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safe
Joined: 18 Mar 2017
Posts: 588
Location: Sweden
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 Posted: Tue Jul 11, 2023 4:56 am Post subject: |
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Was out driving small roads today, its so freaking entertaining! If you're in a higher gear and floor it, it starts to build boost as low as 2500 and at 3500 it really takes off.
Let a friend take it for a drive, he thought it was insane, but manageable.
I found a nice gravel road in the woods today too. Narrow, less than 2 car widths, very twisty up and down all the time, not fast 50-70 km/h top. I'm so impressed by how controllable these cars are. Nice with the "rat rod" style so I don't have to care about the stones blasting the car.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Cedric
Joined: 27 Aug 2004
Posts: 2610
Location: Sweden
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| Posted: Tue Aug 15, 2023 11:47 pm Post subject: |
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Soo,
This weekend Magnus brought his 924 over, which was very fun since its normally in a different part of sweden several hours away. It was great to see the build in real life! Im very grateful that i had the chance to drive it!
From a driving perspective it was very different to my 931 in several ways, it felt very much like a normal 924 n/a below 3,5k ish rpm (but with a noisy exhaust), you cant really guess whats coming. If you keep the throttle planted and it passed 4-4500rpm theres a violent rush up to redline, you really have to keep the eyes on the roads when you drive on twisty country roads like we did  . Ive been in a car with similar weight and over 400hp before and this felt just as fast, especially since it gripped so well and just went. Mine has 0,5bar of boost with 3k rpm already, very different experience for sure. Its noticable thats its a very big turbo in shifts and on/off throttle , mine is very quick to be back on boost but of course the bigger turbo and higher boost levels takes more time. But when on boost the acceleration is so much more brutal than my car thats for sure! The speedometer turns faster than the RPM counter on a normal n/a :woohoo:
Its fascinating though how much hammering this engine takes with this boost (how many std engines can take 2 bars boost/29psi, not many), its not exactly babied, this engine is used all the way. Though for me it might not be the optimal design to have fun on a country road, but its absolutely a proper great achievement on what can be done with very reasonable money and simple tools, i really like the approach in this car. If one would want to add lots of grip and brakes it would end up a very very fast track car, its light and actually the power doesnt overwhelm the chassis at all, but that would probably bring other challenges, and Magnus already have more than one car for track use
Regarding the brakes, i actually didnt push them hard, so I didnt have any issues with the n/a brakes (a little squishy like all na brakes). But i would prefer some semi racing pads to be sure that they are always there when you brake. And a suspension refresh of course, though the suspension wasnt that bad, but considering the high speeds you reach in a second it would be nice to be fully in control. The federal 595 RSR tyres actually gripped up well on the road, surprising amounts of traction !
Made a little quick video, better than nothing .
https://www.youtube.com/watch?v=JswzUStfehU
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1980 924 Turbo
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Beartooth
Joined: 05 Apr 2022
Posts: 206
Location: Roberts, MT
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| Posted: Wed Aug 16, 2023 2:49 am Post subject: |
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Pretty cool that you guys were able to arrange a get-together! The big turbo probably helps a lot. I have no idea what the compressor maps look like, but I'd bet all the 931 turbo variants are way off the "island" (the flow and pressure ratio where it's most efficient) and producing tons of heat if you tried to push anywhere near this kind of boost. Of course, most OEMs lean towards the smaller turbos for more response and user-friendliness; I suspect it also helps keep you from over-revving because it contributes to power drop-off as it approaches red-line. Big turbos have their disadvantages, but especially on a simple non-intercooled setup, I'd go bigger rather than smaller (as safe did here). It'll make more peak power - which normally shouldn't be the target on a street car - while avoiding a potential heat snowball effect you'd run into trying to push a smaller turbo. An intercooler could still help, but I'd guess isn't as critical with all the octane you get from e85. Anyway, definitely a fun project, and I've got thoughts of a similar bare-bones turbo job on one of my Mercedes V-8s (not as much boost though!).
