17 replies to this topic

[carbon]

Just been flicking through the April issue of Mini Magazine, the article on Jonathan Lewis's 1275GT grabbed my attention. He's using a Paul Inch built 1293, and getting 112bhp at the wheels with a 649 cam and 1.5 rockers - so there should be a lot more than this at the flywheel.

 

The really interesting bit is that his car is running with twin 1.25 inch HS2 SU carbs, which are considered by many to be on the marginal side for even mildly tuned fast road engines...

 

So the question is - who's the daddy with twin HS2 power output from A-Series motor?

[KernowCooper]

112bhp at the wheels ? non turbo and SU 1 1/4s ? I know he is a respected builder but surely thats a misprint? given transmission losses then that would indicate appprox 130-135bhp at the flywheel.

 

Given the formula that every bhp takes 2,2cfm of air then the 1 1/4s need to flow 286-297cfm.

Edited by KernowCooper, 21 March 2014 - 07:34 PM.

[grumpy dad]

Hi 

the flow rate of a pair of HS2 

is the same as a single HIF44

you have a longer intake with the HIF44

so you might get a bit more torque

[KernowCooper]

Where are you getting the flow rates for a twin  1 1/4 HS2 the same as a Hif44 good, on the normal maniflod which is a log design maniflod then only one cylinder is on the intake stroke at any one time then the cfm is only what one HS2 can flow not the 2 added together due to the manifold design and siameeese inlet ports, but the true cfm will be lower for any carb once connected to  the head.

 

I read somewhere 85bhp at the flywheel was considdered the upper limit of standard twin HS2 setup

Edited by KernowCooper, 21 March 2014 - 07:59 PM.

[Cooperman]

I think it's a misprint. Never heard of a car producing around 130 bhp at the flywheel on twin Hs2's. It's a struggle even with the 649 and twin HS4's.

[KernowCooper]

Misprint but Paul Inchs phone will be hot no doubt

[grumpy dad]

Hi

 

According to David Vizard's data, the flow rates are:

 

1.25" H2 SU, 110 cfm.

1.50" H4 SU, 133 cfm.

1.50" HS4 SU, 142 cfm.

1.75" HS6, 210 cfm.

1.75" HIF6, 240 cfm.

[KernowCooper]

Yes agreed but on a siameese inlet no one cylinder can see more than the 110cfm of a single carb as the inlet manifold is not normally joined between inlet tracks, they are in essence two carbs on seperate inlets feeding 2 cylinders on a siameese head.

 

The same is true of a 40dcoe stup no 1 cylinder can see more that what ever the cfm is for a 36mm (biggest std choke for a 40dcoe)

 

See page 81 of Vizards Yellow Book info there on twin chokes or twin carbs and what the engine sees

Edited by KernowCooper, 21 March 2014 - 09:05 PM.

[Cooperman]

Even the Swiftune FIA Historic 'S' race engines don't produce that sort of power, even for £20,000 each, with HS4's and a full race cam.

[Earwax]

I would think a misprint or an error repeated down the line.

On flow capacity , i am all with KernowCooper, the twins only see the stated flow rate not double as some expect, and the other factor is the flow through the carb and manifold. Whilst horsepower/bananas is a bit variable on different machines i would have thought that 112 at the flywheel was still too high for hs2s, and i have always pictured 110 flywheel approaching max for twin HS4s.  However back to the original point, it would be good to hear of results for twin hs2s

[mini13]

or just another RR that over reads....

[ACDodd]

Standard twin hs2 and stock factory cast manifold;

100hp @ flywheel, 1330cc stock rockers, stock lift, stock 1.312" intake valve size and one very custom ACD cam.

Ac

Edited by ACDodd, 21 March 2014 - 10:57 PM.

[jaydee]

No doubt that a misprint, but i'd be curious of what inlet, ram pipes and extra work these carbs have.

Both paul inch and jonathan lewis know thir stuff.

[Dusky]

Standard twin hs2 and stock factory cast manifold;

100hp @ flywheel, 1330cc stock rockers, stock lift, stock 1.312" intake valve size and one very custom ACD cam.

Ac

Out of intrest, how heavy were the SU's modified?
And at around what rpm did the top power come? :)

[KernowCooper]

Bit old of a thread but as Cooperman pointed out about the swiftune engine, but back to your question about 13000rpm