Toni Capristo speaking about F355 exhaust and F50 custom made exhaust

Thanks, it's nothing really. Would be interesting to see others try the same to see any variance if any, between different setups.

IMO though, with the valve wired open and limited throttle/rpm required to put exhaust flow through both primary and secondary paths, reinforces my thought that both paths are utilized under wide open throttle.

I can try to set something up to record it on the dyno or street.

Street especially I think even less re-circulation as the diffusers should create a low pressure area at the tips.

Only other thing I think worth mentioning is the stock header collector final ID is 2.3" or 4.15 sq/in area. Stock muffler tailpipe ID is 1.65. That's 2.14 area x2 for 4.28. per bank.

I kept the stock collector final ID and stock tailpipe ID when building my headers and muffler.

Point being, from the factory, the muffler tailpipe area slightly exceeds the header collector area.

 

Last thought is. I can go into more detail but I think the air injection rails and the angle they are tapped into the header flanges can contribute to excessive heat on the exhaust side of the engine. (I've entirely deleted air injection for that reason and because I have no cats)

 

Just to add to this. Agreed. It's not about needing backpressure. It's about optimizing velocity and therefore scavenging. (so basically keeping it on the edge of creating backpressure maximizes velocity/scavenging)

 

Great tests, thanks.

There is flow and then there is exhaust pulsations. Exhaust gases may well flow out of all 4 outlets on full load but when the pulse wave reverses as it reaches the exhaust tip it bounces back up to the headers, creating a vacuum. This is not easy to picture as it occurs extremely quickly during one event of exhaust valves opening. During a full engine cycle the resulting resonance will vary through the rev range and affect scavenging, and the sound obviously.

When some folks make the false assumption that back pressure is needed to improve low end torque, I believe what is needed instead is the resonator effect allowed by a closed valve that will improve scavenging down low, with the opposite effect (loss of power) up top. As pointed out the piping of the main circuits is large enough to flow all the engine can throw at it, but the resonator effect occurring in the Y pipe as the valve is closed may harm scavenging when the engine is running on full load, causing a loss of power up top.

 

PS : stationary testing may not give the same results as tests made with the engine under load.

 

The OEM muffler doesn't have separate chambers for primary and secondary, just different paths. All 4 exhausts connect to the same chamber. If there is re-circulation, it is of exhaust gases, not fresh air. Also, my early Tubi has only a single exhaust chambers and one exit per side, thus any re-circulation would also be of exhaust gases Frankly, if the muffler design is allowing fresh air to be sucked into the exhaust under some circumstances I would consider it a bad design.

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The issue with the stock layout is the gases going round in circles. Any solution that allows breaking this vicious circle should be considered, and whether a twin circuit muffler does or does not draw fresh air in the loop doesn't really matter.

 

Either way you look at it, it's going around in circles. It's just whether that circle is completed inside or out side the exhaust system. Net,net, all the combustion gases generated by the engine are expelled. If fresh air is being drawn in and mix with the normal exhaust it will alter the amount of O2 in the exhaust and could push the fuel trim towards the rich side since the path goes past the O2 sensors. No different than an exhaust leak up stream of the sensors. If it's just re-circulating exhaust gases, it would not have that effect.

Anyway, I though the reason for the bypass was due to noise regulations at the time the car was build.

 

Agreed but if it takes little throttle/rpm to flow through both primary and secondary I don't think more exhaust flow would change that.

 

@johnk...
Under full load the fueling goes to open loop and what the lambda sensors measure is then not considered by the ECU.
When running in closed loop fueling and with the valve locked open rather than closed AND with a twin channel muffler the lambda sensors will measure a mixture enleaned because of outer air being sucked in and the ECU will richen the mixture although it is not necessary - I have done the test several times and it shows right away on the spark plugs.

@INTMD8
Well, at this stage we don't know for sure. We need someone to go to the dyno and be brave enough to place some towel at the exit of the main circuit to see if it gets ingested or not...

 

Well going wot in neutral resulting in a -lot- of visible flow out of both sides is proof enough for me but if you want to check on a dyno I'll await your results

 

And again, if on the dyno my car was pulling outside air through the primaries the wideband probed at the tip would be maxed out lean instead of 13.0 at every tip.

 

Did you measure the mixture anywhere else than on full load?

 

Yes, post #13

 

Ok. It would have been interesting to know the mixture in the mains in closed loop with the valve forced open.

 

Yes, you are correct. My memory wasn't up to snuff. I was thinking the by-pass opened only based on RPM and gear. Checking,the manual I see it also needs to be at full power, which, as you state, is open loop. My error.

 

Oh I see. It was throwing lambda codes (with valve forced open) It was re-circulating at that point, at idle, so it must have been maxing trims positive.

 

I feel a little vindicated.

 

Yes your post #33 is correct (if re-circulating) Seems to be a non issue with a functioning bypass valve.

Others could give it a try on their car to see if it's the same result as all of the exhaust components on my car are unique.

 

Yes, no argument there. However, when I responded to @Timmo, (my post #41), I was thinking that would be a bad idea to wire the by-pass open as some do, but I didn't add that comment. So your comment about throwing codes with it forced open was consistent with that thought.

I could try it on my car but with the vintage tubi muffler both primary and secondary exhaust come out the same tube. So no way to tell what's happening internally. And I wonder if it would be different on a 2.7 and 5.2 since the exhaust paths are so different.

 

Yes, couldn't tell anything on yours. Would be interesting to see others with exhausts that have separate paths. And the Challenge race exhaust.

Certainly could be different on a 5.2 as you said.

 

Had another thought on the re-circulation aspect.

On the stock cats, the inlet tube on the cats extends inside past the outer housing. With some perimeter holes in the end closest to the catalyst core.

So basically a tube within a cone on the inlet side of the cat.

That would function as an anti reversion chamber.

Not the case with aftermarket cats I've seen or of course test pipes.

I think not an issue with a functioning bypass valve but overall may be better to build aftermarket cats and test pipes as an anti reversion chamber as well. (so aftermarket cat, extend inlet deeper, test pipes do a tube within a larger tube for some distance)

 

I don't buy Capristo's theory at all. Manifolds fail because they are made of thin steel, expand when hot, touch neighbouring tubes etc. EGTs are elevated by misfuelling which does happen on older fuel injected cars that don't deactivate cylinders when running issues are detected. The UK roadworthiness test, the MOT, includes emissions testing via use of lambda probes in the exhaust while the engine runs at various RPMs. If unmetered air was making it back up to the manifolds, no 355 would ever pass that test.

Always happy to be proven wrong by some empirical testing, but given that no data is given other than "manifolds fail", I'm disinclined to believe in the recirculation theory.

I still have a Capristo and think it sounds great think.