Flat Top Pistons and Compression Ratio, 1.7L Type 4 Engine

[Mpec2]

I've got a 72 Westfalia with a stock 1.7L Engine. I'm considering an engine rebuild in the future and been reading up about rebuilds and one idea has stumped me a bit.

For the sake of the conversation lets just say that i am leaving the crank, cam, and rods stock and original. Not touching them.

What would be the best route for a rebuild to keep the stock compression ratio?

Lets say i get the 1.7L flat top AA pistons. 90mm bore, 66mm stroke. And i get a set of new AA heads, 55cc's. Both seem to be readily available.

Using the compression calculator on AA's website the required deck height is 0.07 to achieve the 7.3 compression, yet the recommended deck height for our vw's typically seems to fall around 0.04/0.05.

If i go for the 0.04/0.05 deck height i would look at 7.7/7.8 compression ratio? But is that a safe enough ratio when considering longevity?

If not, what typically is a good route to go to achieve that 7.3 ratio.

I am new to these concepts and am just trying to learn as much as possible before diving into a rebuild. Appreciate the insights.

[Wildthings]

You definitely want a deck height of 0.060" or less, if you can get it to 0.040" so much the better IMO. You can step up from regular gas and/or set the timing at 26-27° BTDC @3800+ rpm (hoses off and plugged), if necessary. Note that I run premium ethanol free gas during the warmer months anyway so for me there is no extra fuel expense if my compression is a bit high.

[RalphWiggam]

Depends on the cam.

I used right at about 8:1 on my 72 1.7l build. Heads came from HAM and I run a T4S 9590 cam.

Very happy with my combo.

[Starbucket]

LN Engineering.com/nickies for top quality P/Cs.

[Mpec2]

Thanks guys.

The LN stuff is quite out of my budget, same with the HAM heads unfortunately.

Gotta work within my budget.

Wildthings i read through you're posts actually regarding the 0.040 deck height, which is where i got my estimates from. Appreciate all that info.

LN engineering or AA or whomever really for 1.7L it seems as though flat tops are the only option for new P/C's. I guess the hope then is that during the rebuild the stock pistons and cylinders will be reusable so i can rely on that piston dome for the extra CC's.

[timvw7476]

AA flat tops won't get near the 7.3:1 factory setting CR.
If I had to guess, more like 8.3:1 with the factory headgasket/shims installed.
Which means mid-grade gas or better.

[Mpec2]

Yeahh thats what the math seems to show. Lets assume that my stock chamber cc's end up 52, then with a 0.05 deck height im getting 8.0:1 CR.

I have no problem using mid to premium gas but my concern with higher compression is the heat. Would a compression that high cause overheating issues in a heavy loaded camper like mine?

[Wildthings]

The original timing spec for a domed piston Type 4 engine was 27° BTDC @3400+ rpm, hoses off and plugged, plus or minus a couple of degrees. Using 27° timing with flat tops should give you sufficient leeway in a bus.

[Mpec2]

Is the thought there that by setting the advance on the lower end of the range, i'm getting ignition towards the end of the stroke, which offsets the chance of detonation that would otherwise come from the higher deck height?

[timvw7476]

as far as 'retarding the advance' of the spark event, VW engineered that into these air cooled engines. With the factory settings falling under 28 degrees Before top Dead Center (at speed, i.e.3,400 rpm+). The near-modern water cooled stuff from America all churned away happily with 36 degrees Before Top Dead Center, but they all sported water jackets around forgiving iron cylinders AND mostly iron cylinder heads that could transfer the combustion heat away far quicker that the fins on the alloy VW heads......

The concept of further retarding the spark during advance ignition to prevent detonation is more of an engineering question .....
the answer back then & today for us with the same aluminum pistons & iron cylinders & alloy heads is the already semi-retarded advance timing specs. provided by VW way back. Adding a little more does not hurt, I have no idea if running no more than 24 or 26 degrees BTDC would provide any benefit. Or stave off detonation. This gets technical because it is about utilizing the octane rating of the fuel to extract the utmost energy. Guess wrong & you end up rebuilding stuff. There's no knock sensor/active digital timing retard on these.antiques.

[Mpec2]

Hmmm interesting to know.

