Setting Compression Ratio: Best Practices

[Root_Werks]

While I have my engine out, decided to install thick wall 88's and AA flycut larger valve heads.

Heads have a 58CC and I'm measuring .055 deck height. Using the CB Performance Calculator, my CR is 7.3:1.

My engine is a 2003 1600i, the Mexican VW Owners Manual says the stock CR is 7.75:1.
I'm leaving the stock cam and FI in place.

Bug is driven on pump 89 "E-free" gas, not a hot rod, just commuting and getting places.

Question; 7.3CR okay or should I have the heads flycut to reduce the CC size so it bumps CR back up to something closer to 7.75:1?
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When I set my timing, why do I flush, then take a pee?

[Root_Werks]

I've read a number of threads about CR, they are all over the place and get beyond my engine building skills fairly quickly.

I do believe I'm to understand lower CR's work better with stock cams, especially FI engines.

Picture of stock head next to AA head. Will say I like the castings on the AA heads. All the cooling fins have all the passages. Many years ago when I did new heads, I remember spending an hour or so each head drilling out and cleaning up the cooling fins.

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When I set my timing, why do I flush, then take a pee?

[Floating VW]

[Floating VW]

Also, if you tighten up your deck a little more, say, down between 0.040" and 0.050", you should be able to get away with running a "higher than recommended" compression ratio and still use low-octane pump gas.
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"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[Root_Werks]

[Root_Werks]

I'm using this:

https://www.gofastmath.com/Compression-Ratio-Calcu...Calculator

The only value I'm missing is the IVC, I'll see if I can find out when the Intake Valve closes.

If I plug in a guess value in IVC of .02, it comes back with the correct static CR of 7.3 and dynamic of 7.2.

I think if I can get a correct IVC value, this will paint the picture.

Edit:

I believe the stock FI cam IVC is .037 which gives me a dynamic compression of 7.23.

That's still above 7 and probably fine unless someone chimes in. Easier to flycut new heads now.
_________________
When I set my timing, why do I flush, then take a pee?

[Root_Werks]

Webcams has all the specs (duh), looks like IVC for my cam is .0345 which still leaves dynamic compression at 7.23.

http://www.webcamshafts.com/pages_vehicles/automobile/volkswagen/670.html

Anyone with any experience running an engine on this, please chime in.

Best,
Dan
_________________
When I set my timing, why do I flush, then take a pee?

[runamoc]

[chrisflstf]

Putting big valves in a 1600 with stock cam and low compression is a terrible combination. You dont need big valves

[Floating VW]

[Floating VW]

[frenchroast]

I have a stock cam with a 8.0:1 CR, .055 deck height, and stock valves. Runs great on regular 87 octane gas.

[Root_Werks]

Oops, I meant 34.5% not .0345.

Going to pick up a dial indicator this weekend and measure.

Good info, thank you!
_________________
When I set my timing, why do I flush, then take a pee?

[runamoc]

[Floating VW]

Sorry, I meant to say when the piston is mostly at TDC; in other words, the overlap starts a little before TDC and continues until a little after TDC.
_________________
"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[runamoc]

[mukluk]

[Floating VW]

What he said.
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"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[Root_Werks]

Wanted to post an update on this thread:

Pulled the bigger valve AA heads off yesterday. Either going to have them flycut to reduce the CC from 58 to 50cc's or have my stock heads flycut for the thick wall 88's. If I use the stock heads, might have to shim the Cylinders.

Stock 1600i CR is 7.75:1 and my running 7.3:1 is too low. Going to run 8.2:1 and see how I like that.

Yes, I learned the hard way, stock cam, large valves, low CR don't do well together.
_________________
When I set my timing, why do I flush, then take a pee?

[Root_Werks]

Update:

Been running with the 50cc CC and 8.2 CR which is better.

I decided to pull compression on all 4 cyls. All 4 are 122-125 PSIG which seems a little low for 8.2:1 doesn't it?

Are my big valve heads costing me power? I still have the stock heads. Better to flycut those and put them back?

Plugs are clean, no issues. Just having fun with the engine. At some point, it'll be converted to stand-along FI and after that, swapped out for a 2180cc or similar.

-Dan
_________________
When I set my timing, why do I flush, then take a pee?