Update on 2L Compression ratios

[SGKent]

This post is an addendum to the 1991 technical bulletin that is mentioned frequently on thesamba for 2L bus engines. It does not apply to 914-4, 912e, or the 400 series cars.

Using a late new case and a GD case, both yielded a deck of appx .100" using a .071" shim. If one subtracts the .071" then the deck would be approximately .029" with no shim on these two engines. (One measured .1005" and the other engine .105" but let's call it .100")

Bob Hoover stated that the paper cylinder base shims were .008" . I do not have one here to measure. Assuming he is correct, that would add .008" to the .029" for a total factory deck of .037" with the paper gasket. That is awfully close to 1mm (.03937").

The factory combustion area on a 2L head is about 51 cc +/-. Bore 94mm. Stroke 71mm. Piston dish 15 cc. Came with an aluminum head gasket that was .030". Add .030" for the head gasket to the .037 desk and that is a total deck of .067". Using a compression calculator that gives 7.3 as the calculated compression. That matches published specs for these engines when born.

In 1991 VW published new specs pertaining to the late and FI engines (GD, GE and CV). Technical Bulletin 10-90 T01 . In that bulletin the paper gasket at the bottom was removed and a 1.6 mm shim was added to the base. The head gasket was removed also. 1.6 mm= .063". This changes the deck to .029" + .063 = .092" . The other specs would not have changed. The new CR calculates to 7:0. Cross checking the math, .067" - .008 - .030" = .029". Add the.063 shim and we are back to .092". The other numbers do not change.

What ALL of us failed to notice in the bulletin is that effectively VW further detuned the specs for the CR on late bus engines from 7.3 : 1 to 7.0 : 1. It was more than just a removal of the head gasket and notching of the rod. They also increased the piston to cylinder clearance to stop pistons from seizing. As stated by myself, 2L bus engines run hot.

this agrees with Gene Berg's recommendations as well. Bob Hoover waffled on it but admitted that late Mexican engines were as low as 6.6 : 1 because of fuels in Mexico. You will get more power and efficiency with any gasoline engine if you bump the CR from 7:0 to 7.5 or even 8. You will also get a little more heat. These are considerations an owner must make when building a 2L type four BUS motor.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[orwell84]

I have read some of Gene Berg's writings on compression ratio and was surprised by how low he was recommending for setting it.. It made me realize that I don't really understand the numbers other than the theory. How much power are you potentially gaining by raising the compression ratio from 7:1 to 8:1? How much hotter does the engine run?

[timvw7476]

How much power are you potentially gaining by raising the compression ratio from 7:1 to 8:1? How much hotter does the engine run?[/quote]

I believe that is a loaded question.
Raising CR improves bmep all along the dyno curve, but too much with too little
quality fuel destroys the metallurgy of the engine over a very short time.
Leading to the wish the CR wasn't set that high.
I had no idea VW was going lower that 7.3:1 on CR, but I
ignore bulletins of all kinds. fwiw.
I blame gasoline formulations & re-formulations.

[SGKent]

we can blame many things for everything if we want.

The question of exactly how much would take multiple engines and dyno runs to decide.

GENERALLY small changes don't make a big difference in power or heat. The changes are accumulative - make 10 changes X .5 HP each and it is 5 HP. Air Cooled is a different animal than water cooled because of the problem shedding heat. For example, it is ironic that the P51 Mustang actually benefited from a larger water cooled radiator. The air expanding from heat after passing thru the radiator and out the rearwards facing duct gave the P51 additional thrust and air speed.

The 1991 Technical Bulletin starts with "have been modified to improve heat transfer and increase durability. These changes effect Engine codes: GD, GE and CV." ... The following improvements

I did look up the head gasket on the fiche for those engines, and it is .7mm or .028", close to the .030" in the VR kit now. CR remains 7.3 on the original engine with the head gasket, and 7.0 after the change. The difference of .055" (.063" base gasket added minus the .008" paper gasket) is added to the deck height). That takes it from .037" to .092". Again, these are bus changes only, they don't apply to 914-4, 912e or the 400 series cars.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Zed999]

Using an on line engine calculator I found if I lowered the CR from 8.7:1 to 8.0:1 on my stock cam type-4 I'd loose an awe inspiring 1HP peak.

