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BARknucklehead BARknucklehead is offline
Corrupted by power
Join Date: May 2009
Posts: 55
I need your Small Displacement Turbo Help! (Long)

Hi All,

Big lurker, first time post to the Pelican Turbo forum. I am embarking on a new project that is pretty atypical to most of the projects I read bout here, so am hoping I am not so far in left field that no one can help. I know there is no shortage of informed opinion and knowledge here, so fire away. Thanks in advance.

I recently retired my little MFI ‘S’ motor and since I’ve always wanted to put a turbo motor for my early 911, figured I’d build one in the spirit of my old motor. Because of this, the typical turbo doesn’t appeal to me, meaning 3 liter and up motors won’t meet my desires. Frankly, they just make too much power! Yeah, that shouldn’t be possible, but I said it anyway. Please don’t reply with the standard “Just by a 3.3 and be done with it” comments – they aren’t going to fly; my course is set.

The motor I am building is mostly specced and I already have most the parts to build the shortblock. It will be a small bore turbo, along the lines of the Baby RSR. Basic spec is for an 85mm bore x 66mm stroke, 2.3L intercooled engine. HP target is between 320 and 340HP @ 7500RPM, which calculates out favorably against my goal for boost at a conservative .8 BAR. The motor is being built to rev safely to 8000 RPM and my cam choice makes power to suit (see below).

My final decisions are down to figuring out the turbo and IC configuration. There seems to be a lot of debate between twin turbo configurations vs. using a single unit, especially regarding the throttle response. Seeing as this motor can’t rely on displacement to overcome off boost deficiencies, I want to make sure that my selection maximizes spool up.

During my research, it seems that the preference is for ball bearing turbos over journal bearing turbos, due to the efficiency gains they provide. Unfortunately, when matching my application for twin turbo applications, it seems the only ideally sized turbos that meet my needs are journal bearing turbos.

So the decision looks like it is between:
1. Using two journal bearing small turbos like the GT15
2. Going with two larger, but less ideally sized bearing turbos like the GT25
3. Going with a single ball bearing turbo like the GT28

I was convinced that twin turbos are the way to go, but modern, water cooled turbos w/ variable valve timing like the WRX, Mitsu Evo, Audi and VW 2.0T cars all make excellent throttle response and power from one turbo. From my reading, I notice a consistent theme that most turbos today are most efficient over 1 BAR, typically in the 1.2 BAR range; mine is much lower, so efficiency should be down. Dilemmas, dilemmas…

There is obviously a lot of tweaking that can be done on the A/R and compressor/turbine pitch to optimize any of these, but my question is:

Given the above options, what would you do to maximize spool up for a motor like this and get similar throttle response to what these modern cars make?

As far as full specs, here’s where what I already have:
85mm bore – Nickies w/ 7.5:1 JEs, thermo-coated
66mm stroke – SC rod journal grind, cross drilled
Custom Dougherty turbo cams: GT2 intake with the DC43 exh on a 108 lobe center (@ .050” Duration=248I/242E, lift .485”I/.484”E, 108 lobe centers)
Coated STD main bearings, grooved for cross drilled crank
R&R chromoly rods w/ ARP bolts from LN Engineering, GT3 spec (same size as Pankl Ti-rods)
2.4T 7R case, fully prepped by Ollies: shuffle pins, oil bypass, drilled for cross-drilled crank
Supertec head studs
2 liter heads w/ ‘69S valves, 39mm ports I/E, twin plugged
930 oil pump, race prepped
Properly sized George Narbel headers

Don’t have it yet, but will soon:
EFI - Will use MegaSquirt II Sequencer system (waiting for final availability)
Coil on plug
Crank Fired
Wideband 02, w/ MAP, TPS and intake temp sensor
Apparently this ECU supports knock control and electronic wastegate/blowoff control – would love to integrate this

I am not totally in the dark here. A similar motor to my spec (but with a less aggressive cam) produced this dyno run:

Sorry to be long winded, but thanks again to everyone who battled through.

Last edited by BARknucklehead; 05-26-2009 at 12:25 PM..
Old 05-26-2009, 09:48 AM
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