sizing individual throttles for Fuel injected engine

[reggid]

i have a naturally aspirated engine and want to size the throttle size i should use.

what is the best method? ive seen that hp/cyl is a guide?

so for a engine with 55hp/cyl what size should the throttles be? i don't want too large throttles as it would mean the runners to suit large throttles are too big hurting velocity.

they will be located quite close to the head and tapered runners will be used.

what general method to people use?

[Pumaracing]

For a four cylinder engine on TBs the bhp levels where I consider each butterfly size to start to become restrictive are as follows.

40mm - 200 bhp
45mm - 255 bhp
48mm - 290 bhp
50mm - 310 bhp

You can extrapolate pro rata based on butterfly area to any other size and bhp level. Your 55 bhp/cylinder calculates as 42mm butterflies but you would be fine with the commonly available 45mm butterflies as there is almost no downside from being a little large with TB applications unlike carbs where oversizing can hurt choke signal and atomisation.

[reggid]

So how much a hit would lowend and midrange be with 45's instead of 42's? i can use any of 40,42,45,48,50mm btw

i calculated a head inlet entry port size of 40mm and 48mm entry at the plenum using various means mostly sims to work out correct velocity for wave tuning etc and thus also came to the conclusion of 42mm. but lots of people say go 45mm but i cant help but feel its a little big for lowend and midrange when a 42mm throttle in a straight bit of pipe would outflow the head.

[Snake the Sniper]

From what I understand being slightly under is better than over on ITB sizes. For most use, the smaller sizes will give higher port velocity and therefore better bottom/mid range up to the point they begin to restrict things. Whilst going over size can hurt almost the entire range but give better top end where you will probably spend relatively little time.

What size is you engine, number of cylinders, expected power and usage?

[reggid]

4cyl 130 cubic inch, expected expected power 220hp @ 6500-7000rpm. will be driven regularly on street with occasional club track event

[DVandrews]

42mm will be ideal. I have used them on engines up to 250bhp, I would have a play with trumpet lengths, Emerald do some variable length trumpets which allow you to experiment and also allow a different trumpet lengths on each pair of cylinders.

Dave

[anonymous-user]

Remember, you can move the throttles further upstream or downstream to take advantage of any given diameter, assuming you have tapered runners.

[reggid]

thanks everyone! it is best to start conservatively

[Luther Blisset]

Honda tuners in the US tend to use bigger ITBs than normally advised in the UK with seemingly good results, for 55hp/cyl they'd be using 50mm minimum.

I'm not sure if there's a specific reason for this - maybe they just build dyno queens with poor transient response, maybe it's because they don't have much low end to lose in the first place - but they don't seem to have many qualms about "going large".

If one could get it to fit wouldn't it be better to use (in this case) 50mm ITBs and run a longer inlet manifold to take care of the low end and midrange?

[PeterBurgess]

'they will be located quite close to the head and tapered runners will be used.'

I suppose for good airflow characteristics you should keep TB diameters min to match the port dimenions if the runners are short, avoiding a 'sharp' taper

We managed 240 bhp at the engine on an NA Pinto running 42mm chokes in a pair of Webers so I dont suppose you need huge by large runners?

Maybe the Americans are falling into the bigger it is the more power it makes trap same as the VW aircooled brigade who use 46 mm carbs on 1640 engines revving to a dizzy 5000 rpm

Peter

[chuntington101]

PeterBurgess said:

'they will be located quite close to the head and tapered runners will be used.'

I suppose for good airflow characteristics you should keep TB diameters min to match the port dimenions if the runners are short, avoiding a 'sharp' taper

We managed 240 bhp at the engine on an NA Pinto running 42mm chokes in a pair of Webers so I dont suppose you need huge by large runners?

Maybe the Americans are falling into the bigger it is the more power it makes trap same as the VW aircooled brigade who use 46 mm carbs on 1640 engines revving to a dizzy 5000 rpm

Peter

That is VERY true of the US tuning no matter what engine they are working on! Then when someone comes around with some maths to prove you are better with smaller whatever the get laughed at!

[DVandrews]

The most I have seen on 42mm TBs is 252bhp, this is with direct to head bodies with a relatively short inlet port and a smooth transition to the port shape. Quite often the choice of inlet manifold is limited , sometimes it is integral to the bodies so it is important to view the TB in the overall context of the induction from airhorn to inlet valve. There are some good words on sizing on the Jenvey site, as a rle of thumb I use 42MM up to 220BHP and 45mm thereafter. I've used around 250 sets of 42s and around 50 sets of 45s. Whenever a trade up to 45s from 42s occurs there is a small loss of torque in the lower rev band, often this can be recovered by varying trumpets lengths or fitting unequal length trumpets.

Dave

[reggid]

as far as factory fitted TB if im not mistaken 6cyl S50 BMW use 50mm which is good for 450bhp in a developed engine equivalent to 300bhp for a 4cyl by scaling. The 4cyl S14 2.5L bmw will make 300bhp+ on 48's having come with 45's and 238bhp from factory iirc with quite alot more potential.

Edited by reggid on Thursday 18th July 12:12

Edited by reggid on Thursday 18th July 12:18