150 - 160 bhp xflow - Carbs

[Casbar]

Hoping to re-build my engine to around 150bhp. Seems to be differing views depending on who you talk to, on carb sizes, between 40s and 45s. Is anyone running this kind of bhp with 40's, if so, what choke sizes, jets are you using.

 

What sort of bhp requires an upgrade to 45s

 

Casbar

[Graham Sewell]

From Dave Andrews' jetting program, assuming that you want max power rather than flexibility, 40's can be used on 1700 upto 6900rpm and 1800 upto 6700rpm. It all depends on whether you can get the 150bhp out of the engine within these rev limits - otherwise the engine will be slightly asthmatic above them.

 

If flexibility is the prefered route, then the rev limits are 7400rpm and 7200rpm respectively.

 

As Dave would say, these figures are only for initial reference and not a substitute for a rolling road session.

 

Cheers,

 

Graham

 

 

 

[John]

I have 163 bhp on 40's. Chokes are 34mm with a rev limit of 7300rpm. To make the engine useable at low revs, smaller chokes are better to improve air speed and engine pick up.

 

John

[Robster]

I have forged pistons, accrelite 1800's, but have standard, lightened and balanced rods and crank. Is this safe to 7200 rpm???? What limit would you set the rpm to?

[Graham Perry]

Graham Sewell , Is Dave Andrews jetting programme on his web-site. I have had a look about but cannot see it ?

[Graham Sewell]

Try Jetting program for Weber DCO and Dellorto DHLA carbs.

 

[Roger King]

As a general rule, 45s are preferred for outputs over around 150bhp, but in reality, the difference between 40s and 45s up to around 155bhp are small enough to make the extra expenditure not worthwhile unless you are racing and need every last bit (this is assuming that you already have 40s). If starting from scratch, go for the 45s.

 

It is perfectly possible to achieve more power than this on 40s, but you run into the problem that it is impossible to jet correctly over the entire rev range, leaving you with extreme weakness or richness at some point.

 

All of the above is only a rough guide. You may have two engines that have the same air consumption at max power, but one has much lower internal friction, etc. This engine will show more power, but the demand on the carbs will be the same as for the lower powered unit.

 

Hope this makes some sort of sense.