Don Terrill’s Speed-Talk

Hi All,

I'm looking for a little advice on exhaust system diameters and merge points for my race car project. I'm fairly comfortable with header diameters and lengths, however, our regulations stipulate that we must have a single exit exhaust at the rear of the car. All of the examples of math that I can find seem to be based on either a twin exit system or a twin system with a crossover point.

My calcs (Based on free online calculators) suggest 1.57" (40mm) primaries and 31.5" length, but with 2.7" collectors. I am tempted to buy Pipemax to see if I get a better answer.

My question is, is an increase in tailpipe diameter once the two 2.7" pipes join into one is a good idea?, currently other cars in our race series are using 2.5" collectors and a 2.5" single tailpipe after the two collectors merge..... therefore pumping 400bhp through a single pipe, assume that the gases will be cooler and therefore perhaps thats why the smaller pipe works?

Thanks in advance.

Ollie

I don't know if the latest PipeMax program includes a single tailpipe on a V-8, but I'm pretty sure Burns has designs. Alternatively, here's Brzezinski's stuff: https://www.castheads.com/manifolds-car ... r-systems/

Thanks for your reply, I see from the link that their 'off the shelf' collector goes from 2.5" to 3.5". I assume that the expansion is size is required to create the negative pressure.

I have contacted Burns before, with no reply.

Any further information would be great.

Ollie

Being a 'flat plane' V8 you're obviously treating it as two coupled in line fours phased 90*, so a lot of the potential info you'll see will relate to cross plane V8s and you will have to decide exactly how it applies to you.

Having said that as long as the headers are reasonably matched I'm not sure there's any startling info out there regarding running everything into one pipe, above and beyond keeping restriction low and velocity high, and some kind of merge collector that would suit most any ballpark V8 should work.

Stepping up the pipe diameter, to somewhere approximating the combined cross sectional area, or maybe slightly less, at the merge doesn't seem the worst place to start ..... whether some kind of trick venturi can be put in and make a difference IDK.

IANAE, but ......

Cross sectional area relative to flow and cooling of the gases might be able to number crunched, by I'm guessing you're mostly looking at empirical data, and suck it and see ......

400hp N/A thru a single 2.5" sounds tight but flat plane crank will have a far smoother exhaust flow than a 90* crank V8. This makes a huge difference.
Thats down around .011"²/hp , a 90 deg V8 n/a is not happy at that. Turbo yes ok, a flat plane (im not well versed/experienced in flat plane v8 stuff) with a good set of pipes different story. Stepping out to 3.5" will be too much, staying at 2.5" 'may' be too tight for full hp potential but great for anything below peak hp. In my lack of experience I'd simply experiment. Its not difficult to hit what works.

Calvin Elston is putting 600hp through a single 3” tailpipe on a uneven firing Ford V8. Divide horsepower by pipe area to see if you’re on target. My quick math shows that Calvin is putting more hp / csa than you’ll be.

hoffman900 wrote: ↑Wed Nov 27, 2019 11:09 am Calvin Elston is putting 600hp through a single 3” tailpipe on a uneven firing Ford V8. Divide horsepower by pipe area to see if you’re on target. My quick math shows that Calvin is putting more hp / csa than you’ll be.

Yes "tailpipe" , is much different than choke/collector area of the system. Far cooler gas and more system length/time for pulsations to begin to blend.

Thanks all,

I have no idea what to do now!

oneaves2005 wrote: ↑Tue Nov 26, 2019 12:27 pm Hi All,

I'm looking for a little advice on exhaust system diameters and merge points for my race car project. I'm fairly comfortable with header diameters and lengths, however, our regulations stipulate that we must have a single exit exhaust at the rear of the car. All of the examples of math that I can find seem to be based on either a twin exit system or a twin system with a crossover point.

My calcs (Based on free online calculators) suggest 1.57" (40mm) primaries and 31.5" length, but with 2.7" collectors. I am tempted to buy Pipemax to see if I get a better answer.

My question is, is an increase in tailpipe diameter once the two 2.7" pipes join into one is a good idea?, currently other cars in our race series are using 2.5" collectors and a 2.5" single tailpipe after the two collectors merge..... therefore pumping 400bhp through a single pipe, assume that the gases will be cooler and therefore perhaps thats why the smaller pipe works?

Thanks in advance.

Ollie

You give scant information on your engine and exhaust length for a guy to do some napkin calcs. Is this a road racer or drifter or a drag car or?
It appears the sale on the Speed Talk software bundle is not being advertised but maybe an email to speedtalk might help.
viewtopic.php?f=69&t=57670
Here's a link to the info needed to run Pipemax.https://www.musclecardiy.com/performanc ... e-engines/

Another way to look at it is find out how big the exhaust is on a 200hp 4 banger and doubling the cross section for your application.

One might look up other flat plane V8s like Ferraris or others and see what diameter pipes are used to connect to the muffler, calculate the cross sections and calculate an equivalent single pipe.

Here's a simple chart for at least a reference. https://www.hotrod.com/articles/what-ex ... work-best/

Another way to look at it is find out what the class leader runs for exhaust diameter.

The new GT500 uses 2 3/4" diameter tailpipes after the cats but that's for 760hp.

Vannik’s software might be useful in getting the tuned lengths right.

Thanks, might give pipemax a go - I see they have it on offer for $99 at the moment.

oneaves2005 wrote: ↑Thu Nov 28, 2019 12:10 pm Thanks, might give pipemax a go - I see they have it on offer for $99 at the moment.

Vannik’s software has the advantage that it can be used to explore various non-standard configurations.

MadBill wrote: ↑Tue Nov 26, 2019 10:27 pm I don't know if the latest PipeMax program includes a single tailpipe on a V-8, but I'm pretty sure Burns has designs. Alternatively, here's Brzezinski's stuff: https://www.castheads.com/manifolds-car ... r-systems/

Way back when I tested a whole load of Randy Brzezinski's stuff and it flat works just like he say!!!
DV

Hi all,

Thanks for your help so far.

I thought I might add some detail and see if you guys could help any further.

I have been doing a bit more reading on this forum and on Elston’s website about the exhaust sizing. Which got me thinking about matching the tube area to the exhaust port area at the flange. The area of the exhaust port at the flange is approximately 10.5sq.cm.

Now, I have attached a power graph overlay of two cars with engines the same spec as mine (identical, only variable is manual head porting). The blue line shows the car with a 4-1 system with 1.75” O.D primaries approximately 30-31” long, into a 2.5” collector, both collectors joint o a single 2.5” pipe and exit through a silencer at the rear of the car. The brown line shows a system with the same properties other than the primary diameter which is 40mm O.D.

The cars are used for circuit racing, generally using 4000-7500 rpm. The graphs show WHP, I suspect they are a little optimistic though. The engine is around 250ci.

My view is that the 40mm pipe is too small and the 1.75” pipe is too big. So do I go for a stepped header, as the 40mm pipe matches the port area very well.

I would welcome your thoughts.

Do you have a picture of the entire exhaust system or a generic length of each collector and the rest of the system?