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Wed Dec 09, 2020 7:19 am

How critical is this relationship between intake manifold diameter at the carb flange vs carb venturi/throttle body size?

Something no on ever seems to talk about yet CSP offer 40mm, 44mm and 48mm.

Anyone ever done any back to backs?

Wed Dec 09, 2020 7:27 am

I've been thinking about this lately too. A thread I was digging through a while back hat Pat Downs from CB address it, and he thinks an oversized intake manifold under a smaller throttle was perfectly acceptable and maybe even better ( :? ) although I think that may have just been justification for their one-bore-fits-all approach to their manifolds that they sell.

My gut feeling thinks that a slight mismatch wont be too bad, as the mixture will pull through the airstream with minor turbulence at the dead areas just on the outside edges at the step in the oversized area. I think that it would be interesting to see the flow differences. My inclination would to have the manifold and throttle bore match as closely as possible.

Wed Dec 09, 2020 7:32 am

http://www.wallaceracing.com/runnertorquecalc.php

Wed Dec 09, 2020 7:41 am

Lingwendil wrote: I've been thinking about this lately too. A thread I was digging through a while back hat Pat Downs from CB address it, and he thinks an oversized intake manifold under a smaller throttle was perfectly acceptable and maybe even better ( :? ) although I think that may have just been justification for their one-bore-fits-all approach to their manifolds that they sell.

My gut feeling thinks that a slight mismatch wont be too bad, as the mixture will pull through the airstream with minor turbulence at the dead areas just on the outside edges at the step in the oversized area. I think that it would be interesting to see the flow differences. My inclination would to have the manifold and throttle bore match as closely as possible.

Agree with all of the above. To counter Pat, CSP aren't amateurs, why develop 3 casts when 1 will do?

Wed Dec 09, 2020 7:46 am

having a slight mismatch is not a big problem, but having a huge mismatch definitely leaves room for improvement. The step created by having a slightly larger manifold can help inhibit intake reversion, but if you have a BIG change in manifold cross section/volume right under the carb flange, all you're going to do is slow down your intake charge speed.

like using dellorto 36's or weber 40s on a 48mm manifold..6mm step

versus using dellorto 45s on a 48mm manifold..1.5mm step

the intake tract from the mouth of the velocity stack to the back of the valve is a complete system that relies on air volume and air velocity. Would anyone have a huge increase in cross sectional area/bowl right under the manifold flange at the head? No, they wouldn't.

I don't have a ton of experience with very high rpm, high power dyno-figure-chasing engines, but maybe it makes way less difference with that scenario? I have to imagine for your average 1600/1776cc spirited daily driver engine, 40mm manifolds would work better with 40mm carbs.

Torben has tested inserting a profiled sleeve into the top of large diameter manifolds to decrease the volume, I'm sure he will chime in soon!

just cause I'm bored and for sake of discussion here are some figures

CSA (cross sectional area) of:

36mm = 10.18 square cm
40mm = 12.56 s/cm
45mm = 15.9 s/cm
48mm = 18.09 s/cm

CSA of a 36mm carb on a 48mm manifold is almost HALF (56%) :shock:

Wed Dec 09, 2020 9:20 am

sled wrote: having a slight mismatch is not a big problem, but having a huge mismatch definitely leaves room for improvement. The step created by having a slightly larger manifold can help inhibit intake reversion, but if you have a BIG change in manifold cross section/volume right under the carb flange, all you're going to do is slow down your intake charge speed.

like using dellorto 36's or weber 40s on a 48mm manifold..6mm step

versus using dellorto 45s on a 48mm manifold..1.5mm step

the intake tract from the mouth of the velocity stack to the back of the valve is a complete system that relies on air volume and air velocity. Would anyone have a huge increase in cross sectional area/bowl right under the manifold flange at the head? No, they wouldn't.

