Excess Pressure could cause valve float...

[yellerandahalf]

My machinist told me today that I should be very careful when running high boost pressure in my newly built engine, as it could cause the valve to float, spack pistons, and generally cause all hell to break loose. He said if you get past 14-15 psi, the valves springs on the average motor are not strong enough to overcome the pressure of the air flowing in, and get them closed quickly enough to avoid smacking the pistons. This is also really bad for other things, but the main catastophic thing would be engine siezure and whackage.
But here's the thing: I have heard of several people running in excess of those pressure with the stock head, several insane people running those pressure with stock internals; my question is how. What pressure are you high pressure people running, with what turbos (to get a good idea of flow rate)? I'd like to know if there have actually been valve float problems encountered at high boost, versus just theory. Because it'd be quite a waste to put just 14psi through my, and lots of other peoples' 22psi-safe motor.
Also, is there any way other than failure to detect valve float? I have not heard of any.
-Mateo

 

[BlkZoomZoom]

What motor is he basing this theory on? I can honestly say I have never heard of this problem. Personally unless your running a HUGE turbo I doubt this should be your worries. I'm sure we'll find many more problems to overcome first.

 

[KySpeed]

I'd worrie about crank walk before this. Whats your compression ratio

 

[122 Vega]

I'm pretty sure the FS is a non-interference motor, so you won't hit the pistons with the valves. This can be a problem on higher boost motors, but if you are going to build a motor to support over 14 psi,you should just put new springs in anyways. for etter flow at ranges above 20 psi, a full P&P with better valves and a good three angle valve job. You can also profile your cams and adjust the overlap to reduce the back pressure against the valves.

Britt

 

[Striker187]

i've heard the theory...i thought it occured in high revving engines, not necessarily high boost engines. as for high boost pressures on the stock motor.....an average is probably 12 psi. the highest i've heard of is 18 psi daily, 21 psi at the track. the motor held for a year until a piston ring wore down.....the motor did NOT blow. of course, if you raise the boost, you're taking chances with an unprepared motor. you might get lucky, you might not.

 

[yellerandahalf]

What motor is he basing this theory on? I can honestly say I have never heard of this problem. Personally unless your running a HUGE turbo I doubt this should be your worries. I'm sure we'll find many more problems to overcome first.

This isn't based on any motor in particular, just a very knowledgeable machinist. He, however, does not have any particular experience with this exact motor, but he has been building mazda motors for quite some time.

 

[yellerandahalf]

I'd worrie about crank walk before this. Whats your compression ratio

I'm running 8:1. I don't think I'll be too worried about crank walk, I don't plan on running upwards of 20psi. My concern is this potential problem before i get to that one.

 

[yellerandahalf]

i've heard the theory...i thought it occured in high revving engines, not necessarily high boost engines. as for high boost pressures on the stock motor.....an average is probably 12 psi. the highest i've heard of is 18 psi daily, 21 psi at the track. the motor held for a year until a piston ring wore down.....the motor did NOT blow. of course, if you raise the boost, you're taking chances with an unprepared motor. you might get lucky, you might not.

Yes, this does tend to happen with high-revving engines, that's where the problem originated form I think. Who is doing 18psi? I'd like to speak to him/her. Btw, I am running a prepared motor, at least bottom-end-wise. The head will get lots of work done to it eventually, but that's "stage 3" if you will. I have to get more money before I make that jump.

 

[yellerandahalf]

if you are going to build a motor to support over 14 psi,you should just put new springs in anyways. for etter flow at ranges above 20 psi, a full P&P with better valves and a good three angle valve job. You can also profile your cams and adjust the overlap to reduce the back pressure against the valves.

Britt

Any explanation for WHY putting in new springs is a good idea at that specific point? It'll happen eventually, I'm just worrying if I should do it right versus save up the money and do it a little cheaper later. That is good advice there though, and it is all already planned to be done. Thanks for the help everyone!
-Mateo

 

[122 Vega]

Because you don't want the valves slamming back into the seats at high boost/high rpm. You can actually bend the valves and risk dropping the seat out of the head. It happened to one of my NA race motors, wasn't pretty, the seat exploded and went back up into the intake and into two other cylinders. Probably was caused by the valve springs floating at the high rpm.

Britt

 

[Striker187]

discreetspeed. last i heard he was working on something huge for the car....

 

[DsourceMsp]

I'm running 8:1. I don't think I'll be too worried about crank walk, I don't plan on running upwards of 20psi. My concern is this potential problem before i get to that one.

