Fiats original exhaust system comperd to Vendors (Fiat 124)

[So Cal Mark]

both your sheet and Bernie's dyno sheet show a torque drop off at 4500 rpm. You'll find this is due in large part to an exhaust that doesn't flow enough at high rpm. A good exhaust system will help that curve stay flat to over 6000rpm

[basarabam]

Wish there was an easy way to incorporate a RPM controlled dual exhaust. I might look at some GSXR parts I have, the original exhaust has a baffling flap controlled by the RPM to allow more flow at higher RPM without the loss of velocity at lower. (Kind of like the Ferrari set up). If it was possible to open a 2nd outlet at lets say, 5000 RPM (just throwing a number out) to 50% and 100% at 7000 RPM then this would allow flow plus maintain Velocity. This would all have to be done while on a dyno I guess and adjust the opening % trial and error. Since I am not rich I wont be able to do it but sure would be cool...

I agree with the OP on this. Without a starting point vendors could say they gained 50 HP and who would say differently. Years of working on imports and making them scoot I have seen and heard some funny fish stories. I always laugh when I see some kid with a fart can on his Cavalier with a 500 HP wing on the back. Or how about those little fans you put in your breather pipe to gain 10hp! Many times people "Feel" the difference and there never was one, they will believe their money helped but no evidential proof. (dyno before and after)

I made sure on my Mazda when I was building it that each mod I did had an updated dyno sheet so I could track my progress and my HP to $ ratio, this I passed on to others that were also building cars. I had a buddy with a shop with Dyno in Fargo, ND that was more than happy to let me work off all the time I spent on the dyno in his shop.

I have always looked at HP to cost ratio. If it costs $600 to gain 5-10 hp, think I will be that 5-10 short. Now show me where I will gain 50-75 for $1000, that is a different story. But on the flip side, adding a header back system might gaining you lets say 10 HP with the stock manifold, but gain you 30 with both an aftermarket header and full exhaust, so those 2 things together are better than 10 HP separate. (I am not saying they do, just giving an example)

Like others have said, there needs to be a balance. Thinking throwing a header on to give any substantial improvement might be a stretch. If you are going to run bigger carbs I would say an intake valve upgrade would be well worth and then coupled with a full exhaust system and updating the ignition you might see some VERY nice gains.

They all go hand in hand but I agree with you that there is no hard evidence to support the before and after claims, that I have seen either.

[rodman]

Very nicely said ( basarabam )

[So Cal Mark]

your theory about dual flaps to route exhaust won't have any effect on the Spider. The key area is close to the cylinder head. That's why the step design has proven to be worthwhile. It doesn't hurt high rpm flow while making good gains in the midrange compared to a non step design.
Now, if you're going to leave the tiny restrictive stock exhaust on and choke down a free flowing header, then being able to divert into a larger system would have some benefit. If the system is sized properly, it will make more power throughout the rpm range.

[basarabam]

your theory about dual flaps to route exhaust won't have any effect on the Spider.

Ahhh cool someone has tried it already. Can you share your design and documented findings on how it didnt work? My "theory" wasnt a dual flap, instead one that opened a 2nd path to increase flow and maintain velocity. Again it was just a topic of discussion which I have no proof for or against so hence a "Theory" and not a stated fact such as the comment above...

Would love to see how this is incorrect seeing as how Lotus, Lamborghini, and Ferrari all use this type of system to improve flow without reducing velocity caused by using a larger diameter exhaust pipe. I think something as stated previous coupled with a free flowing header like the one you offer could actually show some great improvements. I agree that the "Flap" design would do little to nothing without improving the entire system, as I stated earlier.