Upgrade Intake Manifold or hotside parts
 

I am currently running a 48 trim Garrett GT2871R on my built motor with 6 speed and 3.9 Torsen. Previously i was running a 2560 on a stock pistons motor with 5 speed and 4.1 torsen. My new setup feels strong but let me down a bit on the dyno. I used a different dyno but the 2871 made less power than my 2560 did. Two different motors two different dynos yes i know. 2560 was run at 15-16 psi 2871 was run at 17-18. Remember its the 48 trim version of the 2871R not the 52.

Im still running the Taco manifold to Artech 2.5 separated gases downpipe to custom catless 2.5 exhaust with big magnaflow. Current motor is as follows

99 motor
8.6 ST pistons
Manley rods
ARP main and head
ACL bearings
Volvo valve springs
Fluidampr
OEM oil pump

My question is. If i wanted to squeeze out a little more power from my current set up, what would be the best bang for the buck?

• Kraken/FM/Begi manifold to 3 inch DP to existing 2.5 inch exhaust (more expensive option)
• Skunk 2 Intake with plenum spacer.

Ken Hill did some dyno testing and got really strong results with the plenum spacer. Though he was running a full 3 inch exhaust. Im not sure if my 2.5 exhaust would diminish the potential gain available from the skunk 2 or not. Of course option 3 is just leave it be and stop burning money on this thing but we all know how that goes.

Looking for thoughts from folks, preferably those with some experience and background to make an argument for one route or the other.

Flyin' Miata said the Skunk2 Intake is good for about 30 HP in the mid 300 HP range based on the dyno they posted. Also, all of that power is 6000-7500 RPM range, looks like they lost a tiny bit of low/mid range power below 4500. So as a wild guess, you might get maybe 10-15 HP on an intake manifold...losing somewhere between 1-5 HP on the low/mid range?

Now, as far as the exhaust manifold + downpipe goes, I'm not impressed by the FM or BEGI designs. The Kracken manifold, however, seems to be the best manifold design outside of Trackspeed Engineering's $3000 EFR setup. As seen on their site, the Kracken manifold is pretty good for the price you pay.

Just going to a better manifold and a 3" downpipe will get you power pretty much everywhere. This will get you better spool and torque in the low 2000-3500 RPM range, better midrange in the 3500-5500 RPM range, and better high end power from 5500+ RPM. Even keeping the 2.5" exhaust and just having a 3" downpipe by itself will get you power everywhere. So, I'd go for a Kracken T25 Manifold and 3" Downpipe. Shipped to my house in the States, it is exactly 740 Euros or about $800. I think the HP/$$$ ratio is much better on the Kracken downpipe+manifold combo.

Remember that exhaust gasses lose velocity and cool down the further away from the turbo you get. You get much more power going from a 2.5" downpipe to a 3" downpipe than going from a 2.5" MagnaFlow muffler to a 3" MagnaFlow muffler. In theory you can take a 4" downpipe, run it a few feet, then funnel it down to a 3" exhaust and you make nearly the same power as a full 4" exhaust. I've seen people do stuff like that on Cummins turbo diesels with great success. The difference between a 4" downpipe + 3" exhaust on a turbo isn't much versus a 4" downpipe and 4" exhaust...at least on a 2000+ ft*lb diesel.

Also, do be aware you are probably going to have to do a retune of some degree when changing the exhaust that significantly. If you do get Kracken's manifold combo, reduce boost pressure by EBC or reducing preload by a mm or two to make sure you don't overboost on your first pull. It would suck to go from 17-18 psi to 20-21 psi and cause some motor damage. Once, I went from 20 psi to 28+ psi on a Ford Fiesta 1.6L with a modified GT2560 by going from a 2 1/4"-ish stock intake to a 3" aftermarket intake. Nearly gave myself a heart attack. If I wouldn't have been spraying straight 50/50 Ethanol/Methanol and watching both MAP sensors, I might have caused some damage.

You can't go wrong with a squaretop from us, 275$ shipped, high resale value if you still want to check out the skunk2.

You didn't tell us the A/R of the two turbine housings.

Both were 64 a/r. Car is mostly street and auto cross so I’m ok with giving up some top end for spool. (Which sounds like a case for hot side versus intake mani).

Also so forgot to mention running 93 pump Walbro 190 and FF640 injectors.

Seems about 300whp or so would be the limit of my current set up from a fueling standpoint.

So how much power did you make on your 2560 and 2871?

I cant see a 2871 with more boost pressure making less power than a 2560 on less pressure.

Exact same hardware setup. Difference being 2560 was stock 9:1 coated pistons with forged rods. 2871 is fully built with the good stuff pistons rods etc 8.6:1

2560 on Ptuning Dyno dynamics made 270 whp 254 wtq at around 15 psi

2871 on York automotive Dyno dynamics made 255 whp 244 wtq at 17-18 psi

i have heard the York Dyno reads lower than most.

As as an extra data point though. At 6500rpm I’m using roughy 10% more injector duty cycle than I was on the 2560 to maintain the same AFR. that leads me to believe the 2871 has to be making more power.

I suppose its it’s possible the York Dyno just reads real low. Apparently their tuner left so now they basically just do Dyno rentals or Dyno days etc. perhaps the calibrations were conservative?

Actually it was about 10% more fuel. Not sure if that means 10% more duty cycle or not.

Also compression teated the motor and it was 178 +\- 2 psi across the board.

That's really strong re compression.

Honestly, sure an intake will help you. But really... Your hot side needs to catch up with your motor at this point. Get the Kraken kit (for the marvelous 3 inch exhaust). Just make sure you don't get the chambered magnaflow exhaust like someone i know.

However, more fuel required for the same afr means more power. Do you have logs for the pulls or other pulls on the street with both setups in similar conditions? Want to run them through virtual dyno and see that that spits out?

You need a 3" exhaust before anything else. Then you need to up the boost on the 2871 some.

i appreciate the thorough response and I agree. The change in exhaust gas velocity is another interesting point. I know wrx go stage 2 with only a downpipe so that line of thinking makes perfect sense. Should mean there are still good gains that can be had with a 3 inch dp to 2.5 exhaust which was my concern

as for the other responses that kind of solidifies it for the upgraded hotside parts. At the end of that day I would prefer gaining power everywhere in the rpm range. Letting my engine breath better and be a bit more efficient also sounds like the right call.

I’ve tried looking through old logs and running them on VD but they tend to be all over the place and I have no idea the circumstances of those logs (was I going uphill or downhill or what)