[RocktimusPryme]

Dyno testing on exhaust options are ubiquitous. You can find plenty of muffler testing, and Ive seen engine masters prove that you actually want a little length of pipe after the header collector to make max power.

But Ive heard it proposed that while, necking down the exhaust near the header (or restrictive bends) will kill a lot of power. That effect is reduced as you get further back from the engine and the exhaust has diffused.

I know some of you have done plenty of dyno work. So Im curious if anyone has real life experience on that? Does power care less about your exhaust choices at the rear of the car than the front?

Im putting together my drag n drive project. It has 3.5 inch pipe into 3.5 inch straight through dyno flows, but if I wanted to make over the axle tailpipes I think 3.5 inch tubing might be a tough ask. So I wanted to know how much power I would be killing if I neck down to 3" after the mufflers.

I am going to put V band clamps on the header collectors for quick removal, so maybe I could have a stinger race pipe you could quick swap on, but I dont think the full 3.5" exhaust with mufflers would cost much, if any HP. So I would rather save myself the work.

[ponjohn]

Supposedly not because the exhaust gases are cooling.

[VCho455]

I've built exhausts that were 3 inches from collector through the mufflers and back to the vertical 90 degree bend in front of the rear axle where I reduced the pipes to a 2.5 inch 180 bend over the axle and then back to 3 inch pipes out the back. There was no real estate over the axle to make 3 inch pipes work without multiple sharp bends. I decided that necking the pipe size down would be better as it was a nice smooth radius as opposed to several sharp bends.

I don't have any dyno data to compare but both cars ran hard enough I didn't feel I left any appreciable horsepower on the table.

[RocktimusPryme]

Originally Posted by ponjohn

Supposedly not because the exhaust gases are cooling.

Thats kinda where I was too, a lot of supposedlies. What gets me though is we can for sure show that mufflers can kill power...... but maybe thats because chambered mufflers actually block flow rather than just reducing diameter.

Before Engine Masters went under I made a facebook request for ideas that they test both that, and to like a 4 foot straight three inch pipe, one with some mandrel bends in it, and one with the same bends but crush bent to see how that affected power.

[Neil Scott]

Depends on power level also, I was able to get 3.5" mandrel bent over the rear on my 67 Bird. The stock gas tank is narrowed 2" on each side and the rear spring perch in moved inward 1". I switched from the 3" when I put the 535 / 800hp motor in. I've since had to go back to the 474 back in. Haven't been back to the track yet. Street driving and have no issues with the tail pipes rubbing.

[scott70]

Years ago I was running my 70 gto with a 455 automatic combo while running all the accessories and weighing 4000lb with me in it. I was running 7.4s in the 1/8th mile with open headers and no exhaust pipes on the car. I wanted to install pipes at this time and get it back on the road so I asked Jim Hand on what he would do.He suggested running 3" X pipe and then use flowmaster reducer cones that necked down to 2.5" and 2.5" dynomax ultraflows mufflers. Then 2.5" mandrel bent Dr gas x tailpipes out the back. I put that exact system on my car and even though it added weight it ran the exact same 7.4's in the 1/8th mile just like Jim predicted it would. Took it to the 1/4 mile track and ran 11.7 at 115 through the pipes and iron heads.

[steve25]

By the time the exh gasses have made there way to getting out of the muffler the gases have cooled and in doing that they have contracted.

Less pipe area is then needed to move out the same volume of exh at that point.

The ultimate is to cut & raise the sides of the trunk to install the mufflers back there.

[Formulajones]

Ive done all kinds of exhaust experiments with dyno and track testing that would take way too much typing to talk about.

To answer the original question shrinking the tailpipe size, is a difficult question and not an easy answer. Power levels are going to have some affect on this so it won't be the same for everyone.

We have a few cars here where I run smaller tailpipes. 2 of them are over 700hp and one is close to 400hp.

