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the JDM tuners mill the 8.5 rotors to 8.3:1 when they want to make big power. RX-7,RX-8‚‚REŽ‚’†S‚ƒŒ[ƒX‚‚ƒtƒB[ƒhƒoƒbƒN‚‚ƒmƒEƒnƒE‚‚‚‚‚‚ƒ`ƒ…[ƒjƒ“ƒOƒvƒƒVƒ‡ƒbƒv “‰
Yeah, I seem to recall a Racing Beat tip from back in the day when all the rotors had the "as cast" rotor recesses was to make sure all the recesses were basically the same and same position apex seal to depression by grinding them.
I guess that would go under the heading of part of the "blue printing" process for a rotary. A lot easier with a CNC mill and program!
Makes sense, there is a whole lot going on with those recesses in the rotor, especially if you have a P-port intake as well.
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Racing beat used Mazda Factory Race 7.5:1 CR rotors from the old turbo race development program even when running Methanol and no intercooler (ie, not running low comp rotors for knock resistance).
Sadly, because of the PDFs I didn't get good pictures of the charts- I will see how screen shots show up.
First this figure 9.11 from page 98 of the 1971 edition shows the curve of Brake Maximum Effective Pressure of the different Compression Ratios of the rotary between 6.5 to 12.6 CR.
BMEP is a theoretical tool to compare the torque a given engine can provide from a given displacement.
Fig 9.11 1971 ed pg 98
For a rotary engine BMEP is very flat from 8.5 to 11 CR with just a slight increase.
Fig 4.37 1981 ed pg 49
This again shows BMEP for 9.2:1 CR compared to 10:1 CR. I found that looked like a peak of 8.6kg/cm2 and 8.8kg/cm2 or 122.3psi and 125.165psi.
Only a 2.3% increase BMEP from 9.2:1 CR to 10:1 CR. 2.3% increase is definitely something, but when you compare that to the torque available from running more boost instead it is not significant.
Also note where the increase is. It is NOT in the low rpm!
Here for fun I have fig 4.34 from 1981 ed pg 48
This shows the BMEP difference between old 12A Leading Deep Recess rotor and Medium Deep Recess rotor (LDR vs MDR).
You can see the increase flame speed of LDR DOES increase low rpm BMEP.
Military put a long delay in plans, but am home now. Hoping to get the garage cleaned up to finish this project. Have an efr8374 and 7670 to try out on it. Thinking the lower pr friendly 8374 may yield the best balance of results since I probably won't be able to run as much boost on the higher company rotors. We will see.
For what it's worth, I've been running 18 PSI for the last 3 summers on my 9.4:1 rotors and 93 octane for no other reason than the abundance of N/A rotors out there. They're all over the place comparatively at least in my neck of the woods.
I know guys running considerably more boost on pump with auxiliary injection on 9.7:1 rotors. The key is in the timing. I thought it was common knowledge by now "It's all in the tune". Make the power with the boost, not the timing.
All I use is Walmart Supertech -20 washer fluid through a rudimentary -in comparison- pre-turbo injection setup by Bret at wannaspeed 'Dudemanownsanrx7'
I've personally never ran a lower compression ratio than 9.4:1, but I have a friend who claims after his switch, his car no longer feels "lethargic".
My car responds so quickly that in 3rd gear, if I mash the skinny pedal at 2000 rpm, once boost hits, it's as if i revved and dumped the clutch. The 160 tread-wear 285 Advan Neovas have no chance in hooking up. The onset of the BW8374 is instant and I believe paired with higher compression rotors, it really compounds the responsiveness of the engine. This of course is just a theory, it could all be in the turbo.
With that said, IF lower compression ratios increase power output per given PSI, and I'm already having traction issues, it's not like I can do much with the added power anyway. Also, if i were to go to a lower compression ratio and it reduced responsiveness, I'm not convinced it would actually result in a faster car.
Military put a long delay in plans, but am home now. Hoping to get the garage cleaned up to finish this project. Have an efr8374 and 7670 to try out on it. Thinking the lower pr friendly 8374 may yield the best balance of results since I probably won't be able to run as much boost on the higher company rotors. We will see.
this thread was a most interesting read and I was just curious if you ever came to any personal conclusion and have anything to add on it?
.
this thread was a most interesting read and I was just curious if you ever came to any personal conclusion and have anything to add on it?
.
Sadly, not yet. Been very engrossed with med school and DCT kits.
At this point, I'm not going to spend time and money tearing down a perfectly good engine. Going to run it as is and hope for decent to good results. Here's to 9.7's and an 8374!
02-06-16 | 12:21 AM
