What does a "good" boost/rpm curve look like?

[amaurer]

I guess the ideal boost-rpm curve would have max boost instantly available at any RPM... but what would a good curve look like that is actually achievable on a 3B with a single turbo?

I shot a video of my boost gauge and tachometer on a 3rd gear pull from 1000 rpm. Here is what I got; would you change anything? Fuel setting is stock. The truck is very nice to drive; I can't feel any lag at all. (This was my MegaCheap build linky)

 

[mainlander]

What do you want the turbo to achieve? Low rpm boost or mid to high rpm boost? Most of u wan the beat o both worlds but it's tough to get it from a stock turbo.

If your happy with it you can just leave it. If you want to try a fee things turn up the full 1/2 turn. Then adjust the wastegate so you get more boost. Then just keep tweaking it fuel/wastegate till you are happy. I wouldn't go any more than 15 lbs of boost and really watch you EGTs. Honestly you'll really be surprised at the difference once your boosting over 10 lbs.

 

[amaurer]

I'm OK with the boost level, I'm more wondering about the shape. Assuming everything else is kept the same, would a different/better turbo make the boost come on earlier, or faster, or better?

 

[mainlander]

Faster is really the only thing that would change with a different turbo. easier an better aren't good way of describing turbo response.

Given you want to keep your settings the same a smaller turbo would respond sooner but that's about it.

 

[Dougal]

For touring I want maximum boost to be available at an rpm below cruise (which for me is 2000rpm).
This means if I hit a hill and drop the revs a bit I don't need to instantly grab a lower gear.

For offroad I want max boost as low as I can get it. I swapped recently from a T25 that had heaps of boost very low to a T25/T28 hybrid that doesn't hit max boost until over 2000rpm. I'm missing the little T25 right now. Sure the T28 hybrid has more top-end, but the low end punch isn't there.

 

[gerg]

You could try swappng out the exhaust housing and putting on a smaller A/R. That is quite easily done. Measure your back pressure before and after and, at that low boost, you shouodnt see a major increase (in back pressure), but it will spool much sooner.Is it a .63?

 

[amaurer]

Its a .49

I don't have a problem with it, per se. I'm just wondering what i means what people say "boost comes on early/late" - is it like an instant jump up to full boost, or a slow crawl, or what.

 

[mainlander]

Instant only happens with a blower but that robs HP.

The smaller the turbine wheel (exhaust wheel) and housing, the sooner the spool up (dependant on the engine displacement of course), the larger the compressor wheel and housing, the greater potential for higher boost. There is of course a balance between the two, and this depends on what type of driving you're looking to do.

My turbo in 4th, starting at 1000 rpm sees 1-2psi instant when I push the peddle down, as the rpm's climb, mine sees 5-6psi by 1500rpm and 12-13psi at 2000rpm. If I'm cruising at any speed and I'm at or just over 2000rpm and put the peddle down, It'll jump to 16-17psi then my engines exhaust pressure opens the flap on my internal waste gate as the actuator spring is not powerful enough to hold the flap closed for any more than 14psi. Now, my actuator arm is also pretty much maxed out so that even if it has opened from seeing 14psi, the waste gate flap is not fully open and more of the exhaust pressure is spinning the turbine, which in turn increases my boost output from my compressor. If I had an external waste gate, I'd be able to set it for 14psi and I wouldn't have the fluctuations due to a week actuator spring (or I'd just learn to be happy with 10psi and call it good, but that ain't gonna happen )

General road driving with highway performance (what I was looking for) wants to see (as Dougal mentioned) max boost reached at cruising speed (mine is 2100 rpm at 100km/hr) as when you see a hill, you can hit the throttle and your turbo should max out and offer the power you're looking for.

 

[amaurer]

General road driving with highway performance (what I was looking for) wants to see (as Dougal mentioned) max boost reached at cruising speed (mine is 2100 rpm at 100km/hr) as when you see a hill, you can hit the throttle and your turbo should max out and offer the power you're looking for.

OK, and when you say "max out" you mean reach peak and remain completely unchanging? Or is there still a slow increase in boost with RPM?

 

[mainlander]

Well, sort of... It sort of depends on the strength of the waste gate actuator spring. But generally yes, you should be able to hit and maintain max boost (set by your waste gate) instantly when at cruising speed.

Internal waste gates can flutcuate with exhaust pressures coming out of the engine dependent on the rpm. Higher rpm will of course see higher exhaust pressures which will push on the waste gate flap and if the actuator spring is not strong enough to hold the flap closed, the exhaust pressures will open the waste gate flap and dump some amount of pressure.
An external waste gate can alleviate this but generally they are reserved for higher boost situations, like 20-30psi or higher...

 

[Dougal]

How are your EGT's? More fuel will bring on the boost sooner, but higher temp is the downside.

 

[amaurer]

1050 or so, preturbo. My wastegate never opens.

 

[Hulsty]

Maybe its time for some fuel adjustments if you want to, will help to bring on boost faster.

For comparisons sake this is ROUGHLY my boost curve from memory during hard acceleration. A Graeme modified CT26 on a 13BT. My cruising rpm at 100km/h is 2200rpm