I don't have a ton of experience with very high rpm, high power dyno-figure-chasing engines, but maybe it makes way less difference with that scenario? I have to imagine for your average 1600/1776cc spirited daily driver engine, 40mm manifolds would work better with 40mm carbs.

Torben has tested inserting a profiled sleeve into the top of large diameter manifolds to decrease the volume, I'm sure he will chime in soon!

just cause I'm bored and for sake of discussion here are some figures

CSA (cross sectional area) of:

36mm = 10.18 square cm
40mm = 12.56 s/cm
45mm = 15.9 s/cm
48mm = 18.09 s/cm

CSA of a 36mm carb on a 48mm manifold is almost HALF (56%) :shock:

Yeah I agree with everything you have said.
I have Big Beef IDA manifolds, and originally on 45mm throttle bodies.
I have since learned the error of my ways and fitted more appropriately sized 36mm ITBs. Thus leaving a cavernous void below my throttle butterflies.

I had a little discussion with Ohio Tom about the FK10 'bark' we're both experiencing, put it down to reversions and I think a large part of that could be related to the airspace in the manifold.

Wed Dec 09, 2020 2:01 pm

Hello.
It is a somewhat clouded area because from what I have learned and seen for the normal street to super street engines there are a lot of variables. In theory we all know what should be best, but theoy does not always compute with real life.
For instance, I have used universal offset manifolds on high compression medium cammed 1200 dual port engines with no noticeable improvement by adding sleeves to decrease manifold volume.
I have had to swop a set of universal manifolds to a set of "carb size (40 mm) manifolds" on a 1776 w. W110 cam, GO3 heads and 8-1 CR as it simply did not pull enough vacum at idle to make it steady. The reduced manifold volume made it possible to pull the ignition timing back to something that looked normal.
I built a 1914 with very good (slightly large intake ports, because I had to continue from what another person had previusly done) Web #110 cam, 9,8 CR 40 Alfa Dells on universal manifolds. This engine was a bit "soft" below approx 2500 rpm. Installed sleeves with 42 mm I.D. and it became better.
That same engine was later rebuilt to a 2054. That engine did not need the sleeves at all. No soft bottom end. In fact it pulled about 3 hp more on top without the sleeves.
Some years ago I built a 2054 with a Scat C45 and 1,25/1,1 rockers, Tims stage II heads, about 9,8 CR, 44 IDf´s and first a CSP Super comp, then later a 42 mm Python. This engine ran absolutelyt best overall with the Super comp exh, but naturally pulled better upper end with the Python. Especially with the Super Comp the engine "snapped" in lack of a better word when it really pulled in the 5400 - 5900 rpm band. It was a good deal less with the Python. I never figured out what caused it, and then the engine was sold so I lost contact with it.

When you go into designing intake and exhaust for a (fast) specific engine the general consensus is adjusting intake for peak hp on the 3rd intake harmonic pulse (or resonanse) and adjust exhaust lenght for the torque area.
However, I have found that with ACVW and to some extend Porsche engines that is not always correct. A street type 1 engine often responds better overall with intake lengths adjusted for the 4rth pulse when we are talking peak below 6000 rpm. And if we are talking peak hp in the 7000 rpm range it likes the 2nd harmonich pulse just as well as the 3rd.
A type 4 engine generally likes long intakes, and I mean long. For some of the high power (100 hp/L or more we are seeing intake lengths of up to 18-19" to get the power curve right through 8000 rpm.

I think one of the reasons to why it works sometimes and not other is that if you are in the neighbourhood of a proper intake length for the intake pulsing versus displacement you do not see much difference. If your intake length is in between so to speak, you can make it better by changing the intake port volume.

Another thing. If your carbs are a little on the small side for the engine underneath it can be beneficial to have a manifold that is too larege for the carb, because the extra volume can really play a role in reducing reversion up through the carb and help the engine make more power.