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1980 931 diamond in the rough |
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Cedric
Joined: 27 Aug 2004
Posts: 2610
Location: Sweden
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| Posted: Wed Aug 16, 2023 5:07 am Post subject: |
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| Beartooth wrote: | | Pretty cool that you guys were able to arrange a get-together! The big turbo probably helps a lot. I have no idea what the compressor maps look like, but I'd bet all the 931 turbo variants are way off the "island" (the flow and pressure ratio where it's most efficient) and producing tons of heat if you tried to push anywhere near this kind of boost. Of course, most OEMs lean towards the smaller turbos for more response and user-friendliness; I suspect it also helps keep you from over-revving because it contributes to power drop-off as it approaches red-line. Big turbos have their disadvantages, but especially on a simple non-intercooled setup, I'd go bigger rather than smaller (as safe did here). It'll make more peak power - which normally shouldn't be the target on a street car - while avoiding a potential heat snowball effect you'd run into trying to push a smaller turbo. An intercooler could still help, but I'd guess isn't as critical with all the octane you get from e85. Anyway, definitely a fun project, and I've got thoughts of a similar bare-bones turbo job on one of my Mercedes V-8s (not as much boost though!). |
That turbo is very big and designed for pretty wild pressure ratios, more like a diesel engine, it still has alot more to give. When you have low efficiency the problem is bigger than just high boost temps, it will also demand alot of drive energy from the exhaust side, which means very high exhaust back pressure, that will both lower the power due to pumping losses but also increase the residual gases in the cylinders which triggers knock, higher turbine efficiency will also lower the exhaust pressure for a specific boost level. If such a turbo wasnt available this whole project would be really tough to do with these kind of numbers, turbo development has come a long way since the 70s/80s, especially at the OEM level. BTW this engine has a good sized fmic, or it would have melted something by now.
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1980 924 Turbo
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safe
Joined: 18 Mar 2017
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Location: Sweden
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| Posted: Wed Aug 16, 2023 6:04 am Post subject: |
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I think that is the compressor map of the slightly larger 257, I have the 252.
But there is still more to have from that one. Bu the look of it the 257 would probably have been better with slightly higher efficiency where I'm at.
Right now I about the intersection of 3.0 and 42.5, maybe, something like that.
I really didn't think I would be so far to the right in the graph...
252:
I did a 13 second pull i 5th a few days ago logging it, the intake temp went from about 20c to 30c after the pull it continued to climb to about 34c for another 7 seconds and stabilized before dropping again. You'll have to convert the C to F but 37C is about 100F for a ballpark.
The tuner had his eyes on the intake temp on the dyno while tuning, he said it looked great.
A bit of lag can be remedied by the ECU. I can activate Rolling Anti Lag and if I get an e-throttle or some other air valve I can activate full anti lag so the boost doesn't drop between shifts.
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Beartooth
Joined: 05 Apr 2022
Posts: 206
Location: Roberts, MT
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| Posted: Fri Aug 18, 2023 3:10 am Post subject: |
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Ahh, missed that it had an intercooler - right there on the first page (no pictures though). 29 PSI non-intercooled did seem a little too good to be true; I just thought that was the e85 advantage, I guess. My truck (diesel, non-intercooled) has some mods to it, and I've peaked out between 25 and 30 PSI, but I can't hold that very long because EGTs get out of hand in a few seconds. I'd add an intercooler, but I don't tow heavy loads and wouldn't realize much of the gains in my typical driving. Anyway, out of curiosity, I went digging for a 931 compressor map. The ones I'm coming up with aren't the best quality, but it looks like you fall right off the island if you try to push much further than stock. An intercooler helps all around, but it looks like you'd still be bottle-necked by the turbo around 250HP (and if you made 250, a turbo upgrade alone would be worth close to 50HP).
Anyway, the turbo you're using is on a different continent than a K26! I found a pretty handy website (maybe it's not news, but it was to me) that you can plug your engine's numbers into and it'll give you estimated HP and superimpose the flow and pressure ratios on a variety of compressor maps. https://www.borgwarner.com/matchbot I've been playing with some (highly estimated) numbers for a 924, and it looks to me like that turbo is a little big, but not oversized (especially shooting for peak power). For what it's worth, all of the turbos I can select there seem a little big for a more typical street turbo setup. Anyway, it's a pretty interesting tool, and I see it's estimating around 300 HP for 29 PSI on a 2.0; hard to tell how accurate that is, but even if it's 50 HP off, that's still double the power on an (internally) stock engine!
