2.75" vs 3" pipe on supercharged vehicles.

[SS_Firehawk]

So I was helping a guy with his Y pipe issues here on this forum the other day. His 3" Y pipe terminated to a 2.75" or 70mm mid pipe. I basically told him it's an insignificant difference on a bolt on car. So over the past couple days I've been searching to find an answer for my own car.

So as many of you all know, my exhaust system is not of the same manufacturer. PPE's mate to the Motordyne Y pipe at 2.5" The Y pipe is also 2.5" and when it converges at the Y and increases to 3". A 3" 200 Cell HFC is connected to the Y pipe. My Tanabe mid pipe is 2.75" and connects to the Motordyne HFC that is 3". Exhaust is then split to two 2.5" all the way out to the tail pipes.

Again, many already know twin superchargers are being installed. 475-525whp is expected. I can't find anything that tells me if this will pose a problem at higher rpm. I understand the Rotrex superchargers like some back pressure to get them up to speed, but is a single 2.75" pipe going to flow enough to keep the top end happy? I'm not stressing about it too much right now until I get the results, but this is something I'm watching closely. I'll send an email to GTM just to see if they think.

Has anyone else seen any tests on this? If it's causing a big restriction, I may have to part with my beloved Frankenstein exhaust.

Edit: Attached exhaust pics. Notice on the mid pipe to HFC connection, the mid pipe is visibly smaller.

[SS_Firehawk]

Doing some basic math, granted I'm no mathematician, the 3" is 15% larger than the 2.75". To go a little deeper, I wanted to find the area of the inner diameter of the pipe to get a more accurate measurement. so I took the known radius, divided by 2, subtracted the thickness of the wall, squared it, then multiplied by pi. For reference, the Tanabe Medallion wall thickness is .047" and the Motordyne wall thickness is .062".

Results:
3" = 6.49sq in.
2.75" = 5.54sq in

With it converging to a 2.75in mid pipe, I'm a bit worried about it. I'm sure the long tubes are helping compensate for the smaller piping, but I have my reservations that this will be enough. I saw a chart on line but it seems to overestimate the pipe size necessary. What is known is that at about 550whp, 2.5" dual exhausts starts hitting a wall. Combined area for a dual 2.5" is 8.86sq in. My hypothesis... I need a different exhaust

[Sh0velMan]

Originally Posted by SS_Firehawk

...

Again, many already know twin superchargers are being installed. 475-525whp is expected. I can't find anything that tells me if this will pose a problem at higher rpm. I understand the Rotrex superchargers like some back pressure to get them up to speed, but is a single 2.75" pipe going to flow enough to keep the top end happy? I'm not stressing about it too much right now until I get the results, but this is something I'm watching closely. I'll send an email to GTM just to see if they think.

...

In a word: No.

Way, way too small for that power level.

[Sh0velMan]

Originally Posted by SS_Firehawk

Doing some basic math, granted I'm no mathematician, the 3" is 15% larger than the 2.75". To go a little deeper, I wanted to find the area of the inner diameter of the pipe to get a more accurate measurement. so I took the known radius, divided by 2, subtracted the thickness of the wall, squared it, then multiplied by pi. For reference, the Tanabe Medallion wall thickness is .047" and the Motordyne wall thickness is .062".

Results:
3" = 6.49sq in.
2.75" = 5.54sq in

With it converging to a 2.75in mid pipe, I'm a bit worried about it. I'm sure the long tubes are helping compensate for the smaller piping, but I have my reservations that this will be enough. I saw a chart on line but it seems to overestimate the pipe size necessary. What is known is that at about 550whp, 2.5" dual exhausts starts hitting a wall. Combined area for a dual 2.5" is 8.86sq in. My hypothesis... I need a different exhaust

Your hypothesis is correct.

Power can be boiled down to airflow. Airflow can be (roughly) estimated by using square inches of orifice area just like you've done here.

A single 3" is sufficient for most N/A builds where your peak power output is around 350-400hp at most.

You could do a single 4" most likely, or a 4.5" but routing a pipe that fat will be annoying under a Z.


A true dual setup running at least 2.5" will serve you best, or a single of at least 3.5-4". (If you can make it work, I'd love to see a big honking 4"+ single on a Z, and hear what it sounds like)

[Sh0velMan]

Oh and your figure for 2.5" dual is a little off.


2.5" Dual = 9.817 sq inches
2.75" single = 5.939 sq inches
3" Single = 7.068 sq inches
3" Dual = 14.136 sq inches
3.5" Single = 9.621 sq inches
4" Single = 12.566 sq inches
4.5" Single = 15.904 sq inches

Edit: re-read your post, I see where our numbers are different, you're subtracting wall thickness. Pipe diameters should be ID rather than OD. So subtracting thicknesses should be unnecessary.

Edit 2: And the reason they over-estimate pipe size is because most of the flow is happening at the outside perimeter of the pipe, so a general rule of thumb is to add 10-15% (with the percentage being smaller the larger the pipe diameter) to the pipe size to allow for turbulence losses at the center of the pipe. Sounds silly, but it's been worked out many years ago by folks a lot smarter than us hahaha.

