Exhaust sizing and flow rates

[Dreamkiller]

Hi everyone, im about to begin to fabricate my exhaust and wasnt sure about what size pipe i should use.
1. Twin 2.5"
2. Twin 3"

Basically the engine will be stock for 6months untill and a pair of turbos will be fitted. Due to this i only want to do the exhaust once.

My goal is to produce a minimum of 800hp at the engine.
I would like to use twin 2.5" so the car can be lowered a little lower and also the space in the engine bay it tight, so that half inch smaller will help alot.

Will a twin 2.5" exhaust support the intended power or should i just go the 3" pipes.

My theory is that people with 100hp+ single turboed v8's generally use a single 4" exhaust. so, using surface area =

1.Single 4inch
2 x pie x radius = 2 x 3.14 x 2" = 12.56sq inches

2. Twin 2.5inch
2 x pie x radius x 2 = 2 x 3.14 x 1.15 x 2 = 15.7sq inches

3 Twin 3inch
2 x pie x radius = 2 x 3.14 1.5 x 2 = 18.84sq inches

What would you do??

 

[R-Jay]

would you really want to drive around for 6 months with an inefficient exhaust?

otherwise for the power you're looking for, i think option 3 would work.... but i think you might want a cross pipe somewhere in the middle....

just wait till jbrady has his say, hehe

 

[Dreamkiller]

My previous post should say "people with 1000hp + not 100.

It couldnt be as bad as the standard exhaust so it wouldnt bother me.
Ive made one side of the extractors so some gain will me made here.

 

[xirforever]

If you had twin 3" exhaust it would be a lot worse than stock exhaust for that 6 months. One reason, backpresure, you would lose a load of torque, and our cars dont have much to begin with

 

[JBrady]

Not to flame you but... your math is wrong...

Pi times radius squared = area

2.5" pipe radius is 1.25 so... 1.25 x 1.25 x 3.14 = 4.9 square inches of area.
3.0" pipe radius is 1.5 so... 1.5 x 1.5 x 3.14 = 7.065 square inches of area.
4.0" pipe radius is 2.0. so... 2.0 x 2.0 x 3.14 = 12.56 square inches of area.
Dual 2.5" pipes would therefore be 9.8 sq/in area.
Dual 3.0" pipes would therefore be 14.13 sq/in area.

While dual 2.5s would work OK... it will not be what I would run. I would rund dual 3" at 400hp per pipe. In fact my 250hp turbo engine responded nicely with a 3.0 upgrade from 2.5". This is ALL for TURBO ONLY. Naturally aspirated should use the SMALLEST diameter that gets the job done.

Another thing worth considering is wall surface area. Pipes flow highest near the center and slowest near the walls due to drag so less wall area for any given flow area will flow higher. A 4.0" pipe also has 12.56" of surface wall. Dual 2.5s have 15.7" of surface wall (actually a little less for both as the ID is smaller than OD but this is for generalization anyway).

Now, I must ask what is the hurry? You will have to fabricate downpipes with your turbosystem so you will be doing some exhaust at that time anyway. In the meantime you may want to merely build the back half of the exhaust. If the rest of the engine is stock this will keep more torque than changing it out now. If you just want to change it anyway I would build the back half out of 3.0 inch and have a muffler shop make a cheap dual 2.25" front system with a cheap X pipe for now.

One more consideration is that http://www.drgas.com/product.asp?id=24 carries oval tubing mild steel and http://www.burnsstainless.com/OvalTubing/ovaltubing.html has oval tubing in stainless and that will allow more ground clearance. You will use larger oval tubing for a given area and you will have more pipe surface for that area but you will get your flow area with more ground clearance.

 

[Dreamkiller]

Sh%t it is too, dunno what i was thinking at the time! sorry.

I am considering doing the 2.25 " pipe from extractors through cheap 2.5" cats and crossover then into 3" pipes through 3" resonators and mufflers.
That way i only need to buy new cats.

Another option is to use the full 3" and incorporate a valve of some sort to create some back pressure, and effectively have a 2.25" exhaust.

