Converting pegaso engine to 36 mm carbs

Hi all,

After gaining a few horses from switching the pipe on my pegaso engined rs 125, my thoughs naturally turned to what the next step was to get me over the magic 50 hp at the back wheel.

A bit of digging on various forums made look at the twin bst 33 carbs that I use for fuelling, which I had assumed would be ok for that kind of power.

looking at my valve sizes (3 x 30 mm) with valve throats at 25.5 mm gives a cross-sectional area of 1531 mm sq .The bst 33's , taking into account the 4mm thick butterfly spindle give a total area of 1444 mm sq.

So I plugged in the figures into lotus tuning software, and sure enough the power from 4000 rpm upwards goes up by around 3 hp. I think this why the engine has such a flat torque curve , keeping 40 ftlb between 3500 and 6200 rpm and without the increase in torque i was expecting around 5000 rpm... the engine wants to but runs out of induction past 4000.

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So.... with all this in mind I start looking at aftermarket carbs, the universal nod seems to go to FCR carbs, which flow more due to flat slide /no butterfly. However I dont have the best part of £800 quid to spend on carbs, and the 33mm I need seems to be rare and not crop up on ebay, and the ones I have seen crop up are 39 mm which I think is massive overkill for this engine.

I'd love to have the money to be able to spend 800 quid to test a theory, but sadly not... :(

Thinking OEM (cheap) I instead looked at 36mm CV carbs, which doing the maths gives me 1746 mm sq taking the butterfly into account which I think will be more than enough.

Fleabay came up with the goods, with some BST 36 s from a 98 triumph speed triple. I went for these in the hope that being the same model of carbs the external dimentions would be the same, and would let me use the spacers etc from the 33mm BSTs .

these turned up yesterday, and a quick look confirms that 36 mm is simply a bored out version of the 33.

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Brackets all fit, I had to make a plug for the fuel rail, shorten one of the butterfly spindles, block off the inside float bowl vent lines and unblock one of the outer ports to allow pipes to be fitted. All I need to do now is sort out how the choke mechanism, and I have a set of bst 36's that will bolt straight into the existing carb inlet boots:

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I've simply swapped over needle jet, needle , spring , main jet and idle jet to get me in the ballpark with jetting, but no doubt it will be miles out, so will sort a trip to PDQ's dyno again to get evrything set up right. I must be their best customer by now!

Will report back with results.....

Do it. Too bad you couldn't find a couple of fcr37

Ok so on the other thread I never replied with what I want to.

In your intake,

Say that these numbers Im pulling out of the are factual not fictional.

we have a cam that's 260* @0.05"

And that we have an intake 18" long.

And the motor turns 9000rpm

And the valve is closed for 0.00473 seconds

And the intake is in tune at this rpm for first harmonic resonance

When the valve closes the air slam in to it and create pressure as this pressure backs up every mm of the port (valve is still closed) the pressure will back up untill it reaches the bellmouth and at that exact instant the valve will open again. (it takes 0.00437 seconds to build max pressure in the intake runner)

This is when its fully in tune. Above this rpm the pressure wave won't build to full potential.

Below this rpm the valve is closed longer. When the pressure wave hits the open end of thr pipe it changes waves and puts a negative wave towards the valve (kinda how an exhaust scavenges huh)

At an even lower rpm pressure builds, escapes, and builds again " at whatever rpm the valve is closed long enouh to allow this"

So lengths of intakes/exhausts are like fues or timers, length is time.

Most intakes don't use the 1st harmonic they become Verry long. And tend to not work well.

They use the 2nd at max revs, 3rd midrange, and if there's a wide enouh operating rpm the 4th at low revs

some race engine builders like to design the intake to resonate 3rd at say 6k make the exhaust scavenge at 7k and make the intake 2nd resonance at 8k. Works Verry well.

Remember a pipe with an open end reverses signal, and a pipe with a closed end reflects signal.

Anyways.....

Good luck numb hands.

Edited by englertracing

Hopefully that makes any sense.

Hopefully that makes any sense.

That does make sense! so what we're talking about arent really what i think of as harmonics of the fundamental resonance frequency ( ie multiples of the main wave frequency), but numbers of reflections of the main resonant frequency that have had time to occur in between inlet valve events? In that case, in answer to your original question, I still have no idea! but now i think I have a handle on what I need to work out...

So in theory, a shorter intake tract would give more points to tune to, provided i could offset the negative effects from the negative reflections, but I assume there is a payoff with a shorter tract length of less inertial ramming at the end of the intake cycle when the piston is on the start of it journey back up the cylinder? Am I thinking along the right lines?

I'll sit down down with my calculator and boys book of maths and see what comes out, and then plug the figures into the simulator to see if i confirms what i work out...

Thanks for the explanation, its hard to find someone who can explain the fundamentals of what going on clearly :thumbsup:

Will report back.

PS, have machined up an adaptor for the choke, just need to strip down and clean the carbs to make sure no swarf etc is anywhere it shouldnt be, then I'll fit them and we'll see whats what!

Well the lower the number you cab use the stronger they are it diminishes. So I'd shoot for using 2 and 3 or 3 and 4

Intake tuning actually works differently than described. When the valve opens, a pressure drop occurs at the valve, which starts a wave of negative pressure moving from the valve to the first major increase in cross section, which in a tuned system would be the mouth of the intake bell extension, wherever that might be. As the negative wave exits the intake mouth, it causes a positive wave to be reflected back toward the valve. If this wave arrives at the valve as it is closing, it can help push some extra intake charge into the combustion chamber.

