One of the biggest things you can do to reduce heat in the engine compartment is to get your headers ceramic coated (they might already be).

Note that not all long-tube headers have equal-length primaries. This is critical to maximize the heat extraction effect. If you bought a high-end stainless-steel name brand like Borla, which would generally cost ~$1500, they are probably equal-length. If you paid more like $500-700, they're probably not.

 

they are borla. their not ceramic coated though.

 

High quality stainless headers like Borla don't necessarily need coating because the material doesn't retain heat like a cast manifold or non-stainless header.

 

True -- stainless steel already does a good job in this regard.

However, coated stainless steel is even better, and if you're looking to evacuate as much heat as possible from the engine compartment, coating the headers is one thing you can do.

I assume, but don't know for a fact, that the Borla headers have equal-length primaries. Anyone know?

 

i believe they are.

 

Did some searching -- based on the photos of the headers on Borla's website, they don't look like it.

Here's a picture of a set of headers with equal-length primaries:


See how the rearmost primary bends towards the front of the car and then back to the collector? That's because the direct route is too short compared to the length of the rest of the primaries.

The Borla looks like this:


Primaries don't have to be exactly equal. Variation of 2-3" is fine on a street car. However, the Borlas look like each is about 3-4" longer than the last.

 

and because its not the same length, it will run hotter?

 

No it won't. The EGT isn't going to be affected by designing equal length versus slightly unequal length primaries. The difference is that if all of the primaries are exactly the same length then each cylinder will be tuned to have the best scavenging at the same RPM. However, if you make the primaries slightly different lengths then each cylinder is tuned to slightly different RPM which will reduce peak power but slightly widen the power band. All of this depends on how much care went into designing the header.

If you are concerned about EGT then valve events, timing, and AFR are going to be much more important than the design of your exhaust manifold. Assuming the EGT is what it is and you are concerned with engine bay temperatures than that is a different matter. Ceramic coating will reduce heat transmission from the exhaust into the engine bay which will keep the bay cooler, if the coating is thick enough. Stainless steel has a little lower thermal conductivity than mild steel or iron in a cast manifold. However the thickness of the wall of the primaries in a cast manifold versus a SS tube header are fairly different as is the mass available as a heat sink. This really only matters near start up as eventually they will both warm to near EGT. It should be easier to cool a tube header as it has more surface area and less mass. Hard to say what effect you would have unless you understand the airflow through the engine bay at varying speeds. It is worth noting however that if the you can transfer heat from a tube header more efficiently than a manifold you are really just stealing energy from the exhaust gas which is counter-productive from an exhaust flow perspective. Best real world example of this is 4th Gen with a rear mount turbo and steel long tubes versus cast manifolds.

 

I'm not referring specifically to EGT, but to overall engine temperature. The scavenging effect that results from equal-length primaries results in cooler overall operating temps, because the headers (by way of the scavenging effect) are basically pulling the heat out of the cylinders. The temperature of the exhaust gas itself will be mostly unaffected.

 

Chris and I talked about this in CamaroZ28.Com Podcast #255...plus we covered some other cool stuff. Check it out!

 

To be precise there are two forms of scavenging, inertial and wave (or resonance). Inertial occurs in the collector or Y or X pipe when one exhaust gas pulse follows the low pressure wake of the previous cylinder or bank. For wave tuning, remember that the pressure wave is reflected in the collector back up all of the primaries... That is to say that if we simplify it to a 2 cylinder engine, you would pick an RPM to tune for in your rev range, then the length of the primary is such that as the wave leaves cylinder one, it travels down primary one, is reflected in the collector up primary two, then is reflected again off of the exhaust port right before the exhaust valve opens for cylinder two. In that manner the pressure right outside the port can become lower than ambient to increase gas flow.

The improvement from scavenging only happens at the specific RPM the length is tuned to improve though. Above and below that RPM you do not get the effect or have the reverse of the effect. In addition, the unequal firing order of the cross-plane V8 also throws a monkey wrench into scavenging as there are the two cylinders per bank that fire sequentially. The Tri-Y header is an attempt to alleviate that problem... crossing the primary from one of those cylinders to the opposite bank's collector is a different, more unwieldy one. The most effective is to go to a flat-plane crank where you have an even firing order, but that isn't a very realistic modification (you could always buy a Ferrari I suppose ).

In short, you have to hold a motor with equal length primaries at a constant RPM in order to get the full effect of scavenging.