Charge piping size/length and its affect on response and lag

[Aaron]

I've been told forever that the length and size of your charge piping will not have an affect, at least a noticeable affect, on a turbocharged car's boost response, lag times, and spool times. However logically, I can't help but think the complete opposite.

Now I haven't had the chance to test a car before and after adding 8ft of charge piping, but every system I see with over 3-4ft of piping also has an almost intolerable amount of lag. For example, a DSM with a FMIC. Maybe that's how they were stock, but even with the stock turbo the one I drove wouldn't spool until post 4,000rpm, and lagged for a good 3 seconds thereon afterwards. The STS LT1 I drove was the same way. Not saying correlation = cause here, but it isn't helping.

My 3.4l DOHC at 10psi, assuming 94% VE. I spent a lot of time, and money, making my charge pipes as short as possible, and have roughly 4" between compressor and intercooler, and about 18" between intercooler and throttle body. All 3" diameter. I know 2.5" would be more than sufficient, but my intercooler is 3", my compressor outlet is 3", and I reduce my 4" thottle body to 3". Plus, 3" mandrel bent aluminum is cheap (I buy cheap eBay intakes for CRXs and crap for their piping, then trash the rest of the kit).

Here's my math on it, tell me if I fucked any of this up.

Flow (CFM) = Displacement (L) X RPM X VE X Pressure Ratio / 5660.

3.4 X 7,000 X 94 X 1.68 / 5660 = 664cfm of airflow, at redline, at max boost. We'll negate the intercooler volume, as that will be constant and I don't feel like calculating it. So 22" of 3" ID pipe is:

pie X 1.5^2 X 22 = 155.5 cubic inches, or .09 cubic feet.

So at 664cfm, it would take approximately .008 seconds to fill my 22" of charge piping, at redline. Hopefully my math is wrong...However, we don't care about redline, we care about spool. So at 3500rpm, my engine is only making roughly 60% VE, and boost is at 10psi (Just spooled).

Airflow is reduced fom 664 to 212cfm, and the time it takes to fill my 22" of charge piping is roughly .025 seconds. Still very much so negligible. Now let's try 8ft of 3" charge piping, which isn't at all uncommon on a DSM with a FMIC or a STS rear mount. The volume is now 678.6 cubic inches, or .39 cubic feet. At 7,000rpm, it takes .035 seconds, at it takes .11 seconds at 3500rpm.

To conclude, at 7,000rpm on a 10psi 3.4 DOHC, there is a difference of .027 seconds in the amount of time it takes to fill a 22" versus a 8ft long piece of 3" ID pipe. And at 3500rpm, the difference is .085 seconds. So less than a tenth of a second when quadrupling your charge pipe volume. SOmeone tell me I f-ed up the math somewhere...

[The Dark Side of Will]

There are two effects that I think you might want to look at.

When you're looking at spool RPM or transient response (especially transient response), don't forget that the turbo is turning a low RPM at the beginning of the wait and a high RPM at the end of the wait. This means that you're average volumetric flow through the fill period is about half of the end-of-fill flow numbers that you're using.
The other thing to think about is that only some of the exhaust energy goes into compressing the air, and some of it goes into spooling the turbo. If you can get MOI numbers for your turbo from the MFG, then you can estimate what fraction of the turbo's power is going in to accelerating itself and what fraction into compressing air (using the ideal HP to compress air formula).

[p8ntman442]

check the math on the flow number because displacement x RPM dosent sound right. One Revolution does not suck in 3.4 liters. Thats your total displacement.

[Mach10]

Yeah, man... I'm pretty sure a 4-stroke engine doesn't pull it's displacement per revolution...

2 engine revolutions complete one full 4-stroke cycle, which at 100% efficiency would be 3.4L, c'rect?

[Shaun41178(2)]

3 inch charge piping is huge man. Damn. It will def have more lag then if you had no piping and intercooler on it. Will be more laggy then 2.5 inch piping too.

2.5 inch piping has no problem flowing enough air for 500 hp which I can't see you making more then with your boost levels

I would seriously consider 2.5 inch for you but you have your 3 inch already done so just use that.

[The Dark Side of Will]

Aaron, how did you derive the 5660 number? That's where the /2 would be to account for the engine being a 4 stroke.

Also, I think that 94% VE is a bit high.

[Pyrthian]

3 inch charge piping is huge man. Damn. It will def have more lag then if you had no piping and intercooler on it. Will be more laggy then 2.5 inch piping too.

2.5 inch piping has no problem flowing enough air for 500 hp which I can't see you making more then with your boost levels

I would seriously consider 2.5 inch for you but you have your 3 inch already done so just use that.

ya ya
using 3" instead of 2.25 or 2.5 means you are adding volume. volume needs to fill. yes, at this point - the amount is pretty small - but, if you are going for the instant - you want to have a little volume as possible, between the compressor and the throttle plate. and that means as short AND as small dia as possible.

[eHoward]

Whenever someone tells me something ludicrous, i say "No. you're wrong."

I've been told forever that the length and size of your charge piping will not have an affect, at least a noticeable affect, on a turbocharged car's boost response, lag times, and spool times.

[Kohburn]

there is no need for the charge pipe to be as large as the exhaust tubing after the turbo. the exhaust is flowing a much higher volume of air in a less dense state.

[crzyone]

I put a larger intercooler on my GTR. The stocker was 2.25" thick, my new one is 2.75" thick and on top of that the bars that channel the air are also larger.

I did not notice any change in response from my turbos, but I did notice a big difference in how much air my BOV's are recirculating when off the throttle. They are much louder than before.

I have 2 small T25 turbos with ceramic exhaust wheels for fast spool.

One of these days I'll get a video from inside the car. I will take a vid from a rolling start in 1st gear to show its response. The turbos make max boost around 3300rpm all the way to 8000rpm.

Of course more piping and larger intercoolers make for more lag but I don't think its a huge difference. Those STS turbos that mount at the back of a car or truck don't seem to have a bunch of lag either, and they have to deal with building exhaust and charge pressure.


Massive intercooler 3" piping to cooler and 3" out.