Throttle body sizing

chodjinn · Post by **chodjinn** » Tue Nov 18, 2008 1:27 pm

I'm trying to size a throttle body for my engine. Curerntly have 2x 55mm Jenveys.

Pi x r squared = 0.0023799 square metres per throttle

therefore total area = 0.00476

I figure a single 80mm throttle will be the equivalent size =

Pi x r squared = 0.00504 square metres

Good enough?? Seen a few second hand throttle bodies between 70-80mm that seem to do the job? I dont want a silly size throttle and i think 80mm should be the absolute max?

topcatcustom · Post by **topcatcustom** » Tue Nov 18, 2008 2:48 pm

Dont forget about the restriction in the AFM!

chodjinn · Post by **chodjinn** » Tue Nov 18, 2008 3:23 pm

No AFM on my motor here mate!!! Dont need one!

topcatcustom · Post by **topcatcustom** » Tue Nov 18, 2008 5:33 pm

doh!

Just outta interest, as I have not decided to build an injection (complicated) motor yet, how does the ecu know how much fuel to squirt in when there's no afm?

chodjinn · Post by **chodjinn** » Tue Nov 18, 2008 6:19 pm

Uses a MAP sensor built into the ECU mate, run a vaccuum line to the Megasquirt connection nice and easy. I've still not wired mine up yet, and i've had it for over a year!

minorv8 · Post by **minorv8** » Sat Nov 22, 2008 8:49 am

I don´t get it. US engines came with quite small throttle bodies and still produce quite good power. I believe 5 litre Fords had something like 54 mm throttle body and they are good allegedly up to 300 hp. Why does Rover need huge throttles, e.g. 2x65 mm and upwards ? Surely the throttle body cannot be the bottleneck ?

Has anyone actually measured the vacuum in the plenum at WOT (if the TB actually is restricting, you should see a change in vacuum) ?

chodjinn · Post by **chodjinn** » Sat Nov 22, 2008 10:33 am

2x65mm would be masive, isn't that what the twin plenums were? Very large. I'm not going larger than 80mm and no smaller than 70mm.

r2d2hp · Post by **r2d2hp** » Sat Nov 22, 2008 11:06 am

I tend to agree with you I dont think it is, and its a myth in most case that the AFM is a restriction as well.

Its just most aftermarket ECU cant use an AFM.

On the emerald ECU it does not really use map sensor. In the main it just based on throttle position and input from the WideBand Lambda sensor. The Map sensor is only configured when +ve boost comes in as a correction. Thats the way Dave Walker maps them or has done mine.

If you want to know chapter and verse on throttle bodies, afm and flows then give Mark Adams a call, he will keep you straight

kokkolanpoika · Post by **kokkolanpoika** » Sat Nov 22, 2008 3:14 pm

minorv8 wrote:I don´t get it. US engines came with quite small throttle bodies and still produce quite good power. I believe 5 litre Fords had something like 54 mm throttle body and they are good allegedly up to 300 hp. Why does Rover need huge throttles, e.g. 2x65 mm and upwards ? Surely the throttle body cannot be the bottleneck ?

Has anyone actually measured the vacuum in the plenum at WOT (if the TB actually is restricting, you should see a change in vacuum) ?

If remember right my friend 5.0GT (efi) mustang throttle body is approx 60-65mm?

And i have hear/or read somwhere that rover std plenum is slightly resistive even if it was 72mm? Because plenum floor or is not sou "airodymaical"?
That´s why they do carpon units and 2-3 throttle system.. The carpon plenum is mutch smoother? inc JED aluminium douple or triple unit?
'
I have chek the vacuums with 72mm plenum and stage 3 4,6engine.. Haltech will show approx 980-1000mbar @5000-6000rpm.. Not stable reading.
And with my siamesed plenum (2x65mm) it will show stable ~1013mbar @5000-6000rpm

std 65mm plenum is good for 280hp and 72mm is good for 320hp..

chodjinn · Post by **chodjinn** » Sat Nov 22, 2008 5:09 pm

so 80mm should be good for my power aims then!

kokkolanpoika · Post by **kokkolanpoika** » Sat Nov 22, 2008 5:23 pm

I think Jeremy Clarkson 5,2litre Rover SD1 run 78mm carpon unit? And power output is 360hp..? Why they want to but too big throttle in it, if it make no sense?

dnb · Post by **dnb** » Sat Nov 22, 2008 5:43 pm

minorv8 wrote:Has anyone actually measured the vacuum in the plenum at WOT (if the TB actually is restricting, you should see a change in vacuum) ?

Yes. The restriction cost over 50 bhp on my car.

Here's a couple of graphs of datalogs from dyno runs on my car. They are before and after the twin plenum upgrade (with two "massive" 63mm TBs instead of a single 65mm) . The "after" graph shows evidence of clutch slip too.

Before:

After:

K4000JB · Post by **K4000JB** » Mon Nov 24, 2008 6:20 pm

i was taking to a mate about this the other day and we came to the conclusion the more power came from having multi throttle bodies due to the air being more evenly distributed to the cylinders

Wotland · Post by **Wotland** » Mon Nov 24, 2008 6:45 pm

Just for exemple, this one uses twin 55mm if my memory is good :

chodjinn · Post by **chodjinn** » Mon Nov 24, 2008 8:54 pm

currentl have twin 55mm throttles so a single 80mm will be good enough for me. For the time being . . .