50 PMOs on 2.8

[911 SLANT]

Can 50 PMOs work on a 2.8 motor with J&E 9:5.1 pistons, MOD S cams, 40/35 ports, Rarelyl8 headers and M&K GT3 sport muffler? What would I have to do to run these?

[SoyRacer]

Work - yes

Work well and make power where you need it - No

40mm are ideal and 46mm can sing with some tuning time though.

[1QuickS]

With 911S cams in a 2.8 you will want 36mm venturis which would be 40mm throttle bores.

[RWebb]

use the 50's whenever you rev it over 14,000 rpm

[911 SLANT]

Cant the Venturis be changed to smaller ones? Reason I asked is I can get a good deal on 50mm PMOs. I thought I could change venturis and jets to work with my 2.8 Would I be better off running 46mm PMOs with 36 venturis?

[stownsen914]

For any street use, 50s are too big for a 2.8. You'd risk the engine running poorly at anything other than high RPM.

1QuickS is a (humble) 911 carb expert, in case that isn't obvious.

[911 SLANT]

I spoke with Paul (1QuickS) personally at the Phoenix Club swap meet and he recommended 46mm PMO with 36 venturis.

[Gordo2]

Originally Posted by 911 SLANT

Cant the Venturis be changed to smaller ones? Reason I asked is I can get a good deal on 50mm PMOs. I thought I could change venturis and jets to work with my 2.8 Would I be better off running 46mm PMOs with 36 venturis?

Venturi can be swapped but the throttle bore is fixed.

Think of where the throttle plate / butterfly opens and closes against the interior wall of the throttle bore - the diameter measured at that point is the carb's size.

You could try to drop small vents in a larger bore - but the combo would be difficult if not impossible to get to acceptable flow / performance. Think of the flow: 50mm initial opening /top of the carb, choked down to 36 with the vents, back out to 50mm at the throttle plates, back down to XXmm at the manifold and down to XXmm at the intake/intake valve. Fuel/air charge flow would be like sprinting from ride to ride at an amusement park.

PMO comes in 50's, 46's and 40's. Correct sizing per motor is essential to solid performance, and the sizing recommendations are science and experience based (not me - the pro's...). Sometimes you have to stick to what's known to work.

Good luck,

[1QuickS]

I'm adding 38mm in 46mm bores if you expect to make peak HP at 6600 RPM, should have proposed that earlier in addition to the 36mm recommendation. The 36mm recommendation was based upon some customer feedback and the 38mm recommendation is from the venturi sizing chart from my website.

A larger throttle bore than recommended (46mm) will soften throttle response at lower engine speeds. I have run 46mm PMOs on my 2.3S motor (43mm throttle bores in my Webers) when I was doing some testing for PMO and I didn't think the larger bore size was an issue of concern.

Weber's recommendation for venturi size is .7 to .9 times throttle bore diameter which for 50mm bores gives a venturi diameter range from 35mm to 45mm with 40mm being the optimal venturi diameter. So, 38mm venturis in 50mm throttle bores should be fine.

The link is to a throttle body sizing tech page from my website which should be of some help:

Performance Oriented

[911 SLANT]

But, 38mm venturis would be to large for my 2.8 build? If so, and I go with 46mm PMO. How would you set these up for my 2.8 build Paul?

[1QuickS]

Again, have a look at my web site discussing venturi & throttle bore selection. Venturis are selected based upon what RPM the engine makes peak HP. 911S cams in a 2.0 made peak HP at 6600 RPM. I do not know what RPM 911S cams would make peak HP in a 2.8 but I think it would lower that RPM from 6600 but I do not know for sure.

If your engine did make peak HP at 6600 RPM and you used the venturi sizing chart from my website then you will see 38mm venturis are what you want. Then, based upon Weber's throttle bore sizing you will find that throttle bore diameters from 54.3mm to 42.2mm would be fine with 47.5mm being the best. (These bore sizes are calculated by dividing 38mm by .7 to .9 which is Weber's recommended sizing formula.)

Since you have access to 50mm PMOs at a good price then you should be OK to use them.

Richard Parr recommends 38mm venturis in 46mm throttle bores for a 3.0 with 911S cams so there is that. A 2.8 is not that much smaller so I would not worry about 38mm venturis in that.

A 2.8 will have quite a lot of torque at lower RPMs so unless you want to have more midrange power the 38s in 50s should be fine. However, if you are more inclined to desire a broader torque curve then 36s in 40mm bores would be good; I have a customer with a 2.7 with 911S cams generating 200HP at the wheels with 34mm venturis in 40mm bores and another with 36mm venturis in 42mm throttle bores making 210HP at the wheels. So, 38s in 50s would be OK but better would 38s in 46s and most probably for real world driving (90% of your driving time is street driving) 36s in 40s.

[lvporschepilot]

Originally Posted by 1QuickS

I have a customer with a 2.7 with 911S cams generating 200HP at the wheels with 34mm venturis in 40mm bores and another with 36mm venturis in 42mm throttle bores making 210HP at the wheels.

is that single plug or twin with higher compression?

[1QuickS]

Single plug, 9.8:1 compression