Lessons Learned:
Adding the butyl layer to the camping mat to the heat shield there is about 12 mm of insulation. The cap to the overflow bottle for the engine cooling system sits high and fouls on the insulation. If you don’t mould the layers of insulation to the contours of the bonnet you will find the insulation may foul on other things under the bonnet ie the intake pipe with the air flow meter, which sits high.
As mentioned in the prior post I was not happy with the bulkhead vent seals. As anticipated the lip on the bulkhead vent pushed the seal down therefore there was no seal on the outside of the lip on the high side of the vent. I removed the 12 mm thick seal on the high side of the vent and replaced it with a 9.5 mm thick seal. It now clears the lip and should seal better.
I noted that the seals on the sides and low edge of the vent flap had the impression of the lip from the bulkhead. That caused the seal to form 2 hills and valley effect. I suspect that the seals were not sitting flush to the opposite surface thus not sealing properly. I nicked the seal with a razor blade on the line of the lip impression. That allowed the seal to resume its shape and theoretically will permit the lip it sit inside the nick.
Overall impressions:
The change in db readings after the bonnet install are not significant and suggest no improvement. The aural experience suggests otherwise. The truck at 60 km/h, 80 km/h and 100 km/h is significantly quieter. The engine rumble is still there but the peaky noises are subdued. The highway cruising experience much more tolerable. The wife gave it the tick of approval. The reason for the insulation exercise in the first place was because my loving wife refused to yell over the cabin noise on a trip back from Newcastle one weekend.
The best bang for buck is the cabin. That area gave the greatest reduction in db reading. The bonnet and rear seat floor were marginal improvements. That said every area focused on was an improvement (however marginal) and the whole is greater than the sum of its parts. Doing the cabin and not doing the door seals or bulkhead vent seals won’t be as effective. The bonnet is probably not worth the effort for reducing in-cabin noise unless you drive on the highway with the vents open. At that speed the noise is generated by wind turbulence more so than the engine. I recommend you replace the bulkhead vent seals if you have an aged vehicle. There was a significant improvement in noise from fixing the seals (especially after I nicked the seal and fitted a seal that bridged the upper bulkhead lip).
For a simple comparison what to expect the aural noise now in the cabin at 100 km/h is the same aural noise level that I experienced at 80 km/h prior to insulation.
Overall cost – 20 rolls of insulation at $23 a roll ($460) + 2 cans of Quik grip spray on glue at $18 per can (say $40) + heat insulation mat at $100 + roll of 3 mm insulation applied to the door seals ($25) + about 3m of 12 mm seal for the bulkhead seals at $10/$12 p/m (say $30) = $650 ish.
