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Thread: adventures in turbo tuning
01-23-2011, 04:50 PM #1
adventures in turbo tuning
It's been awhile since I've posted anywhere. Been out of the country and bla-bla-bla. It is time to turn my attention back to my smart.
I'm starting this thread as a continuation to my "Udderly Custom" thread on SCOA. I though I'd do it here because SCOA and I have drifted apart. That's all that really needs to be said.
My goal is to walk through the process of tuning a car with an aftermarket turbo. I started this on my car, and think I now know what needs to be done to finish it. I also think I now have the tools. I may ultimately fail, but will document it here for the entertainment and derision of all who care to follow along.
This will probably take a few months to get through because I'm doing a lot of traveling for work lately, but I need to get this out in the public domain to put some steel in my spine to actually get started.
First, a recap of what I have now. If you've read the Udderly Custom thread on SCOA, this will all be review.
I have a 2008 Cabrio. Last summer I had Larry "Screaming Cow" LittoDvl install a turbo. The turbo was the base kit from Jack (www.smart-turbochargers.com) which has at its core a Garret GT12. I chose the GT12 because I wanted something that would spool faster and have not been disappointed with how the turbo gets on boost from low RPM.
In addition to the GT12, Larry custom fabricated a air-to-water intercooler, and also installed larger injectors and a Split Second FTC-1 controller from SFR. Larry did a great job making it all fit and work together.
Nevertheless I have felt that there was more in the system than the sum of its parts. My 1/8 mile times were nothing to brag about and the car occasionally "bucked" or "stuttered."
I started playing with the tune in the Split Second box and promptly fried the plugs. but I also don't think that was all me, but I'm getting ahead of myself.
To top off today's post, I want to show this teaser:
That's the end goal, and the graph is proof it can be done to a custom turbo'd smart.
Next installment is "why we need to tune." Hopefully sometime this week.
PS -- here's a link to the full res graph
Last edited by geosynch; 01-29-2011 at 08:26 PM.
01-23-2011, 05:49 PM #2
Thank you geosynch!
I am looking forward to reading the info as I am sure many others will as well. Glad to have you here and sharing your positive energy with the community.
01-23-2011, 06:39 PM #3
Got tired of looking at pyramids Geo ?
01-23-2011, 08:31 PM #4
Not tired of looking at pyramids, but eventually the credit card companies catch up. You can go on the lamb for only so long. ;-)
01-24-2011, 03:53 AM #5
I'll add my welcome and am looking forward to reading about the progress. I have learned a lot reading your other thread on the other site.GetSmrt
01-25-2011, 03:14 AM #6
Looking forward to your adventures, as I am a new enthusiast at this and am also working towards something that involves tuning.
01-27-2011, 08:04 PM #7
Hello again. Last time I posted I said I was going to discuss why tuning is required when an aftermarket turbo was added. And I will. But I think it'll take a few posts.
The last few days I've thought about how I might concisely state why tuning is required. I don't think that can be stated simply, other than "because I said so" which is weak.
So, I think I'm going to approach this in steps. I hope I don't bore people, but it is important we get the science behind it correct.
So, step one is a discussion on stoichiometry. From Wikipeida:
Stoichiometry is a branch of chemistry that deals with the quantitative relationships that exist between the reactants and products in chemical reactions.
In this case it is octane (gasoline) and the oxygen that is in air. Let's peel back another layer of the onion. Again, from Wikipedia, for the reaction of hydrogen (H2) and nitrogen (N2), the stoichiometric ratio is exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of ammonia NH3, or, mathematically, N2 + 3H2 → 2NH3
For octane and oxygen, the reaction is a little more complex. Stated chemically, octane is CH3(CH2)6CH3 and oxygen is simply O2. But when you take 14.6 parts (by mass) air (of which oxygen is about 21%) and one part octane and you get perfect combustion, the result is:
2 C8H18 + 25 O2 ----> 16 CO2 + 18 H2O
Thanks to physics forum for doing the math for me.
Anyhow, the point is the the AFR of 14.7 has a basis in real chemistry. With perfect combustion, the resulting products are simply CO2 (carbon dioxide) & H2O (water).
So the stoichiometric ratio is, by one measure at least, the optimum ratio. But for modern cars with a catalytic converter, the stoichiometric ratio is important for another very important reason. If the mixture is too rich or too lean, modern catalytic converters simply don't work. Take a look at this graph:
which I got here.
All rolled up in to one, this entire post can be summed up as, 14.6:1 is the optimum ratio of air to gasoline really because of two (related) reasons. First because with perfect combustion, it yields the reactants of (just) CO2 and H20. Second, because catalytic converters simply stop becoming effective as the AFR becomes too rich or too lean.
Next post; why the correct AFR is 12:1 under boost.
Last edited by geosynch; 02-08-2011 at 09:45 PM.
01-29-2011, 01:43 PM #8
12:1 under boost
Anyone who has been following the turbo thread on SCOA (or anywhere else) knows that under boost the AFR should be in the range of 11.5:1 to 12:1.
Why is that?
I confess that getting a clear answer was elusive. I'm not questioning that it is the correct answer, but the goal was to find a good scientific reason. The best AFR being 14.6:1 for a n/a engine has a good solid scientific foundation. I couldn't duplicate that for under boost; at least not with only a few hours of internet research.
What I did find is summed us thusly.
As the AFR gets leaner than 14.7:1 the flame burns hotter. Anyone who has stoked a fire or has trimmed an acetylene knows that adding oxygen to the combustion process makes it hotter. So as less fuel is in the mix, the flame is hotter. I think this is really a secondary effect, but confess I'm not positive.
Primarily the act of compressing air makes the temperature go up. Even though we aren't dealing with an ideal gas, the Ideal Gas Law is still a good model for air -- the temperature of the air goes up proportional to the pressure. I'm pretty sure this is the major issue for forced induction. You not only have the piston compressing the fuel/air mixture, but also the turbo.
As temperatures go up, the risk of detonation goes up. Detonation is when the air/fuel mixture ignites (due to high temps in the combustion chamber) before the spark from the plug does. With detonation, there are two flame fronts (one from pre-combustion for lack of a better term and one form the spark plug). If this condition persists it will ultimately cause piston damage, valve damage and spark plug damage. Modern engines have knock sensors that will retard the timing when detonation is detected, but this will only take you so far.
Enriching the AFR for a forced induction motor is the primary weapon to avoid detonation. Why an enriched mixture cools the temperature in the combustion chamber is where I'm a little fuzzy. I've read three different opinions, and frankly none of them were very authoritative, but they all sounded plausible. I suspect it is a combination of all three phenomenon:
1- Burning cooler because of the oxygen content is lower than 14.6:1
2- The extra fuel adds a boundary layer between the metallic surfaces
3- The fuel is denser than the air and isn't nearly as susceptible to increases in temperature as air
BTW, the boundary layer is what keeps rocket nozzles from melting... Just thought I'd throw that in there.
Bottom line is that under boost, the AFR should be about 12:1 because we're keeping things cool to prevent detonation. Detonation will destroy an engine.
To sum up the last couple of posts:
Under non-boost, AFR should be 14.6:1. Under boost, the AFR should be 12:1.
Next up: How do we do that?
Last edited by geosynch; 02-08-2011 at 09:46 PM.
01-29-2011, 06:44 PM #9
Geo, one correction to make. the ignition is retarded when detonation is detected,not advanced per your text.
01-29-2011, 08:24 PM #10
I knew that! Dunno what made me type it ass-backwards.... I fixed it in the text though.
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