I have heard of a few stories about crank walk in Dsms, and every single one of those stories the car was in this range we are talking about (19-22psi). Anyone every seen the mitsu symbol under the hood? (doh)

 

[yellerandahalf]

Because you don't want the valves slamming back into the seats at high boost/high rpm. You can actually bend the valves and risk dropping the seat out of the head. It happened to one of my NA race motors, wasn't pretty, the seat exploded and went back up into the intake and into two other cylinders. Probably was caused by the valve springs floating at the high rpm.

Britt

ouch! ::cringe:: that sounds like something i don't want to do.

 

[yellerandahalf]

So the general consesus is it's cool for a while, but it's definetly a good idea not to run high pressure for too long? And ideas on how long? How long has discreetspeed been running his motor?

 

[Striker187]

about a year before the ring went. i dunno what his status is at this point.

 

[jflo]

he's shooting for 400hp also. not coming back until he reaches that

 

[yellerandahalf]

he's shooting for 400hp also. not coming back until he reaches that

sweeeeeet

 

[SpeedProtege]

My machinist told me today that I should be very careful when running high boost pressure in my newly built engine, as it could cause the valve to float, spack pistons, and generally cause all hell to break loose. He said if you get past 14-15 psi, the valves springs on the average motor are not strong enough to overcome the pressure of the air flowing in, and get them closed quickly enough to avoid smacking the pistons. This is also really bad for other things, but the main catastophic thing would be engine siezure and whackage.
But here's the thing: I have heard of several people running in excess of those pressure with the stock head, several insane people running those pressure with stock internals; my question is how. What pressure are you high pressure people running, with what turbos (to get a good idea of flow rate)? I'd like to know if there have actually been valve float problems encountered at high boost, versus just theory. Because it'd be quite a waste to put just 14psi through my, and lots of other peoples' 22psi-safe motor.
Also, is there any way other than failure to detect valve float? I have not heard of any.
-Mateo

The amount of misinformation out there is alarming. I would not worry at all about the valves coming open due to air pressure let say your running 20 psi. lets say the total area on the back of a valve is 2 sq inches. that amounts to 40 lbs. ever try to squeeze a valve spring? pretty stiff, 40 lbs is not going to do much.

Valve float occurs when your springs arent stiff enough to return the valve to a closed position as the piston is racing up on the compression/exhaust stroke. This happens when you rev way past redline, and your stock springs are perfectly fine for the stock rpm range.

If your valves are floating and its a non interference engine, it will cease to increase rpms and sound like s***/fall on its face.

And... crankwalk... every dsm'rs nightmare. both low psi and high psi engines have suffered. any engine can actually suffer from it, its just more prevalent in the 2nd gen dsm engines. crankwalk is actually thrust bearing failure. some of the theories floating around are inadequate oiling due to poor oil squirters and beefier pressure plates. high boost pressures have nothing to do with it.

and who do you race your spec miata with?

 

[yellerandahalf]

we race the SARRC races, mostly the more southern ones, as we're out of Raleigh. My dad and I run a red, old, beat-up one. it's a ton of fun. Thanks for your input on the valve float issue, that's a very good point.

The amount of misinformation out there is alarming. I would not worry at all about the valves coming open due to air pressure let say your running 20 psi. lets say the total area on the back of a valve is 2 sq inches. that amounts to 40 lbs. ever try to squeeze a valve spring? pretty stiff, 40 lbs is not going to do much.

Valve float occurs when your springs arent stiff enough to return the valve to a closed position as the piston is racing up on the compression/exhaust stroke. This happens when you rev way past redline, and your stock springs are perfectly fine for the stock rpm range.

If your valves are floating and its a non interference engine, it will cease to increase rpms and sound like s***/fall on its face.

And... crankwalk... every dsm'rs nightmare. both low psi and high psi engines have suffered. any engine can actually suffer from it, its just more prevalent in the 2nd gen dsm engines. crankwalk is actually thrust bearing failure. some of the theories floating around are inadequate oiling due to poor oil squirters and beefier pressure plates. high boost pressures have nothing to do with it.

and who do you race your spec miata with?

 

[jred321]

I have heard of a few stories about crank walk in Dsms, and every single one of those stories the car was in this range we are talking about (19-22psi). Anyone every seen the mitsu symbol under the hood? (doh)

if you look at some of our electronics there's probably some mitsu symbols there. i wish our motor was a mitsubishi.

like speedprotege said, crank walk on dsm's had nothing to do with boost pressure or really power output, although when some people upped their power output crank walk occurred quicker, but probably would have occurred regardless.

for the valve springs, there are equations and stuff out there that you can do to determine the actual correct spring rate for what you're planning on running, but i wouldn't worry about it for now, especially with the non-interference engine and the main risk is basically just running like s***, not really breaking stuff from valve float