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Assumptions about lack of air

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In the attached image are a stock dyno run, and then a run with mods to exhaust and intake. Big gains down low and almost nothing up top. Would it be fair to assume it's not getting enough air at higher RPM hence the smaller change, or is it normal to get much larger gain down low? This shows a nearly 90% increase at 3000rpm, and 4% at 6500rpm

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Is there a little more info around these two dyno runs? What engine for example? It's possible that the issues restricting the high rpm performance are just down to airflow through the head along with the stock cam profile. It would however be unusual to see such a massive improvement in low rpm performance with the changes you've listed.

The two runs are on the same dyno, one last year, one last week. Engine is a Nissan vk45, exhaust pipe is same diameter, same pieps except no cats and no mufflers, and some grinding work on the exhaust manifolds clearing up the factory rough welds and big lips where pipes meet, like 4mm high. plus a cold air intake and re-calibration of the MAF table to suit the intake. Paak power went from 212kw to 216kw at the wheels but as per the graph there are a lot larger gains down low.

I have attached a very crude drawing (I cant do art) of the exhaust manifolds, the bits circled in red are what I ground down, and a before and after pic. But I wasn't really expecting much from it, I was just I had them off so decided to do it, if I hadn't pulled them of I would never have notice how bad they were

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I'm a little surprised that those changes netted the improvement down low. I think you'll find the factory exhaust manifolds will definitely be holding you back at high rpm. Beyond that it's hard to say specifically since I'm not familiar with that engine.

Ok, so the results up top are more normal then? The butt dyno and the cars traction control light are agreeing with the dyno results so I don't have any reason to doubt the machine or operator.

I have looked at getting custom headers made, but the NZ$3000-ish price tag has put me off. Its a lot of money if it only comes back with 3 or 4 percent increase. My money is going towards suspension, brakes and tyres next. I hope to get into some grassroots motorsport with the car before the end of the year, a bit of circuit racing :)