Downpipe/hotside sizing formulas?

[tt335ci03cobra]

Does anyone know a relatively involved sizing calculator? One that takes into account displacement, rpm, turbo load, torque/hp etc? I'd like to upgrade my restrictive 2.5" dual downs/hotside to possibly dual 3.5" to properly breath out the current flow (800/800 16psi, 890/915 20psi) and in anticipation of larger turbos flow of 1000-1200whp/900-1100wtq(comp 62/60 tbb .70 a/r to replace the hp kits 57/48's .63 a/r journals)

I think 3.5" downs and hotside should be good with the current turbos but perfect for 1000-1200whp, thoughts or suggestions?

Current setup:

[email protected]/1, 375whp/400wtq na

-Modified dual in dual out hp twin 57 kit with 57/48 .63 a/r journal bearings.

-stock 03 cobra heads/cams but I'd like to port for 300-330cfm this winter and add mmr stage 2-3 cams/possibly refresh the rings/seals/gaskets/rotating assembly if compression falls under 165 in any cylinder by then.

Any thoughts or suggestions would be greatly appreciated. A few people have recommended 3" and some even said 4".

I know a lot of 4.6 guys run 3" very well but I think the 5.4 needs slightly larger but it's just a guesstimation. Thanks for any input.

 

[Turbo98]

I'm not sure there are any forumulas for that. The bottom line is the larger the better. So go as large as you can/want.

 

[tt335ci03cobra]

Ok good to know, thanks

 

[itSSlow98]

You really cant go to big on the downpipes, the name of the game is move it out as fast as you can. You can go too big on the cold side and uppipe though because you want velocity in those areas.

I would do 3.5"s for downpipes if it were me. I dont think youll see much if any gains with a 4 over the 3.5 so make it easier on yourself fabrication wise.

 

[tt335ci03cobra]

Thanks! I'm thinking I'll need custom headers as well, I used the area of a circle to get 1/3/4" primaries being equal to dual 3.5" piping and 1/7/8" primaries being equal to dual 3/3/4" piping. Only thing is I haven't done/can't do the volume maths of runner length vs down and hot piping until I take some guesstimates and measurements with the car on a lift...:-/

Do you think "mid length" (8-14") custom mandrel bent headers(manifolds) will be scavenged/flow properly through the sizes equal to them via the area formula of (d^2 x Pi x 2)/ 4 that I used earlier to find equalities? Would I need more dual exhaust diameter depending on primary lengths for proper flow? Ie more volume in a long tune 1/3/4" primary than a short tube 1/3/4" so larger diameter hot and down piping would be needed?


The setup I'm pretty sure on for now is either (all stainless/hot pipe wrapped, turbo blankets) 1/3/4 into 3.5 or 1/7/8 into 3.75 with custom e cutouts right after the down pipes but before the crossover (probably a custom narrow h or wide x) as well as egr and possibly back pressure bungs then same size piping all the way back, race style cats and high flow, possibly bullit style mufflers for weight/space/(will be super quite with turbos/cats for a traditional muffler) out to custom 4.25" tips older bassani slant tip style angled inward 30*

Solid size wise? I basically want emission/hassle legal with cutouts closed but early full dump open for max power. I think closed the cats only need to be good for low boost/5-600whp so I should be able to find some that can survive.

 

[Turbo98]

So did you make those power numbers you posted? 890rwhp from twin 57's is very good with stock heads/cams. I've never seen anybody make that power with those turbos-assuming they are the Garrett turbos that came with the HP kit. BTW, what do you mean by 57/48? Just curious.

 

[tt335ci03cobra]

So did you make those power numbers you posted? 890rwhp from twin 57's is very good with stock heads/cams. I've never seen anybody make that power with those turbos-assuming they are the Garrett turbos that came with the HP kit. BTW, what do you mean by 57/48? Just curious.

Ya, we got those numbers with 20.x psi on a 5.4 with a custom dual in dual out intercooler setup.

Here's one showing the backpressure killing it off at 5k or so. Mind you I'm at 4300ft so there's a 10-13% correction factor on these numbers depending on the day but we only have 91 so on 93+ the numbers would be very similar.

