Twin 40's On A 1.9 8v

[taffycrook 2]

I have a set of twin 40's on a 205 manifold and as I have just bought a 205 it would be rude not to fit them to the thing.

My problem is the 205 is a 1.9 now we all know that 45's would be better, but how much will the 40's allow me to get out of the 1.9. I have 34mm chokes. But i am sure i have some 35mm chokes somewhere.

I plan to get around 150-160 bhp from it to make trackdays more interesting. This is the upper limit for 40's but its cheap power, as I already have them.

Will i get it with 40's and just a cam swop and some head work.

Ps the engine made 133 bhp last time on the rollers so its a good un. Most 1.9's make around 120-125 on these rollers and 90-95 @ the wheels.

[jonD6B 0]

What's been done to the engine so far to enable it to make 133bhp on thr RR?

I was once told that 40's are only good for a realistic 140-150bhp but don't quote me on that as I really don't know for sure.

[taffycrook 2]

As far as I know the engine is stock apart from the exhaust and air filter.

As I said it is the upper limit for 40's but as I already have them they are going on.

[RobMGti 0]

Had this conversation a few times whilst on here and ive been asked about it in pm's. Ive had 40's fitted to a standard 1.9 and a hiflowheads stage 3 with a 285 cam (not the 270 originally supplied) the first lump i got an even 140bhp and with the second lump i got just over 160.

 

they CAN provide the power your after.

 

Rob

[taffycrook 2]

Cheers Rob, I was thinking about this and the smaller carbs will make for a nicer more flexible engine than the 45's which tend to be over carb'd on 8v motors. After all most 2.0 16v motors use them to good effect and we all know how much more they can flow than the 8v motors.

I am not seeking ultimate performance, but it may show others that usable power isnt that hard to find or need to be as expensive as they think. As always I will post my results good or bad on here.

Looking forward to this project now!

[TT205 42]

Not carbs, but I was told that 40 throttle bodies would not be a limitation on an 8V - in fact in the past the 'throttle body kits' for the 8V were 40's not 45's

[taffycrook 2]

TB's are not as dependant upon size for their function and performance.

A 40mm hole will flow x and a 45 mm hole will flow y amount of air, the air speed in the bodies doesn't matter too much as it will speed up in the ports. This enables suppliers to stock fewer sets as 45mm will do almost all engines up very high performance 4 pots to stand road motors.

Carbs on the other hand will suffer if they are too small or too big , choke size is critital.

A mildly tuned 8v 1905cc motor is not breaking any performance records, but still needs a fair amount of air to make all its beans this is why 40mm carbs are considered too small as the choke limits the air flow and even if you fit 36mm chokes they do not flow as much as 36mm chokes in a 45mm carb, plus they reduce the signal and can lead to poor running. This is why it is common practice to only fit 34mm chokes in 40's.

[RobMGti 0]

^^ Yup

 

All true, apparently the Venturi (swirl) effect is effected massivley when using 36mm chokes on 40mm carbs compared with using 34mm chokes.

 

For the record Taffy my engines were all done on 34mm

[saml666 0]

I'm about to break with tradition and put 40's (with 35 or 36mm chokes, can't remember) on my Mi16 engine.

 

Basically yes, they'll still give 160ish bhp but more importantly more grunt, a more usable engine than one on 45's. Oh yeah, my Mi16 is internally std, otherwise I would definitely need 45's (before everyone points it out for me)

 

The guy supplying my carbs was adamant that it would be a better, stronger engine with power where you need it for the road. He practically refused to sell me 45's before I'd at least tried the 40's (and yeah, he had at least 3 sets of 45's there so wasn't just playing for convenience!)...

 

Anyway, just my 10p worth - going for a strong std Mi, maybe big head/cams next year...

 

Sam

[notamondayfan 44]

ummm ive always been a bit confused about which carbs to run.

 

origionally i was going to run dellorto 40s, simply because a mate found a decent pair in his uncles garage and gave em 2 me!

 

now a set of 45s have become available for around £150, so definatly within the budget.

 

but then theres the ever growing popularity of using R1 carbs, which again are 40mm, and can pick up a pair for under £200.

 

btw, my 8v will be heavily modified, using forged pistons, lightened and balanced crank, and a big valve head, cam, etc etc

[mfield 20]

Having used both 40's and 45's i have to agree that on a mild 8v 40's are the better choice. I would however recomend 32mm chokes and not 34mm, my 40's had 30mm chokes and low down the car really pulled well but ran out of puff by about 5.8-6k. With the 30mm choke the top speed was only 125mph( 122 on the gps ) but it got there quick and pulled just as strong 100-110 as it did from 80-90. The reason i say go for 32mm chokes is for that bit more power your looking for with out losing the low down torque. When i changed to 45's with 36mm chokes the car was actually slower untill into the higher revs, maybe 34mm chokes in a set of 45's would be the best option and something i wanted to try but never saw a set for sale

[boldy205 75]

quote taffy crook: this is why 40mm carbs are considered too small as the choke limits the air flow and even if you fit 36mm chokes they do not flow as much as 36mm chokes in a 45mm carb.

this may be a silly question but why wont they flow as much if you have the same choke size?

