TheSamba.com :: View topic - Dynamic compression ratio inside, 87 octane?

Fri Jan 21, 2011 7:59 pm

Hey,

I just finished putting together my short block.

--------------
1679cc
-DPR 69mm CW crank
-DPR rebuilt/ballanced stock rods
-DPR ballanced/resurfaced 12# flywheel
-Mahle forged machine in 88s
-Deves piston rings
-DRD L3 heads
-CB Performance Cheater Cam, #2280
-CB Performance 28mm lightweight lifters
-Case machine work by Brothers VW Machine including bronze lifter sleeves

I will be running the stock dual 32pdsit carbs and a NOS stock exhsaust.
----------------

I am really happy with how everything is going together. The heads have 54cc combustion chambers. The cylinders deck out at .052 without any shims. This gives a static compression ratio of 7.8:1 and with the CB cheater cam it works out to a dynamic compression ratio of 6.8:1.

This seems like a good place to be to me. Safe deck but good squish.

Think I will be able to run 87 octane if I assemble the top end as-is?

Fri Jan 21, 2011 9:24 pm

0n my recent trip to arizona in my bus I got 25mpg on a tank of 89 and from then on ran 87 and got 27, my comp is 8.2 to 1 and I have a litle more cam tha you.

Just make sure yout fuel air ratios are good.

Sat Jan 22, 2011 12:40 am

Double check your calculations, i get 7.73 comp with the specs you listed and the only cam specs i could find close to the advertized duration numbers of the cheater cam was an intake closing of 65* which gives a 6.15 dynamic at 154 ft elevation in Salem.

This is less than the stock dynamic so you will be plenty safe. could even bump it up a bit.

Also, with a 54 cc chamber, i bet those heads still have the step in them so your squish isn't as good as you think. double check that, you could be closer to .080 deck with that step.

all and all sounds like a build you will be very happy with. You could even go with a higher lift cam (1.4 rockers maybe) to take full advantage of the flow of the heads.

Just my thoughts :D

Sat Jan 22, 2011 7:11 am

Well, I used CB Performance's calculator to get my static compression ratio of 7.8:1. I used a random DCR calculator on the net for that one.

My cam spec card states that the intake closes at 35 degrees and not 65 degrees.

Sat Jan 22, 2011 7:18 am

I just found a different DCR calculator and according to it, my DCR is 7.29:1?

http://www.wallaceracing.com/dynamic-cr.php

Sat Jan 22, 2011 7:25 am

You built a good engine. Run it like it is and everything will be fine on 87 octane. On that small of an engine I would not even worry about using some V8 dynamic compression ratio calculator. Your stock exhaust is hurting you. You are throwing away 10 HP. Switch to a cheap 4-into-1 extractor and you will notice an increase in power and MPG.

Sat Jan 22, 2011 7:29 am

vwracerdave wrote: Your stock exhaust is hurting you. You are throwing away 10 HP. Switch to a cheap 4-into-1 extractor and you will notice an increase in power and MPG.

But, it was expensive, I already bought it, and I like the look!

If it turns out to be a problem, I can easily swap it out later.

FYI, running the stock exhaust is why I picked the L3s. My thought pattern was that I would be able to run the stock exhaust with stock size valves.

Sat Jan 22, 2011 11:18 am

rizzag wrote:
My cam spec card states that the intake closes at 35 degrees and not 65 degrees.

that 35* is the .050" number. you have to go by the advertized duration which is the difference from the 222* to the 274* right? thats 52* more :shock: so half of that needs to be added to the 35*....

just trying to help...

Sat Jan 22, 2011 7:26 pm

rizzag wrote: vwracerdave wrote: Your stock exhaust is hurting you. You are throwing away 10 HP. Switch to a cheap 4-into-1 extractor and you will notice an increase in power and MPG.

But, it was expensive, I already bought it, and I like the look!

If it turns out to be a problem, I can easily swap it out later.

FYI, running the stock exhaust is why I picked the L3s. My thought pattern was that I would be able to run the stock exhaust with stock size valves.

In stock heads the ports are the restriction,they underflow the valves posible flow by about 25%, so now your port restriction is gone the valve are flowing there full potential.

If the stock valve size could use nothing more than the stock exhaust why would DRD bother to port them, you know it makes sence :wink:

Sun Jan 23, 2011 7:36 am

krusher wrote: rizzag wrote: vwracerdave wrote: Your stock exhaust is hurting you. You are throwing away 10 HP. Switch to a cheap 4-into-1 extractor and you will notice an increase in power and MPG.

But, it was expensive, I already bought it, and I like the look!

If it turns out to be a problem, I can easily swap it out later.

FYI, running the stock exhaust is why I picked the L3s. My thought pattern was that I would be able to run the stock exhaust with stock size valves.

In stock heads the ports are the restriction,they underflow the valves posible flow by about 25%, so now your port restriction is gone the valve are flowing there full potential.

If the stock valve size could use nothing more than the stock exhaust why would DRD bother to port them, you know it makes sence :wink:

Yes, it definitely makes sense. I think I am over thinking all of this and should just put it together!