 

[JBrady]

Sh%t it is too, dunno what i was thinking at the time! sorry.

I am considering doing the 2.25 " pipe from extractors through cheap 2.5" cats and crossover then into 3" pipes through 3" resonators and mufflers.
That way i only need to buy new cats.

Another option is to use the full 3" and incorporate a valve of some sort to create some back pressure, and effectively have a 2.25" exhaust.

Dreamkiller and xirforever (and anyone else still using the term "backpressure") please read my posts on this subject. Backpressure is BAD... small pipes create velocity. Using 3" pipes with restrictors will not do this.

Now, I would recommend starting with metal substrate catalysts. They are MUCH better flowing than standard ceramic cats and are now inexpensive at only $135 each. The core size is the same for 2.25, 2.5 and 3.0 inch so you can buy the 2.25 and easily modify them to work with a 3" system when the time comes. http://www.rpmoutlet.com/powercat.htm

So, from the extractors you want 2.25" pipes into the catalysts, followed by a decent 2.25" x-pipe, I would run several feet of 2.25 off the x-pipe and then you can use 3" mufflers and tips. When you do your turbo you will run 3" downpipes to the same catalysts (modified for 3" in/out) and then 3" pipes to the mufflers.

 

[gowest]

This is my HKS dragger muffler I measure it today and the mid section is 75mm or 3 inches and the rear dual pipe is measure 60mm or 2.5 inches.

 

[JBrady]

This is my HKS dragger muffler I measure it today and the mid section is 75mm or 3 inches and the rear dual pipe is measure 60mm or 2.5 inches.

gowest, thank you for posting the picture and the measurements of that exhaust system. I am assuming it is for either an LS400(Celsior) or SC400 (Soarer).

That said I must make some clarifications. I commonly see metric exhaust tubing in the 50mm, 60mm, 75mm sizing. I do not know if it is easy to find in between sizes such as 55mm and 65mm. I say this for a VERY good reason... 50mm is 1.96", 60mm is 2.36" (not 2.5") and 75mm is 2.95". These sizes do not provide for many choices. 55mm would be 2.16" and 65mm would be 2.56". SAE standard sizes give 2.00", 2.125", 2.25", 2.50", 2.75", 3.00" (with 2.125 and 2.75 being much less common). IMO most people us much too large of exhaust pipe and sacrifice low and mid range torque/power/response.

Now... since the stock pipe size is 50mm with (on my LS400) a center resonator that is also ONLY 50mm OD and the rest of the single center pipe at 60mm... we should look at how much of a difference the HKS system really is. To do this lets look at the total cross sectional area there is between these sizes. Lets also use the ID (inside diameter) as that is what the gases actually flow through.

To increase from a 50mm pipe to a 60mm pipe you have increased the flow area by almost 50% (47.4%) and this will DRAMATICALLY reduce gas velocity and subsequently effect the scavenging. This usually kills off low and even mid RPM torque/power and may not even increase high RPM power (could even be less) and will definitely increase noise levels.

I have suggested that a good pipe size upgrade from the 50mm would be 2.25" and in fact I would like to test 2.125" but it is not commonly available. It is not JUST diameter but design of the pipes that determine flow capacity/restriction. Back in the carburetor days people would mill of the choke horn, polish the barrels and streamline obstructions... this would increase the carbs flow dramatically even though the butterfly valve size would stay exactly the same size.

My whole point is that the HKS exhaust looks nice but on a stock motor would probably decrease vehicle performance. It would be nice for a turbo or supercharged engine. Remember my LS400 makes about 300hp through a SINGLE 50mm center resonator and the GS400 is rated at 10hp/10tq higher with a single 2.36" (60mm) center resonator. Increasing to a 75mm center and 60mm duals is probably way to big for even these 300hp engines let alone the original 235hp 1UZFE engines.

 

[legacy191]

Thanks Brady for that insight... I just purchased a 2000 GS400 and one of the first upgrades I was going to do was upgrade the exhaust. Where I live at, everyone thinks I'm nuts just for thinking about it. I know the post is old but it has great insight on what i want to do...

