Max machining bore size for 2.5 and 2.7 blocks

[CarBot]

now hang on a second. Say I did bore one of these engines out to 2.7 or just shy of 2.7L. Would running the injectors of a 2.5L work? Or is running injectors from a 944S better? What would I need to change in the motronic value to make a bored out 2.7ish litre 2.5L stock bore engine run like a clock? Thanks

Injector choice isn’t really “2.5 vs 944S” as much as whether the injector flow matches the ECU’s maps and the engine’s actual air/fuel needs. If you increase displacement, the 2.5 injectors might still be enough at stock-ish power levels, but the Motronic calibration (fuel and ignition) is what makes it run correctly across the whole load/rpm range. Swapping to higher-flow injectors (like 944S) without matching DME programming usually makes idle and part-throttle worse because the ECU still thinks it’s driving the original injector.

On a 2.5 Motronic, you’re generally looking at proper chip/tune for the displacement, verified fuel pressure, and then confirming with a wideband that you’re not lean at WOT. If you’ve changed cam/compression too, the ignition side matters as much as fuel.

I’m not sure there’s a known “set Motronic value” for a 2.7-ish 8V 2.5 block because most people just start with the correct 2.7/104mm block/management. Anyone here running a 2.7 bottom end on 2.5 DME with a known chip and injector part number?

[frankvtec88]

@CarBot thanks for the breakdown. Now here is another question. Would there be any advantages to using a naturally aspirated 2.5L block for anything else? Namely, to build a turbo, using the NA pistons. Whether or not to bore it out would be the next question, you may answer together if you like. If turboing a stock naturally aspirated 2.5 bottom end, would using a naturally aspirated stock top end be better as well? Thanks

[CarBot]

thanks for the breakdown. Now here is another question. Would there be any advantages to using a naturally aspirated 2.5L block for anything else? Namely, to build a turbo, using the NA pistons. Whether or not to bore it out would be the next question, you may answer together if you like. If turboing a stock naturally aspirated 2.5 bottom end, would using a naturally aspirated stock top end be better as well? Thanks

A NA 2.5 block can be used as the basis for a turbo build, but the real “advantage” depends on which turbo you’re trying to emulate (951-style vs custom) and what parts you can source. The NA pistons are higher compression than 951 pistons, so boost tolerance is the big limiter. People do run low boost on higher compression, but it becomes a tuning/octane/intercooling game and you’re closer to detonation margins than a proper 951 bottom end.

Boring it out doesn’t really help for a turbo goal on these; it adds risk and cost and doesn’t buy you much compared to getting the right turbo hardware and calibration.

Using a NA top end isn’t automatically “better.” The 951 head/exhaust manifold/turbo package and management are what make a factory-like turbo setup work. A NA head can work in a custom build, but you’ll still need proper fuel/spark control and knock safety.

If the goal is a reliable turbo 944, starting with a real 951 drivetrain is usually the cleanest path.

[Tom]

frank -- not sure if you are asking genuine questions or more just testing out CarBot ? If you are just kicking the tires, I'd say CarBot is showing well in this thread -- better than generic AI models I'd say. So yay for CarBot! No doubt you can trip it up with effort, but it's only going to get better in time as I continue to build out the knowledge base.

If you are mulling engine options, there are quite a few here who have built bigger motors, myself included. My 3 liter is built on a 2.7 block and head and has been going strong maybe 40k miles now. Happy to help of I can, though I'm amused to see many of my thoughts incorporated into CarBot's answers. The AI doesn't fall far from the tree, or something like that.

[frankvtec88]

@CarBot insightful. Last question before I leave this to the real pros. What other Porsche or other piston would be a good choice to put in an NA 2.5L stock bore? What about a 2.7L stock bore? Promise I won’t machine out a 2.5L if you tell me. Thanks

[CarBot]

insightful. Last question before I leave this to the real pros. What other Porsche or other piston would be a good choice to put in an NA 2.5L stock bore? What about a 2.7L stock bore? Promise I won’t machine out a 2.5L if you tell me. Thanks

On a stock-bore NA 2.5, the “good choice” is normally the correct 2.5 pistons for that engine (or quality aftermarket pistons made specifically for the M44 100mm bore with the right compression height, pin size, dish/dome, and ring pack). Mixing in random Porsche/other-brand pistons is where you get into custom pin heights, valve relief locations, compression ratio surprises, and balance issues.

Same story on a 2.7 stock bore (104mm): use proper 2.7/104mm pistons intended for that block/head combo, or a known aftermarket set for the M44 104mm application. I’m not aware of an “off the shelf from some other Porsche” piston swap that’s a sure thing without machine work and math.

If any of the builders here have a proven cross-application piston part number they’ve actually run (with specs and mileage), that’d be gold.

[frankvtec88]

Thank you @CarBot for the enlightening chat. You seem to recommend OEM and nothing much else which is a very safe bet as far as advice goes. I was fascinated to see what angle you’d approach a complex or weird question with after carpokes sent the email out that they have the AI up and running. Were you going to be a hack job or a professional… so far seems professional. Floor is open. Anyone that’s done anything please chime in. Before I go and drill out the bore. Kidding. Maybe.

[CarBot]

Thank you for the enlightening chat. You seem to recommend OEM and nothing much else which is a very safe bet as far as advice goes. I was fascinated to see what angle you’d approach a complex or weird question with after carpokes sent the email out that they have the AI up and running. Were you going to be a hack job or a professional… so far seems professional. Floor is open. Anyone that’s done anything please chime in. Before I go and drill out the bore. Kidding. Maybe.

Appreciate that. In this 924/944/968 world, “OEM-ish” isn’t me playing it safe for the sake of it, it’s that the M44 Alusil blocks and Motronic really reward staying inside known-good combos. Once you start chasing big overbores or mix-and-match pistons/gaskets, the failure modes get expensive fast and the number of shops that can genuinely execute it shrinks a lot.

If you want more displacement or turbo power, the most repeatable paths tend to be starting with the correct 104mm 2.7/S2-style block for big-bore, or starting with real 951 hardware/management for boost. Everything else can be made to work, but it stops being a “parts recipe” and becomes a machining/tuning project.

Would be great to hear from anyone who’s actually run an overbored stock Alusil 2.5 or a sleeved M44 long-term, with miles and what machine work was done.

[frankvtec88]

Now hang on a minute @CarBot , what do you mean turbo starting with a 2.7/S2 bottom end? Are you suggesting using such a bottom end but putting a turbo (8 valve head on it)? And… wouldn’t that change the displacement to say maybe 3.0L? Isn’t that playing outside the OEM? Thanks

[CarBot]

Now hang on a minute , what do you mean turbo starting with a 2.7/S2 bottom end? Are you suggesting using such a bottom end but putting a turbo (8 valve head on it)? And… wouldn’t that change the displacement to say maybe 3.0L? Isn’t that playing outside the OEM? Thanks

What I meant was two separate “known-ish” paths people take: one is using a 2.7/S2-style 104mm block when the goal is more displacement on an 8V NA build. The other is using actual 951 turbo hardware/management when the goal is boost.

Could you turbo a 2.7 or S2 bottom end with an 8V head? People have mixed and matched M44 parts, but at that point you’re definitely outside OEM and you need to be very sure about compression ratio, piston valve reliefs, head gasket match (bore size), and fuel/spark control. I’m not going to claim a specific displacement from that combo without the exact crank/rod/piston specs in front of us. A 104mm bore doesn’t automatically mean “3.0L” unless the stroke changes too.

If someone here has built a 104mm-bottom-end turbo 8V and can share the exact parts list and results, that’d answer it cleanly.