Hey, where'd my vacuum go?

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dingo
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Re: Hey, where'd my vacuum go?

Post by dingo » Sat Mar 24, 2012 10:54 pm

SlowLane wrote: idle curiousity).

'idle curiosity' nice one !!
'71 Kombi, 1600 dp

';78 Tranzporter 2L

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RSorak 71Westy
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Re: Hey, where'd my vacuum go?

Post by RSorak 71Westy » Sun Mar 25, 2012 1:39 pm

Yes your observation is correct, more ignition advance results in more rpm and more power, the problem with VW engines is at 3000 rpm you dont want more than 30 degrees of total advance otherwise things tend to run too hot. With more initial advance unless you limit the advance built into the distributor you will go over the 30 degree limit at speed.
Take care,
Rick
Stock 1600 w/dual Solex 34's and header. mildly ported heads and EMPI elephant's feet. SVDA W/pertronix. 73 Thing has been sold. BTW I am a pro wrench have been fixing cars for living for over 30 yrs.

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Amskeptic
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Re: Hey, where'd my vacuum go?

Post by Amskeptic » Sun Mar 25, 2012 4:35 pm

SlowLane wrote: Here's my hypothesis: when the timing is set to 7.5 degrees BTDC, the combustion impulse is stronger than when it is set to 5 degrees ATDC. The engine is more "eager" at the more advanced timing, and so requires more restraint (via the restriction imposed by the idle speed screw) to stay down at the prescribed idle speed. Since the volume being moved by the air pump is (more or less) equal for the two ignition settings, the increased restriction at the more advanced setting results in a higher vacuum.
That is not it.

The whole *reason* for the retard was to cram more volume through the engine . . . this serves to dilute the emissions, it is why VW Solex Bosch went to all of that effort to *make the engine weaker* at idle so they could crank open the idle air circuit. Have you compared the exhaust pulses during the retard at 800 rpm vs no retard at 800 rpm? Twice as much puff. Your diminished vacuum is because yes, you need more opening at the throttle when retarded, which drops vacuum right there, and the exhaust pulses are stronger and stop the intake scavenge at overlap.
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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SlowLane
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Re: Hey, where'd my vacuum go?

Post by SlowLane » Sun Mar 25, 2012 5:13 pm

Amskeptic wrote:the exhaust pulses are stronger and stop the intake scavenge at overlap.
Okay, I'm going to have to call you on this one.

Firstly, it would seem to me that a stronger exhaust pulse would result in better scavanging, since it's the velocity and inertia of the exiting exhaust gasses which draw in the initial intake charge. More energetic egress, more eager ingress.

Secondly, how exactly does retarded timing make for a stronger exhaust pulse? Wouldn't the strength of the exhaust pulse for one cylinder depend on the characteristics of the power stroke in the following cylinder? And wouldn't the power stroke have more authority with advanced timing than with retarded.

I dunno, maybe I'm the one who's retarded here, but something ain't adding up.
'81 Canadian Westfalia (2.0L, manual), now Californiated

"They say a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance."
- Terry Pratchett

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Amskeptic
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Re: Hey, where'd my vacuum go?

Post by Amskeptic » Sun Mar 25, 2012 6:02 pm

SlowLane wrote:
Amskeptic wrote:the exhaust pulses are stronger and stop the intake scavenge at overlap.
Okay, I'm going to have to call you on this one.

Firstly, it would seem to me that a stronger exhaust pulse would result in better scavanging, since it's the velocity and inertia of the exiting exhaust gasses which draw in the initial intake charge. More energetic egress, more eager ingress.

Secondly, how exactly does retarded timing make for a stronger exhaust pulse? Wouldn't the strength of the exhaust pulse for one cylinder depend on the characteristics of the power stroke in the following cylinder? And wouldn't the power stroke have more authority with advanced timing than with retarded?

I dunno, maybe I'm the one who's retarded here, but something ain't adding up.
A retarded spark moves the entire combustion event towards the exhaust valve opening
(the theoretical most efficient start of the combustion event is 4* BTDC at idle with the combustion pressure pulse hitting the piston at 10* ATDC), if you start the combustion event at 10* ATDC, it is ending near 25*ATDC.

Your "better scavenging" only occurs at a higher rpm, it is all totally useless at idle.
An authoritative power stroke is one that hits the piston just over center. As mentioned above, the combustion event should be *done* at 10 ATDC. Starting combustion at 10*ATDC makes for a weak push on the piston, which is exactly what they wanted so they could add air flow through the engine without having an idle speed that was too fast. Late ignition also yielded a big blast of hot air pushed out of the exhaust which served perfectly the need to light the afterburners in the exhaust ports on the '73 and '74 smog pump buses. That air pump was blowing fresh air on the retarded timing combustion gases coming out and doing some final oxidation. BMW did this to great effect all the way to 1980. They called the exhaust manifolds "thermal reactors".
Colin
(p.s. it is more exciting than that. Exhaust gases are "condensing" as they cool from a 1000* to 6-5-400* in short order. It is the condensing of the hyper-light hot gas to cooler smaller gas that draws the intake charge. Of course if the exhaust gas is still literally burning on its way out the door, that scavenge is not happening)
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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