Exhaust System Design

[FlowSpecialist]

As requested in the exhaust flow question thread here is an article on my own rules for exhaust system design for primary pipes, secondaries and muffler flow.

All pipe sizes referred to are internal pipe diameter because this is the appropriate measure for determining flow characteristics. The o/d for imperial pipe will usually be 1/8" larger because most tubing is 1/16" wall thickness.

PRIMARY PIPES

For primary pipes (headers) the chart below is based on the power output of a 4 cylinder engine or one bank of a V8. If you want to convert to another engine configuration then divide by 4 to get the power per cylinder.

Header I/D...Power
1 1/8"............125
1 1/4"............150
1 3/8"............185
1 1/2"............220
1 5/8"............260
1 3/4"............300
1 7/8"............350
2 0/0"............390

If the secondary pipes are a 4 into 2 into 1 configuration then the I/D of the secondary pipe should be 25% larger in diameter than the primary and the collector should be 50% larger in diameter than the primary or use the muffler chart below.

If the secondary pipes are a 4 into 1 configuration then the I/D of the secondary pipe or collector, which will also be the exhaust system pipe for a 4 cylinder or one bank of a V8, should be 50% larger I/D than the primary or use the muffler chart below.

For the final system pipe size of a V8 where both banks join together to go through a single muffler or set of mufflers then refer to the chart below or use the rule 50% larger in I/D than the collector on each bank.

SYSTEM PIPES AND MUFFLERS

The chart below shows the maximum flow potential and power potential at various pressure drops of the final system pipe sizes and mufflers. Again sizes are I/Ds. This assumes 100% efficiency for the mufflers which will only be achieved with good internal design.

The rule for pipe or muffler flow is you need between 2.2 and 2.5 CFM per bhp measured at 25" of water pressure drop. In the chart below I've used the average of these which is 2.35 CFM per bhp.

Diameter I/D...Flow 20.3"...Flow 25".....Power
1 1/2"................220..............244..........105
1 3/4"................299..............332..........140
2 0/0"................390..............443..........185
2 1/4"................494..............548..........235
2 1/2"................610..............677..........285
2 3/4"................738..............819..........350
3 0/0"................878..............975..........415
3 1/4"...............1031............1044..........485
3 1/2"...............1196............1327..........565
3 3/4"...............1372............1523..........650
4 0/0"...............1562............1733..........735
4 1/4"...............1763............1956..........830

Example. You're designing a system for an 800 bhp V8. The power per bank is 400 bhp so the primary pipe should be 2" I/D. The secondary if it's a 4-2-1 system should be 2 1/2" and the collector should be 3". The muffler flow chart agrees that a 3" pipe will cope with 400 bhp on each bank. The final pipe size if both banks join into one should be 4 1/4"

Example. You're designing a system for a 200 bhp 4 cylinder engine. The primary should be 1 1/2" I/D. The secondary if it's a 4-2-1 system should be 25% bigger which is 1.875" or 1 7/8". The collector should be 50% bigger than the primary which is 2 1/4". The muffler flow chart agrees that a 2 1/4" system should cope with 200 bhp.

Dave

[MAP]

Greetings Dave,

Excellent! Thanks for that information. One question - why are the diameters not a function of tube length? Is it because the diameters are predicated on a presumed average section length, or is it because the length dependence is inherently not significant? The latter seems unlikely.

Best,
Mark

[1989GTA]

Thanks Dave

Your findings are very close to what others have posted.

[Brian P]

The diameters are not a function of tube length because the diameters are essentially a function of how much gas flow has to pass through them, and that's in proportion to the horsepower.

The tube lengths are important also, of course, and it would be interesting to see what the rule-of-thumb is here ... it'll probably be different for various cylinder arrangements and firing orders.

[paulzig]

Does this formulae take into account the weight of a vehicle, which may change load characteristics and maybe necessitate a larger diameter pipe?? As an example:

I'm doing a Windsor Boss engine of 427 cu.in for a 3400lbs car making around 650bhp and I'm worried an 1 7/8 primary may choke the engine with the weight..

[FlowSpecialist]

Originally Posted by paulzig

Does this formulae take into account the weight of a vehicle, which may change load characteristics and maybe necessitate a larger diameter pipe?? As an example:

I'm doing a Windsor Boss engine of 427 cu.in for a 3400lbs car making around 650bhp and I'm worried an 1 7/8 primary may choke the engine with the weight..

Why do you think vehicle weight makes the slightest difference to an engine?

Dave

[FlowSpecialist]

Originally Posted by MAP

One question - why are the diameters not a function of tube length?

I see no reason why they would be. Tuned length, if that's what you mean, is an entirely different topic.

Dave

[paulzig]

Just thinking that maybe the extra load of a larger vehicle may put a different load on the engine when running the quarter, thus choking it if it has a smaller sized primary, dyno figures are one thing and 1/4 mile performance is another... Just thinking out aloud tho LOL...

I've seen cars that have the right sized primary but yet perform better in terms of mph when equipped with a primary that is theoretically too big.

[FlowSpecialist]

The only load you can put on an engine you do by opening the throttle and making it produce bhp. What, if in fact anything, the engine is dragging around after it it neither knows nor cares about.

How does the engine know how much the car weighs when the engine is on an engine dyno?

Dave

[paulzig]

True... Interesting, stuff tho, learning a lot from this thread