Pop off valves ??

[marcus]

ok now that Sparky has decided to help some of us non technical folk out a bit I want to jump in and ask a question.

I have been watching Indycars for the last 8 years and I guess F1 when turbo's were used there as well but I dont understand what a pop off valve does??..I believe it limits the amount of boost or something like that and the term to do with bar or something like that..(Im not quite sure on the last thing something like 4 bar of pressure or something??)

but my question is what does a pop off valve do and what does bar of pressure actually mean??

CHEERS
Technically not inclined to the slighest Marcus

[KC]

The popoff valve works just like a wastegate valve on Rally cars in that it vents boost pressure from the engine. Instead of it just relieving unwanted boost pressure, the popoff will release the engine plenum chamber pressure if it exceeds the maximum allowed boost pressure for the race. This results in an immediate reduction in about 350 horsepower. it takes just a second or two for the valve to close but the turbo needs a few more seconds to rebuild the boost level back up again and it will cost the racer greatly in speed.

The pressure in the intake plenum of the engine can measured in several ways.

CART uses a set level of barometric reading or bar. Bar expresses the relationship between pressure, temperature and volume of air.
The typical popoff setting for road course and street course racing is set at 42 bar, or approxiamtely 28 pounds / square inch. The larger ovals are limited in boost level to around 32 bar or around 15 pounds / square inch.

[Sparky]

Welcome to the Forum, Marcus!

See? Now that wasn't painful, was it?!

Anything you want to know, just ask. ANYTHING...

[koenda]

KC, I don't know if I read it correctly and understand it.
But bar is just the unit of pressure. It's 100,000 Pa or approx. 1 atm. (which is 1.013 bar) Yes, I'm from Europe, so I'm using the SI
42 bar seems a lot to me because usually the max. pressure inside the combustionchamber is about 200 bar. But I guess you forgot the dot..
(Getting a book to check it.... 1 Pa equals 0.145 x 10^(-3) lbf/in², so 1 bar equals 14.5 lbf/in². yep, you've forgotten the dot )

[ma]

The units used are Inches of Mercury. 30.0 Inches Mercury = 14.7 PSI , which is also atmospheric pressure at sea level.

So, at 42 inces of boost, pressure is only 5.88 lbs above atmospheric, not much! The same boost is used on both road courses and ovals.

One bar is approx. atmospheric pressure, which means that on the old turbo F1 cars, when qualifying boost was at times as high as 8 bars (before restrictions), the boost was as high as 117 lbs !

No wonder the engines were only good for 10-15 laps !

[Sharky]

Jon Beekhuis (commentator for CART on ESPN and ex driver) said something pretty interestion on RPM2night the other day. It seems that champ cars run about 1.3 atms of pressure or something like that....anyway....very close to the atmospheric pressure. Anyway....in the past few months there has been a lot of talk about how to reduce the speed of the cars and one obvious suggestion was reducing the boost of the turbo but the turbo manufacturers complaines because they said that if they reduced the boost they would have to develop a whole new turbo system because since the pressure would be even closer to the atmospheric pressure they would be better off running axial compressors instead of radial compressors (like nowadays).

Just a note. An axial compressor would be kind of like a fan that would just "blow" air into the engine while a radial compressor is like....mhhhh....how to describe without a pic....it's more or less like the inside of a water pump that really "sweeps" the air into the engine.

[marcus]

ummm..thanks guys..still a bit to techy for me but im reading it several times and understanding it more each time I read it.

one more question..

what would an champcar engine be like without a turbo charger ???

or is that basically what the IRL is ???

[KC]

I stand corrected on the units. They do indeed use Inches of Hg for the units when describing 42 inches of boost.

[Sharky]

Nope....they world't be as underpowered as IRL cars because I think that IRL cars restrict the ammount of RPM's to 12000 or something like that. A champ car would still be able to rev up more and therefore produce more power. HGowever....they would lose a great deal of power. Somewhere in the vicinity of 150 to 250 HP.

BTW....I once heard a rumor that during last year's off season penske had ran one of their cars without the pop off valve (therefore unlimited boost) and the engince produced something close to 1100 HP!!!

[Liz]

Well, then, isn't the popoff valve supposed to stop what happened to Paul Tracy's car when that Plenum thing blew forty feet in the air and took half his car with it? Why did it not work - did the pressure build up too fast or was the valve defective, maybe?

[ma]

Liz:

Most likely it was a catastrophic mechanical failure of the engine internals - most likely the crankshaft - not an over-boost problem. The plenum housing is probably capable of withstanding over 250 psi of pressure - way, way, way, more than the turbo is capable of producing.

