MGB & GT Forum

Hi everyone,

My current MGB engine is getting due for a rebuild. I envisage that, once rebuilt, the engine will have the following specs (some of which will be carried over from the current specification):

- 40 thou overbore;
- 12H 2708 cylinder head i.e. 1.625" (43mm) inlet valves, 'open' combustion chamber design, standard head thickness less skimming for squareness, all ports smoothed internally and matched to manifolds;
- Approximately 9.2-9.5:1 compression ratio;
- Sand bent extractors, 2" tailpipe, single silencer;
- Customised distributor;
- Local 98 octane unleaded petrol (no ethanol.)

I have a pair of HS6 carburettors (which I'm not currently running on the car) and am wondering which needles and springs would work as a good starting point. The SY needles quoted in the old special tuning documentation have a fairly aggressive (rich) profile and might be too much. Can anyone offer any suggestions?

Forgot to mention will probably get a Piper BP270 cam...

I was just going to answer that without a cam the needles would be too rich. Now that you've included the cam info, I'd try the SY and see.

If you need a needle that approximates the SY until about half way up the needle and then stays leaner when the SY gets richer, you can try an RH or SM from Moss (TR3 rich and normal, respectively) (or an SW, even leaner - TR4A)

---

Bill Spohn www.rhodo.citymax.com/carstuff.html
Current: 1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1957 Jamaican MGA (200 bhp)1965 1971 Jensen Interceptor (350 bhp)
2009 Pontiac Solstice GXP Coupe (375 bhp)
2007 BMW Z4M coupe (340 bhp)
Recent: 1969 MGC roadster (175 bhp),Jensen CV8 (375 bhp),
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
North Vancouver BC

Many thanks, Bill. Much appreciated.

I don't have gander as to what you would need for your street engine as for HS6s, but I used this needle spread sheet extensively for the race engines with twin HS6s, and it's a nice tool that will let you compare several needles at one time .
A the top of the page, select HS6 .100" needles, and have fun.

http://www.mintylamb.co.uk/suneedle/

---

Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Nice tool, indeed. I remember coming across it a little while ago. Generally I've plotted my own spreadsheets in Excel for comparison, using the needle profiles quoted in those old SU tuning books from the 'seventies. But this program makes it much more immediate, provided the profile you want has already been added!

Just saw another thread where people were suggesting that 1 1/2" carbs were capable of flowing more than enough air and fuel for all but the most wildly modified B-series engines. I don't have access to a flow-bench, but my father once owned a mildly breathed-upon MGA Twin Cam which, of course, had 1 3/4" carbs standard. It was fairly docile (but not sluggish) below 3200 rpm, but we always attributed that to the (relatively) small capacity as opposed to it being over-carburetted at these revs. With this in mind, would HS6's really hurt my MGB's performance at moderate speed and load conditions if I mounted them on the engine I mentioned earlier in this thread?

If you are a typical driver that spends 95% of their time in the lower rpm ranges, you'll lose some bottom end with the HS6 conversion. If you are the type that is always wringing it out and runs a lot of the time between 4,000 and red line, the HS6 would be a good choice for you.

I've always considered it a poor trade if you are giving away performance for bragging rights. The larger carbs aren't going to kill the bottom end, they'll just make it a bit more sluggish. Be honest about what you'll be doing with the car and do what makes sense. Given the stuff you've put into your engine, it seems to me you may very well run it in a way that you'd get some benefit from the larger SUs.

BTW, the Twin Cam was no more sluggish than the MGA 1600 pushrod - in fact they power curves pretty much match each other in the bottom half. But when you got up past 4500 and the pushrod curve starts to 'turn', the Twin Cam just stays a straight line up past 6700.

---

Bill Spohn www.rhodo.citymax.com/carstuff.html
Current: 1958 MGA Twincam (race car (170 bhp)),1962 MGA Deluxe Coupe (98 bhp)
1957 Jamaican MGA (200 bhp)1965 1971 Jensen Interceptor (350 bhp)
2009 Pontiac Solstice GXP Coupe (375 bhp)
2007 BMW Z4M coupe (340 bhp)
Recent: 1969 MGC roadster (175 bhp),Jensen CV8 (375 bhp),
1969 Lamborghini Islero S (350 bhp), 1988 Fiero GT turbo (300 bhp)
North Vancouver BC

Put on HS4's tuning will be easier,throttle response will be sharper.
Your engine build is basically a warmed up road engine. Does't require 98 octane fuel and HS6 carbs.

---

LNDRVR4X4.COM
Home of Project "INCARN8'


1957 Series 1 Land Rover electric VEHICLE CONVERSION

FIXITUPCHAP.COM
FIXITUPCHAP INCORPORATED

RD3 Radar/ Electronic Warfare Technician
VIETNAM 1969-1972