I tried different things with all of them. One interesting test I did if you exhaust is in question is run electric cutouts. When done nicely using extensions and making a straight shot from the collector they can be beneficial. Strangely the 400 HP car responded well to the cutouts open and I e of the 700 hp cars didn't care, it was literally the same. I expected the opposite

In your case going from 3 1/2 to 3 likely isnt going to matter. That's how our 700hp cars are done. My 400hp car goes from 2 1/2 to 2" tails which was done for appearance reasons. That one responds to the cutouts open.

I would do what works for your application and not worry too much about it

[ta man]

How much horsepower?

[RocktimusPryme]

Builder estimated over 700. I personally don’t think it has enough cam for 700. But it’s 650+ all day.

And there may be ample room. It has a cell so no tank in the way. But room directly over the axle seems tight to me. We will see. I wanted to know my options.

[64speed]

Just did this. None felt on the ass dyno

[Scarebird]

Originally Posted by Formulajones

...My 400hp car goes from 2 1/2 to 2" tails which was done for appearance reasons. That one responds to the cutouts open...

How much does it respond?

I noted the sound volume is much different 2-1/4" vs. 2-1/2". I am not sure if sound volume is a consideration for the OP.

[RocktimusPryme]

Originally Posted by Scarebird

How much does it respond?

I noted the sound volume is much different 2-1/4" vs. 2-1/2". I am not sure if sound volume is a consideration for the OP.

Not really. This is my nostalgia straight axle gasser, 66 fairlane for Gas B under Dragweek rules. I didnt mention the engine/car combination in the original post because its not pontiac, but I think conversations like this transcend brand and are engaging for everyone. I hated it when I cranked it up first fire open header, but even now with just like a yard of straight pipe into the dynomax ultraflows, I feel like its an approriate level of loud for the type of car it is.

Its more safety, and the design of the floorboard.

1. I dont want to dump in front of the rear wheels, both because I dont want to smell it, and if disaster strikes and it dumps fluid out of the exhaust I dont want it to be on the tires.

2. This car has a clearly defined pocket in the floorboard meant to fit mufflers that are in a good spot to put pipes over the axle, if they fit.

Right now because the angle of the headers, the straight pipe makes the mufflers hang way too low, so it needs to go to an exhaust shop anyway because I hate welding in pie cuts. Somebody else can do it.

[JLMounce]

Although theorized around an incompressible fluid,, bernoulli's equation probably has some effect on this.

You're likely creating a pressure drop near, or inside the muffler before the neck down to 3" pipe. This will increase exhaust velocity upstream, nearer to the header which can actually help with savaging on a normally aspirated engine.

I had to ask AI about a lot of this as I'm not smart enough to completely wrap my head around all of this in an authoritative way, but it seemed to make some sense. On your setup specifically the choke point created by necking down from a 3.5" pipe to a 3" after a muffler likely creates no negligeable impact on power production. At least not in an rpm range that you're likely going to be operating in.

The caveats here are built around how much exhaust gas cooling actually occurs, most of which happens in the muffler. If you're talking about a small bullet style muffler that is just cutting the sound a bit, that's different than putting a 20" case muffler on the car. However it sounds like you're putting a fairly standard packed style muffler on the car. Unless you're spinning the engine to 8000 rpm, I doubt there's much loss in power that will be presented and certainly not so much that couldn't be overcome by taking out a bit of weight somewhere else.

[Formulajones]

Originally Posted by Scarebird

How much does it respond?

I noted the sound volume is much different 2-1/4" vs. 2-1/2". I am not sure if sound volume is a consideration for the OP.

9 HP on a chassis dyno.
The 2" tails also did cut down on noise compared to the 2 1/2

Interestingly my Chevelle making 786hp did not care. It's 3 1/2" through an "x" and a nice neck down to 3" going into 3" dynomax mufflers and 3" torque tech tail pipes. So I actually neck that down at the muffler inlet before tailpipes. 3 1/2" electric Cut outs are straight pipe off the collector and 20" long. Back to back runs at the track through exhaust and then cutouts open and it was identical. Within a couple hundredths. Literally ran back to back to back and 6 passes never varied more than 3 hundredths whether open headers or not. It just did not care. You'd think it would with that much power but not always the case.

