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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Wed Jan 25, 2023 9:47 am Post subject: Setting Compression Ratio: Best Practices |
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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? _________________ When I set my timing, why do I flush, then take a pee?
Last edited by Root_Werks on Wed Jan 25, 2023 9:57 am; edited 1 time in total |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Wed Jan 25, 2023 9:55 am Post subject: Re: Setting Compression Ratio: Best Practices |
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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.
_________________ When I set my timing, why do I flush, then take a pee? |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Wed Jan 25, 2023 10:19 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Root_Werks wrote: |
I do believe I'm to understand lower CR's work better with stock cams, especially FI engines. |
Yes and no.
Do some research on the difference between "static" compression and "dynamic" compression.
You'll want to keep your DYNAMIC compression above 7.5 to 1 for optimal performance and efficiency, and less than 8.5 to 1 unless you want to use high octane racing gas. _________________ "It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works." |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Wed Jan 25, 2023 10:28 am Post subject: Re: Setting Compression Ratio: Best Practices |
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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. _________________ "It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works." |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Wed Jan 25, 2023 10:45 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Floating VW wrote: |
Root_Werks wrote: |
I do believe I'm to understand lower CR's work better with stock cams, especially FI engines. |
Yes and no.
Do some research on the difference between "static" compression and "dynamic" compression.
You'll want to keep your DYNAMIC compression above 7.5 to 1 for optimal performance and efficiency, and less than 8.5 to 1 unless you want to use high octane racing gas. |
Thanks! Yes, was reading up a bit between Static and Dynamic. I'll keep researching.
Any assistance/advice is greatly appreciated. I could have the engine assembled and back in the Bug within hours, but want to ensure I'm not really missing an opportunity to get things set correctly. _________________ When I set my timing, why do I flush, then take a pee? |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Wed Jan 25, 2023 10:54 am Post subject: Re: Setting Compression Ratio: Best Practices |
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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? |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Wed Jan 25, 2023 12:21 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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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? |
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runamoc Samba Member
Joined: June 19, 2006 Posts: 5601 Location: 37.5N 77.1W
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Posted: Thu Jan 26, 2023 7:51 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Quote: |
"dynamic" compression |
I have 10:33 to 1 Close as I could get to 10:5 to 1without going over. Static. on my 2180 because of the cam. Seems that once every revolution the cylinder has both the intake and exhaust valves open at the same time. This lowers the dynamic CR. I use hi-octane gas anyway so I can set the timing to 32° _________________ Daily driver: '69 Baja owned 44 yrs - Plan B: '81 Rabbit Diesel LS Deluxe - Plan C: '72 Ghia
Yard Art: 2 Sandrails
Outback: '69 Ghia - '68,'69,'70,'72 Beetle - '84 Scirocco, GTI - Pair of '02 Golfs-
VW Wiring = It's just wires |
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chrisflstf Samba Member
Joined: February 10, 2004 Posts: 3446 Location: San Diego
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Posted: Thu Jan 26, 2023 8:52 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Putting big valves in a 1600 with stock cam and low compression is a terrible combination. You dont need big valves |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Thu Jan 26, 2023 6:43 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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Root_Werks wrote: |
Webcams has all the specs (duh), looks like IVC for my cam is .0345 which still leaves dynamic compression at 7.23. |
I'm not sure what you mean by 0.0345. To me, the IVC is expressed as an angle, like 35 degrees or 42 degrees, etc.
The best way to find the IVCA of your cam is to clock it yourself. It's very rare that the advertised angles on the cam card and the actual angles of the cam are the same. For example, my own cam was advertised as having an IVCA of 35 degrees @ 0.050", while the true angle of the cam is 30 degrees @ 0.050". I was actually happy to discover this, as the earlier closing angle gives me a slightly higher dynamic compression ratio (at the expense of high RPM air flow, which I don't really care about).
Clocking the cam is easy, all you need is a dial indicator on the cam and a degree wheel on the crank pulley:
Your engine will certainly run on less than 7.5 to 1 dynamic compression (most every stock ACVW engine ever built has less than 7.5 to 1), but you will be sacrificing some much needed fuel efficiency and torque. _________________ "It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."
Last edited by Floating VW on Thu Jan 26, 2023 7:13 pm; edited 1 time in total |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Thu Jan 26, 2023 7:12 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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runamoc wrote: |
Quote: |
"dynamic" compression |
. . . Seems that once every revolution the cylinder has both the intake and exhaust valves open at the same time. This lowers the dynamic CR . . . |
Not exactly. The intake valve opening angle has very little effect on the dynamic compression, compared to the intake valve closing angle.
