This post is meant to be informative and helpful and is not meant to put any turbo builders or tuners down. But knowledge is power! Just this week I was sent a few screen shots and tagged in a few posts about some other turbo competitor customers complaining about turbo lag and expectations... I know the opposite also happens and have talked with many tuners/turbo builders about this subject and was actually asked to make this post for customers.
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So how much lag is normal? And do bigger turbos spool just like stock with the right combo? Tow just like stock? What should you expect as a customer? I have a few graphs and some data that should help shed some light on this subject.
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I know every turbo company strives to put out the best product and try to have an edge over the competition. There are pros and cons about each product but there is no getting around the fact that larger turbos spool slower and are less efficient at lower rpms. Much in the same way that smaller turbos run out of steam and can't make the top end power that a larger turbo can make.
Please don't muck up this thread with turbo bashing... I have ran and dyno'd them all and would consider most of the turbo builders friends. We all see the same issues.
Please don't muck up this thread with turbo bashing... I have ran and dyno'd them all and would consider most of the turbo builders friends. We all see the same issues.
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First off is tuning is going to make the BIGGEST difference. If your tunes are not dialed in and your tuner does not know what they are doing, then the info below is useless because your experience will likely be WAY WORSE if not dialed in. The examples below are assuming the truck is dialed in with good tuning, no issues/leaks, a 100% truck. Turbos can add a little lag (typically 2-300 RPMS) … crappy tuning can make your truck UNDRIVEABLE!
Check out the 1st graph. Same truck, no changes except for a tune. Not a live tune, but simply a different tuner. No revisions, nothing. Simply switching tunes (The tune that ran bad blamed the turbo and never asked for a datalog, they did send a few revisions but didn’t help). Crappy tuning can also hurt your turbo very quickly.
Check out the 1st graph. Same truck, no changes except for a tune. Not a live tune, but simply a different tuner. No revisions, nothing. Simply switching tunes (The tune that ran bad blamed the turbo and never asked for a datalog, they did send a few revisions but didn’t help). Crappy tuning can also hurt your turbo very quickly.
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In the 1st graph you will see the difference good tuning can make. At 2000rpms the difference in simply changing tunes added 250hp and 700tq, you would not believe they were the same truck before/after. Good tuning is the NUMBER 1 biggest thing you can do for your truck.
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So assuming the tuning is dialed in and everything is running right? How much lag is normal and can be expected?
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Check out the dyno graph attached comparing 3 common turbo options.
Red – 2011 Turbo with 2011 HPFP 450hp and 1035tq
Blue – 2015 Turbo with 2011 HPFP 521hp and 1113tq
Green – KC s1 Turbo with RCD Stroker 653hp and 1378tq
You can see the 2011 turbo spools up super-fast but also falls off quickly in power
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The larger stock 2015 turbo adds a little lag and gains about 75hp on the top end
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The KC s1 with stroker pump adds even more lag over the 2015 turbo but is capable of adding 200hp on the top end over the 2011 turbo when paired with a stroker pump.
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The main issue/problem people have is in the lower rpms. Look at the orange line showing that the KC s1 will lose 225ft/lbs of tq when compared to the 2011 turbo. That is a pretty big difference. Not only will there be a power loss with the larger turbo but it will also run less efficient, worse MPG, and higher EGTs in that rpm range.
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The crossover point is usually around 1800-2100rpms where you will start to see the bigger turbo perform similar… but the smaller turbo is still in its peak efficiency range in the middle of the compressor map. It's not until you get above about 2000-2300rpms where you will even start to see the advantage of the larger turbos. Above 2300rpms you will make more boost, less smoke, cooler egts, more power, etc.
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So you are essentially trading 225tq at 1500rpms for 200hp at 3000 rpms and adding about 200-300rpms in turbo lag at lower rpms
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If you are still reading this far, then I added 1 final graph. Same graph as before but with KC s2 with on a 2011 hpfp struggling to hold 1300us with a weak pump. The main take away is to see that the KC s1 holding rail at 1800us with a stroker pump will spool faster and make more power than KC s2 struggling to 1300us with a 2011 pump. So sizing your fueling/turbo properly will make a huge difference in the overall performance of your truck.
