Turbo swap

MikeT · Post by **MikeT** » 16 Sep 2007, 12:43

I've read about turbos being a compromise between spool up time and max exhaust flow. i.e A small turbo will spool up quickly but restrict maximum exhaust flow whereas a larger turbo will take far longer to spin up but allow better exhaust flow. Is this correct?

What category would the xantia diesel turbo fall into? My guess is the former (small) ergo an exhaust restriction at full power?

deian · Post by **deian** » 16 Sep 2007, 13:28

that is correct, ans yes i would say the turbo diesel turbo's would be small so they can spool up quickly, they don't need to be big because the engine doesn't rev that high anyway.

my xm 2.0i turbo has a small turbo too, as do the saab cars

cars with twin turbo's have the best of both, the small one to help reduce the turbo lag time on the bigger one.

a good example would be mazda rx-7s

caveman_si · Post by **caveman_si** » 16 Sep 2007, 13:38

The xantia is fitted with either the garrett or the kkk turbo. The kkk is supposedly slightly smaller quicker spool but lee top end. Where as the garret is slightly bigger so a smidgen slower to spool up but more top end.
But there is next to nothing between them.

The main beneifit of the garrett over kkk tho is its easier ability to crank up the boost pressure. Its adjustment is easier to do.

AS for fitting another type of turbo it would be possible but you will get more guaranted performance by adjusting the boost pressures and improving the cooling of the air.

MikeT · Post by **MikeT** » 16 Sep 2007, 13:52

As long as it's not restrictive to the exhaust, that's what I'm really getting at. I hope to increase induction and cooling greatly and it would be ideal to sort any exhaust restriction when it's dismantled for repair.

deian · Post by **deian** » 16 Sep 2007, 15:06

I reckon the 2.1 is better for that kind of modification as it is a 12v engine, I would assume it has 2 exhaust valve per cylinder for better exhaust breathing.

If a turbo is better at restricting the exhaust then surely it means it will spool up faster? This is where a waste gate comes into play I think.

Post by **CitroJim** » 16 Sep 2007, 17:38

caveman_si wrote:AS for fitting another type of turbo it would be possible but you will get more guaranted performance by adjusting the boost pressures and improving the cooling of the air.

I'd agree si.

The turbo is a small A/R job and similar to that on the Activa engine. The beauty of a small A/R turbo is they spool up fast and at low engine RPM thus giving plenty of low-down grunt with very little lag. The penalty is that they run out of puff at about 5000 rpm and then start having a negative influence on performance by being restrictive on exhaust flow and heating the inlet charge as the turbo compressor moves away from it's area of optimum efficiency.

If you placed a larger turbo on the engine you'd end up with lag and a loss of low-down power compared to what you have. The high-rpm efficiency of a diesel is severly limited by pumping losses as a result of the high compression ratio on a diesel engine so the net gain would be very little or less. Pumping losses are significant as considerable power is nneded to overcome the resistance imposed by the compression ratio at high rpm. Diesel engines are best at low rpm and can never be screamers because of this so all mods have to be planned to make best power between 2000 and 4000 rpm.

Long ago I did all the calculations for the Garrett T25 (A/R 0.63) on the Activa, that showed it went a bit squiffy at 1 bar boost and at around 5500rpm. At this point it was heading toward surge conditions. My research showed very clearly that the best and easiest mod was to increase intercooler efficiency to reduce charge temperature as low as possible. The difference in calculated BHP between no intercooling and 80% efficient intercooling was quite staggering. This was for a petrol engine but I believe the theroy is just as applicable to a diesel.

So, keep your existing turbo, go for just under a bar of boost and up the size and efficiency of the intercooler. Also, a nice free-flowing exhaust helps enormously.

Dei, yes, the 2.1 will be a better bet being a 12V engine but it needs a pump swap though. The Lucas Epic is not amenable to tuning in the same way a Bosch pump is.

Yes, the wastegate dumps excess boost pressure and its very presence on the turbo shows the engine and turbo are capable of more than it is currently set for.