[getlost4x4]

I found these headers that are supposed to fit the 3.5L V8. the price is nice at $195

http://www.summitracing.com/parts/HED-39800

looks like this guy swapped in a BOSCH 4.6L and used the headers listed above. so they should fit the engine, but will they fit in the bay and do you think they are worth it over the stock headers?

http://bahnstorm.com/rover/engine.htm

 

[leftlanetruckin]

Looking at the pictures of the headers on the motor, I dont think they will fit the engine bay. The 4.6 manifolds stay pretty close to the block, and for good reason! Space is tight right there.
Of course, anything will fit with enough time/patience/skills/money, but I dont think the gains would be worth the hassle myself.

Martin

 

[LarryS]

i think you'd want a tri-Y if anything (which the stock is slightly already, though far from ideal, particularly the 5 & 7 cylinders) - you may even find it down on low/mid end power over stock with a 4-1, and a combination of 2 silencers to stop the popping - if that bothers you.

 

[getlost4x4]

yeah, tri-y is much better. they don't look like they would fit in the stock location anyway.

maybe just replacing the stock cats, and then doing a 2.5-3" exhaust system might be the ticket.

there must be a formula to calculate the optimum exhaust size for the engine. i've had a few exhaust systems done, but its always just been a guess as to the right size.

 

[LarryS]

there are formula's of course but they are long and tedious and still a compromise and whole books are written on the subject, the one thing that is significant on a crossplane V8 (as most are) is the need for a full dual exhaust with a balance pipe [H pipe (old skool) or the more modern X pipe]...just how on earth can you package that under a P38 :think:

 

[leftlanetruckin]

I look at mods this way, for everything...
What is the PIA vs benefit ratio.
In this case, I think the gains are minimal on the exhaust manifolds versus the total PIA it would be to get a set in there. The crossover pipe is a good idea, but making it all fit, especially without losing precious ground clearance, would be a major challenge indeed.
I would aim for a free flowing exhaust after the manifolds, and call it a day.
I have ran 3" on motor swaps though, and actually LOST power/torque, so beware of going too big. I think 2.5" from the manifolds back would be ideal, with either high flow cats, or otherwise (cough cough). Having wheeled open top Heeps with no cats though, I would at least fit a high flow one myself. Cost is minimal and packaging is a non issue.
Just my .02 from over the years

Martin

 

[glenhendry]

...the one thing that is significant on a crossplane V8 is the need for a full dual exhaust with a balance pipe...

Interesting Larry, I thought that all Australian spec P38s came with single exhaust out the back, mine is joined into a single system soon after the cats. Are you saying this is a bad thing for the engine breathing? IIRC Holden V8s (in old aussie sports cars) used to prefer single exhaust under certain circumstances due to backpressure etc?

Since only one pot is firing at any time, I cant imagine why a single system is a major detractor of performance?

 

[LarryS]

yep, mine a single side exit too. One pot is firing at any one time but the crossplane crank and the firing order produces uneven pulses in the exhaust, its this that actually gives the "V8" sound, and those uneven pulses aren't the best for efficient scavenging if you just join them any old how, a good dual exhaust should be worth perhaps 10-20hp.

I've yet to see any evidence back pressure is a desirable thing perhaps there are other reasons why the Holden example gives better performance with one exit.

The fact the P38 doesn't have a full dual exhaust is probably packaging - i've looked a while ago and even started a solidworks model, there might be a way but it's very tricky.