Ive got 2 1.8 cvh engine n a 2.0 twin cam engine which engine would be better for modifying in my sierra sapphire

 

id say the 2.0 dohc iv had a few of both not seen eny 1.8 cvh tuned but seen quite a few twin cams with turbos and about 180 bhp but im no expert just had standerd ones twin cam with four branch manifold and scorpion exhaust an induction kit made a bid diferance in power delivery

 

Neither are great for tuning options. There is nothing at all really for the 1.8 and the dohc isn't great other than the turbo kits. The single cam pinto 2.0 has lots of options (my dad has a mad 180bhp one in his Westfield) but don't think there's much for the dohc.