Thread: D2a V8 Catalytic Converters.

On the D2 Terry?

ramblings in NO particular order--

cats are basically a can with a mesh across it,, great for flow,,,,

The D1 primarys are much better (longer the better)but they dont have to squeeze cats in,,,

so if you must have them,,, hi-flo cats,

any improvement has to be around the manifold/primarys area,,

not sure how (why??)you attach cats to extractors(sort of defeats the purpose)

the main exhaust flows well enough, and is large enough, I took the main muffler out, (had no mufflers at all on the D1) all the pressure waves in the world would be over by the muffler stage,,

but in reality its all a case of diminishing returns,,
you could build a great new system, pay some guru a fortune and get what,, maybe 10 hp. maybe.


This makes interesting reading,,

remember Yamaha's EXUP?

The myth of exhaust backpressure [Archive] - My Pro Street


like I said,, just ramblings

Pedro,

thanks for that, I don't really want to spend a fortune more then I have to on an exhaust to try and improve the power, that is if there are decent improvements to be had in the first place?

That is why I was hoping to get some advice from people who had done this before and know the do's and don'ts so I can spend the right money the first time for the best gain.

Again any assistance would be appreciated.

cheers,
Terry

The best way to explain it is like this i hope i get it right and yes it is back pressure kind of

I dont like to call it back pressure more of a build up

Changing a certain part of an exhaust changes the way the engine performs

For example putting Bigger free flow cats will give you better low end as its easier to breath a low revs
Once the revs build up the fact that the rest of the system hasn't changed causes the pressure to be pretty much the same

If you change the size of the pipes then every thing changes as the amount off rev required to build the same pressure in the pipe is higher
so at low revs it has no restriction pretty much the same as no exhaust at all if you go to big

It probably doesnt make sense but its the same for all engines the exhaust isn't just there to keep it quiet it does help with performance

2 strokes are an easy way to see this in action as they pretty much have to be spot on are they run like crap
The pro's change the length of the exhaust as they change the gearing as it makes a huge difference to the performance characteristics of the engine

If you know anyone with a go kart go out with them one day and ask the to change the length of the flex pipe that joins the exhaust to the header

Thats why changing the rear muffler to a piece of 2 or 2 1/2 in pipe makes so much of a difference
length is every thing race cars point out the side just because the exhaust is so short not cause it looks cool

If it rambles sorry

Sorry had to add a little edit here Pedro_The_Swift is spot on for a performance motor just not really a daily driver where economy is part of the equation

Thanks for that Steve,

I do actually know a little bit about 2-strokes and have some idea about how exhausts and back pressure can effect performance especially in racing conditions.

What I'm actually asking is rather, if chasing engine performance on a D2a V8 what is the best and most cost effective way to go?

Is this best achieved through modifying the exhaust or chipping it, or both? If the exhaust how far to you need to go? Or is it a waste of time and money for the proven returns available?

I intend to fit a snorkel soon as well, does this have a positive or negative impact on how the engine breaths and performs?

I'm not after redline horspower improvements I'm just after improved bottom end and midrange (hopefully), a little bit more on top wouldn't go astray either if possible.

There must have been over the years dozens and dozens of D2 V8 owners on this website that have had exhaust mods done that were hoping to achieve exactly the same thing I'm asking, so is there an accepted path to go down or supplier?

cheers,
Terry

Cheapest way to improve the note and give a noticeable change to the feel would be to change the rear muffler for a pipe or a bigger free flow muffler

Dont expect to pop a wheelie but you will notice a difference

Ill be doing this this week i bought one of those cheap mufflers of ebay
I put a 3.9 in place of the 3.5 and the muffler on there has 2 one inch out lets and the cheapy has 2 three inch out lets

The reason i went for the cheapest one i could find is it will probably get squashed anyway with winter approaching and the last one cost 200 buck

As for getting more power theirs not really a lot you can do to a EFI motor without spending quite a bit of money and the first place you would start would be chipping and then build your exhaust to match

OR

The best places that i know of to look for performance parts for a Rover motor is
Real Steel
RPi Engineering - V8 Engine - Carburetion and fuel Injection

4 strokes don't need back-pressure. Thats something I hear all of the time and it still makes me wonder. 4-strokes work on a scavenging principle where the flow of the exhaust from one cylinder is used to pull exhaust from the next cylinder in the cycle. Thats another reason why 4+ cylinders into one header has the pipes merge into the collector in a pattern that makes the next cylinder to exhaust adjacent to the previous where the pipe enters the collector. For the system to work it needs to have velocity of the gas flow, but it doesn't need back pressure as such, so stuffing it with cats, mufflers and chicken wire wont give you bottom end power. If the velocity slows from a larger collector then the scavenging becomes ineffective which drops the power at the bottom end. The flow increases and the restriction drops in the top end rev bands with a bigger collector which then makes it produce more power in the higher revs. There is a heap more to exhausts though, with differences between 4 to 1 and 4 to 2 to 1 exhausts, and IIRC the 4 to 1's are better for revving and the 4-2-1's are better for torque although this depends on a other factors. The problem with the Disco2 exhaust is that it restricts the gas flow through the collector so it makes it hard for the gas to get out and makes scavenging worthless. The cat will only affect flow btw. What I would do as a first step if you want to keep things simple, is to just remove the restriction where the manifold and exhaust meets with a D1 flange and then immediately merge this into one after the flange and then into the cats. In reality this will probably be a total PITA and not worth the time and money when you could probably do it other ways that net better results for cheaper. Alternatively you may be able to use a D1 set of manifolds if they don't require any additional D2 fittings etc and then run the cats behind the motor instead of beside the motor as how most V type engines in cars do it. Or D2 extractors.

2-stroke exhaust systems work on a completely different system to a 4 strokes which is why they are all designed with expansion chambers. These work on the principles of pressure waves that are reflected within the expansion chamber, and the pressure wave is used to repressurise the combustion chamber with expelled but unburn air-fuel mixture. The exhaust lengths are to do with the timing of the rebounded pressurewave and it returning to the exhaust port - so a shorter exhaust works better at the top end and a longer exhaust will have better low rev power. No relation to 4 strokes at all.

I was only showing it as an example not a rule

I wasn't disagreeing with any of the principles you have described

But i cant understand why you would think there is no build up of pressure in your exhaust pipe on a stock system

If there was no pressure it would be free flowing and any modifications would be pointless as free flow means no restrictions

I'm not sure I wrote that

There is a pressure build up in the stock exhaust system.


Oh Terry, I also don't think that the snorkel on a D2 gives power anywhere in the rev range. IMHO it reduces it despite talk of ram effect etc. Thats at least my experiences with the TD5, but I would assume it would be the same for the V8.