if you had the choice in a kit car for a turbo application would you rather have the power higher up the rev range or down the bottom end with a little less lag?

i have the choice of exhaust housing size im swaying towards the larger as the car is light so loosing a little bottom end wont be that much of an issue i think and i would like the car to make peak power well up the rev range,

although im open to suggestions

thanks

with my turbo'd hillclimber, i made the decision on the grip available. When on List 1A road tyres, bottom end grunt in a light car is fun, but ultimately useless, look at the V8s out there, always slow on the clock for anything sub 50mph when they start to get some grip over the torque.
I ran mostly on super sticky soft Kumho V70As List 1Bs, and they were much better at dealing with the grunt low down, in fact i altered my setup so that the engine spooled earlier.

Now on full hilllcimb slicks and theres ooodles of traction, so it'll handle lots of low end and top end grunt, no issues.

for a road car though, a turbo that comes in with a bang is hard work and potentially ego and car damaging, ensure the guy doing the mapping is told to make the car drivable rather than going for 'pub' numbers. Dave Walker at Emerald always maps mine, and the pub numbers are hit within 20 mins of getting the car on the dyno, the next few hours is mapping out the nasty turbo spiky delivery. Makes the car much faster on track and much better to drive

turbo setups that work ace in the donor car, are often crap in a car half the weight, which are usually traction limited. Might be different if you have a dax Quadra i guess.

a smooth power delivery is better , even if the turbo spools earlier .

the T4 turbo on my locost doesnt come in with a bang - more of a wallop

quote:

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Originally posted by Volvorsport
a smooth power delivery is better , even if the turbo spools earlier .

the T4 turbo on my locost doesnt come in with a bang - more of a wallop

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if a mapper is being honest with their client, they will tell you that they just map to obtain correct AFR's throughout the rev and boost range, the turbo and wastegate is completely unconnected to the ECU unless you are using the ECU to control boost in which case you may be able to alter the gain (how quickly the wastegate opens) a little but thats about it. You can make slight differences in boost by running it lean or rich at certain points in the map but that doesnt give the best power, fuel economy and safety. Its all about the turbo size, turbine and compressor wheel trims, exhaust size/design and inlet track/restrictions. The only way to make a meaningful difference to bringing in the boost early is to run anti lag if your ecu has the capability, but its a bit brutal for a road / track car and is mainly used on drag cars to spool the turbo at idle when on the start line.


Go for the bigger housing if your engine has a little grunt when off boost you are likely to get a smoother boost curve if its not too big.

[Edited on 3-12-10 by Antnicuk]

i cant see any point in using an ecu that doesnt control boost electronically, anything else is half a job. You can bring the spool on earlier than with a traditional boost signal from the comp housing.
It also helps taking the boost signal from the plenum rather than the comp housing too, as its a true indication of what the engines seeing boost wise being post I/C. Often say 10psi at the compressor housing can be 2-3psi different at the plenum.
using a decent boost solenoid and wastegate (in my case external wastegate) allows the torque curve to be made much neater. i have 3 maps switchable from the dash, all 3 are very different to drive as are the torque and power curves.
The difference of a decent ecu, decent boost and wastegate control and mapping conduscive to be fast on track cannot be underestimated.

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Originally posted by CNHSS1
i cant see any point in using an ecu that doesnt control boost electronically, anything else is half a job. You can bring the spool on earlier than with a traditional boost signal from the comp housing.

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It doesnt really make a lot of difference if the boost is controlled electronically by the ecu or a good electronic boost controller, the result is the same, by adjusting the 'gain' you can make slight adjustments to how the turbo comes on boost. It is done by either holding the wastegate shut until late on when the max boost limit is reached and then allowing it to open quickly or by letting it open gradually as it would do if there was no electronic control, which builds the boost more gradually as the door/valve starts to open a little as boost is increasing. The ultimate boost reached if the setting is the same will be the same regardless of gain but by upping the gain to have the boost coming in sharply can often lead to boost spike as the wastegate doesnt open quick enough to relieve the turbine wheel of the exhaust gases.

Ultimately these are only tiny changes in turbo response regardless of how many maps you have saved in your ecu, the only way to get a desired boost curve is to get the correct size turbo and also the correct exhaust/inlet to compliment it. Sizing a turbo is a bit of a dark art unless its a common engine. On a cosworth, you can pretty much predict exactly what will happen with any given turbo, on a rotary, its a case of suck it and see........ hence i'm about to fit my third turbo.

ah, i see, you wanted it all, you must be female chaps then

yes, holding the wastegate shut rather than relying on the spring in the actuator to dit it, is what makes the difference, also using an external WG helps here, as due to their design they use the pressure to keep the valve shut as well as open it when necessary so much more accurate boost curve and less likley to spike.
on lots of setups, spring only OEM actuators are pretty ropey imho, fine for road use where the way it opens isnt that important, but on track can make the world of difference.

ecu control of boost is a must, anything else is a compromise. For sake of a boost solenoid and a metre of tubing, youd be mad not to use the benefits, oh, in my humble opinion of course.

The question posed was on boost spooling, not ultimate boost achieved, which as you say will be the same, irrespctive of method. Its a bit like comparing an Astra and an Aston Martin at 100mph, both achieve the figure, its how, and how quickly they achieve it, thats of interest.

as you say, these are small changes, wont bring a T4 in at 2500rpm on a 1300cc motor, but i was assuming (dangerous i know) that the turbo was pretty well suited and only the small change to the housing size was being discussed as per OP.
no amount of spooling trickery will help if the turbso not sized correctly, but if it is, then some of the suggestions here will help.

worth looking into split scroll turbos too, plenty on 'tinternet to keep all occupied.

a friend is running a protortype vari-vane Turbo Technics turbo on a nissan 1800cc twink, makes boost at 1500rpm, with full boost about 2,2k holding hard until 7,5k. Few teething problems which are being sorted, but sees just under 300hp before lack of traction on the rollers made readings pointless. It has no turbo'kick' and is much closer to the superchargers ive driven, pretty seamless relentless power from low rpm, but without the parasitic losses of some superchargers.
geoff at TT is running a pair on his monster Sierrra v6 sprints/hills car which makes boost at just very low rpm!

but im sure you all know of this anyway....

[Edited on 6/12/10 by CNHSS1]