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Static and Dynamic Compression Ratios
https://www.slantsix.org/forum/viewtopic.php?t=45603		 Page 1 of 2
Author:  DadTruck [ Wed Jul 13, 2011 11:29 am ]
Post subject:  Static and Dynamic Compression Ratios
any ideas on where to set the compression ratios for improved performance and still be able to run regular gas?

using the Pat Kelly calculator to work through some options for the D-150 build.

stock motor ( 57 cc combustion chambers, pistons down .160, 3.510 head gasket bore and .045 thickness) and stock cam net
static compression 7.97
dynamic compression 7.31

stock motor and the Oregon Cam 1527 nets
static 7.97
dynamic 7.08 intake valve closes later, so the dynamic compression is less

Oregon 1527 and removing .100 from the combustion chamber volume nets
static 9.3
dynamic 8.3
would that have a chance of running on regular gas?

Oregon 1527 and removing .080 from the combustion chamber nets
static 9.06
dynamic 8.03
would that allow a some distributor advance?

Oregon 1527 and removing .060 from the combustion chamber nets
static 8.7
dynamic 7.76
that has the dynamic compression close to the stock levels,, so there would not be much performance gain.

Is there a dynamic ratio break point for regular gas?

thanks
DT
Author:  DadTruck [ Thu Jul 14, 2011 11:12 am ]
Post subject: 
found some answers here and a few other places

from this forum this posting gives some insight
http://www.slantsix.org/forum/viewtopic ... ompression

that was basically re-stated on a in another forum where it was mentioned
that for street dynamic ratios of 8 to 9:1 are ideal,, with 8 favoring street and regular gas and 9 favoring high performance street and premium.

as running regular is the priority for this build I will plan to the conservative side of the compression and leave plenty of room for additional ignition advance,

I am still trying to find a good correlation between measured compression in psi and dynamic compression * atmospheric pressure in psi,,, but my calculated figures are low,, maybe its the heating effect?

regards
DT
Author:  Doc [ Thu Jul 14, 2011 11:40 am ]
Post subject: 
Here is my opinion on cranking cylinder pressure readings and the fuel you need to use:

Anything under 100 psi = rebuild the engine.
100 to 140 psi use 87 octane
140 to 170 psi use Premium (92 octane)
170 to 200 psi use Race gas (100+ octane)
200 to 240 psi use Super Race Gas (114)
240+ alcohol

These are just general groupings and there are many other factors that influance when the engine starts to detonate.
I have successfully run engines on pump gas that had 170 psi cranking compression.
DD
Author:  SurfRodder [ Thu Jul 14, 2011 11:52 am ]
Post subject: 
Doc,
What do you think about E85? What if one were to mix E85 50/50 with premium or something along those lines... I will be able to get E85 on base relatively soon so that might be an option for my sister's car. will running E85 or some mixture containing it be better for the 8.11 DCR you calculated for that engine?
Author:  DadTruck [ Thu Jul 14, 2011 12:05 pm ]
Post subject: 
Doc
so how does one predict "cranking pressure"?
Author:  Doc [ Thu Jul 14, 2011 12:29 pm ]
Post subject: 
Use an on-line calculator.
Try this one:

http://www.rbracing-rsr.com/comprAdvHD.htm

I get around 155 psi for SurfRodder's RV10 RDP combo at 1000 ft elevation.
DD
Author:  SurfRodder [ Thu Jul 14, 2011 1:06 pm ]
Post subject: 
next time im up there...likely next weekend, i'll see what it actually is at sea level and we can verify the calculator.
Author:  Doc [ Thu Jul 14, 2011 1:42 pm ]
Post subject: 
That will be a great cross-check...
That calculator said 159.5 psi @ 0 feet (sea level) and 158.5 @ 200 ft.
As we know, checking cranking compression (cranking pressures in psi) is somewhat of an "art" in it's self.
DD
Author:  SurfRodder [ Thu Jul 14, 2011 1:51 pm ]
Post subject: 
Quote:
That will be a great cross-check...
As we know, checking cranking compression (cranking pressures in psi) is somewhat of an "art" in it's self.
DD
I think I can manage that... You should see the gage deflection on a charging pump in a nuclear power plant!!
Author:  DusterIdiot [ Thu Jul 14, 2011 6:39 pm ]
Post subject:  Well crap!
Quote:
240+ alcohol
I always thought it was 21+ alcohol...unless Sandy is around :roll:


:lol:

Good info, I hadn't thought about a calc for cranking pressures...
Thanks!

Hmmm...

-D.Idiot
Author:  sandy in BC [ Thu Jul 14, 2011 8:34 pm ]
Post subject: 
You only have to be 19 in BC........
Author:  DAlexanderW [ Fri Jul 15, 2011 3:42 am ]
Post subject: 
18 in Quebec...
Author:  theomahamoparguy [ Fri Jul 15, 2011 4:37 am ]
Post subject: 
Ive heard that mid to upper 7's is the limit for pump gas on the dynamic comp, and still have some "breathing room" course there is options. addvancing/retarding cam, thicker head gaskets etc. if you get close to where you want to be. My advice is to stay .2 under ideal from the calc estimates. eg; calc says 7.8 dynamic.( your estimated self imposed limit adjust the setup to try for 7.6.) You dint wabt ti be on the edge jus in case.
Author:  theomahamoparguy [ Fri Jul 15, 2011 4:44 am ]
Post subject: 
Ahh, alchohol. Yed it works but be prepared for some prep to the fuel system in terms of preventing degradation of rubbber etc, Also, gonna need to jet it up alot due to alch stoich and you might like to up the compression to take full advantage of the newer octane, (also to get any MPG out of it). Otherwise its like cheap racing fuel (or Jack Daniels)
Author:  DadTruck [ Fri Jul 15, 2011 5:19 am ]
Post subject: 
Doc

can you expound a bit on your method for checking cranking cylinder pressure?

I have seen posts where some folks say the throttle must be open, others say it does not matter,, cold vs warm engine, plugs out,,,,

thanks
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