The Triumph Dolomite Club - Discussion Forum

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 Post subject: HS8 carbs
PostPosted: Wed Apr 11, 2018 12:59 pm 
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Would a pair of HS8 instead of HS6 carbs do anything to improve power, or is it just a waste of money?

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 Post subject: Re: HS8 carbs
PostPosted: Wed Apr 11, 2018 1:03 pm 
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You'll need a fatter exhaust and HS8 with the low profile chamber.

Edit: See post viewtopic.php?t=26252#p250691

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Current fleet: Triumph Dolomite Sprint '75, Daihatsu Fourtrak, Honda CG125, Yamaha Fazer 600, Shetland 570

Disposal fleet: Golf GTi 16v MK3 Anniversary

Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated it on pickup from dealer, was made out of magnetic metal as only car I've ever been crashed into, 4 times), Golf GTi MK3 16v x 3


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 Post subject: Re: HS8 carbs
PostPosted: Wed Apr 11, 2018 1:47 pm 
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I have a straight through exhaust system, if that helps

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 Post subject: Re: HS8 carbs
PostPosted: Wed Apr 11, 2018 3:25 pm 
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They may give you a bit more power but without changing to a different camshaft you won't be utilising the carbs to their full extent. Plus you will loose MPG.

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 Post subject: Re: HS8 carbs
PostPosted: Wed Apr 11, 2018 4:54 pm 
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I was told once to think of an engine as an air pump. Air is sucked in and expelled. The amount of air sucked in on each stroke is dependent on the volume swept by the piston, the design of the cylinder head, the profile of the camshaft, and the design of the exhaust.

To get adequate vapourisation of the fuel from the carburettors the air must be speeded up in the carb venturi. This is why the venturi is known as the choke. By squeezing the air into a narrower channel it must speed up, a bit like putting your finger over the end of a hose pipe. Generally the carbs fitted as standard are matched to the engine so that happens optimally. If you fit bigger carbs without altering the cylinder head, and/or camshaft, the air won't speed up as much, fuel vapourisation will be poorer, and the engine will be less efficient. It may produce less power than standard and it will almost certainly waste more fuel.

It is generally incorrect to think that bigger carbs mean more power. Here endeth the first lesson.

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 Post subject: Re: HS8 carbs
PostPosted: Wed Apr 11, 2018 5:32 pm 
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Changing the choke (venturi) size changes a lot more than just adding some extra BHP, the size is chosen together with the jet/needle to distribute evenly, to provide the right amount of torque as well as BHP at the right engine speeds whilst maintaining the correct AFR. I can think of a number of mates who have stuck twin 45 Webers on low capacity motors and found that the performance was way worse than stock because as said above, the AFR was not only incorrect but fuel was unable to atomize correctly.


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Current fleet: Triumph Dolomite Sprint '75, Daihatsu Fourtrak, Honda CG125, Yamaha Fazer 600, Shetland 570

Disposal fleet: Golf GTi 16v MK3 Anniversary

Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated it on pickup from dealer, was made out of magnetic metal as only car I've ever been crashed into, 4 times), Golf GTi MK3 16v x 3
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 Post subject: Re: HS8 carbs
PostPosted: Thu Apr 12, 2018 8:11 am 
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Quote:
I was told once to think of an engine as an air pump. Air is sucked in and expelled. The amount of air sucked in on each stroke is dependent on the volume swept by the piston, the design of the cylinder head, the profile of the camshaft, and the design of the exhaust.

To get adequate vapourisation of the fuel from the carburettors the air must be speeded up in the carb venturi. This is why the venturi is known as the choke. By squeezing the air into a narrower channel it must speed up, a bit like putting your finger over the end of a hose pipe. Generally the carbs fitted as standard are matched to the engine so that happens optimally. If you fit bigger carbs without altering the cylinder head, and/or camshaft, the air won't speed up as much, fuel vapourisation will be poorer, and the engine will be less efficient. It may produce less power than standard and it will almost certainly waste more fuel.

It is generally incorrect to think that bigger carbs mean more power. Here endeth the first lesson.
Wot, no homework sir? :)

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1972 Spitfire MK IV
1972 Stag
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 Post subject: Re: HS8 carbs
PostPosted: Thu Apr 12, 2018 8:26 am 
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Well right from the start I don't have any figures or charts to rely on.
However when I got my sprint it was on 1 3/4 SU's
I went to 2" SU's and made alloy mounts and opened up the manifold to suit (chain is as strong as its weakest link). This worked very well and the engine revved far more freely beyond 5000 than before.
Then I put on a set of bike carbs from a Kawasaki TR6 (4x 38mm Keihlins) and put them on a cobbled up home made manifold and these went just as well as the 45 Dellorto's that I finally settled on. All these combinations returned 28 mpg and my driving was mixed in as much as I only give the car some beans when its safe and I could enjoy some power. At other times I drove carefully to save fuel.
Sometimes I think that tuning cars is a waste of money as the roads are so clogged up with traffic and the road surfaces so potholed and rough that unless doing track days or similar why waste money and time.

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 Post subject: Re: HS8 carbs
PostPosted: Fri Apr 13, 2018 2:51 am 
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I suppose it depends on what you define as power. As Jon says it will allow the engine to rev more freely at the top end where 1.75in carbs give a restricted feel. However, from my experience you lose drivability at lower revs. Depends on what you want. My 2in SUs are now sitting on the shelf and the originals are back on. 99% of the time the car is now better to drive.

Incidentally my 2in ones had a different choke setup which was very difficult to link up and get working. Thus it was also harder to start and horrible to drive until it warmed up.

Geoff


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 Post subject: Re: HS8 carbs
PostPosted: Fri Apr 13, 2018 8:27 am 
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Thanks for your input guys, appreciated

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1972 Spitfire MK IV
1972 Stag
1980 Sprint


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 Post subject: Re: HS8 carbs
PostPosted: Fri Apr 13, 2018 10:10 am 
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Some HS8 carbs have different pistons and iirc these were of a Rover. Full open was piston 3/4 open. These carbs were too big for that engine but Jags had also HS8 so we too. These pistons were a different casting and had a notch that limited the travel in the bowl. Had a customer once with tripple HS8 on his XK what was not running ok. Did find the issue and he said they were from a Rover.

Jeroen

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