Ok, question here. I'm still about 2 years away from opening up the VQ and/or boosting it (non-chemically).

I've been doing lots of research on various compression ratios versus boost amounts. I set a goal at 600 RWHP. Whatever FI, internals setup used has to make that. Now, here's the dilemna. Use something like the APS kit, which can support 18 psi on the VQ at 3.5L. Punch the VQ out to 4.2 or 4.3 L and ratios make that boost into 12.6 psi. At 12.6 psi, can you get away with 11 or 11.5:1 CR on a mix of higher octane gas?

If so, then you could have a monstrous 11.5:1 CR motor off boost, and a ridiculous 11.5:1 @12.6 psi motor on boost.

The equation for "dynamic compression ratio" is DCR = (boost/14.7+1) * static compression ratio. From what I understand, the DCR supportable by the engine is related to the octane rating of the gasoline used.

Here are some examples.

Stock motor (10.3 SCR) at 8 psi = 15.9:1 DCR
8.5:1 SCR motor at 18 psi = 18.9:1 DCR
11.5:1 SCR motor at 12.6 psi = 21.3:1 DCR
8.5:1 SCR Supra/civic/whatever motor at 29-35 psi = 25.2-28.7:1 DCR

So, does anyone know any relationship between octane rating and DCR?

 

Mostly depends on the ocatane avalble to you. General rule is :
1CR ~ 3 octane pts.
1 octane pt ~ 1psi of boost.

All in all you generally get more power with a lower CR and higher boost as long as the intake and exhaust component do not restrict flow and charge temperature is kept in check. I am assumeing that the APS turbos can still make useable boost over 18psi, we will soon know.

 

SO I caould Run 93psi!!!!!!! on 93 octane!








J/K

 

No the rule of thumb means a "delta change" in 1 "Static" CR point. So for example if the OEM SCR is 10.3 :1 and you drop the SCR down to 9.3.1 you reduce the octane needs by about 3 pts. So were you needed 94 you could now run 91 with similar knock resitance. Of course this is simply ballpark , all other paramater being the same. Over the last decade, combustion chamber designs (lower squish volumes have allowed higher SCR and maintained knock tolerance), have skewed this rule


Sorry I mis-typed my last post, the rule of thumb is :

1 SCR "decrease" ~ 3 octane pt "decrease".
1 octane pt. "increase" ~ 1psi boost "increase".

So following the crude rule of thumb: a 1 SCR reduction may allow an additional 3psi of boost to be run , all other parameters (timing, IC efficiency, A/F etc.) being unchanged.

 

So 12lb's of boost on 91 octane will not be a problem with 9.0 to1 Compression ? Of course with some timing retard

 

Not efficient. If you get reflashed w/ a snail strapped on, your A/F ratio will drastically change if you remove the belt to run "off boost." The car will be overly rich and will run like $hit.

Always more efficient to have low compression/high boost. As opposed to high compression/lower boost btw.

8.5 is usually the magic number in boosted applications. 12#s on 9.0:1 is pushing it. I had 7lbs on 9.3:1 (94 octane which is not available in any local Sunocos anymore) w/ inj swap and THAT was pushing it. Dont get me wrong, 12 lbs on 9.0:1 and even 10.0:1 can be done. You'll have to kill the timing in the upper RPMs (which you should do on even 5lbs!), maybe swap inj's and probably run 100 octane though.

 

I agree that more boost makes more power than more compression and lower boost. However, if I have a power goal, I want the minimum boost and max compression I can run to get there. I'm not going to spend $15-20K on an engine to build a damn highway running car (see any of the cars on T04R.com).

I'm not talking about pulling off a belt. I'm talking about having an 11.5:1 compression ratio motor that spools the APS turbos by 3000 RPM. I am planning on running 97-98 octane at a minimum. I am having a hard time believing that this can't be done. I have seen too many race motors running twice the boost with the same compression ratio.

I just saw some Evo in a magazine that traded in his 8.5:1 for 9:1 and upped the boost from 19 to 32 psi. I understand the 8.5 thing, but I don't want a soggy motor if I can spend the same money and get a hard-hitter. I'm not going to strap on turbos that will flow 900 hp worth of air and put in ridiculously low CR pistons so I can keep turning it up. I want to be able to start off with it turned up and adjust CR to match.

But thank you all for the input, I appreciate it. I'm printing off this formula to go do some more figuring.