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safe
Joined: 18 Mar 2017
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| Posted: Fri Aug 18, 2023 4:59 am Post subject: |
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| Beartooth wrote: | Ahh, missed that it had an intercooler - right there on the first page (no pictures though). 29 PSI non-intercooled did seem a little too good to be true; I just thought that was the e85 advantage, I guess. My truck (diesel, non-intercooled) has some mods to it, and I've peaked out between 25 and 30 PSI, but I can't hold that very long because EGTs get out of hand in a few seconds. I'd add an intercooler, but I don't tow heavy loads and wouldn't realize much of the gains in my typical driving. Anyway, out of curiosity, I went digging for a 931 compressor map. The ones I'm coming up with aren't the best quality, but it looks like you fall right off the island if you try to push much further than stock. An intercooler helps all around, but it looks like you'd still be bottle-necked by the turbo around 250HP (and if you made 250, a turbo upgrade alone would be worth close to 50HP).
Anyway, the turbo you're using is on a different continent than a K26! I found a pretty handy website (maybe it's not news, but it was to me) that you can plug your engine's numbers into and it'll give you estimated HP and superimpose the flow and pressure ratios on a variety of compressor maps. https://www.borgwarner.com/matchbot I've been playing with some (highly estimated) numbers for a 924, and it looks to me like that turbo is a little big, but not oversized (especially shooting for peak power). For what it's worth, all of the turbos I can select there seem a little big for a more typical street turbo setup. Anyway, it's a pretty interesting tool, and I see it's estimating around 300 HP for 29 PSI on a 2.0; hard to tell how accurate that is, but even if it's 50 HP off, that's still double the power on an (internally) stock engine! |
I saw that tool, but to fancy for me, I don't understand the parameters , they didn't include the old K26/27/29. Garrett have something similar.
I wanted to make 400 hp and guesstimated it would need 2 bar to come near, then choose this turbo.
I wonder what a 931 head would do thrown into the mix....
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Beartooth
Joined: 05 Apr 2022
Posts: 206
Location: Roberts, MT
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| Posted: Sat Aug 19, 2023 3:54 am Post subject: |
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I've seen a couple guys here who know the 924 far better than me suggest that the 931 head would be the choice of the two, but it seems the only real advantage is that it's more resistant to detonation - I do remember ideola saying that the idea that it flows better is mostly a misconception.
I've always loved tinkering on engines and reading about them (especially the stuff I couldn't afford); I've read Corky Bell's book cover to cover a couple times, for one. Not having built a turbo setup from scratch, however, I probably know just enough about compressor maps and the nuts and bolts of turbos to make me dangerous. Anyway, the calculator seems more pessimistic than what I'm seeing from your dyno result: it's showing it'd take around 40 PSI to hit 400 HP. That's using 85% for the Volumetric efficiency - which, if I back all the boost numbers to zero, puts the predicted power right at 125 HP. With a lot of work, you might get closer to 90%, but as Mike911's porting thread demonstrates, that's no easy task. Anyway, 400 is asking a lot of a relatively agrarian architecture (let alone the drivetrain) but then you've already come this far, so... if you're willing to push the rest of the way, Godspeed to you!
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safe
Joined: 18 Mar 2017
Posts: 588
Location: Sweden
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| Posted: Sat Aug 19, 2023 4:38 am Post subject: |
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| Beartooth wrote: | I've seen a couple guys here who know the 924 far better than me suggest that the 931 head would be the choice of the two, but it seems the only real advantage is that it's more resistant to detonation - I do remember ideola saying that the idea that it flows better is mostly a misconception.
I've always loved tinkering on engines and reading about them (especially the stuff I couldn't afford); I've read Corky Bell's book cover to cover a couple times, for one. Not having built a turbo setup from scratch, however, I probably know just enough about compressor maps and the nuts and bolts of turbos to make me dangerous. Anyway, the calculator seems more pessimistic than what I'm seeing from your dyno result: it's showing it'd take around 40 PSI to hit 400 HP. That's using 85% for the Volumetric efficiency - which, if I back all the boost numbers to zero, puts the predicted power right at 125 HP. With a lot of work, you might get closer to 90%, but as Mike911's porting thread demonstrates, that's no easy task. Anyway, 400 is asking a lot of a relatively agrarian architecture (let alone the drivetrain) but then you've already come this far, so... if you're willing to push the rest of the way, Godspeed to you! |
I'm already over 400, 420ish crank.
