exhaust size

[israelcj7]

hay every one,
i have a 4.6 carb'd stroker with a dual exhaust on a cj7 and i want to add an x pipe.
my setup is a hadman headers for 4.2 that i change the head plate to fit the 4.0 head, 2" pipes from the headers to the mufflers(speedrex 2.25in/2.25out) ,and 2.25 pipes from the mufflers all the way back.i want to do the x pipe in the section between the headers and the mufflers and i was thinking about changing the back pipes to 2",now is it worth it? or maybe just an x pipe and leave the back pipes as it is?
if it's any help my engine setup is:
258 crank and rods
0.30 pistons, polished but not dished (13.0 cc dish)
7120 head ,home made p&p, stock valve size,
stock 4.0 95 cam
hei dis' and splitfire wires
msd 6a and msd blaster2 coil
hadman headers dual exhaust
cliford intake and holley 4 barrel 470 cfm carb
home made cold air intake
electric fan

thanks

lishay

[1bolt]

Well we have it on good authority (see the sticky thread in Performance toward the end) that an I6 will perform best with a split dual exhaust that should be 123 into one and 456 into the other; split all the way to the tail pipes. No H or X pipe. The reason as best I can determine is that an I6 does not have a complimentary firing order like a V8 does. Basically you get no help from the opposing banks exhaust just the oposite you get reversion (one exhaust fighting another). Because the firing order leads to having an exhaust pulse right as another exhaust valve is closing during "overlap". The spent cylinders exhaust valve is nearly closed as that same cylinders intake valve cracks open, leading to exhaust pulse from 1, 2 or 3, being pushed into the closing exhaust and out the opening intake valve of 4, 5 or 6).

This is probably one reason why there's always built up gunk in the first couple inches of a 4.0 intake manifold. Also might explain why Renix intakes always seem to be more caked than HO intakes, (in my experience having torn down dozens of them) the renix exhaust has all 6 primaries (most of them less than 6" long) merging into a single log.

I am too lazy to actually figure out which cylinders would interfere with which just for this example, but I hope you get the idea (maybe someone else will wrap their brain around it and work it out for us) but the gist is that most V8's will benefit from a cross over pipe of some sort, while a straight 6 wont. You can see how the factory headers have evolved and seem to try and keep the front three isolated from the back three further and further as the years progressed, from Renix log, to HO with two smaller logs that only merge at the down pipe, ending in the cast iron 99+ exhaust manifold that keeps them split past the pre-cats.

So keep it split, as for size... IMO 2" is fine all the way back, and may lead to better scavenging in the low and mid RPM's while 2-1/4" will probably breath a little more freely at upper RPM's neither will have a big difference over the other. a couple footpounds given up for a couple hp or vice versa.

[israelcj7]

you say that a headers split to 123 to one and 456 to the other is the best setup,my headers is split to 126 to one and 345 to the other(258 headers), is there a big difference in performance between them?

thanks for the reply.

lishay

[1bolt]

honestly if I could answer with certainty I'd probably be making a living in the racing industry and not here offering opinions. I do remember reading somewhere that a configuration like you describe (Banks makes a header like this) may have benefits different from typical high RPM horse power that most aftermarket manufacturers are trying for. So that setup may be to enhance low end torque or mid range power at the expense of higher RPM horse power.

There's a considerable amount of in depth details about things like asymmetrical headers, 180* headers, balance tubes etc. in David Vizards books on How to build Horsepower, which is probably where I read about the configuration you mention. Only problem is he's nearly always talking about V8's and although he spreads things around and often applies principles to other engines to illustrate points I don't recall him ever touching on the ideal exhaust setup for a typical I6 firing order... Now that I think if it though I seem to recall him talking about Jaguar I6's in one of his books. Should be the same firing order so the same principles would apply. I'll try to track it down and see if there's any nuggets.

It's important to realize that most people view power as something that is added to by engine mods. When it really should be looked at as something that is MOVED around (tuned for specific uses) On an engine that is well built It's very difficult to add low end torque without losing higher RPM (peak) hp... And it's very easy to lose lower RPM torque by doing things that increase that peak hp.

On the other hand lots of header designs have no regard for wave tuning, and do things for reasons like fitting the primaries under intakes, how it looks etc. In addition to other practical matters of fitting things, they might also configure things to fit the longest primaries into the smallest area etc.

[Cheromaniac]

you say that a headers split to 123 to one and 456 to the other is the best setup,my headers is split to 126 to one and 345 to the other(258 headers), is there a big difference in performance between them?

The former would make more sense if the cylinder firing order is 1-5-3-6-2-4

[israelcj7]

thanks guys you have been a great help