Question: noise level of a flat 6 on open pipes?

911hillclimber · Post by **911hillclimber** » Mon Jul 27, 2009 9:32 pm

Does anyone racing have a dB level for a flat 6 on open headers please?
Graham

Lightweight_911 · Post by **Lightweight_911** » Mon Jul 27, 2009 9:42 pm

I can attest that they are LOUD !!!

I had a pair fitted to an early 911 that I imported from the US many years ago & a similar set on the 914/6 GT ex-race car I imported last year (2.5 litre twin-plug engine).

Out of interest I checked the sound level with a digital noise meter ...

... the reading was 140 db at ~ 5500 rpm !!!

Gary71 · Post by **Gary71** » Mon Jul 27, 2009 9:54 pm

140 db

Staggering. Maybe that lump of stainless I carry around is doing something then!

911hillclimber · Post by **911hillclimber** » Mon Jul 27, 2009 10:13 pm

So... a bit too much for the 110dB on a hillclimb then..
Saves me making some to try.
Thanks!
Graham.

Matt black70 · Post by **Matt black70** » Mon Jul 27, 2009 11:13 pm

Hi Graham,I have a db meter if you want to borrow it.
Matt

Luke · Post by **Luke** » Tue Jul 28, 2009 9:19 am

you could try something like this to bring the noise down (i think Andy had some fitted to one of his beasts in the past, but my memory is not what it was...)

http://www.supertrapp.com/product_secti ... p?CatID=30

911hillclimber · Post by **911hillclimber** » Tue Jul 28, 2009 12:33 pm

Luke:
Clean forgot about Super traps, so thanks. Discarded them first time round on cost, but new they seem a good way to go as they are adjustable and can be very short in dimension too.

The Silencers I have now are cross-over, but i can see almost straight through them so cant be too restrictive?

A compression test might be a good next step. Maybe the engine is worn.

MikeB · Post by **MikeB** » Tue Jul 28, 2009 3:56 pm

Graham

How about two Formula Ford 1600 silencers, they are reasonably short and working on the theory that you have two banks of 1600 cc each, they should be able to cope

911hillclimber · Post by **911hillclimber** » Tue Jul 28, 2009 4:46 pm

I think they are not 2'' pipes, but will check.
Dare I say it, they are pig ugly too!

But cheap enough to try. The silencers i have are very straight-through, so can it really be reasonable to think they are restrictive?

The power would be down if the gasses were hindered in some way, not that the sound waves are being absorbed?

The inside of the pipe in the silencer (Custom Chrome) are serrated to pass gasses into the silencer fibres, but the mass of the gasses are straight out the 2'' pipe passing through the silencer box.

I need to do the back to back test on the Rollers.

MikeB · Post by **MikeB** » Tue Jul 28, 2009 5:04 pm

Graham,

Are you thinking your current silencer system is sapping your power?

If so I would doubt that it is making much difference, as long as there is no major restriction in the system. If there is a problem with the exhaust it will be towards the manifold end of the system, and only then if you have used tubes that are far too small.

Having cobbled together several systems for various hillclimb cars using the "that sort of looks right" attitude, I was most surprised when an engine builder commented that my system ( designed for an 1100 engine) supplied more power than his when fitted to a 1400 unit.

So if the primary pipes are slightly bigger than the exhaust port on the head, I would reckon your system wouldn't be too much of a problem.

I'd look elsewhere.

911hillclimber · Post by **911hillclimber** » Tue Jul 28, 2009 9:40 pm

Interesting...

The system started with a set of tired 2.4 early headers.
I have the same set-up on my hillclimb/road 3.2 on stock injection in my 1973T. The silencer is a works 'Liestricht' (spelling) 3 pipe system and it is great. I guess about 220 bhp but don't know for sure.

With this in mind I found the stock 2.4 headers would not fit because of the tube lengths needed for the equal length design, so I cut them up to fit and collect into the stock 3 into 1 collector. They don't look too bad, but certainly are not equal length!

For hillclimbs the noise limit is 110 dB@ 4500rpm, so I chose these big silencers, 2'' pipe in, and 2'' out.

After Bob has rolling road tuned the webers the ends of the stainless silencers are yellow (oxide tone on the steel) all over, so they have been hot. The inlet side of the silencers are still 'natural' polished.

The headers are fully wrapped incidentally to the connection to the said silencers.

The headers during cut-n-shut:(forgive the welding please)

MikeB · Post by **MikeB** » Tue Jul 28, 2009 9:51 pm

Graham

The only time I had a problem with an engine not producing power, was an Alfa Guilia 2 litre, which had a modified head and a competition exhaust. However, it would not rev and the power flattened off at 6000. It turned out the while the inlet was nicely modded, the exhaust ports were std. Opened them up and the thing flew round to 8000.

What did Bob think of your power curve? Was it just low everywhere?

911hillclimber · Post by **911hillclimber** » Tue Jul 28, 2009 10:11 pm

Bob was 'dissapointed' by the result. He was expecting more, but would not predict how much.
Max power (145kW) (197bhp) @ 6350
Max Torque 265Nm @ 4580

I felt the same but tried to put a brave face on it.

From the details in the Anderson book, I was expecting about 230 bhp/Lbft

I actually do not know the condition of the 3.2, but the engine/box came out of Terry Davison's Nomad Sports racer and was rebuilt some time ago by Crispin Manners, so I expect it to be good condition, but a compression test might be a good idea.

The webers are ultrasonically cleaned, 34mm chokes and fresh jets/emulsion tubes etc. The manifolds are 40mm bored straight through PMO's and the fuel pressure is 3.5 psi max.

The result is weak, and I have every faith in Bob after knowing him since 1989, so I'm sure he has squeezed every HP out of the thing.

I should remember that even at 200 bhp it is in a car weighing just 549Kg so = 364 bhp/ton

The gearbox is of greater importance and is my first priority at the moment. This issue just bugs me.

I think the box need TLC on 3rd and a Wevo conversion inside, but no real money available right now.

MikeB · Post by **MikeB** » Tue Jul 28, 2009 11:06 pm

OK I have a Weber jetting programme. Tuning your engine for Power it reccommends a 38/39mm choke, for a Flexible engine it says a 37.

My 2.2S engine is running 32 chokes in PMOs, so 34s in a 3.2 are starting to sound a bit narrow.

If it is the choke sizes, your engine would probably go quite well up to the mid range, producing the sort of power expected.

My programme reckons that the 34 mm chokes will start to restrict the engine by 5500, so at 5000, you are probably getting the sort of power a 3.2 should produce, but from there on the engine will be starting to get restricted.

I've found this programme to be pretty accurate in the past, so I would put my money on you inlet side of the engine being why you are not getting the max power you expect.

911hillclimber · Post by **911hillclimber** » Wed Jul 29, 2009 6:09 am

I asked several people about choke sizes for the engine and it's use. Hillclimbs need torque low down so that was my goal.

PMO supplied the chokes, and they are 36's.

Anderson's book says 36 iirc and Bob said 36 too.

I do remember thinking there was very little venturi shape to them when they arrived.

Despite all this the engine pulls very strongly and the throttle response is very sharp, but I'm used to softie injected engines nowadays!

This brings me back to the headers again.

They are the small tube dia for the 2.4 so markedly smaller than the giant 3.2 ex ports.
I am considering for the winter garage scene making a set of 1.625 ID manifolds using Turbo Thomas parts.
There is little room between the exhaust head sode and the lower chassis rails of the car, and 'SSI' header 'depth' is about the most I can squeeze in depth wise, but things are tight in width too.
Any headers made will not be equal length.

Thanks for the help,
Graham.