Have gt got their bhp figures from a boy racer mag?

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tommoore

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Is it just me, or are stock bhps are a little high on some models, followed by ridiculous gains from mods such as exhausts.

Take for example the hyundai coupe gen2/tiburon turbulence. It had around 140bhp. Gt6 gives it 152. Then by adding racing exhaust, manifold and decat, it's got over 170! I've owned 3 of them, decatting would loose power (as it does on many small capacity na engines), the rest would be lucky to take a stock engine past 150bhp.

Then a triumph spitfire. By decatting a car that would never have had a cat, then remapping a car that has carbs, i got a hell of a power gain!
 
Hate to break it to you here but a catalytic converter actually saps some of the horses from a car so to de-cat a car it will show a slight improvement IRL also a racing manifold would allow exhaust gasses to flow more freely also adding a power increase IRL too and even a high performance exhaust system or even a tuner type fart cannon can add an easy 10bhp or there abouts IRL. How do I know this ? I'm a qualified mechanic and have worked on my fair share of modified cars and a few rally cars.
 
I have to agree with OP, some of the cars BHP are way off and the tuning give too much BHP i mean this is simulator it should have exact or at least close to real life BHP numbers
 
ghostrider135
Hate to break it to you here but a catalytic converter actually saps some of the horses from a car so to de-cat a car it will show a slight improvement IRL also a racing manifold would allow exhaust gasses to flow more freely also adding a power increase IRL too and even a high performance exhaust system or even a tuner type fart cannon can add an easy 10bhp or there abouts IRL. How do I know this ? I'm a qualified mechanic and have worked on my fair share of modified cars and a few rally cars.
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On turbos yes. On NA no. May stick a couple hp right at the top on a dyno queen, but generally de-catting and/or going to big on exhaust will loose to much real world performance. On my last coupe, I got gains by removing my 2.5" sports exhaust and fitting a smaller diameter 2" system. And gains of 10bhp for a catback on NA? Complete crap. With a manifold, may just make double figures. The local corsa and saxo owners must love your garage.
 
big diameter exhaust witout cat, very low backpressure ,loose Torque on low and mid RPMS (it depends on Engine and exhaust)

big diameter exhaust + cat you have some more backpressure ,because of the cat.

A good diameter exhaust (it depends on the Engine and Torque curce) ,without Cat is the winner .Turbo or not.

its my job :)
 
TCSTigersClaw
big diameter exhaust witout cat, very low backpressure ,loose Torque on low and mid RPMS (it depends on Engine and exhaust)

big diameter exhaust + cat you have some more backpressure ,because of the cat.

A good diameter exhaust (it depends on the Engine and Torque curce) ,without Cat is the winner .Turbo or not.

its my job :)
Click to expand...


Exactly. With added to highly tuned engine, big exhausts may unlock more bhp, on a high revving, lightened and balanced NA tune, low rpm torque isn't needed. But just plonking a big exhaust on a low tuned 2.0 road engine won't improve anything. Thats my point here. The bhp figures go up far to much for the mods done. And if you then play around in car settings, where theres a graph, de-catting adds power and torque at all revs. If I'd previously had a stage3 NA tune and sports computer mods, then yes 20bhp from a full exhaust system would be believable.

Also theres the weight. If I've replaced a stock exhaust with a titanium one, it's going to make the car lighter by a surprising amount. Certainly more than lightweight windows
 
Sorry that's not quite correct. Exhausts on normally aspirated performance cars are tuned to create a resonating pulse through the exhaust manifold as each cylinder fires, which drops the pressure in the branch of the manifold for the cylinder which is about to fire. It's a scavenging effect which means that the low pressure in the exhaust manifold draws gasses through from the cylinder. They do this on the inlet side as well which is why you have things like Helmholtz resonator chambers on inlets which most boy racers throw away and lose power. Anyway, the point is that this isn't specifically to do with 'back pressure' rather that the diameter of the exhaust along with the length of the branches of the manifold and the distance from the collector to the first 'obstruction' in the exhaust system, are all factors in creating this scavenging effect. In some cars when you remove the cat, you disrupt this effect, removing some of the scavenging effect from the manifold and losing a little power. But it's not a loss of 'back pressure' so much as the disruption of the resonance of the exhaust which causes the problem. This is why, for instance, you get different power characteristics if you have an exhaust manifold where all four branches meet at the same point (4-1) compared to one where the branches go to two, and then to one (4-2-1), as well as with different lengths of the primaries on the manifold. All of these things will have an impact on where along the rev range the exhaust scavenging effect will occur. On a tuned NA engine this would be very carefully calculated using huge mathematical equations. The tuner would also use the appropriate length trumpets on the throttle bodies to ensure that the exhaust characteristics are matched by the inlet characteristics. Plus you'd also be looking at things like valve overlap, cam timings, and other bits and pieces to support the setup and give the best possible efficiency.

So basically, on some cars the removal of the cat will cause a drop in power because it has ruined the resonance of the exhaust. In other cars, it won't be a factor, so will allow for an improvement in the power. I guess int he case of GT we should just assume that the exhaust parts are all very finely tuned and well made, and not the kind of thing a boy racer has asked some bod at Powerflow to knock up for him, with no idea as to the results.
 
Seeing as the final HP amount for 'most' cars is quite realistic, I'm ok with the fact that some individual mods may be a little off
 

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