The V8 Owners Forum -

Ok, so open headers aren't practical for regular driving, but can a good full system beat them on the drag strip? Lets say, just for example ( ) that the car in question is a Jag XJS, with a big block lump putting out somewhere between 600-900bhp (nitrous not included), could a full exhaust system handle full power or would it just bog down with back pressure?

Of course the simple (and planned) solution is to fit a pair of cut outs to divert the manifold to open side pipes for racing, but I'm intregued as to whether a full and legal (i.e. quiet) system could let the engine work at full potential.

Does pulse tuning come into the equation with these sort of engines or is it barely noticeable when a 9l engine is blowing out burning fuel?!

Pulse tuning is a big part of getting any engine to work well unless you have a big supercharger on top, even then it will add alot by scavenging the cylinders. On a 9L+ engine in an enthusiastic state of tune it will be contributing between 70 and 200 bhp depending if you are talking 600 bhp or 900+
Exhaust systems are a huge subject in them selves... but simply for maximum power you want a tuned manifold, 4 into 1 with a short secondary from the collector (about 12 to 18 inches set up using a bar of soap at the track) then followed by a tapered "megaphone" section, about 14 degrees out to twice the diamiter (these will make a nice soothing bass rumble sound at about 6500 revs that will be a joy for all to behold . There are variations (have a look at a set of Nascar headers on Ebay, but they are developed with serious budgets!)
You can quite easily build somthing like this and incorperate it using cut outs and a full system.
The individual header pipes on big boys dragsters (top fuel, funny car etc) are like that because of the down force produced, they loose a few hundread BHP by not using a full manifoold, but with 6500+ bhp the priority is traction not getting even more power.
Mike

After the "megaphone" does it stay that larger diameter to the rear of the car or do you mean exit there? If a good system is worth that much extra power then I'm not bothered by cutouts! What about a balance or X pipe? Not that I know if there would be room.....

all right you have asked for it now!
12 to 18" secondary, then the cut out, then the megaphone section then fresh air and enjoy! a good straight through systen with correctly sized boxes should drop only 3 to 5 % over over such a system, and would go somthing like 12 to 18" secondary, into a small ish first box, straight through (say volume 3 times engine capacity), then a length of pipe about 3 to 4 times the secondary then another straight through box about 5 to 10 times engine capacity then 8 to 12" of final tail pipe. An cross pipe is a good idea, about 3/4 diamiter of the secondary and attached just after the collector. An x pipe join would be at the end of the 12 to 18" secondary, and come out as another 12 to 18" bit (same length as the secondary)and into the uncorking/cut out valve and into the megaphone bit and the other leg being the first box on the system. hope this makes sence.
Mike

I believe I have actually found a man who is a book on all things V8!

Sorry Mike I slightly missread your first post, so for most power you dont want a full system, and the most important thing is really just making sure all the primaries are equal lengths, then into short secondaries, followed by a nice megaphone?!

..And a full system (very good one) will drop 3-5% power aye?

And boxes with 10 times a 9l engines volume..? I dont have to repeat the maths but how the hell would you ever fit 2 90l mufflers under the car?! A 40gallon drum in the boot maybe with 2 ins and outs?!

I thought nascars used a X and found this pic on google:

[quote="topcatproduction"]I believe I have actually found a man who is a book on all things V8!

Sorry Mike I slightly missread your first post, so for most power you dont want a full system, and the most important thing is really just making sure all the primaries are equal lengths, then into short secondaries, followed by a nice megaphone?!



Not quite correct
You wat equal length headers if you are looking for max power at X rpm with the length varied depending on what revs you need to use

Some say to get a better spread of power you need to use headers of variable lengths and then you can also pair them eg 1 and 4 vs 2 and 3 and equalise total volume on these etc.

Biggest thing in getting more power is getting a smooth tranfer at the connections be it 2 into1 or 4 into 1 etc,


Oh yes and if you are making your own pipes and need to ceck lengths simple way is blank one end and check volume of water that will fill each header. Oh and another old trick to stop pipe "crushing" when you bend it fill it with water and freeze it - the ice will crack but still hold the circular tube shape

Ian

Hi Ian and Tom,
to quote myself in my first post
" Exhaust systems are a huge subject in them selves... "
I was trying to keep it very basic, Ian you are compleetly correct about Primary lengths spreading the power if you need to, you can also use 4:2:1 headers, crossover set ups and stepped primaries, it all gets to a point very quickly where you need an engine dyno, big budget and is well beyond the individual at home. Just trying a 8:4:2:1:2 cross over system for mine has me stalled, and all I have done is try and build it to calculated lengths off verious calculators off the web I've avoided fancy tapered pipes and used ready made bends, it is a big subject.
If I were you Tom I would go for equal length headers, and a ballance pipe, or x pipe like in the Photo you have. The last box can be less than 90 litresand still give good results, but it was partly to demonstrate the point that theory is one thing, what you end up with is always a compromise (that secondary on the system you posted is more than 18" but it had to fit around the transmission (it also highlights one reason few people put 9L engines in road cars, it is a big job to get it all in there).
There are some very good books on tuning big blocks out there, admittadly mostly about BBCs but the principles apply equally to fords 460s , it is the details that differ.
Mike

Ok to keep it within the law from a volume point of view would I need 2 mufflers and if used full length pipes instead of side exit could I get away with straight through's?

Hi Tom
can't say for definate, as it depends on alot of things (not least exhaust valve opening point, age of vehicle and engine etc, but an X pipe should in itself reduce the output by about 8 to 12 dB, the first boxes certainly need to be straight through, the last as big as you can fit and preferably straight through. You can always tack weld temporary bafels in the tail pipe to get you through any noise test.
Mike

Just build a decent system with as much silencing as possible if you have room.

IMO most open headers sound shite, unless they are making a few thousand HP.

It would be easier, and I guess by using decent mufflers it would reduce losses too, what sort of diameter pipes would you want to use on a 557ci bbf, power in the range of the first post with the possible use of n20 in mind?

This you will need to work out when you have a spec, but start about 2.25" to 2.5" primaries, into 4" to 4.5" collector maybe even out to 5". Yes you can get silencers that big, just not alot of choice!
Mike

the sound of 8 primaries each the same size as the main single pipe on range rovers etc sounds ridiculous but its what I suspected!!

Hi Tom
I think I said some time ago you need to change your thinking if you are going to a big block . You have a lots of stuff to get used to . I would really get a book on tuning one (mopar, chevy, ford or Buick) doesn't really matter which one to start with, just get some idea of the issues, and scale.
Maybe avoid the ford FE series, they have specific issues around the oil system that are not relevent to other engines.
Mike
PS you could always turbo charge it, that reduces the sound level alot (12 to 18 db) and would give you a usefull amount more torque depending on how much boost you used

I can appreciate your engine may be bigger.


But my dual 3" with 2.0" primaries works fine. Yes, I have measured a restriction in the exhaust, but not enough to worry me, and certainly not enough that I'm going to build another exhaust until I become rich.

At the end of the day, you can only build a system that will fit under the car. So while 3.5" dual system would probably be ideal....will it fit ?