It is true, of course that the positive pressure, along with the inertia of the moving air stream will "pile up" on the closing valve, and that a wave will run back from there prior to the next intake opening, but that has much less to do with the tuning effect.

Intake tuning actually works differently than described. When the valve opens, a pressure drop occurs at the valve, which starts a wave of negative pressure moving from the valve to the first major increase in cross section, which in a tuned system would be the mouth of the intake bell extension, wherever that might be. As the negative wave exits the intake mouth, it causes a positive wave to be reflected back toward the valve. If this wave arrives at the valve as it is closing, it can help push some extra intake charge into the combustion chamber.

It is true, of course that the positive pressure, along with the inertia of the moving air stream will "pile up" on the closing valve, and that a wave will run back from there prior to the next intake opening, but that has much less to do with the tuning effect.

I suppose I had it backwards then. and what you described would be the first harmonic. And would take a long intake tract.

Any updates numbhands?

Apologies, I've been a bit busy with work and stuff, but have managed to run it up with the new carbs. Oddly enough it feels ok, idles fine and seems to be a bit revier at the top end ( took it to 8000 rpm by accident, oops! and no rev limiter it seems...)

Not had time to take to the dyno yet, I've just fitted some adjustable maxton forks to replace the crappy standard ones, and that meant new brakes and some new brackets to fit the the old mudguard. Will get to it in a week or so hopefully!

Thanks for the theory on intake tuning guys , much appreciated!

Does anybody have a fomula for working out the correct intake length? I've looked at a few online calculators, but I'd like to know the maths behind whats going on.

I have had a play at working it out , assuming the shock wave is moving at the speed of sound and reflects once for the first harmonic, then (and this is where I get unsure) three times for the second and 5 times for the third...is this thinking along the right lines?

Anyway tuning for 6000m rpm I seem to be getting a length of 490 mm for the fourth harmonic,686mm for the third, 1.143 mm for the second and 3430 mm(!) for the first....Can anyone check these figures for me to see If I'm near or if I've got the concept completely wrong! :blush:

Cheers guys,

WavePulseRPMChart.jpg

this chart is less than accurate because it doesnt take cam timing into account, a 180* cam will surely behave different than a 280* cam

blairs formulas, which i had to dig out of a milk crate.

Induction wave velocity in meters/second

Square root of the quantity [1.4x287 x induction temperature (kelvin)]

ram peak RPM point

[induction wave velocity M/S x dimesionless peak ram factor] / Induction Length(millimeters)

You input these dimensionless ram Factor numbers into the above formula, solving the formula provides you with the RPM for each input ramming peak.

Dimensionless Ram Factor numbers for each of the ram peaks

Ram Peak #1 = 8900

Ram Peak #2 = 6600

Ram Peak #3 = 5150

Ram Peak #4 = 4150

Ram Peak #5? =3300 (Ram peak 5 not provided by blair but from 84 dave)

Ram Trough points

[induction Wave velocit M/S x Dimensionless Trough Factor] / induction length mm

Same as above but for troughs (where the resonance is working against you)

Dimensionless Trough Factor

Trough #1 = 12000

Trough #2 = 7600

Trough #3 = 5700

Trough #4 = 4500

Trough #5 = 3650

I have to give credit to 84Dave from 4m.net a stock car racing site for providing these formulas as well as a great write up. I printed it and have a few copys, but all data on 4m.net has beel lost many times due to hackers or viruses. So to provide anyone searching for it who may have used it years back I have to give dave credit for his great write up which was called 84dave induction and exhaust tuning

and also Professor Gordon Blair, they are his formulas.

Thanks for the info englert!

Since I have a limited amount of room to space out the carbs to lengthen the inlet tract, I've done the maths for an increase of +10 and + 20 mm as well as what I have at the moment.

With my current length of 265 mm, the second harmonic is only effective at 8542 rpm ( way above my redline ) and this only drops to 7943 at 285 mm so will be tuning to third and four harmonic.

275mm gives me a ram peak 3 at 6423 rpm with the wave working against me at 7190, which is ok as the torque is rapidly falling away by then anyway, and the 4th ram peak helping at 5176 and hindering at 5612.

With 285 mm peak 3 is at 6198, trough at 6860 , peak 4 at 4994 and trough 5415.

My thinking is to try the standard length (265) with new carbs, get them set up and get a baseline reading before changing any lengths.... booked in for the first of march, so I'll try to get the spacers I need machined in time, otherwise it'll have to wait for another session.

I shall post results!

Tanks again for the help, everyday is a school day, :thumbsup:

If you can only get one set done I would pick the +20mm.

What is your redline? 8000?

Redline is 7500, but it's shaking so hard by then I dont like to go there too often!

Machined up some 10 mm spacers today:-

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I've built in a O ring to each, so when I make the next two they'll be "stackable" and simply clamp together using longer bolts in the existing holes that currently hold the carb rubbers to the head, that way its a quick swap to check the effects of +10 +20 mm. Hope it warms up a bit for friday, -1 at the mo...

-1?!?!? Holy sheet. F that.

Hope you have a hell of a heater in the shop.

-1?!?!? Holy sheet. F that.

Hope you have a hell of a heater in the shop.

Celsius, about 30.

Yup, I'm a metric boy! not that cold really, i'm just a wuss... ;)

Well 30f is too cold for me anyways, hell 50f is cold to me here in California.

Anyways numbhands, I'm excited to hear your results!

How about a dyno run video?

How about some wide open pass videos?

Well 30f is too cold for me anyways, hell 50f is cold to me here in California.

Anyways numbhands, I'm excited to hear your results!

How about a dyno run video?

How about some wide open pass videos?

Will do mate! :thumbsup:

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