The overlap of the intake and exhaust valves mostly occurs when the piston is hovering near TDC. Compression only occurs when the piston is on its way up AND the intake valve is effectively closed (which is why the IVCA is so important).
The overlap does help the engine breathe at high RPM though, as does a later IVCA. _________________ "It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works." |
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frenchroast Samba Member
Joined: October 13, 2019 Posts: 679
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Posted: Thu Jan 26, 2023 8:04 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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I have a stock cam with a 8.0:1 CR, .055 deck height, and stock valves. Runs great on regular 87 octane gas. |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Thu Jan 26, 2023 9:48 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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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? |
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runamoc Samba Member
Joined: June 19, 2006 Posts: 5601 Location: 37.5N 77.1W
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Posted: Fri Jan 27, 2023 5:54 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Quote: |
overlap of the intake and exhaust valves mostly occurs when the piston is hovering near TDC. |
What do you mean mostly? They overlap every time. _________________ Daily driver: '69 Baja owned 44 yrs - Plan B: '81 Rabbit Diesel LS Deluxe - Plan C: '72 Ghia
Yard Art: 2 Sandrails
Outback: '69 Ghia - '68,'69,'70,'72 Beetle - '84 Scirocco, GTI - Pair of '02 Golfs-
VW Wiring = It's just wires |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Fri Jan 27, 2023 5:01 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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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." |
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runamoc Samba Member
Joined: June 19, 2006 Posts: 5601 Location: 37.5N 77.1W
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Posted: Sat Jan 28, 2023 5:45 am Post subject: Re: Setting Compression Ratio: Best Practices |
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Quote: |
They overlap every time. |
The guy at aircooled.net told me that's the reason the static CR was so high and not a static 8.5-1 is because of the cam. Sounds good to me. Can you explain why can I set my static CR at 10.5-1 and the engine not blowup on pump gas? _________________ Daily driver: '69 Baja owned 44 yrs - Plan B: '81 Rabbit Diesel LS Deluxe - Plan C: '72 Ghia
Yard Art: 2 Sandrails
Outback: '69 Ghia - '68,'69,'70,'72 Beetle - '84 Scirocco, GTI - Pair of '02 Golfs-
VW Wiring = It's just wires |
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mukluk Samba Member
Joined: October 18, 2012 Posts: 7028 Location: Clyde, TX
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Posted: Sat Jan 28, 2023 3:31 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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runamoc wrote: |
The guy at aircooled.net told me that's the reason the static CR was so high and not a static 8.5-1 is because of the cam. Sounds good to me. Can you explain why can I set my static CR at 10.5-1 and the engine not blowup on pump gas? |
You can set your static CR to 10.5:1 and it runs pump gas ok because your cam's intake valve closing point is sufficiently late that the dynamic CR is low enough to safely operate on pump gas. Typically a DCR somewhere between 8.0:1 and 8.5:1 is considered the maximum for pump gas.
Static compression ratio is simply the total swept volume of a cylinder divided by the unswept volume. Dynamic compression ratio is the swept volume of a cylinder while the valves are closed divided by the unswept volume. This is why DCR is always less than SCR. DCR is generally what determines the octane requirement of an engine.
While your engine SCR calculates out to 10.5:1, because the intake valve doesn't close until the piston has traveled up the bore a ways, the engine has an effectively smaller swept volume and thus a lower "real" or DCR. To get the engine's DCR back up to an acceptable level when using a cam with a later IVC point, you build the engine with a higher SCR. This is why SCR recommendations are based on what cam you are installing -- if you had installed a different cam with an earlier IVC point, you would have had to use a lower SCR. _________________ 1960 Ragtop w/Semaphores "Inga" |
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Floating VW Samba Member
Joined: April 28, 2015 Posts: 1597 Location: The South Zone
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Posted: Sat Jan 28, 2023 7:51 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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What he said. _________________ "It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works." |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Fri Apr 28, 2023 9:47 am Post subject: Re: Setting Compression Ratio: Best Practices |
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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? |
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Root_Werks Samba Member
Joined: December 31, 2007 Posts: 813 Location: San Juan Islands
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Posted: Tue Oct 17, 2023 4:48 pm Post subject: Re: Setting Compression Ratio: Best Practices |
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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? |
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