Exhaust crossover on straight six

[jagman]

Anyone know if there's any advantage in using a crossover on a twin pipe six cylinder exhaust? Aim is to reduce backpressure and increase cfm capacity. Engine uses two cast iron manifolds (3-1, not headers so the system is not tuned.
Thanks Gus

 

A balance pipe bridging the banks of a V6 or joining the front and rear three of a straight six makes a HUGE difference to the Volumetric efficiency curve. On an engine with lots of overlap/ Variable Can phasing , it will tend to have most effect at lower to medium engine speeds. It has a much much more profound effect then joining the banks of a V8. As a general rule, for a V6 or straight six you want to keep the "banks" seperated as far back as possible. The 1970's BMW 320/6 and 323i comparison serves to illustrate this point, where the 323i had alot more specific low speed Vol. eff and a much greater area under it's torque curve all out of proportion to it's higher CR and thi despite having the same valve/ports for bigger cylinders

 

[Whittey]

I was under the impression that a balance pipe was to balance out the uneven exhaust pulses of a V8. None of the Fordsix crowd put in a balance pipe, street driven or racing.


-=Whittey=-

 

[Marquis]

A Balance pipe is placed very strategicaly on the exhaust system made my Arvin Merritor, used on the latest Euro Spec E46 M3. It's positioned roughly near to the rear seats. I seriously doubt this is an NVH sound quality "device", as the pipes could have easily have been made to run through a common central silencer with perforations used to provide an acoustic link, but minimising actual dynamic pressure pulses.
From my V6 ( very similar in terms of firing order and gas exchange to a straight six)dyno experience, I've typically used a short communication very far back , near to the rear differential. And I varied the position to the rear tank too. This had the effect of moving a peak/trough feature further down the engine speed range as I moved it further back-granted it was a very local effect. On the other hand, the fact that the engines I worked with had Variable Cam phasing, with quite alot of over lap at low speed- it may have less of an effect or a different effect on an older fixed cam timing engine with less of a scavenging effect.

Unless one has a VERY VERY well correlated GT Power or Ricardo WAVE model, I would reccomend using a dyno and actual test hardware to position this for your requirements as it is very dependent on temperatures and even back pressures.
You may find that for your specific application It's not worth the bother.

On V8s I've worked with simulation has shown that a balance pipe help low speed vol. eff, however I have yet to ever have any data from actual test that supports this.

We STILL use an acoustic link between the banks via gas exchange in the silencer for sound quality Noise Vibration Harshness ( else the cruciform crank V8 sounds like an uncultured 50's american car or a Corvette on side pipes...)

Some collegues of mine removed the balance pipe on a Porsche 928 V8 and found it DID lose a lot of Low speed torque....but none of this can be quantified.

 

[L79]

Look at

http://www.castheads.com

and check out his exhaust systems with 2-into-1 collector. If we assume he didn't make up the data, it would appear that joining the pipes can make a large difference on a V8 when exhaust manifolds are used rather than headers.

 

[Marquis]

L79 I looked at that site you talked of. Yes, I've always suspected that on a cruciform crank V8 linking the banks made a big difference, it's very different to a Straight six/V6. On a 100 Bhp /litre V8 project worked on I experimented with tuned length exhaust systems with different connectivity. Specifically I noticed that having each bank go 4 into 1 gave similar top end to having the manifold tubes connected such that there is effectively a 270 degree interval between adjoining cylinders. The main thing to rememeber about a cruciform crank type V8 ( as opposed to a single plane crank) is that they are even firing (every 90 degrees) as a whole engine but UNEVEN firing when you consider one bank (this phenomenon makes the V8 unique and is responsible fo it's deicious burble!) As such you want to minimise interactions between close firing cylinders. With this in mind it's not difficult to see why the 4 into 1 per bank arrangement had a nasty hole in the VE curve at low speed, and when checked I found a great deal of "anti tuning", where at that particular engine speed there is high residual exhaust gas Backflow into the cylinders from ineffective scavengining/adverse tuning. This was exaserbated by the high horse power per litre requirement needing long cam durations (by OEM standards anyway) and the resulting overlap.

And unequal length primaries may be fine for a modified road car but they are far from ideal "no no" for a production car for a variety of reasons,including emission compliance and smooth idle quality. (it's sometimes unavoidable though due to packaging)