Dynamic Compression change with cam swap vs Octane

[Andy]

I have been searching the internet for answers to this question but only found information that applies to V8s which run at higher compression than aircooled VWs

I am trying to wake up my 2110 a bit, it has an engle 120 that gives a dynamic compression of 7.2:1 that I run on 98 ron fuel, I am thinking of swapping to an fk8 cam, this will drop the dynamic compression to 6.97:1.

What is the range of dynamic compression ratio that the engine works well at and at what dcr would it be appropriate to use 95 ron fuel.

Should I expect to see an increase in performance, swapping cams even though the dcr will be lower with the fk8 cam.

Thanks. 
 

[Peter]

the more dynamic compression, the more power you will get,
but you can only raise it up to a certain point ( just below  the limit of detonation )
watercooled engines can get away with higher compression because the water draws away the heat from the chamber better ( so reduce the chance of detonation )
then aircooled heads.

[red baron]

more cr leads to more heat and more pressure above your piston. so keepin it cool up there is one way to prevent knocking. the other way is high octane fuel, cause it self ignites under bigger pressure/temp than fuel with less octane.

where do you guys get these dynamic compression ratios from?

[TexasTom]

I like to think dynamic compression ratios in the 7.25-7.5 range approach the limits for pump gas in an air-cooled engine here in the states ... many variables though.

[Zach Gomulka]

where do you guys get these dynamic compression ratios from?

X2, I want to know how to calculate this!

[Lids]

http://www.empirenet.com/pkelley2/DynamicCR.html

[red baron]

okay i got this tool now, but the research about cam specs is on the plan...

thx

[TexasTom]

Here's another calculator ...

http://www.kb-silvolite.com/calc.php?action=comp

[Andy]

Here's another calculator ...

http://www.kb-silvolite.com/calc.php?action=comp

I think that is the calculator I used, I did the calculations a few weeks ago and wrote them down.

I've seen the figure of 7.3 as one to look for. What would the effect of running at 7.0 be in regards to loss in performance, I presume the engine would run slightly cooler.

I was asking about the compression for the lower octane fuel as in the near future I will need to build an engine for a bus, I intend it to be quite mild so it doesn't justify the extra expense of the high octane fuel.
 

[RFbuilt]

there was a thread, about compression and cam,
that Doug berg and mr "the other one"  was chiming in on, with good info

DB mentioned about "customizing"  your own fk8 or englew120  whichever floats the boat,

with  things like adjusting lobe centers, some dur.  etc  to wake up the motor (better dynamic comp)

as a newb, well thats how i understood their info.. i hope i wasnt too far off

- on a sidenote, our cam guy that makes/grinds our Honda inline 4 motors,  made me a cam
   as R10 (R from their brand Rocket, and 10 as a denote)
  lift of .430 and dur. @ 0.050 to be 243* on 107 LC
  similar to a 110 juz enough to wake up my 1679cc project "stocker"

  i believe with your 2110 andy, a "tweaked" 120  would do wonders, provided u know where or what rpm band u wana play around in.  etc  , im sure the likes of Webcam can tailor to ur needs,


as a newb LOL everybody feel free to correct me on my understanding
and as a newb, im too shy to mention my friends business which he can custom tailor a cam hehe

Ralf

[RFbuilt]

but then again an engle w120 cam is quite common.. and the likes of the fk8  or similar cam will def wake up a 2liter  under the right circumstances , so  custom cam's  may not be the first option  when you can get an "off the shelf"  cam already

beauty of our VW hobby,  a billion choices when it comes to camshafts,induction and or exhausts hehe 
let alone head options too!


Ralf

[Andy]

The thing is I don't really know enough to specify a custom grind cam. I also have an fk8 ready to go so I can't justify the expense either. I am just hoping that dropping to 7.0 dcr isn't going to affect its performance too much.

[RFbuilt]

doug berg, mentioned sumthing in the region of

knocking a bit off the lift (or was it duration lol)
and tightening up a lil the lobe centers 

to liven up the dynamic comp..



if i understood that correctly.. 

then if u go fk8 and it lowers dynamic a bit (but surely livens up ur top end
then DB's way sounds like it will let u have your cake and eat it to?


but ofcourse if im misunderstanding the wisdom's of the veterans here  sum1 should smack me upside the head



pls sum1 correct me   

[Dougy Dee]

I ran an E120 straight up in a 2110 with 8.5 Static CR and it was a HUGE disappointment. At 9.5 it came alive and liked 91 octane.

Prior to that I ran the FK8 with about 8.0 and it was a disappointment as well. I would have bumped the compression on it but it was starting to lose it's nose....
T
he only common denominator was that it wasn't the cam, it was the low CR that didn't do it for me.

Whatever calculator you use, there are several, plug in 9.5 static with a 51 IC @ .050 event and shoot for that DCR.

Disclaimer: Your results may vary. They may be better.

[Andy]

I ran an E120 straight up in a 2110 with 8.5 Static CR and it was a HUGE disappointment. At 9.5 it came alive and liked 91 octane.

Prior to that I ran the FK8 with about 8.0 and it was a disappointment as well. I would have bumped the compression on it but it was starting to lose it's nose....
T
he only common denominator was that it wasn't the cam, it was the low CR that didn't do it for me.

Whatever calculator you use, there are several, plug in 9.5 static with a 51 IC @ .050 event and shoot for that DCR.

Disclaimer: Your results may vary. They may be better.

I have read that Jim Ratto describes a 120 cam in a 2110, as a good combination for a bus, I tend to agree as my engine is too mild for my liking 

Guessing that your rods were 5.4s you dcr works out to be just over 7.5:1, with a static of 9.5:1. interestingly I had to take about 9cc out of the combustion chamber volume to raise the compression from 8.5 to 9.5.

I am beginning to doubt that putting in an fk8 without bumping the compression will be worth it.

Anyone got an fk8, if so what compression do you run, and how do you rate its performance.

[RFbuilt]

maybe u could bump ur cr and go w125  straight up also? 

looks like ur displacement is fine for that..

lets wait for Jimm Ratto  on his experience with the fk8  and others

[K-Roc]

Hey you guys just something to think about when usung these Dynamic Comp. calculators, or even some engine simulation programs.   Notice that they do not take into consideration the actual V/E ( Volumetric efficency ) of the engine, Which also changes with RPM....  You really need to know exactly what your V/E is as well to make an even good guess as to what your Dynamic compression will actually be..  Calculating static comression is one thing but dynamic is a whole nother can o worms 

K-Roc,

[RFbuilt]

good stuff K roc!