Sarek

Hi all,

I've snooped around a bit and haven't found the answer so I hope you don't mine me asking seeing as a lot of you seem to upgrade to 2.2 or 2.5. If you increase engine capacity but don't change the turbocharger, how much does it lower the boost threshold?

Is it directly proportional to the increase? (ie does a 25% increase in capacity make the turbo kick in at 25% lower rpm)?

tweenierob

I havent got my maths head on unfortunately.... so cant work out percentages..

My last turbo was a std TD05, i changed to a 2.5engine which changed my 1.5bar of boost from around 3200 (approx) to 2200 or something stupid like that, the boost hit was ridiculous, in a good way.

Rob

Sarek

Wow, that's a huge difference; I'm dead curious to find out the maths behind this. It's as if the rpm drop is roughly proportional to the square of the capacity increase. I'm sure there's more to it than that and there are many other factors but as a rough guide this could be interesting.

If anyone has results from other capacity changes, please post them up - it would be interesting to see if it fits the theory!

john banks

1 bar dropped from 3100 to 2500 RPM for me on the 20G by swapping from 2.0 to 2.5. Which is just what you would expect. In simple terms the spool up should be proportional to the capacity, and this does indeed seem to be the case.

Bear in mind gearing and RPM limits, a 2.0 revving to 9000 RPM with good cams could have a similar power band compared to a 2.5 revving to 7000 RPM, as could a 2.2 revving to 8000 RPM.

I try to avoid having any bothersome lag at all at least in higher gears by half of redline RPM for a nice road car.

Hoppy

Just had a 2.5 block and TD05 fitted to my UK00. Still running-in but I get 1 bar at exactly 2,500. Then it really starts to pull very hard

It's much stronger than any 2l I've driven, including a lot of time in a WR1 (lovely car, though) and would pull the **** off an FQ340.

Richard.

MR_MAJ

Guys i dont want to hijack this thread or anything but can i just ask how youd go about doing the 2.5 conversion? What do you need?

john banks

The search facility for "2.5" or "EJ257"

ozzy

hope you've got deep pockets http://bbs.scoobynet.co.uk/showthrea...&highlight=2.5

Stefan

madou

Do I remember correctly that you started the path to the dark side with just a TD04 hybrid on a standard 2.0 engine ...

Sarek

That's interesting. That is more like what I would have expected until I read about others' experiences.

Do you know of any technical articles about this sort of thing? Anywhere? I've done quite a bit of googling but have never managed to come up with anything useful.

I hate lag! Personally I don't see the point in having a car that goes like holy whatever from 5000rpm upwards but has very little below that. Fine for the drag strip but I prefer to use my car on the road.

john banks

It can feel better than it is. For example, hardly anyone drives an Impreza below 2000 RPM as they feel rough compared with many modern normally aspirated four cylinder motors. So if you start making significant boost even from 2000 RPM the car feels incredibly flexible as a road car. Note where VAG motors make their torque peaks as stock. I think that is how a turbo motor should be, trouble is we all get greedy for power and make them laggier, which makes them less linear, power bands are narrower, more difficult to control in low grip conditions etc. One man's meat, another man's poision. Hence why it is helpful to drive examples of cars similar to your planned mods to see if you get on with them. A nice 2.5 setup running 400 BHP can feel no less flexible than a typical consumer 16 valve engine, even the variably valve timed stuff that claims 80-90% of peak torque quite low down, but a good 6 or 8 twin turbo motor is truly inspiring IMHO.

Hoppy

Madou, no darkside but I've had my car since new and gradually tweaked it over the years. Yes, I had a TD04L hybrid on JB's recommendation and have been very happy with that, but you've got to move on

In fact, I've pretty much followed John's lead as we seem to be Scooby contemporaries and our cars are (or were once) identical. I've still got some of John's cast-offs under the bonnet but he's left me way behind now, mad power-junkie that he is

I also moved to the 2.5 on John's suggestion, after toying with costly 2.2 and 2.33 options. Then the American 2.5 block became available at a really good group-buy price - like £1,200 delivered. Whichever way you look at it, if you want a lazy motor with plenty of torque, this is the way to go.

I'm presently limited by the 5-speed gearbox which cannot take much more than 300lbs/ft, but the upside is that it stops you from getting a masssive turbo with all the problems that John outlines above (not to mention potential engine failures). I'm hoping to map it for 320lbs/ft and maybe 350bhp, and I can tell you now that it will be a very easy to drive yet mightily fast road car

I need to put a few more miles on it yet, and then an EcuTek remap which will make it really sweet. Then I'll write it up in the Projects section - The Lazy Man's Guide to 350bhp. Just read ScoobyNet, buy the right bits when you spot a good deal, then get someone with spanners to bolt it all together. Job done, and not that expensive.

Cheers,

Richard.

Sarek

I totally agree!
I would love to do that but unfortunately I don't think there exists a car of the spec I'm thinking about

madou

Thank you for the post, did not mean darkside in any negative way, I cannot resist moving on either ! Look forward to your write up in the projects section

Sarek

Thanks for all of your posts so far people. Has anyone else got any experience of this? (boost threshold change with increased capacity and same turbos)