[udidwht]

Keep in mind that many 1.7s and 1.8s were rebuilt to 2.0 engines over the years. The technical bulletin would apply if the shop or person was aware....or cared enough.

Also anyone with the AMC heads....

A .028 machined step is part of the combustion chamber in the AMC head (replaces the removed head gasket). That will play into the math for deck height.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[SGKent]

Also keep in mind that air to fuel management systems were primitive by today's standards. I was speaking with a good friend last night about his father's experience developing the Formula Super Vee engine for VW. He didn't get a handle on head and oil temps until they thermal coated the heads and pistons, then the problem went away. Too bad VW didn't just release new heads that were coated to stop the heat issues. Honda, Toyota etc take what they learn in their formula racing programs and turn it into production performance.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[SGKent]

Noticed last night that in Richard Atwell's notes on the 1991 Technical Bulletin he indicates that the CR was lowered from 7.3:1 to 7.0:1 with that change.*

I posted this morning for California bus owners to post NOX numbers from smog tests and head temp if they know their CR, and maybe the deck in the thread at https://www.thesamba.com/vw/forum/viewtopic.php?t=742104

* Richard Atwell ~ The gasket (~0.80mm before crush) in not the same thickness as the 1.6mm spacer VW recommends using which begs the question: has VW also recommended lowering the compression ratio or were there further instructions? Well, first consider there is was also a paper gasket at the base of the cylinder and if that measured 0.80mm also then there should be no change to the CR but Wilson says the paper gasket should be 0.20mm thick and so is the metal base gasket included with the rebuild kits so it appears VW did recommend a slight reduction to the compression because overall the deck height has increased by 0.6mm (approximately 7.3:1 to 7.0:1).
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[raygreenwood]

[SGKent]

Ignoring the effect of cam timing and ignition timing, the ratio of pressure increase in a cylinder when a fuel mixture in a gasoline engine burns is about 7 x 1. That means the initial pressure is multiplied by about 7X which pushes the piston. If the initial pressure inside the cylinder is 7.0 : 1 CR x 14.7 psi seal level pressure then the initial pressure is more or less 102.90 PSI. X 7 when the mixture burns the internal pressure would be 720.30 PSI. If the CR is 8.0 then the pressure inside the cylinder is 8 x 14.7 psi or 117.60 psi. Multiple that x 7 and the pressure is 823.20 PSI. That is an increase in about 103 pounds per square inch pushing on the piston each time it fires, although the initial difference from 7:0:1 to 8.0:1 is only about 15 psi. That difference goes into mechanical energy which lowers the exhaust gas temps leaving the head. BUT the additional energy from the compression goes into heat that is shed thru the fins on the cylinders and heads. Since there is not 100% efficiency the head and cylinder temps will climb. That also raises the efficiency of the burn which adds even more power. So while raising compression does increase power it comes with risk of pre-ignition and detonation. Pre-ignition will burn a hole in the center of the piston, detonation destroys the ring lands and outer edge of the piston. It is subtle and accumulative. It is worse when the piston accelerates slower like on a bus vs a sports car. We lack the cooling ability that water cooled engines have. Also our engines were designed when lead was in the fuel. That lead helped slow the burn. Modern fuels lack the lead and are easier to pre-ignite and detonate than fuels when our buses were new. I speculate it is likely why VW lowered the CR on late 2L bus engines. Likewise, Mexican bugs run mid-6's in compression because their fuels were worse. To get more power and efficiency one adds compression just short of an engine destroying itself. To run cooler one lowers the compression. From what I understand, somewhere around 3 or 4 :1 the cooling effect is lost and the engine gets hot again because the mechanical energy is insufficient to push the pistons successfully. Old modern A's had that issue until their CR was bumped up into the mid 4's to 5. Water cooled cars in the 70's were running 9 or 10 :1. Today with modern computers to monitor everything including variable combustion chambers even the CRs can approach 13 to 14 on daily drivers. My MDX runs about 11.5 or 12:1. That was only found in racing motors in the 1970's and 1980's. In fact, anything above about 11:1 was close to the edge of destruction on late engines. The dome in pistons was sometimes used to help the engine flow more air and fuel thru it. It had more purpose than just compression.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[raygreenwood]