I don't dare go much further, yet. I'm out of clutch and injectors. Drive train is probably holding on for dear life!! Upgrades are needed, but 500+ crank is doable with this turbo.
I got another block prepared with forged pistons, haven't decided on rods yet but it will get new rods to support a lot more. Turbo gearbox and torque tube too.
But first on the list are a bunch of small things and suspension and brakes, safety third priority!
_________________
/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Cedric
Joined: 27 Aug 2004
Posts: 2610
Location: Sweden
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| Posted: Sat Aug 19, 2023 5:19 am Post subject: |
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13s pull in fith gear was an interesting run, thats a long time on wot
Be careful with the matchbot, its a very very very crude tool, easy for me to say who used to work as senior engineer on the subject. But its really easy to get lots of nonsens stuff from it if used wrong, Some of the inputs are in reality outputs from your turbo match, not something you can just type in regardless of turbo. Could be a little bit useful if you have good data for some simpel analysis, hp is not one of the good outputs.
I have fairly detailed model of a 931 engine in the simulation tool that pretty much every OEM engine is made with, and used it for trying lots of different turbos, cams, exhaust manifold designs etc. Sadly i dont have access to it anymore, friends have to help me with that, otherwise i could have done way more fun test on all parts of the engine.
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1980 924 Turbo
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Lau Christophersen
Joined: 17 May 2013
Posts: 108
Location: Denmark/Copenhagen
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| Posted: Sun Aug 20, 2023 8:56 am Post subject: |
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| safe wrote: | | Beartooth wrote: | Ahh, missed that it had an intercooler - right there on the first page (no pictures though). 29 PSI non-intercooled did seem a little too good to be true; I just thought that was the e85 advantage, I guess. My truck (diesel, non-intercooled) has some mods to it, and I've peaked out between 25 and 30 PSI, but I can't hold that very long because EGTs get out of hand in a few seconds. I'd add an intercooler, but I don't tow heavy loads and wouldn't realize much of the gains in my typical driving. Anyway, out of curiosity, I went digging for a 931 compressor map. The ones I'm coming up with aren't the best quality, but it looks like you fall right off the island if you try to push much further than stock. An intercooler helps all around, but it looks like you'd still be bottle-necked by the turbo around 250HP (and if you made 250, a turbo upgrade alone would be worth close to 50HP).
Anyway, the turbo you're using is on a different continent than a K26! I found a pretty handy website (maybe it's not news, but it was to me) that you can plug your engine's numbers into and it'll give you estimated HP and superimpose the flow and pressure ratios on a variety of compressor maps. https://www.borgwarner.com/matchbot I've been playing with some (highly estimated) numbers for a 924, and it looks to me like that turbo is a little big, but not oversized (especially shooting for peak power). For what it's worth, all of the turbos I can select there seem a little big for a more typical street turbo setup. Anyway, it's a pretty interesting tool, and I see it's estimating around 300 HP for 29 PSI on a 2.0; hard to tell how accurate that is, but even if it's 50 HP off, that's still double the power on an (internally) stock engine! |
I saw that tool, but to fancy for me, I don't understand the parameters , they didn't include the old K26/27/29. Garrett have something similar.
I wonder what a 931 head would do thrown into the mix.... |
I have a 924 N/A block with original pistons mounted with 931 std. Head. The block was milled down I think 2mm to increase the compression ratio to something like 8,4 : 1.
But it is still only on the pallet waiting for the rest of the car to be finished.
At the moment mounted with a turbo from a 2,9 liter diesel engine (kia carnival 2,9 geri)
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76 924 (PARTS)
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77 924 converted to 924S
77 928
79 931
79 931(PARTS).
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Beartooth
Joined: 05 Apr 2022
Posts: 206
Location: Roberts, MT
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| Posted: Tue Aug 22, 2023 3:13 am Post subject: |
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| Cedric wrote: | 13s pull in fith gear was an interesting run, thats a long time on wot
Be careful with the matchbot, its a very very very crude tool, easy for me to say who used to work as senior engineer on the subject. But its really easy to get lots of nonsens stuff from it if used wrong, Some of the inputs are in reality outputs from your turbo match, not something you can just type in regardless of turbo. Could be a little bit useful if you have good data for some simpel analysis, hp is not one of the good outputs.
