Re: N/A maximum? Is turbocharging the only way?
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if it helps at all, im running an 1.8L in my 1600 na, with a tubular mani, straight through, and backbox, JR ignition leads, blitz sus air filter and heat shield, 14.5degrees timing, organic clutch, always ran on super, threw down 151bhp, albeit still on the 1600 ecu, using a flyin miata adaptor plate to use the stock ecu and maf etc
151 bhp, is this a dyno'd figure at the wheels or an assumption at the flywheel?
What 1.8 are you using?
If its a dyno i'd like to take a look at the graph.
14 degrees timing actually yields less max power than 10, this has been proven on rolling road results, it just moves the torque curve. The only bits on your list that will give more power is the manifold and exhaust, maybe 5 bhp on an NA.
The air filter is not the major restriction on the intake side, its actually the MAF. As long as you are drawing air from the engine bay the heat shield wont make much difference.