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Tue Jun 24, 2014 10:59 am

Shooting for a static compression ratio to get the right dynamic. My understanding of engines mostly comes from liquid cooled American iron, where dynamic ratio of 7.5-8.5 to 1 is a good target for pump gas.

Here's what I have:
2180
Thick 92s
Scat Volksracer 82 crank
4340 H-beam 5.4 rods
ACN L5 heads, 55cc chambers.
1.25 rockers

Planning on using a Web 163 cam, adv. duraction is 288, 250 @ .050.

Intake opens 17-degrees BTDC and closes 53-degrees ABDC.

Exhaust opens 53-degrees BBDC and closes 17-degrees ATDC.

I live at around 6,200 feet elevation but figure 5,000 for most driving. I don't want too much compression at lower altitudes but don't want to pay too much of a price for the effect high altitude has on power production. A common mod for 2-stroke dirt bikes here is to mill the head to get some snap back that high altitude robs. Don't know if that works or if it's even a good idea on air-cooled VWs.

Any and all advice is welcome. Thanks.

Tue Jun 24, 2014 11:22 am

A. I'm an idiot who has only built three engines and is still trying to figure this stuff out -- there are people on here that build three engines before breakfast and you should listen to them.

B. I got fantastic performance out of a 8.3:1 at 5K - 7.5K elevation but I kept having trouble with the studs pulling and stretching -- changed to 8.1:1 and the problems went away.

Tue Jun 24, 2014 5:37 pm

think on this fer a bit. the dynamic thingy is for ??? chevy ford? 1.6 ratio rocker1.7? 1.8?1.9? 2.2ratio rocker???? and your vw has what rocker ratio???1.1,1.25???? your engine sees real time valve opening&closeing rates,not what is on the cam spec sheet. and thus a mild vw cam could be an aweffull wild v8 cam. if your going to use that thingy I would atleast convert the cam figures to realtime figures t the valve with the 1.6 or 1.7 ratio that is comenly used on old v8 stuff. or back down the v8 figures to the vw ratio & see where you stand.
there can be a whole lot of differance @.050 with a ratio change. hell even aluminum pushrods can change that figure by 3 degrees on each end.
1.25x.050= .0625 , 1.7 x.050= .085 so....half of [email protected] is now gone so what did it loose in running at valve degrees???????????? I used that thingy on some blower motors in years many years past.& I didnt pay much attn to it then either. but it is a good selling feature for weaker pistons that need to be warrantyed due to....well...BOOM!!!!good for the seller that is.

Tue Jun 24, 2014 5:45 pm

I run ANC L5 heads, super squish pistons, over 12:1 compression, and an 86A webcam 116LC -2 retard. .045" deck. Static is like 12.3 or so. It has so much cranking compression that it can hardly be rolled over to adjust the valves. My starter hates me. Runs super cooler after lots of tuning. Heavy baja, 100+ degree weather, head temps peak around 340F under #3. 91 octane fuel. I drive it from 2500 to 8000 ft all the time.

Just throwing that out there. The KB Silvolite calculator puts my dynamic at around 9 to 1. This combo has a narrow tuning window to get it perfect so it isn't for most people.

Tue Jun 24, 2014 7:27 pm

The few I've spoken with that know what Dynamic Compression is, and have built Aircooled VW Engines, usually tend to stay below 7.5 Dynamic. Their reasoning is a Watercooled Head can run slightly higher Dynamic due to the added cooling. The site I've used with success is from United Engine

https://www.uempistons.com/index.php?main_page=calculators&type=compMachine;

I run 9.3 Static in a 1776, and with the cam, makes around 7.6 Dynamic. This is around 800-1000 ft above sea level. With being much higher above that, you may be able to raise that to the 8.5 Dynamic you've used with Watercooled experiences. I'm not sure. Maybe some of the higher elevation guys will chime in. If I was running that combo, I would run around 9.5 Static to get 7.5 Dynamic.

Tue Jun 24, 2014 8:01 pm

Shoot for 9-1 even, and shoot for .050 deck of course.
Altitude does allow for more compression to a degree, but a degree less then you'd expect by simple air density. So much is about TEMPERATURE

"Dynamic" should be called cranking compression, because that's what it was called before the monkeys thought they discovered fire.

Ever see a pearl yellow early bug drivin around in golden a few years ago?? That was my brother. He drove it up to wyoming every weekend.
Ok get to work..........20cc of aluminum is a lot of chips :wink:

Wed Jun 25, 2014 3:08 am

Yes, the monkeys have it arse-about again. Dynamic, containing the root dyno=power, suggests it would be effective compression at the max VE, as that is where max power occurs...

But we all knew what he meant.

Wed Jun 25, 2014 5:34 am

Thanks for the info, folks. Gives me something to think about. I've never seen the yellow bug that I can recall, Modok. My shop is in Golden at 6 & 40, but I live in Morrison.