They made 864whp? on a stock 4.6 terminator with 24psiback when fastlane built the kit out of an hp racing tuner kit. That car made 700whp on a safe tune in Ohio on 93 on 18? Psi, here on a really safe/conservative tune on 16.x Psi with 91 it made an uncorrected 650?whp and corrected 756 on an 85* day with 14%

The numbers were somewhere around those but it's been years and I can't place them perfectly.

Only thing fastlane could have done any better on this kit IMO was exhaust, hot, and down pipe sizing. It's all 2.5" and even 3" would have been a great choice. With the 4.6 it would get minor back pressure issues up here after about 20 minutes of harder driving. Rare but occasional.

Here's a dyno pic comparing the 4.6's power band to the 5.4's, both on pump gas with the same boost

Early on, around 4k the 5.4 has a solid 110whp on the 4.6 but the exhaust chokes it to only 818 vs 756 (corrected 10-12% numbers) by redline.


Anyways if you think my numbers are too high because of correction factor, most shops up here correct 18-22% haha, I raced a "750whp" 8psi 2011 camaro ss on my wastegate spring tune (600corrected, 530~ uncorrected) and walked it. I read I think 4-5psi and he was claiming I lied/was a cheat and upped my boost, I said fine, and took him for a ride, showed him spring, 10, 13, and 17psi, he quietly shut the hell up after.

 

[Turbo98]

Ahh, well, I thought those numbers sounded high. There's no sense getting into a discussion about it but those turbo correction factors are pretty crazy. It's been proven they are way too high.

Anyway, I'm sure it will be a nice set-up.

 

[prostkr]

The last 5.4 build I worked on I used 1 7/8 primary tubes (it was with GT500 heads). They matched the port perfect once you oblong and square them to fit the flanges. I used 3" collectors into the turbos and transitioned up to 4" as quick as I could after the turbo. This car has twin 76mm turbos. Its in the early stages of testing, but so far seems to be working well. It made 1250 at 25psi through an auto. We shall see in the weeks to come how well the kit does.

 

[86merc]

For down pipe size a good rule of thumb is 1.25 times the turbine wheel diameter. Error on the side of larger tubing. I am sure the increase from the 2.5" will help you out.

You can check out TheTurboForums. And they can give you all kinds of formulas to calculate the tubing sizes. Much of it has to do with the heat and energy doing the work versus flow. By design the turbine impeller and housing as the most restrictive part of the system anyways. So huge headers and cross overs are not needed. With the overly large size pre turbo piping you lose energy (heat) due to gas expansion.

Again, you can calculate all that out if you have all the info and understand thermodynamics. But in the end you will find it just isn't worth all the work. haha Plenty of proven combos out there to compare. And talk to your turbo company. They have a lot of experience and are usually willing to share their findings.

 

[Turbo98]

Going larger and larger on the downpipes is going to start getting heavier too. Obviously you also have to take into consideration cost and space limitations and realistic power goals. I have 3" downpipes and they are good for over 1100rwhp through an auto. Your max power will be determined more by the size of the turbos-not so much by the displacement of the engine.

I couldn’t really follow exactly what you were saying regarding pipe sizes. What applies to a n/a exhaust application really doesn’t apply to a turbo application. You didn’t mention the up-pipe (pipe from the header to the turbo). You really can't equate the header primary sizes to the downpipe size (or the up-pipe size for that matter) because you have a turbo in-between. The exhaust is restricted and is forced through the smallest part of the system at the turbine wheel and that's where the energy is converted and the work is done. So exhaust flow before and after the turbo is different. Also, considering this, the up-pipe can be pretty small. In fact if I'm not mistaken, Precision's v-band inlet is only like 2" in diameter. So your exhaust "restriction" and major determining factor of back pressure will be at the turbine housing and you need to choose your a/r and compressor wheel size accordingly. As mentioned, opening up the exhaust after the turbo will free them up also.

Regarding the exhaust note…. On one of my previous set-ups, I had turbos, cats, and Dynomax Bullits. It wasn’t quiet. Just FYI (much louder in person)
Dyno '98 Turbo Cobra 4.6L with Full Exhaust - YouTube

Oh, also with cutouts, you’re downpipe size becomes a little less of a factor.

 

[86merc]

Aluminum exhaust, post turbo, is a nice upgrade to save weight as well.

 

[tt335ci03cobra]

Awesome info and help guys, thanks!!