So perhaps i can make the flat top pistons work? If i set reasonable deck height, set timing around 27ish, and run better fuel and properly set the carbs It's reasonable?

Note that this wont be a daily, maybe 2000 miles a year? Mostly highway mileage to some local camping spots (i live right outside chicago).

[Starbucket]

You could lower the compression by cutting some cylinder shims out of 0.020"-0.030" gasket paper to put between the case and cylinders.

[Wildthings]

Back in the good old days of crappy fuel that caused the buildups of carbon-lead deposits on the piston tops and heads, it was likely that the fuel mixture would be set off by the glowing deposits on a hot engine before the spark plug fired. This could cause the burn to start way before it was needed or wanted and often caused severe engine knock as the burn was occurring before the combustion area had been squished down and shockwaves were in front of the flame front causing the air-fuel mixture to explode versus burn. Today's fuel don't have lead and tend to burn cleaner so these thick deposits do not occur in most engines.

The other form of knock is because the spark plugs are firing so early that again the burn is occurring before the combustion chamber is squished down. The goal it to have the burn essentially completed, reaching the cylinder walls by around 13° ATDC IIRC. This 13° ATDC applies to most all engines, aircooled or water cooled. The initial advance, centrifugal advance, and vacuum advance all work together to make this happen.

Low octane fuel does not make the fuel burn faster, but it does make it more explosive which means uncontrolled combustion out beyond the flame front.

High compression means the AF mixture is hotter at the same point of upward piston travel which can accentuate preignition caused by hot spots, or can mean that the mixture once set off by the plugs has gotten hot enough before it has completely burned to explode in front of the flame front.

Engines with decent squish areas, limit how far the flame front needs to travel to burn the majority of the fuel and once the fuel is burning it will not tend to explode, so squish is important. A tight deck also adds to the turbulence in the cylinder which can make the flame front advance more quickly, again fuel which is burning does not tend to explode.

I think it would be interesting to somehow duplicate the piston and combustion chamber design of a Vanagon WBXer engine in an air cooled motor. The smallish diameter deep dish couple with the wide squish area seem to squelch any tendency for these engines to knock. I have never had any of my various WBXer engine exhibit knock no matter how far the timing was advanced, and even when the initial timing is set to the most retarded end of the book range the engines tend to run very well.

So long as an engine has a sufficiently tight deck, I see no problem with running flat topped pistons. If someone has problems with and engine with flat topped piston either knocking or running hot, let us know.

Note that 20ish years ago I bought a set of AA flat topped pistons, where either the pin height was wrong or the cylinders were the wrong length and it ended up with a deck height of around 0.080". It was very hard to control knocking on that engine.

[Mpec2]

Thanks Wildthings, the more i learn about the technical details behind these engine builds the more interested i get. Looking forward to working through this build. Im relatively confident now that i can get the flat tops to work nicely.

Now onto figuring out heads

It seems to be generally recommended to just buy new heads (AMC with replaced hardware for example) vs getting a valve job on original?

[Wildthings]

[Synergy Solutions]

In another avenue of protecting the engine, a j+s safeguard would be a way of dynamic detuning if your grade of gas (say during a road trip) became to low and you didnt know. Automatically it will pull timing if it begins to detonate.
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Im a Mobile Aircooled VW mechanic, and work on old and new cars alike. Specialty in Camper busses/Vans, TDI, and FI diagnosis and repair.

(206) 841 7324

[Mpec2]

Interesting technology, wasn't aware that was available.

I think i'll grab a new pair of AMC heads, replace the exhaust valves. and run it with a CHT gauge and monitor accordingly. In a few years, when i do a full top-down rebuild i'll upgrade all the hardware, but for now ill have to be budget friendly and take the risk.

If anyone would be interested i can post some pics of the heads when they arrive and do a general look over. Seems like its always a topic of conversation so someone could have a basis for AMC heads in 2024.

[APPLEGREENVW]

[Mpec2]

I do have the original heads, and they do not appear cracked.

The reason i'm considering the AMC's is the shop around me (the only one that has answered me at least) wants $500 + cost of all hardware for valve work. And they're not even a shop that really works on old air cooled VW's.

[Starbucket]