As for the barking "off cam", that will always be the case, to an extend. The more cam duration the worse it is. But It should be possible to reduce it by changing the intake and/or exhaust length. Not always easily doable and not always in the general interest if it messes with the upper end powerband. I guess it comes down to what you want the most.
In my experience it also has something to do with cam design and most likely precission of the grind, because I have noticed that in general the Engle and Scat cams are the "worst". Steve Long, Web, Raptor and Nowak cams are not nearly as bad.

Wed Dec 09, 2020 2:50 pm

...aaaaand there we go! :lol:

somewhat related but not, I've always been curious about throttle body sizing versus carb throat sizing. Assuming individual throat for cylinder...A throttle body obviously does not have to have an internal main venturi or auxiliary venturi to induce proper fuel flow and atomization, and as a result has much less obstruction. Does the throttle plate size NEED to be smaller then? How much smaller? Can it be the same size bore as if it were a carburetor?

Luke, why did you make such a jump from 45mm bodies to 36mm bodies? I ask because a 36mm carb (like lets say a 36 DRLA) would generally only be suitable for smaller displacement engines.

Wed Dec 09, 2020 3:25 pm

There are more than one factor in that equation. But if you take a 40 mm universal Dellorto as sold through say CB for many years with a 34 mm venturi and want to equal the flow with a TB. Same stacks and approx same distance from bell mouth to butterfly, the TB would need to be about 38,5 mm.
If you take a 40 mm alfa Dellorto or Alfa Weber style carb and want it toflow the same as a 40 mm TB you will need to have a butterfly around 41,5 mm.
These are average numbers.

Wed Dec 09, 2020 3:40 pm

Alstrup wrote: Hello.
It is a somewhat clouded area because from what I have learned and seen for the normal street to super street engines there are a lot of variables. In theory we all know what should be best, but theoy does not always compute with real life.
For instance, I have used universal offset manifolds on high compression medium cammed 1200 dual port engines with no noticeable improvement by adding sleeves to decrease manifold volume.
I have had to swop a set of universal manifolds to a set of "carb size (40 mm) manifolds" on a 1776 w. W110 cam, GO3 heads and 8-1 CR as it simply did not pull enough vacum at idle to make it steady. The reduced manifold volume made it possible to pull the ignition timing back to something that looked normal.
I built a 1914 with very good (slightly large intake ports, because I had to continue from what another person had previusly done) Web #110 cam, 9,8 CR 40 Alfa Dells on universal manifolds. This engine was a bit "soft" below approx 2500 rpm. Installed sleeves with 42 mm I.D. and it became better.
That same engine was later rebuilt to a 2054. That engine did not need the sleeves at all. No soft bottom end. In fact it pulled about 3 hp more on top without the sleeves.
Some years ago I built a 2054 with a Scat C45 and 1,25/1,1 rockers, Tims stage II heads, about 9,8 CR, 44 IDf´s and first a CSP Super comp, then later a 42 mm Python. This engine ran absolutelyt best overall with the Super comp exh, but naturally pulled better upper end with the Python. Especially with the Super Comp the engine "snapped" in lack of a better word when it really pulled in the 5400 - 5900 rpm band. It was a good deal less with the Python. I never figured out what caused it, and then the engine was sold so I lost contact with it.

When you go into designing intake and exhaust for a (fast) specific engine the general consensus is adjusting intake for peak hp on the 3rd intake harmonic pulse (or resonanse) and adjust exhaust lenght for the torque area.
However, I have found that with ACVW and to some extend Porsche engines that is not always correct. A street type 1 engine often responds better overall with intake lengths adjusted for the 4rth pulse when we are talking peak below 6000 rpm. And if we are talking peak hp in the 7000 rpm range it likes the 2nd harmonich pulse just as well as the 3rd.
A type 4 engine generally likes long intakes, and I mean long. For some of the high power (100 hp/L or more we are seeing intake lengths of up to 18-19" to get the power curve right through 8000 rpm.

