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Improving the driving experience


John Bonnett
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It has been quite a while since I added an update on the Lightweight but I just wondered if my latest campaign to reduce cabin noise level and thus improve the driving experience might be of interest.

I've tried to carry out the work methodically taking noise measurements at each stage in order to assess the effects. I started with the bonnet putting two layers of material; one to remove the drumming and the second to damp the noise. I felt for a long time that the booming sound was coming from the bonnet. It's worth noting that the decibel scale is logarithmic so a reduction of just one is a ten times improvement.

So, here are the results;

Speed (mph)                          Before insulation (db)               Bonnet insulated (db)

50                                                       83                                                 82

60                                                       83/84                                           84

70                                                       85/86                                           84/85

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Following on from the above, I next tackled the gearbox cover, bulkhead and scuttle (inside the cabin). The gearbox cover was treated with a high temperature material and supplied with fixings that increase its adhesion rating from 100C to 130C. On the bulkhead and scuttle I applied two layers; a Dynamat like product followed by a 10mm thick closed cell foam. This produced a further significant improvement. The last photo shows the footwell works started.

Speed (mph)                           Before insulation (db)                     Bonnet insulated (db)                     Gearbox cover/bulkhead (db) 

50                                                            83                                                      82                                                              79.7

60                                                            83/84                                                84                                                              79

70                                                            85/86                                                84/85                                                        82

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One thing I forgot to mention is the total sealing of the bulkhead and scuttle from the engine bay. The PeaceMat underlay has covered every possible leakage point and this coupled with the insulation on the inside and outside of the gearbox cover should mean little or no heat transfer from the engine bay. The effectiveness of the materials is demonstrated by touching the outside of the bonnet above the exhaust manifold after a run; stone cold.

I've not finished yet. The floor area is next. It already has a layer of Dynamat which damps the drumming but needs ccf to reduce noise. That will be the final job. I'm very pleased with the result so far which has definitely made conversation much easier and the drive more pleasurable. 

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10 minutes ago, Ben Caswell said:

So how much does all the insulation weigh is it still a light weight?

For sure, there's a weight penalty Ben particularly with the PeaceMat XS which is exceptionally heavy. However, as the car is used predominantly for road trips rather than racing I don't think the additional weight will be apparent. What is apparent is the reduction in the noise level which is the priority.

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On my last Herald rebuild, plus on my GT6 rebuild - and my intention is the same for the current restoration - I used a roll of the old felt and bitumen adhesive, cut and glued to the vertical surfaces in the bulkhead - behind and under the dash, and the footwells, plus modern car underfelt for under the carpet. (totally waterproof and costs pennies)

The sound reduction was amazing especially on the GT6 and I could actually hear the CD player. I didn't insulate the rear area so a lot of exhaust noise, but I think in a Herald if the panel behind the rear seats is insulated it will keep a lot of noise out of the cabin.

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That's extremely interesting John, something that's on my list to do. Looks like the tunnel/bulkhead is the biggest winner. Like you I'm not bothered by the weight. Just got a new exhaust (standard) sounds great out side the car, but I'd like to hear it inside! Which products exactly did you use and where did you get them? This would make an informative article for the Courier. 

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9 minutes ago, dougbgt6 said:

That's extremely interesting John, something that's on my list to do. Looks like the tunnel/bulkhead is the biggest winner. Like you I'm not bothered by the weight. Just got a new exhaust (standard) sounds great out side the car, but I'd like to hear it inside! Which products exactly did you use and where did you get them? This would make an informative article for the Courier. 

Doug, I'll drop you a PM. I'm not sure what the rules are about mentioning suppliers. But suffice to say that they are a UK firm delivering to mainstream manufacturers including Bentley. They offer a huge array of products for different applications but I was given good advice and guidance as what to use for my job. 

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John,

I did similar to my GT6 using Silent Coat sound deadening panels. As far as I can see they are similar to dynamat but cheaper.

I havent measured the outcome as you have done but I found the results to be very good. I can still hear the engine note and the exhaust just as you would wish, but it's no longer like sitting in an oil drum. It's possible to conduct a conversation and listen to the stereo at most reasonable speeds. Unwanted heat in the cabin has also been almost eliminated.

 

 

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Definitely a win win Dave; heat and noise reduction in one hit.

I'm on the last bit of the job now fitting the PeaceMat XL to the floor. I used a simple punch to make the holes for the seat mountings which works just as well on carpet. The punch is just a piece of tube sharpened on one end. Hit it with a mallet with a piece of wood underneath and it made a clean cut.

 

 

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Interestingly but disappointingly, none of the last lot of insulation has made any difference to the noise levels. The noisiest part of the car is near the roof which is not good news bearing in mind the amount of work needed to address the situation. It does have a layer of Dynamat but nothing else. Next highest are the footwells despite having a lot of attention and material applied. 

Overall, there has been a significant improvement in noise reduction but I am a bit disappointed that there hasn't been a bigger improvement. Not a huge cost; less than £150 for the materials but a lot of physical effort to apply them. I was hoping for a better return for my labours.

In conclusion, throw everything at the roof, bulkhead, scuttle and gearbox cover and not worry too much about the rest of the car. I hope this write up and my experiences will be of interest to anyone contemplating doing the same job.

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