What are non reversal cones?

[A900ss]

What are non-reverse cones?

I’ve been looking at the ACT catalogue and their manifolds contain non-reversal cones?

What are they and what to they do (power and noise I guess)?

Thanks

[V8 GRF]

They maintain the back pressure lost when the cats are removed and as the name suggests prevent gasses coming back up the pipe.

[Sardonicus]

V8 GRF said:

They maintain the back pressure lost when the cats are removed and as the name suggests prevent gasses coming back up the pipe.

You never want back pressure on a 4 stroke they are anti gas reversal (limit turbulence) cones and IMO debatable if a worthwhile improvement, there are other area to address on the RV8 that are much more pressing

[spend]

V8 GRF said:

They maintain the back pressure lost when the cats are removed and as the name suggests prevent gasses coming back up the pipe.

More like just aiding the flow transition IMHO... 1 of 4 small pipes firing into big chamber means (flow by CSA) gas suddenly decelerates, the cones should make this transition smoother. With pre-cats in place you had no option but to just blast the front of the cat.... with the effective blockage creating back pressure (which whatever anyone says I cannot see being beneficial in any way or form ~ you want it pulling the charge through 90% of the time!)

[nickb134]

Back pressure on a 4 stroke is a bad thing. The cones may be there to keep up gas speed as spend says and aid cylinder scavenging/refilling although this is doubtful given the way the manifolds are designed. You really need a bunch of snakes on the engine to achieve cylinder scavenging. (Not easy on a dual plane V8 but I have seen it done)

[V8 GRF]

nickb134 said:

Back pressure on a 4 stroke is a bad thing. The cones may be there to keep up gas speed as spend says and aid cylinder scavenging/refilling although this is doubtful given the way the manifolds are designed. You really need a bunch of snakes on the engine to achieve cylinder scavenging. (Not easy on a dual plane V8 but I have seen it done)

Probably used the wrong words there but I thought the 'back pressure' probably better described as negative pressure kept the flow in the right direction,ie. away from the cylinders which I thought was part of something that the cats do?

[davettf2]

Peeps, The exhaust, if properly tuned does two things: It scavenges during the exhaust stroke to maximise the extraction of the burnt gases, and secondly on the inlet stroke, during the time the exhaust valve is open (overlap)puts a pressure pulse back through the system to prevent fresh fuel being wasted out of the exhaust. The effects are much more noticeable in single and twin cylinder engines.
dave

[spend]

davettf2 said:

...on the inlet stroke, during the time the exhaust valve is open (overlap)puts a pressure pulse back through the system to prevent fresh fuel being wasted out of the exhaust

Whats the point of that ~ its just stopping the inlet working....

You need overlap to overcome the inertia in the inlet (start the ball rolling) as the momentum in the exhaust will pull through ~ but if your exhaust stays open too long it's absolutely counter productive as you can start pushing back up effectively.

Scavenging is good, but back pressure is not going to help it, at all. A highly tuned motor is always going to pull excess fresh air+fuel into the exhaust ~ thats how you ensure you get maximum bang in the cylinder '.'.

eta: only fixed quote mark

Edited by spend on Tuesday 14th January 11:41

[A900ss]

Do the above posts mean non reversal cones are good or bad?

Edited by A900ss on Monday 13th January 22:11

[SILICONEKID346HP]

A900ss said:

Does the above posts mean non reversal cones are good or bad?

It means you can spend money for nothing if you want

[macdeb]

I have them as my understanding was they aid cylinder scavenging. Do they work? Couldn't tell you.

[Goaty Bill]

spend said:

Scavenging is good, but back pressure is not going to help it, at all. A highly tuned motor is always going to pull excess fresh air+fuel into the exhaust ~ thats how you ensure you get maximum bang in the cylinder '.'.

^This is how I have always understood it^


I would be guessing here; but perhaps the guys at ACT have thought about this? Maybe even tested it?
Before I appear to be a sycophant of ACT; I have absolutely none of their products, nor have I ever owned any.
Some of the products look very useful, some I am more dubious about. However my dubiousness without personal experience or empirical data has no real meaning does it?

I do know two other TVR mechanics that swear there limited testing of non-reversal cones made a noticeable difference to two separate cars they were installed to.
Proof? Not in my book either, but as evidence? yes.

I would imagine, as stated above, they should be smoothing the exhaust gas flow, and assisting in creating a negative pressure in the collector.
My next guess would be that this effect would likely be lessened as the RPMs climb.

As nickb134 indicates, the exhaust system while being vastly superior to the old cast iron 'log' type, falls a good deal short of being an optimum design.
Just look at a Clive F exhaust.

'Testing' we did on BBCs many years ago; firstly replacing cast iron exhaust with a budget tubular exhaust provided massive improvements. Secondly replacing the budget tubular exhaust with expensive tuned tubular headers of near equal lengths (BBCs had idiosyncratic intake and exhaust properties) produced an improvement, but nothing so noticeable as the first replacement had.
The laws of diminishing returns.

[domV8]

I would have thought that good scavenging requires the flow to maintain speed..?

Presumably, if the exhaust gas slows down as it enters the space where the precats were, this will have a detrimental effect on scavenging - so the cones are there to maintain gas speed as said above, by effectively reducing the cross-sectional area the gas passes through as it goes from 4-1, from the precat-bucket-size to something more appropriate...

I would hazard a guess...

Dom