will pop off a spring and test it for peace of mind. I’ll also order some shims to get it within .060 to bind height… Like every other time though, I really don’t think the valve train is the issue.
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07-14-2017, 01:28 PM
#8
Only reason I mentioned height is your original question focused on the spring force at your maximum lift. Without know the actual seat value, the open value can't be computed.
As for blowing the valve open with boost, shouldn't happen unless you are running crazy boost for some reason. However, you might also lose some control over the closing with too low of a seat value. Pushrod flex can contribute to this as well at higher RPM.
As for blowing the valve open with boost, shouldn't happen unless you are running crazy boost for some reason. However, you might also lose some control over the closing with too low of a seat value. Pushrod flex can contribute to this as well at higher RPM.