[SS_Firehawk]

I included the .062" standard wall thickness. for the dual 2.5". Tanabe uses .047"

Edit: had to edit my post because I saw you saw that I included the wall thickness lol

[Sh0velMan]

Originally Posted by SS_Firehawk

I included the .062" standard wall thickness. for the dual 2.5". Tanabe uses .047"

Yeah I caught that after the fact, tubes in my experience have been sized at the ID though, not OD. That could of course vary depending on manufacturer, material and intended application.

[Sh0velMan]

I also should add that since you have the parasitic losses at the center of the pipe, running a single that has "mathematically lower" area doesn't necessarily mean it will flow less than a dual with a moderately larger mathematical area. For example a dual 3" vs a single 4".

I doubt in most setups you'd see a measurable power difference between them, all other things being equal and Y pipe/Merge collector design being optimal. I know several guys locally that run a 4" flattened pipe rather than 3" duals and they all indicate identical power response while saving significant weight.

[elperuano]

Lotta work n different parts for an exhaust system. Hope it all works out for you.

[SS_Firehawk]

Bringing this back again as I was doing a bit more research on it.
I was trying to determine the maximum flow rate of different exhaust diameters as well as HFC diameters.

Found somewhere in the internets that a good estimate for flow rate per diameter of tubing is 115 x sq in. Not sure how truly accurate it is, but cross referencing with what manufacturers are quoting, it's within .05%. I included dual 2.5" results because I know that it can support the power requirement.

So some quick number crunching gave me this info
dual 2.5"= 1128CFM
single 2.75"=683CFM
single 3"= 812CFM

Now with regards to the flow rate of the Motordyne HFC that I have... It's 3" going in, expands to 4", then back to 3" and is 200 CPI. I found one test with an Xforce 200 CPI 3" cat with a 4" body that hit 750 CFM. XForce HFC. I've seen lower numbers on other cats but there isn't as much info about the testing methods, most between 478 & 596 CFM. Motordyne claims very minimal difference in power output between their straight pipe and HFC, maybe 2-3whp. So I'm leaning towards the 750 number.

So with this information, is the 3" HFC the larger restriction or the 2.75" mid pipe?
The more and more I think about it, I question if this exhaust can handle the 500whp I'm hoping the car will make. The only thing that gives me a glimmer of hope are the results GTM achieved with the stock exhaust and their SC kits. If it doesn't, I'll probably jump in that Fast Intentions line, add mini cats to the Y pipes and 12" resonators. Hate selling parts tho...

[SS_Firehawk]

Some more number crunching. Here are the maximum flow rates for the C30 supercharger range. Keep in mind, my car will have two... Not entirely sure which size GTM is using yet. This should be a good indicator of what range I should be looking at when running them. If these stand true, I think I'm F'd... Keep in mind these are maximum values (blowers spinning at maximum rpm) and will only see restrictions high in the rpm range.

C30-64 = 450.268
C30-74 = 484.904
C30-84 = 519.54
C30-94 = 640.766

I converted the numbers from KGs to CFM
Reference link: Rotrex C30 Technical Datasheet

[Nut_N_Much]

Would you be interested in a full 3 inch FI system ? I have one that just needs tips. Man you have really crunched numbers.. I wish I could add to this but my experience is from watching or building an exhaust because the Teams Engineer wanted it that way. Tom would say the back pressure helps with the torque curve. Not enough gave high HP but lower torque numbers. Those were naturally aspirated V6 & V8 engines.

[SS_Firehawk]

Originally Posted by Nut_N_Much

Would you be interested in a full 3 inch FI system ? I have one that just needs tips. Man you have really crunched numbers.. I wish I could add to this but my experience is from watching or building an exhaust because the Teams Engineer wanted it that way. Tom would say the back pressure helps with the torque curve. Not enough gave high HP but lower torque numbers. Those were naturally aspirated V6 & V8 engines.

Lol what happened to the tips?
Resonators?
The exit diameter of my LTH's are 2.5", not sure if running 3" will make a diff over 2.5". Might flow too well for the SC's to get into it's efficiency range. I'm walking a tighrope of providing enough flow, but not to the point where it's detrimental lol. I think my current exhaust will be great till about 6500-7k rpm, if power starts dropping off, I know for sure it's the exhaust. The stage 1.5's and stage 2 SC's keep pulling till redline with torque staying flat, even rising slightly.

[roplusbee]

I like the fact that you are actually doing your own research and not just blindly listening to what a manufacturer/builder is telling you. Since GTM is doing the work, you could have then test it with one of there 2.5" or 3" systems. That would give you a good indication of what you want without having to take a hit.

[Nut_N_Much]

Originally Posted by SS_Firehawk

Lol what happened to the tips?
Resonators?
The exit diameter of my LTH's are 2.5", not sure if running 3" will make a diff over 2.5". Might flow too well for the SC's to get into it's efficiency range. I'm walking a tighrope of providing enough flow, but not to the point where it's detrimental lol. I think my current exhaust will be great till about 6500-7k rpm, if power starts dropping off, I know for sure it's the exhaust. The stage 1.5's and stage 2 SC's keep pulling till redline with torque staying flat, even rising slightly.

Had to make the tips fit the Zele Kit. Its non-resonated 3 inch. That car now has a Zele Exhaust on it.. I'm supposed to pick it up this weekend but I had to go out of town. I intended to put FI tips on it and sell it. Would be a lot easier and cheaper to just sell as is. I wanted to install it on mine but I don't make enough boost to justify a 3 inch, I would loose HP with it.

Holy crap thats a huge oil cooler..