Author Topic: downpipe - optimum size? (Read 4984 times)

mk1vdub · « **on:** June 06, 2006, 06:56:46 am »

I've just had my universal 2.5" exhaust pieces delivered to make up my exhaust

however I'm not sure I can make a good enough downpipe in the 2.5" the 90 degree bend just isn't really tight enough - it needs to turn pretty quick to clear the gear linkage.

this is whats on there at the moment by the way - a total bodge with screw fitting gas pipes..!

the actual outlet on the turbo (k14) is only about 2.25" in diameter, is there any point in being bigger than this? obviously I'm going to be 2.5 for most of the rest of the way back as thats what I've got, but, I've got a bit of 2.25" which I've collected which will make the first turn from the flange perfectly....

do I persevere with the 2.5" or just use the 2.25" and go to 2.5" further back?

all opinions welcomed

regcheeseman · « **Reply #1 on:** June 06, 2006, 07:53:04 am »

Loving the gas pipe!

Where did you get the universal kit from and how much.

I'll be in the same boat too and already have been weighing up the options.

That 90 deg isn't mandrel bent is it, the 180 deg 2.5" stainless section I bought from powerflow looks to be a fair bit tighter bend.

The stuff from these guys looks good..

http://search.ebay.co.uk/_W0QQfgtpZ1QQfrppZ25QQsassZaQ5fsoundQ5fsolutionQ21

They were offering a kit

2 x 90 deg mandrel bends
3 x 45 deg mandrel bends
1 x 1.5m length tube

for £90 including p and p in 316 2.5" 16swg stainless

Full list of bits here... http:// www.dynoflow.co.uk

GypsyR · « **Reply #2 on:** June 06, 2006, 11:27:49 pm »

At the time I needed/wanted it I couldn't find any 2.5" pipe that bent tight enough either. I went with the 2.25" I could find down to the flex joint. At the flex joint I ran 2.5" all the way back. Though I've nothing to compare it to, I like it just fine and doubt I'll ever mess with trying to go full 2.5". It's working for me as is.

dubCanuck1 · « **Reply #3 on:** June 07, 2006, 03:44:25 pm »

What about bending it towards the firewall a little before sending it down? There seems to be a little more room back there to make subsequent bends.

Just a thought.

mk1vdub · « **Reply #4 on:** June 08, 2006, 06:58:22 pm »

Hey reg,

the bends are from jetex: http://www.jetex.co.uk/

cost me about £170 including an over axle piece specifically for the mk1, which is not listed as a universal piece but they took one out of a system for me! also a flexi which again they didn't list, but they use in a downpipe they do for vag 20vts, wich was nice.

as far as I know the 90 is mandrel bent?

yeah I actually found that place a while ago while resarching myself, and was certainly considering it, the reason I went with jetex in the end was they did the bends in aluminumised (aswell as stainless) which means I can weld it up myself by just grinding the top layer away, where as I'd have to get a few bits for the mig and some stainless wire to be able to weld the stainless, also jetex do 30 and 60 degree bends as well as the 45s and 90s, their pieces are also swaged at one end so will slot together, which I saw as a usefull advantage. So adding all those factors together swung it fairly heavily towards jetex for me.

GypsyR - thanks for your input, thats good to know

dubCanuck1 - yeah I've pretty much looked and thought about every angle down there, I'm sure I could do 2.5" I'm just wondering weather its worth the hassle...?

hillfolk'r · « **Reply #5 on:** June 08, 2006, 09:06:37 pm »

ive got a homemade 2.5 downpipe,its a little tight,but it fits,clears the shift linkage,no prob(wrap part of the dp,so it doesnt melt the shifter bushings) dont worry,measure twice,cut once

935racer · « **Reply #6 on:** June 15, 2006, 02:50:32 pm »

This is what you need my friend, wide radius 2.5" mandrel bent.

935racer · « **Reply #7 on:** June 15, 2006, 03:06:29 pm »

Oh yeah and I like to go a little bit bigger than the turbo outlet because the hot air is still expanding right after the turbo, it doesnt cool down until later, running a 2.5" dp with a 2.25" exhaust makes more sense than vice versa

hillfolk'r · « **Reply #8 on:** June 15, 2006, 05:44:01 pm »

sometimes summit has both 2.5 tight and"not so tight" radius manderal bends
tight is like 3 inch radius,the bigger radius stuff was like 6 inch,the 6 inch rad 2.5 inch pipe will fitthat dp looks like mine,but its nicer,cause its coated+nicer welds,i reused my orig turbo exh flange,welded right to it,and a stainless flex at the bottom of firewall(like a early dual dp)
also aircraft use slip joints to eliminate cracking,but i never had one crack on me yet,although it would be esy to fabone of those in the downpipe too
what about (i think they are called) vortex generators? little vanes to change the flow thru the pipe so it doesnt "barrell roll" down the pipe, thats the slow way for the exhaust to exit the pipe,but it "tumbles" out instead,thats faster
probably not exactly the correct explination,but im sure someone will pick this up and run with it a little more

935racer · « **Reply #9 on:** June 15, 2006, 11:03:34 pm »

My bends are a 4" radius pretty much the widest you can go without hitting the shift linkage.

jwspin · « **Reply #10 on:** June 18, 2006, 05:08:47 am »

the smoothness of bends and transitions is more important than the size. if you are going to go with a pipe thats bigger than the turbo outlet you should have a transition area. it causes turbulence when you go from a 2.25 to a 2.5 directly. same idea as port matching a head.

-jared

mk1vdub · « **Reply #11 on:** June 20, 2006, 06:20:50 am »

Thanks for all the replys guys,

yeah unfortunately the 90 degree bends I've just aren't a tight enough radius, and there really wasn't all that much choice anyway (i'm in the UK by the way)

935racer - that is definately what I need, but as I've already bought my bits, I can't really afford to spend money on it.

I could fit my 2.5" by putting a 'kink' in the gear linkage to clear the downpipe, but taking what jwspin said into consideration I think I'll use the 2.25" peice, and make a smooth transition up to 2.5" a bit further back.

now another question - where is the best place for the flexi?
I basically see two options, one coming vertically down soon after clearing the shift linkage, the other horizontally at the beginning of the tunnel....?

ta muchly

935racer · « **Reply #12 on:** June 20, 2006, 03:09:45 pm »

Quote from: jwspin

the smoothness of bends and transitions is more important than the size. if you are going to go with a pipe thats bigger than the turbo outlet you should have a transition area. it causes turbulence when you go from a 2.25 to a 2.5 directly. same idea as port matching a head.

-jared

Its actually not too important when we are talking about EXPANDING exhaust gasses comming out of the turbo than to a downpipe. If the exhaust was flowing the other dirrection and was "hitting" the edge of the turbo that would deffinately be a cause for turbulence. But as its moving the other way and expands so rapidly after the turbo it doesnt really matter. I have made and tested both 2.25" and 2.5" downpipes, as far as the butt dyno goes there wasnt much difference, but I lost about another 100 degrees in EGT temps from having that extra .25", and I am sure there was a decent decrease in backpressure, when I did this testing I unfortunately didnt write down my rpm vs. boost pressure :?

Anyways idealy in a perfect world I agree there should be a 2.25" smooth transition to 2.5", ( an exhaust adapter works perfectly), but the added cost with no noticeable difference makes it impracticle for most people.