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I have been looking around at a few turbos both new and second hand. I am wondering what is a good AR number to have for the 1.8 (NA8)

Reason I asked is that I have looked at several T25's/ T25G's and they have all had different AR numbers.

Isn't it the bigger the number the turbo won't really start making boost (considering the 1.8 motor here) higher in the rev range?

Zen I did a bit of reading ( Maximum Boost by Corky Bell )
and in there I found this graph
and the turbo I used on my car was a T25 with an A/R of .80

AR is a ratio not just a number.

Two turbos with the same AR but different wheel sizes will have completely different characteristics.

It all depends on how you want your engine to perform... Do you want plenty of power or do you want less lag and better response? Like the piccie above says a higher a/r will give more power and more lag and a lower a/r will give less overall power but the power will come on earlier in the rev range. This is all on the same size turbo. If you get a bigger turbo with the same a/r as a smaller turbo it will spin up slower and come on boost at higher rpm but give more power. So if you get a bigger turbo for a smaller engine you need to compensate by getting a turbo with a lower a/r if you want the boost to come on at about the same rpm...

There are also other factors that affect this like rotational mass, bearing type, and compressor to turbine sizing...

So as the T25's you are looking at are quite small you can afford to have a higher a/r and still have very streetable power. I would recommend from 0.8 - 1.0 a/r.

For most daily drivers having the boost come on about 3500-4000 rpm provides a good 'streetability', however people's preference will differ.

For me I want the turbo to come in a lot lower than that. Even looking at the dyno graphs of the AVO stage 1 for the 1.8, it looks like it comes in at around 2K. This would certainly make stringing together a set of tight winding mountain corners together a lot more enjoyable. Rather than coming on boost when you don't need it - ie entering the corner.

My personal philosophy is that I want mine with power in low to midrange and a reasonable boost in the top end. Driving in traffic - I don't spend my time upwards of 5500 rpm much at all. But I can see though, for those that take their MX5's to the track, then mid and top end would be preferable.

I've noticed on the HKS kit for the 1.8 MX5, the turbo specs are T25G 60T A/R0.64, while for the 1.6, it isT25 60T A/R0.63

So these turbo's should spool up rather quickly.

If a turbo comes in at 2000rpm, then it is far too small... However, if you rarely drive above 5000rpm, and you want a bit more torque/power in the bottom of the rev range, the turbos you are looking at in the 0.6 - 0.8 range are definitely the go. They will run out of puff high up though.

Once again, everyone drives differently, and you should get a turbo that compliments your driving style.

So you are saying a Garrett T28 with an A/R of 60 is going most likely going to die out on the top end?

saboteur wrote:So you are saying a Garrett T28 with an A/R of 60 is going most likely going to die out on the top end?

Depends on the wheel size and trim . there are so many variation for the Garrett range. I know that a GT25-60 ( like used on the SP's) will not losse performance in the top end on the 1.8 but it won't come on till 3200ish RPM

saboteur wrote:So you are saying a Garrett T28 with an A/R of 60 is going most likely going to die out on the top end?

There is a lot more to it than just a/r like Russellb has said. A T28 is bigger than a T25 so it will spin up slower (all other things being equal). A low a/r of 0.6 will help offset the larger size. This turbo sounds like a nice one to put on a 1.8 actually...

Russellb wrote:

saboteur wrote:So you are saying a Garrett T28 with an A/R of 60 is going most likely going to die out on the top end?

Depends on the wheel size and trim . there are so many variation for the Garrett range. I know that a GT25-60 ( like used on the SP's) will not losse performance in the top end on the 1.8 but it won't come on till 3200ish RPM

Spot on...

You can get a .80 AR turbo that spools quicker than a .42, it all depends on the frame and trim of the turbo, so you can't just compare ARs, like i said earlier it's a ratio not a number....

OK, misunderstanding here.

I wasn't talking about making full boost and torque at 2000rpm. Agreed, that would be just dumb and top end would most likely be non-existent or very unsatisfactory.

I was more leaning towards having the turbo just starting to make some boost at low rpm, NOT full boost, just Starting to make boost.

Just that I have seen some dyno sheets that show power and torque coming in with such a huge hit at around 3000rpm. OK, great if you are going in a straight line. But I would think this would be crap if you are trying to string together some tight mountain range corners which is one of the enjoyable aspects of driving an MX5, ripping it through some great right corners and revelling in the handling. Not going \"oh crap\" (and then doing it) because boost just came on big time at the wrong time going into the corner and having the back end step out to hit a guard rail or something.

Maybe what I have seen is bad tuning?

I have also seen the AVO dyno sheets and they appear to be more progressive in power and torque delivery.

Well that is fair enough and I understand those concerns. Most proper sized turbos will start to make boost around 2000rpm or so if that is when you plant it, but the power will come on fairly suddenly once it winds up to full boost which could be anywhere between 3000 - 4500 rpm. And the more power your car produces generally the more sudden the 'boost hit'.

As you mentioned it can be softened with good tuning, but only to a degree.

Turbo cars just take a lot more care with the throttle. In a normal mx5, the power is low enough that you can plant it in most circumstances and not break traction, making it fairly forgiving. But you have to be much more careful with your throttle positioning with a turbo and much more aware of your engine. That is my opinion anyway.

Turbos ARE for straight line speed. They won't make you go any faster through a corner, just out of one, and you have to be more careful as you accelerate out as well.

I think if you are concerned, get a smallish turbo (T25 size is good), Run it at reasonably low boost to begin with, and go to a tuner with a good reputation.