There has been an ongoing debate about the pro’s and con’s of swapping the 8th gen Civic Si intake manifold onto the 9th gen Civic Si without any real concrete testing. Just butt dyno reviews, bromancing and numbers being thrown around with no context. So basically your average day on an enthusiast discussion board.
We’ll be having none of that here — I requested a 2012+ Civic Si that had a Full Race exhaust and Full Race 3″ catless downpipe, running the stock intake as our “base” to start from. We also got the the PRL SRI for the stock IM & RBC so that testing would stay consistent — and we tested the PRL SRI before installing the RBC IM.
So in short the testing involves:
- PRL SRI on stock intake manifold.
- RBC intake manifold w/ PRL SRI (to see difference over stock manifold).
- ZDX throttle body.
Now the ground rules are simple:
- The vehicle must be fully retuned after each major modification change on the vehicle.
- No “snorkel modding” the intake out of the engine bay to artificially reduce intake air temps (reducing air temps will indeed increase HP — the goal of this test isn’t to show you this). The goal is consistent and realistic testing (particularly to demonstrate differences from mod to mod).
- Two to three pulls are done on the “final” tune to ensure the engine has “settled” and the pulls are consistent between attempts — maintaining this requirement ensures comparisons between the various mods we are testing are consistent.
Long and productive Saturday: eight hours without the car leaving the shop dyno and over 70 dyno pulls later, we had concluded testing.
Now on to the results.
Stock tune vs VitTuned
I baselined the car on the stock tune and we got just shy of 180whp (the dyno baselines 162-165whp for a bone stock 2012 Si). Not bad at all for two simple exhaust bolt ons. The stock intake had been retained and this example demonstrated why I recommend keeping the stock intake if you can’t afford FlashPro/Tuning yet — the car actually runs mostly OK with the factory airbox on the vehicle. Obviously doesn’t make “best power” for the mods, but the car drives and performs well day to day.
I proceeded to fully tune the car — and the power went up nicely with a cleaner power curve throughout the rev range, stopping just shy of 190whp — with gains of 11-14whp through the top end over the factory tune.
Stock intake vs PRL SRI
Back to the laptop I went and more tuning commenced. I was pleasantly surprised by the solid low end gains from 1700 rpm til 2500 rpm — as much as 12 ft lbs of torque to the wheels will definitely be something you can feel during normal stop and go driving. Slight loss from 2750 rpm to 3000 rpm though — nothing major. And no real gains until after ~ 5700 rpm, with a maximum of 4.5whp was had from 6750 rpm til 7000 rpm. Not a bad gain for a simple mod — I’ve seen much worse performance from some intakes on this platform (worse than stock intake at times).
And now the RBC intake manifold!
On to what we’ve all been waiting for! I dug back into the engine bay and worked on installing the RBC intake manifold PRL graciously supplied for testing — as well as their adapter for the kit. This install is a bit more involved than the SRI and required a larger variety of tools — and about 2-3 hours of shop time to install.
Once the intake manifold was on, the RBC IM version of PRL’s SRI was bolted up and the coolant system was burped. This step is very important — the coolant system must be properly burped. I’ve had customers send me datalogs with 280 degree Fahrenheit coolant temps after doing work on the car that involved draining the coolant system — which just guarantees a blown head gasket and very costly repair. I recommend using this kit, or something similar, to assist with purging the coolant system of all air: Spill-Free Funnel.
- Below 2100 rpm there is as much as 12 ft lbs of torque lost when using the RBC intake manifold.
- From 2100 to 3500 rpm there are minor torque gains (1-6wtq) when using the RBC intake manifold.
- From 3650 rpm until 5750 rpm there is nothing but bad news when using the RBC intake manifold — as much as 15 ft lbs of torque lost!
- After 6200 rpm is some good news — we begin to see minor gains, based on “peak” numbers, we only got a 6whp gain using the RBC.
- At ~7150 rpm there is a 7whp gain.
- At 7500 rpm there is a 11whp gain.
So what can we gather from this? There is a hefty trade off when using this intake manifold on the 2012+ Civic Si. You are basically sacrificing a lot of mid/low end for a powerband that carries better after 6000 rpm.
So pick your poison: what are you using the car for?
Racing? Then technically speaking this car will be a bit faster when keeping the revs above 6000 rpm.
Daily driven stop and go “fun” car? The torque with the stock IM might benefit you more.
The choice is yours — as with everything in life, we do what we do with our toys for our own pleasure and enjoyment.
Wait, let’s make a joke and put a huge TB (ZDX/J37) on the car and see what happens?
Now I really have no idea how TB swaps got so popular on bolt on motors. The simple fact is this — items like throttle bodies, injectors (yes I’m looking at the guys claiming RDX injectors are necessary with an RBC IM swap), etc, are nothing more than SUPPORTING modifications, and ONLY benefit you when the motor has a flow requirement that is now surpassed by the items on the car. To say the stock 9th gen throttle body is a restriction on a bolt on 9th is simply a JOKE. The following comparison demonstrates as much. For the marginal gains (1hp) that is had up top with the TB, as much if not more is lost in the mid/low end.
But so and so put a TB on and it pulls so hard…. sorry, please schedule an appointment to have the butt dyno re-calibrated.
Hopefully this has been an insightful test for us all.
What’s all this cost?
- RBC Bored to 70mm for ZDX and CNC Bored for 9th gen injectors – $420
- PRL RBC Adapter Kit – $135
- ZDX TB (when purchased as kit option from PRL) – $220
- PRL SRI – $200
- Shop labor (if not installing on your own) — 3-4 hours ($240-$320 here)
I’d like to give a big thanks to PRL Motorsports for supplying us with all the goodies for this test.
Thanks Ernesto for supplying the test vehicle — enjoy the mods and the tune!