I have fairly detailed model of a 931 engine in the simulation tool that pretty much every OEM engine is made with, and used it for trying lots of different turbos, cams, exhaust manifold designs etc. Sadly i dont have access to it anymore, friends have to help me with that, otherwise i could have done way more fun test on all parts of the engine. |
Yeah, I agree on matchbot. Even the best calculator is just a yardstick - doesn't mean anything until it's running, and the figures (especially HP) are just ballpark stuff. I do like that it has so many inputs - in an ideal world, as long as you know what to input, you would be able to get a very accurate output (even HP). Things like volumetric efficiency, temperature and elevation (I live at 4000 feet, and we have roads that top out over 10,000). That said, you have to be pretty smart on the motor itself to get that close on the numbers you input, and then if the math behind the corrections is sloppy, the best you'll get is some rough estimates. Things like matchbot are fun though; what I'd do would be to get a basic plan together, then run my own numbers before I put any money down. I'd love to see the simulator you built if you ever find it. I once built my own calculator too - just using a spreadsheet, and it was pretty rough, but I definitely learned some things just piecing it together.
| safe wrote: | ...
I got another block prepared with forged pistons, haven't decided on rods yet but it will get new rods to support a lot more. Turbo gearbox and torque tube too.
But first on the list are a bunch of small things and suspension and brakes, safety third priority! |
I'm not sure this is "good idea" territory, but I'd be tempted to keep pushing the stock bottom end until something breaks... Obviously, it's not something you'd want to do unless you consider it a throwaway motor (and even then, you're playing with fire). I'm assuming you've seen the tread talking about using a Mitsubishi rod and (I think) Nissan pistons. The ideal thing would be a custom billet crankshaft to match the rod length and compression height and give you more stroke, but that's big money. That said, the rest of the car is important too - keep going too far, and you're basically riding a rocket! Brakes of course, but just plain nuts and bolts (especially in the suspension) can let you down at the most inopportune time. A transaxle upgrade is probably needed at some point too - not a lot between "cheap" and "ideal" there.
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safe
Joined: 18 Mar 2017
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Location: Sweden
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| Posted: Thu Aug 24, 2023 10:01 pm Post subject: |
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| Beartooth wrote: |
I'm not sure this is "good idea" territory, but I'd be tempted to keep pushing the stock bottom end until something breaks... Obviously, it's not something you'd want to do unless you consider it a throwaway motor (and even then, you're playing with fire). |
The thought has cross my mind... But the engine is pretty healthy, it had minimal wear in the cylinders, pistons that were in spec and the head is slightly ported with new guides. So the head I want to reuse with a better bottom end.
I might have pushed it further if the clutch wouldn't be so close to it's limit. Pushing it more would require higher revlimit and that is a nono for me on the stock stuff.
With lighter forged pistons, stronger rods and balancing of everything I would be fine with pushing 7500 and increasing boost on the top. From an engine perspective that is, still terrible for the gearbox!!
| Beartooth wrote: | | [I'm assuming you've seen the tread talking about using a Mitsubishi rod and (I think) Nissan pistons. The ideal thing would be a custom billet crankshaft to match the rod length and compression height and give you more stroke, but that's big money. |
Yes, I've seen it. But I think I'll go with a set of Auto Verdi rods, if I can get hold of them.... or perhaps the rods Mittelmotor sells that are reasonable in price. Pauter is an alternative to.
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/Magnus, Stockholm Sweden
=======================
Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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Borken
Joined: 23 May 2015
Posts: 5
Location: Falkenberg, Sweden
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| Posted: Wed Jan 03, 2024 4:48 pm Post subject: |
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Where did you find that silicone bend that connects to the throttle body?
Is it a standard 90° reducing bend? What dimensions?
Great build BTW!
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safe
Joined: 18 Mar 2017
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| Posted: Wed Jan 03, 2024 5:42 pm Post subject: |
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| Borken wrote: | Where did you find that silicone bend that connects to the throttle body?
Is it a standard 90° reducing bend? What dimensions?
Great build BTW! |
Thanks!
Yes, it's a standard 3" -> 2.5", probably, nothing special anyway. It stretches over the throttle body fairly tight.
I bought most of the silicone hoses from do88.se or Biltema (the straight bits).
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
Porsche 931 -79
Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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924 FAQ (Technical)
