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 Post subject: b2x5 engines
PostPosted: Sun Feb 14, 2021 11:30 am 
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Joined: Thu Aug 20, 2015 6:36 pm
Posts: 208
Car Models: CD Carlsson, 9-3 Turbo
Just going to talk about my experience with my tuned oem b205 engine. Just something for people to read and i think it will have some good information for anyone who is looking to tune the chocolate box t7 engines. Will also explain why i think the b205 is a tad stronger.

Feel free to correct me on anything i get wrong. Always learning :)

Have to remember that there is going to be alot of variations in peoples hardware setups which i think is another factor. Some setups such as high boost tdo4s (every b235 then :roll: ) stress the engine alot more with high iats and egts.

First of all how many b205s do you hear of popping vs b235s? All i ever hear of is b235s breaking pistons around the ring lands, and half of the pistons you see show signs of knock or have melted. (Little 5cm tdo4s giving really high egts does not help)

B205 8.8:1 compression ratio (not 100% sure on this but seen 2 sources on it now)

B235 9.3:1 compression ratio

that is a good difference in cylinder pressure. 8.8:1 is going to swallow boost easier than 9.3:1

The second thing is the piston crown thickness between them. B205s have almost flat top pistons with just a 50p sized dish in them. They have a thicker crown compared to the B235s where about 70% of the crown is dished and is lower. I reckon this makes the b205 stronger not because of outright strength but the ability to handle heat better than the 235 pistons.

It works on the same principle of getting rid of any sharp edges in a combustion chamber to deter knock. A thin piece of metal is going to heat up faster and reach its melting point faster than a thicker piece. (Have to assume all the 2x5s use the same material for the pistons) It could even be that b235 ring gaps were too tight from the factory and are butting up when they expand but this would be abit odd for just the 235s to get this.

So knowing those two things a tuned b235 and b205 with an identical setup at say 1.5bar of boost will not be under the same stress as each other. Just to makes things worse for the 235 its compressing another 300cc worth of air/fuel.

One thing that is not going to help either engine though is poor flow from tiny exhaust housings from tdo4s... High egts making things hot. Again the b205 should be able to take a little more.

I got my b205 at 107k miles, Upped the power to 320hp/430nm. I waited for it to blow but it hasn't happened with 2.5 years and 40k miles of daily driving and many pulls up to 7.1k rpm, there hasn't been a squeak from it. In the early days of ownership i even had a chain tensioner let go and the exhaust valves smacked the pistons... got the dents to prove it. still didn't break anything.

Afr's did spend some time in 10s due to a fault but are now in the low 11s. keeping them cool is the key but im confident that my setup even with this richer tune would of blown a b235 a long time ago.

Now the car has a gt28rs and im hitting 60-125mph in 10.4 seconds. Mair is trying to sit at 1450 with spikes of upto 1560 (The map is aiming lower with mreq at 1365). So we are looking at around 330hp and 470nm at 1.4bar and probably 360hp and 550nm if its perfectly dialled in with more boost. Mostly gained torque at the moment as i think my old 19t (1.6bar) was at the 325 mark in its final days. dynoed at 318hp with horrific fueling.

We'll see how long this setup lasts but whatever happens i will tear the engine down and see what kind of wear it has or what broke.

Whats peoples experience with the b205? anyone blown one up? if you have show the setup and any data with that. I wont ask if anyone has blown a b235 up because i know they have.

1990 Black CD Carlsson T5+B234
2001 Red 9-3 "Kabel" 325hp

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