Starting to consider downpipe issues along with a turbo/injector upgrade this summer, hopefully. I just saw a pic of a 4" downpipe (forget the brand) on a 1st gen on another thread here a couple days ago. It appeared to be connected to a smaller (3-3 1/2") turbo outlet, such as on the H1C or HX35. Just a fitting flared right out to 4". What is the point of installing this downpipe if the turbo housing outlet is not at least 4"?

Seems to me your exhaust flow potential is going to be limited to whatever is the smallest restriction in the system. That potential flow will vary depending on where the restriction is located. The effect being more restriction towards the turbo or less flow restriction effect towards the tailpipe, taking into account the exhaust cooling as it travels farther from the engine, or something to that effect.

Am I getting this?

 

The sooner you can get to full diameter the less restriction there will be.

 

Understand. So what or where do we define full diameter? I guess what I was trying to get at in my usual obtuse manner in the first post is that it seems to me like the definition of full diameter would be the turbo housing outlet. No?

 

Think of this example.

Compare the backpressure after the turbine in these two situations:

1. Full 3" exhaust
2. No exhaust

They both will be exiting the turbo at 3", but the full 3" exhaust will give much greater backpressure. That's an exaggerated example though; going to a 4" downpipe will be far less dramatic.

 

I have some similar ??? , with my 4" turbo back system , [ an other post here now ] , something els to consider may be the venturi effect , [ gases passing through that reduced area speed up , so that poses the ? if you take out the 3 1/2" going to the 4" , is that going to reduce the spooling of the turbine ?

 

That's a good question... I wish we had some aerodynamically-inclined people in this thread.

When I personally go to a 4" exhaust, it will not include the downpipe. I'll upgrade to a 4" DP when I go to a turbo with a 4" outlet. I don't think it's worth paying $150 for a DP only to upgrade it later.

 

I dont know how the venturi effect plays into this, but I know I dropped 200deg peak EGT in my nonIc 91 when I went with a full 4" over the stock 3" system. It seems to me the faster you can reduce back pressure, the faster the turbo will spool, and thus get you more air into the engine.

Daniel

 

This is a very interesting discussion that requires some real thought. This is just my two cents to get the ball rolling and somebody might know more on the topic so please chime in. First you have to think about how exhaust gases exit a turbo. If no pipe was on the end of the turbine the gases would spiral out of the end and rapidly spread out in diameter like a tornado turned sideways. This means that the gases will be able to take up the cross sectional area of a pipe even if it increases in diameter rapidly and thus reduce back pressure. In terms of venturi effect in the dp we need to remember that the turbo will eliminate individual exhaust pulses from each cylinder. Therefore the spool up time will be more benefited by drastic reduction of backpressure than trying to maintain high velocity with a smaller diameter. If individual pulses were the case than you would want to evacuate them as quickly as possible to eliminate the negative pressure wave behind them from causing restriction to the next pulse and if there is valve overlap (from high duration cam) possibly reduce intake efficiency. This is the reason behind tuned length headers and is more of a naturally aspirated engine discussion. For us, as little back pressure as possible.

 

I fly helicopters and the only reason why Helo's fly is because of air movement in the right places. Yes if you look at the diagram from a cross view you can see that how the 3inch to 4inch bevel is helping the air be in a less restricted place. Letting the HOT molecules separate will let them cool off and when you have heat at one end and cool at the other end pushing it it will escape faster. Even if you just have a 4 inch downpipe added to a stock exhaust system (3") then it will lower EGT's and let your turbo spool up faster, because of the less restrictiveness when you hit the 4". If the turbo had a bigger exhaust housing then the spool would have to be bigger and it would take even more time for it to spool up.

 

Hi All,
I also fly Helicopters. PM me and let's talk.
As for the aerodynamics,
Realize that you need the small restriction to start with for spoolup. That only takes the distance from the turbine to the outlet. After that, you need the open spaces of the 4". Think AUGMENTOR.

Mike

 

Off topic a bit but I just have to know!! have any of you guys ever started it with the down pipe off? it must sound pretty cool!

 

In my opinion, I feel a 4" downpipe is absolutely NOT necessary until you install a 4" turbo, hx40, htb2 etc.

 

Very VERY loud!


That's my thought as well...

 

I did a test drive with just a down pipe and the 4x4 shift lever out. VERY, VERY LOUD, sounds great, but driving like that would not be fun.

 

The reason for the larger down pipe is to reduce back pressure as soon as it leaves the turbo. If it did not help at all then how do you explane the drop in EGT's and the faster spool up with the larger DP. On the same note look at the size of the holes on the exhaust manifold flange. 40's and lots of other larger turbos use the same flange. If you should not go any larger then the smallest part you could probbobly run 2 1/4" pipe. I dont think so. the exhaust has to move fast to get the turbo to spool and then you need to help get the gasses away as easy as possible.