matching throttle bodies to engine size

[Kev_205]

Now i've been pondering. How do you match individual tb size to the engine? Most 2 litres I see run 45's but some go for 48's and touring cars are yet bigger still at 50mm. I'm going to stick my neck out and say its cam profiles rather than cylinder capacity but if so is it just "try different combinations on the rr" or is there some complicated maths to chew on?
Then throw in optimum trumpet length to boot - but thats a lot easier to test on the rolling road

[graham bahr]

it could be argued that you only want slightly bigger than the port size, seeing as throttle bodies dont suffer anythng like the flow restrictions of carbs.

[imull]

it will depend on the speed the engine goes round at, the cam profile, the profile of the throttle bodies/carbs and a variety of other factors. Basically, air at STP cant travel faster than the speed of sound (not strictly true but more or less so). So you design your inlet system so that it is big enough that no matter how much air is needed it doesnt meet this requirement...

my 998cc imp for instance runs on twin 40 Webers but with a range of choke depending on what cam i want to use (or so the previous owner says)

[richard_sykes]

For a Carb the formula is, (cfm)

((rpm x displacement)/3456)x1.1

e.g. Chevy 350 (built for 7000rpm)
((7000 x 350)/3456)x1.1 780cfm

A 650cfm carb is used. (Big engine and RPM!! so dont worry about big figure )

Thats just an example, not sure whether there is one for TB`s.

Going for the size smaller but closet is better to apparently.

(from Auto Math Book)

[Richy_Rich]

Is that the maximum rpm of the engine? The reason I ask is that the max rpm is often out of the power band, so would it make sense to susbstitute the rpm at which you expect(want) most power in the equation?

[zefarelly]

all my engine is calculated . . . the trouble is where do you start ?

for me I have a few restrictions, so theres your constants, and work from there.

valve size may restrict it? , as will maximum port size? I can't get my inlet ports more than about 33-34mm at the manifold face without hitting fresh air/water jackets etc and homologated carbs are 40mm. inlets are 40mm

i DON'T THINK i'D GAIN MUCH FROM 45'S AS A CONSEQUENCE . . . .ALTHOUGH THROTTLE BODIES . . . . hmm

[graham bahr]

hmm, generally throttle bodies dont need to be anywhere near as big as carbs, a 40mm throttle body will have a 40mm hole all the way through and the omly restriction being the throttle plate and spindle, assuming its not a roller, barrell or slide type, but a 40mm carb will also have an aux venturi, and a choke which on a 40mm carb cant be any bigger than 33mm if you want it to work properly, so in reality you would need at least a 45mm carb or probably 48 to flow as much air as a 40mm throttle body.

most throttle body sizes are based on carbs and as such are bigger than they usuallly need to be, but fortunatly for us fuel injection is so much better at atomising fuel that we dont get the same downsides as when you over carb.

zef you would probably gain a bit at high revs from 45's but it would probably cost you a whole bunch of power mid to low revs, now if you could find a pair of 42mm carbs..........

[graham bahr]

the stroke of an engine also tends to have an effect on carb/throttle size, long stroke engines will often work much better with smallish carb/throttle sizes than short stroke engines

[richard_sykes]

The rpm in the formula is the mid power band. I believe (not too sure) that the cam shaft (when you by a fast road or race cam) is what really selects where the power band is.

The rpm then I guess should be mid power band?

Not sure I will have a better look.

[greenamex2]

I tested my race engine on 42, 45 and 48mm throttle bodies on an engine dyno.

The 48's gave over 12BHP more than the 42's with no loss anywhere else in the rev range. The 45's were somewhere in between.

That was on a slightly oversquare 16 valve 1600 reving to 9500 but has the largest volume intake ports as standard in the known universe.

I ran it on 45mm carbs for a while and it was awful.

[graham bahr]

Originally Posted by Denis Bassom

I ran it on 45mm carbs for a while and it was awful.

i'm not supprised, most people think carbs are good until they try the same engine on throttle bodies, with the last engine i personally (rather than customers and friends) had on 45 mm carbs i thought was good at the time even though it wouldn't take big throttle openings below 5,000rpm, until i put it on bodies and found much to my amazment that it would pull full throttle from 2,000 rpm, i knew it was going to be better but didn't realise just how much, on a full race cam it gave better drivability than sidedrafts and a road cam.