[Sharky]

Liz. The pop off valve has no safety purposes....aside from slowing cars down. What happened to Paul Tracy during Fontana was a completely independent problem.....from what I heard the pressure increased dramatically in one part of the engine but ir had nothing to do with the pop off valve. The only purpose of the valve is to keep drivers from using more boost than the one CART allows.

And just in case some of you are wondering if somebody could tamper the valves. The valves are controled by CART officials and they are only installed minutes before the race or qualifying. They are installed under the closed eye of a CART official and nobody but these officials is allowed to touch them.

[KC]

What happened to Tracy's car is that on the large ovals, where the engine runs at high peak rpms constantly, it can produce a massive fuel surge when feathered. The fuel metering system cannot act quickly enough to a lift off and fills the plenum with fuel that the engine is not going to use. The same thing happens in NASCAR on the restrictor plate tracks except the fuel is dumped over the side of the carburettor. The very bad part about this situation is that it also produces a massive lean surge in the engine and raises combustion temperature to the point that the piston can begin melting. CART's use of methanol as a fuel helps out a lot because it has low volatility and does a lot to cool the enigne as it is injected in. The WC cars use high octane race fuel and burn up pistons all the time at restrictor plate tracks.

[Liz]

Erm, what does "feathered" mean in this context? When you "feather" an airplane engine you shut it off. But it can't mean that.

[chunder]

Great chat topic guys.
Just to let you know that in the early pop off valve days 1987 (I think) the drivers used to link a piece of tube to the valve so that they could hear when it was going to pop. Derek Warwick in particular demonstrating this on an old Grand Prix programme.
I know that an Indy Mercedes ran a single cam engine at Indianpolis once much like the old Buick powerplants. Unser Jr and the Penske team walked the race and Lord only knows ahat power that beastie was putting out.
In qualifying trim the BMW four could kick out 1300+ hp. I always found it strange that the Beemer was the most powerful motor being only a four pot. Any thoughts.
Also in 1988 Honda were reputed to have found an ingenious way round the valve, maybe explaining their dominance in the '88 season.

[Sharky]

Hi chunder! Welcome to the forum. I've heard about the tube but from whet I know they still used it....at least in 1996...drivers were supposed to hear a "whistle" when the pop off valve was about to open and therefore take the necesary measures to avoid that from happening. I'm not sure if they now use the telemetry readings now though.

As for the BMW engine. Remember that it was 1300 hp that you could only use for a couple of laps max. Although that's still a very impressive figure. Don't you just wonder how much power would they be able to get from an engine nowadays with the technology available now?

BTW....I think this is a pic of the engine
pic

[KC]

In racing parlance, they mean to lift off gently when they speak of "feathering" the throttle. Apparently when things are just right, a slow lift off of the throttle can cause this plenum build up and explosion.

[Marshal]

Originally posted by Sharky
Don't you just wonder how much power would they be able to get from an engine nowadays with the technology available now?

Honda reckoned that they were just scratching the surface of the potential when turbos were banned, so, 2500 bhp in qualifying and 1500-1800 in race trim (unrestricted this is). My, wouldn't that be exciting. With current aerodynamic rules I reckon they'd be pushing 260mph at places like Hockenhiem......

[chunder]

Ok then guys, let's get seriou about this. What is the most power you have heard about from a turbo engine in racing (excluding dragsters).
I would say maybe the Donohue 917 CanAm cars or some of the late eighties F1 cars. Maybe some the Pikes Peak guys could shed some light.
As far as the proposed Hockenheim thesis, I agree but maybe the aerodynamics would have something to say about 260mph. Would look damn impressive though. Imagine the brakes needed.
Back to the BMW motor. Heard a story about Fabi setting a qualifying tiome at Monza and the thing grenading on the cool off lap. Can you believe the expense of building an engine to do two laps.

[Sharky]

Well....aerodynamics would surely keep an F1 car from showing all it's potential with that kind of power....but I think that the 260 Mph mark could be easily reached. But once again that makes me wonder.....if you installed that same engine in a GT1 car.....with much beter aerodynamics....

[koenda]

chunder:
i've read the book "Unbeatable BMW, 80 years of racing history". In this book they described the BMW engine.
It had the most power because it could withstand the most boostpressure. With the boostpressure-regulation BMW suffered the most because the V6 engines could run at higher rpms and thus produce more bhps.

The picture is definately a BMW, the "laydown" version.

[koenda]

of course I should have given the correct title

Unbeatable BMW, Eighty Years of Engineering and Motorsport Success, by Jeremy Walton.