[taffycrook 2]

Its not a silly question.

Its about the overall restriction to flow both have an aux venturi in the way and on 45's there is more room for flow araound it than on 40's added to the fact that the throttle plate is larger also there is more volume in the 45's.

Webber must of spent millions developing the dcoe and they fit 34mm in 40's as that is the biggest choke that works to max effect.

Flow bench tests have proved that you gain little to no more by fitting 36 mm chokes in 40's apart from upsetting the fuel signal across the venturi.

 

I'm about to break with tradition and put 40's (with 35 or 36mm chokes, can't remember) on my Mi16 engine.

 

Basically yes, they'll still give 160ish bhp but more importantly more grunt, a more usable engine than one on 45's. Oh yeah, my Mi16 is internally std, otherwise I would definitely need 45's (before everyone points it out for me)

 

The guy supplying my carbs was adamant that it would be a better, stronger engine with power where you need it for the road. He practically refused to sell me 45's before I'd at least tried the 40's (and yeah, he had at least 3 sets of 45's there so wasn't just playing for convenience!)...

 

Anyway, just my 10p worth - going for a strong std Mi, maybe big head/cams next year...

 

Sam

 

If you are using more than 34mm chokes please read the above.

 

I have run a mi16 on 45 as a daily driver on 45's and it was amazing.

I have set up many and 40's will strangle the life out of it. Has the fella who is setting it up/supplying the carbs done many?

My results have been along the lines of

35mm chokes on 45's 178 bhp

36mm chokes on 45's 185 bhp standard engines

38mm chokes on 45's 192 bhp repro cams

38mm chokes on 48's 225 bhp rally cam mapped ign

 

Head and cam depending on which one, is going to be far too much for a set of 40's.

In fact with that set up I would save money and not bother with 40's as the std injection system will produce more power, the cat cams inlet only profile or repro cams on a otherwise std mi on injection will net you close to 180 bhp.

Add a mappable ign set up and that figure will rise plus it will be far nicer to drive. Much cheaper and easier to live with than carb's.

Edited May 12, 2006 by taffycrook

[saml666 0]

Head and cam depending on which one, is going to be far too much for a set of 40's.

 

Chap, my head and cams are standard - I bought my pug for a bit if a laugh a few weeks ago as its been 10 years since I had one before. I'm only doing the Mi conversion as the 8v died just 200 miles after I bought it, and a friend had an Mi16 engine in his cellar . . . the devil makes work for idle thumbs and all that.

 

You're quoting Max BHP figures and thats not really what I'm after. I respect Dave's (the supplier) advice as he's built and raced dellortos for years (incl some Mi16 engined single seaters). We know that at the top end 45s would make more power but we're going for a stronger more tractable 16v lower down.

 

(oh yeah, I ditched the injection as didn't have loom plus lots of other bits and I like the look / tidiness of carbs)

 

Maximum BHP figures are probably more important to you as you're looking for good results on the track where you can hold it right up in the revs for longer but I'll be doing more road-work where unless you're truly insane you just can't keep it singing at 7,000 ALL the time.

 

I've got a 4.6 V8 running less outright BHP than it could if it were really stretched but it pulls one hell of a lot harder throughout the rev range, in real-life driving conditions its faster through the gears than similar cars with higher max power figures ... and ultimately thats what I want.

 

Cheers for the info, you've obvisouly done your homework - I think we're building different horses for different courses...

 

Take it easy, Sam

[taffycrook 2]

Anyway, just my 10p worth - going for a strong std Mi, maybe big head/cams next year...

 

Sam

 

Thats the head, cams reference I was refering too.

Only one mi16 I have put on carbs is a rally car the rest were all road cars.

Tests proved that even the choke size made a major difference to power and more importantly torque outputs on these motors. My car which was a every day driver it was my only car, was tractable and very mild mannered untill you wanted it to be an animal. The throttle isn't an on off switch.

It managed a sub 14 sec quater and topped out at around 140 mph.

That is good performance from a scrap yard engine, and a set of £300 carbs. Plus it would allow further mods without changing the induction system.

I am not saying your way is wrong, it might work very well but in my experience it won't be the best use of rescources.

My advice would be to look at standard fit carbs from the 70-80's fiats mg's ford's lotus supercars etc this is where most of the development was done, check out the sizes compared to cyl cc it will give you better idea of whats what.