Tony T

 

[JBrady]

Thanks Brady for that insight... I just purchased a 2000 GS400 and one of the first upgrades I was going to do was upgrade the exhaust. Where I live at, everyone thinks I'm nuts just for thinking about it. I know the post is old but it has great insight on what i want to do...

Tony T

You are welcome. I have not yet done my exhaust upgrade for many reasons but a big one is that I want to have a dyno facility ready to test both a 2.5" and a 3.0" center pipe on my 290++hp VVTi engine in my 1999 LS400. I would also like to test fit a GS400 stock pipe (it may bolt up to the LS400) as it has the larger center pipe (2.36" vs 1.97"). The exhaust is virtually the only difference between the 300hp/310tq rating of the GS400 and the 290hp/300tq rating on the LS400. If it does fit I may be able to have a full catalyst to center section Y pipe built for both models.

 

[legacy191]

I will Keep you informed on the outcome..


TonyT

 

[GSMnow]

I am a little off topic here, but since I will be building an exhaust system from scratch for a 1UZFE in a Celica, I was hoping for some JBrady wisdom.

Here are the facts. The engine will be a totally stock 1993 LS400 motor. If they fit, I was hoping to use a set of JBA shorty headers for a Tundra 4.7L
http://www.jbaheaders.com/headerdetail.asp?PartNum=2010SJT
The tubes are a bit large at 1.5 inch each, but not silly. They are tight to the block so I was hoping they might help some clearance problems. They claim a 30 hp increase on the Tundra. Will they be worse than stock manifolds? From there I was going to run either 2 or 2.25 inch pipes (welded up mandrel bends) to a pair of CatCo convertors. They are ceramic matrix, but fairly free flowing and about $80 each. I will look into the metal matrix also. My 83 Celica shares the floor with the 82-85 Supra, and so it has space for a cat on either side. 22RE on the left or 5MGE on the right. Putting the cats there will give about 18 to 24 inches of pipe from the headers to the cats. Assuming the cats are 12-14 inches long, it will then be about another 15 to 20 inches til I reach a "Y" pipe behind the trans. I am sorta forced into a single exhaust from there as the car is quite low and there is only one notch in the rear cross member to clear a pipe. I was planning on 2.5 inch after the "Y" and I already have a 2.5" DynoMax UltraFlow straight through stainless steel muffler from my previous turbo 4 banger.

http://www.dynomax.com/ultrafloss.stm

I believe I have a 14 inch long 2.5 inch center in side out unit. It fits nicely in the stock muffler location. This setup will have quite low back pressure abt should hold fair velocity. Looking at the DynoMax tech tips, they recomend 2.25 inch duals and 2.5 inch single for a 250 hp 4.0L motor.
What do you think? Will this system be too low of velocity? The car should be about 2800 pounds race ready with driver and has taller gearing so it will be turning a few revs moe than in the LS400.

Gary M.

 

[JBrady]

Your plan seems good. Cannot say about the JBA header fitment. Most likely you will have to modify one or more tubes. If it were mine and I was modifying the headers I would probably just make them into long tubes and buy/build/use good collectors. More time and money but should work well. As far as their claim of 30hp... hard to say... definitely WAY better than the stock manifolds. Your single after the cats should be fine. I would probably go ahead with the 2.25" pipe for the dual section.

 

[GSMnow]

Thanks, I did do a bit of reading before coming up with this plan, but it is always great to have an experienced person confirm the calculations. My turboed 22RE calculated out to about 300 hp at the crank. This was figured from fuel consumption and 2nd gear acceleration runs. Wheel power was in the 250 hp range. IT weighed 2700 pounds (without driver) and did 0-60 in about 5 sec. flat. I am hoping to get close to this with the 1UZ in CA legal trim. The JBA headers are CARB legal (on a Tundra) and the use of CatCo or Random Tech cats would be also as the car and engine both have over 100,000 miles on them. But my ace card is the conversion is being done in Illinois and I will bring the car out after it has passed the IL IM-240 test. This may get it grandfathered in, but I am going as close to CA legal as I can without being silly.

Gary M.