I think one of the reasons to why it works sometimes and not other is that if you are in the neighbourhood of a proper intake length for the intake pulsing versus displacement you do not see much difference. If your intake length is in between so to speak, you can make it better by changing the intake port volume.

Another thing. If your carbs are a little on the small side for the engine underneath it can be beneficial to have a manifold that is too larege for the carb, because the extra volume can really play a role in reducing reversion up through the carb and help the engine make more power.

As for the barking "off cam", that will always be the case, to an extend. The more cam duration the worse it is. But It should be possible to reduce it by changing the intake and/or exhaust length. Not always easily doable and not always in the general interest if it messes with the upper end powerband. I guess it comes down to what you want the most.
In my experience it also has something to do with cam design and most likely precission of the grind, because I have noticed that in general the Engle and Scat cams are the "worst". Steve Long, Web, Raptor and Nowak cams are not nearly as bad.

There we go :shock: :shock: :shock:

It's gonna take me a while to digest that, thank you as always!

sled wrote:
Luke, why did you make such a jump from 45mm bodies to 36mm bodies? I ask because a 36mm carb (like lets say a 36 DRLA) would generally only be suitable for smaller displacement engines.

A 36DRLA would be running a 28/30mm venturi
A 48 IDF/IDA could be running a 36mm venturi
I think that is where you might be better to draw some similarities.
Convention seems to be to have the butterfly around 0.85 x intake valve size. 42x0.85 = 35.7mm

I wanted some manners and my 2276 was never build for all out top end. 1500-3000rpm is important to me (despite fitting an FK10 :) )

Small pedal movements meant big changes in manifold vacuum, I made a progressive linkage which helped in terms of how linear it pulled but they made tuning with speed density (manifold pressure) very difficult, they just wanted Alpha-N (throttle position). Which is fine in a race motor.

I spoke with Paul at VW Speedshop, thinking I was going to order some ~40mm but he said the 36s would be best for my engine. He has made over 170BHP on them and I wasn't going to argue.

Wed Dec 09, 2020 5:17 pm

Well, now you have them, so use them. But you are forgetting one thing. A set of 36 mmTBs can flow to 170 hp and change. BUT! as displacement increases you run out of air much sooner than you´d expect, and then it becomes more difficult to get the cylinder filling needed for say 170 hp. If the engine is smaller you can keep air flow up longer up in the rpms and take advantage of the rpm to pull more air through. Remember, a 36 mm venturi can flow significantly more than a 36 mm straight pipe.

We used the same principles when we did some limited displacement track racing Alfa Romeo Alfasuds way back when. Back in those days it was a challenge to pull more than approx 130 hp through a set of 36 Alfa Dells. With some carefull thinking and a couple of tricks, that did get banned after the first season. I wonder why :roll: we pulled over 140 hp @ 7400 rpm with a 3300 rpm powerband. That was just over 100 hp/l. Today we could probably make it even better, but those classes are long gone.

Wed Dec 09, 2020 5:22 pm

With a carburettor you also have the secondary Venturi which is restrictive because it is in the center of the throat where the bulk of the intake charge wants to travel .

So a 36mm throttle body will flow way more than a carb with a 36mm main Venturi and a secondary Venturi . That is why JPM developed their Venturi for IDA’s

It’s also my understanding that Throttle plate size is also relative to manifold length. If you have the throttle plate nearer the port and longer velocity stacks to tune the intake length then it needs to be smaller than if you have long manifolds and shorter velocity stacks because of the taper in a manifold .

Wed Dec 09, 2020 7:08 pm

Throttle plate diameter is dependent on distance from the valve. The intake port needs to taper out to the stack, the closer the plate is to the valve the smaller it can be.

With an IDA the auxiliary vent sits above the Venturi doesn’t really become a restriction until the vents are big (48mm). In some situations the throttle plate opening and the disruption from the plate becomes the restriction.