[SGKent]

98 octane? Maybe AV gas or Sunoco 100 racing gas if you can afford it and don't get caught. The average premium in California is 91 now. We have 87, 89 and 91. There are a few stations that sell 95 Octane but it has more alcohol in it. Pure Gas is hard to find here - we have state mandated blends for summer and winter. State mandated everything is what you get when the Getty family is in control.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[raygreenwood]

[SGKent]

Ray - I had the experience of having a nicely tuned 1971 1600 bus engine. It was about 8.5:1 dual carbs and a mild street cam. Went like a bat out of hell but about 40,000 miles was all it would last before a stud pulled, or the case cracked, or a valve acted up, or something started to go wrong. So eventually I took the bus engine back to total stock for a 1971. The power was gone, it was gutless, but it lasted and lasted and lasted.

If you look at the type IV engine in a 914-4 or 912e vs the bus you see the same pattern. The bus engine is way detuned. I believe VW understood by the late 60's that buses had issues with reliability when they were tuned the same as a car was. Lowering compression would be an easy way to detune if pistons were seizing in buses.

That said - I remember reading an article written by a VW engineer about 1978 or so, in which they said they were about done with the air cooled engine in North America. The reason he gave was that in order to meet continued emission standards the engines had to run too lean for the air cooled engine to live. They believed that with the lean mixtures it would be impossible to cool the engines enough to keep NOX down and keep parts from melting. The 1991 Tech Bulletin that lowers compression starts with, " 1. To eliminate seizing of pistons.... "
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[raygreenwood]

[SGKent]

[SGKent]

my final thought on this today is that there are people with a deck around .060" to .065" with higher CR that seem to run cooler. My guess is that the original engines that were .059" deck and 7.3:1 ran fine. My guess is that some people who did not maintain them blew head gaskets from overheating their engines when pushing too hard. At the same time there were probably some lean failures like my friend in his Vanagon, who no fault of his the AFM was too lean. If enough engines came in with a heat related failure VW probably did not want to do an individual analysis on each one to determine a specific fix for each failure. These are assembly line vehicles. VW likely came up with a one size fits all solution which was to lower compression ratios.

Today we are restoring these buses one at a time. Each one is done on an individual basis.We can't even get the factory parts for them like a dealer would have in the late 80's early 90's. We each need to make an individual choice whether we want to follow this fix. Myself I have removed the head gasket because I believe it is a point of failure, however I am choosing not to go to such a large deck and low compression number. When the engine is built, and broken in, I will post about how it did compared to my current engine which is built to the 1991 Tech Bulletin specs. That engine runs hot in my view.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[orwell84]

I doubt that a lot of original drivers had the kind of understanding of the aircooled engine many of us do now. We are driving around with manuals in our heads, gauges and a thorough understanding of the engines limits. Back in the day they were just another car and people drove them as such.

[SGKent]

[60vwnewengland]

Hey all, I asked this question in my Cape Cod Sage thread, but thought I’d ask here too.

Im rebuilding a GD motor, stock crank etc. lower end was fine, just replacing pistons, cylinders and had the heads rebuilt. I’m fitting the new Brazilian Mahle Pistons and Cylinders. I measured the deck height w/o shims and I’m getting about 030”.

What size shim am I looking for here? .040” for a total deck height of about .070”?

Or

Per this thread here, I should get either a .060 or .070 shim to get a total deck height of .100 for 7.3 compression?

Yes?
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77 Westy - Cape Cod Sage
LLAP