[JLMounce]

Originally Posted by Formulajones

9 HP on a chassis dyno.
The 2" tails also did cut down on noise compared to the 2 1/2

Interestingly my Chevelle making 786hp did not care. It's 3 1/2" through an "x" and a nice neck down to 3" going into 3" dynomax mufflers and 3" torque tech tail pipes. So I actually neck that down at the muffler inlet before tailpipes. 3 1/2" electric Cut outs are straight pipe off the collector and 20" long. Back to back runs at the track through exhaust and then cutouts open and it was identical. Within a couple hundredths. Literally ran back to back to back and 6 passes never varied more than 3 hundredths whether open headers or not. It just did not care. You'd think it would with that much power but not always the case.

This probably tells you more that the 2.5" section of your exhaust system on your 400hp car isn't sufficient for the amount of power it makes, and necking down to 2" tail pipes added pumping losses that the engine can't overcome. Thus the cutouts are helping in that situation.

I have mandrel 2.5" pipes on my bird, behind the 580hp 465 and I know for a fact it's probably killing 20-30hp.

Your bigger hp cars are also street cars and I believe you've mentioned they have street car style exhaust systems, likely with sizeable mufflers. In this case your exhaust systems have done the work they need to probably by the time the exhaust gasses reach the mufflers and so having a smaller span of pipes out the back likely isn't having any effect on your higher HP engines.

[Formulajones]

I would agree a 2 1/2" system would be a choke point for 500+ HP.

On Dad's GTO making 724 HP it's a similar deal but his cutouts make a difference on the dyno.

His is much like my exhaust. 3 1/2" from headers into a dr gas "x" and the "x" is what necks down to 3" at the exit, then into 3" mufflers and 3" torque tech tailpipes. Pretty much exactly like mine but with mine knecking down right before the muffler and his knecks down with the Dr gas "x".

His electric cutouts are 3" grafted into the 3 1/2" pipe coming off the header collectors about 18" back on the dyno his car picked up power with the cutouts open where as my 786 HP engine didn't

I find that pretty interesting. There are other factors involved here we either don't consider or don't care on a street car.

[steve25]

Info from a David Vizard article states that to support 570 hp a muffler will need to flow 625 cfm @ 1.5 hg or 20.3 inches of water on a flow bench, and so will the tailpipe.

If a motor dyno’ed at 500 hp with open exh then two mufflers that flow 550 cfm will keep the hp loss to about 5 hp.

To equate to open pipe hp of a motor the whole system , collector to end of the tailpipe will need to then flow 2.2 cfm per hp.

A pressure wave terminator box will make everything down stream appear invisible to the motor, but the minimum volume is 8 Xs that of one cylinder up to a max of 15 Xs.

[HWYSTR455]

For a car that will see considerable track time, I think cutouts are a great solution. You can experiment with sections of pipe on the cutout for fine tuning too.

I have 2.5" the whole way on my 535 (GTO), but am considering swapping out the mid-section w 3". For what I use the car for, mostly street with maybe an occasional track run, isn't really making it a priority at the moment.

I like the twin tips in valance cutouts, think the 3" replacement ones don't look 'proportionate', and not many options for tailpipe size choices. 2.5" is what will stay.

Not really interested in cutouts myself, I can live with the loss of power, there's enough spare in my personal opinion for how I use the car. But I'm pretty sure I will go to a 3" mid section. I know it will help, and for reasons stated, not concerned with exactly how much of a difference.



.

[JLMounce]

All valid points. There's a lot of reasons to stay with a smaller exhaust system if you don't specifically need it, even knowing the power loss.

In my case I keep the 2.5" system in place for ground clearance and noise suppression. My frame sits right at 4" from the ground and with the 2.5" system, the frame is still the lowest part of the car. I'm also able to get tail pipes all the way to the back without needing a narrowed tank.

Since it's primarily a street car, it's traction limited as it is now, without an additional 20-30 hp.