As for the original question I believe to large manifold below the plate adds volume and reduces velocity. There is a company called EFIHardware out of Australia that builds taper bore throttle bodies. I had their 55/53/50 tapered, which is 55mm at stack, 53mm at the plate and 50mm at the base. 53mm throttle plate with the disruption flows equivalent to the 50mm bore. They worked really well, but I’ve move to a bigger set.

Wed Dec 09, 2020 11:01 pm

Tried to search, but couldn't find it. Pat had a pissing match with someone once about this.

Wed Dec 09, 2020 11:30 pm

Yeah, I remember that.
Tho I'm impressed, everybody pretty much has it nailed about the taper and sizing, nothing to add.

except you all forgot one thing.

Where is the fuel injector? :lol:

Thu Dec 10, 2020 12:13 am

modok wrote: Yeah, I remember that.
Tho I'm impressed, everybody pretty much has it nailed about the taper and sizing, nothing to add.

except you all forgot one thing.

Where is the fuel injector? :lol:

Right below the throttle plate.

Thu Dec 10, 2020 3:36 am

BELOW the throttle plate, interesting...
On the one hand it's aimed so the spray would have to traverse the high velocity streams from the throttle, which is probably good.
on the other hand it's pointed to the area of lowest velocity in that extra large area under the throttle body.....potentially not good.

Thu Dec 10, 2020 4:12 am

UK Luke 72 wrote: sled wrote: having a slight mismatch is not a big problem, but having a huge mismatch definitely leaves room for improvement. The step created by having a slightly larger manifold can help inhibit intake reversion, but if you have a BIG change in manifold cross section/volume right under the carb flange, all you're going to do is slow down your intake charge speed.

like using dellorto 36's or weber 40s on a 48mm manifold..6mm step

versus using dellorto 45s on a 48mm manifold..1.5mm step

the intake tract from the mouth of the velocity stack to the back of the valve is a complete system that relies on air volume and air velocity. Would anyone have a huge increase in cross sectional area/bowl right under the manifold flange at the head? No, they wouldn't.

I don't have a ton of experience with very high rpm, high power dyno-figure-chasing engines, but maybe it makes way less difference with that scenario? I have to imagine for your average 1600/1776cc spirited daily driver engine, 40mm manifolds would work better with 40mm carbs.

Torben has tested inserting a profiled sleeve into the top of large diameter manifolds to decrease the volume, I'm sure he will chime in soon!

just cause I'm bored and for sake of discussion here are some figures

CSA (cross sectional area) of:

36mm = 10.18 square cm
40mm = 12.56 s/cm
45mm = 15.9 s/cm
48mm = 18.09 s/cm

CSA of a 36mm carb on a 48mm manifold is almost HALF (56%) :shock:

Yeah I agree with everything you have said.
I have Big Beef IDA manifolds, and originally on 45mm throttle bodies.
I have since learned the error of my ways and fitted more appropriately sized 36mm ITBs. Thus leaving a cavernous void below my throttle butterflies.

I had a little discussion with Ohio Tom about the FK10 'bark' we're both experiencing, put it down to reversions and I think a large part of that could be related to the airspace in the manifold.

FK10 bark? I have a 10, curious to what mean

Thu Dec 10, 2020 4:20 am

Induction noise is the bark, I've tried both the csp manifolds and cb's generic offset on my IDF's. This is with 40's and my butt dyno didn't notice any change in power but the idle mixture changed ever so slightly, like an 1/8 turn in on the csp version

Thu Dec 10, 2020 4:29 am

modok wrote: BELOW the throttle plate, interesting...
On the one hand it's aimed so the spray would have to traverse the high velocity streams from the throttle, which is probably good.
on the other hand it's pointed to the area of lowest velocity in that extra large area under the throttle body.....potentially not good.

Brilliant :oops:

The injectors fire a fine mist not a jet, and I'm at around 3% duty cycle at idle, so I dare say atomisation is pretty decent. Think of my big beefs as plenums 8)