Supercharger inlet design and its effect on powerband

[ejm211]

I love this forum! I get lost in here on my lunch breaks, what an informative source.

I have an interesting question that I've searched on but have been unable to find a conclusion.

I'm currently working on a supercharged MB engine with the drive pulley in the front and the throttle body in the rear. The issue is that due to space, the stock system utilizes a very small throttle body (74mm) and a y-pipe splitting to dual intakes.

Engine overall

Hard to see but at the rear of the blower sits the drive by wire throttle body and the y-pipe.

Now, an 82mm throttle body is available through MB and is a bolt on affair, with results in the 35-40whp range. Obviously 40whp out of a throttle body proves the stock unit is severely under sized. There have been a couple people to fit a 90mm BBK DBW throttle body (with an additional 35-40whp increase!), but it's a very tight fit.

Here's is where the issue lies. I'd like to see if I can run a 95mm or even a 102mm DBW throttle body but the space is simply not there behind the blower. Should I settle for the 90mm or can I build an extension and bring the throttle body away from the blower inlet?

Below is similar to the stock configuration, albeit with a much shorter snout (pay attention to the inlet only)

Current inlet design

While this is what I had in mind (similar blower as above but a different end casting style)

Revised inlet design

Relocating the throttle body to the passenger side of the blower would give me enough room to run the larger throttle bodies but at what expense? What will this increase of air volume post throttle body do to the power curve? I actually expect this to be easier to tune since the air would be less volatile when moving through the throttle body.

Any insight? Any input is greatly appreciated

 

[Lou Scannon]

The main issue I foresee is transient response. This is due to the plenum volume between the throttle and the intake valves. If you're willing to sacrifice sharp throttle response, or at least experiment to find out if it's OK or not with the increased volume, then give it a try.
Secondary issue is: relocating the throttle body upstream of the blower by itself increases the required size of the throttle body, just to stay on par with a TB downstream of the blower, due to the difference in air density and hence volumetric flow, so you will not gain as much as you might expect with this approach. As a first approximation, the pressure drop in the throttle body will be a square function of the velocity through it. The velocity will be inversely proportional to the air density, which will roughly proportional to the absolute air pressure.
Third potential issue is, is the blower designed to run in a significant vacuum, i.e. at idle & part load?
There are other potential system and application issues depending on the specification and function of the original and substitute throttle bodies. E.g, idle speed control, electrical interface, turn-down ratio; which I'll leave to you to investigate.


"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[bentwings2]

I have had supercharged cars for a long time. My streetrod has a 6-71 blower and 3 750 cfm carbs. Far more primitive than your system. I just looked at the race car computer info and I see 20 pounds boost in .05 sec after the throttle is opened. I don't think it is even open all the way in .05 sec. that's pretty fast for leg/foot movement.

So with that, unless you have a good data recorder I don't really think throttle response is much of an issue. If it were just NA then you would have problems but not with PD blowers.

If your blower is just a positive displacement type, as soon as you open the throttle past an air flow point greater than the blower can displace and the motor can digest you will have boost. Since you have already determined an intake restriction, additional area will help. If it happens to be a foot or so away again I don't think it will matter. If it were one of our Jurssic age carbs, yes there might be lag as well as a flat spot....worst case a huge backfire. You only have to worry about flowing air thru the blower not air and fuel.

If your blower is a screw type (looks like it is) then you already have compression or boost available. The excess is being bled off by some kind of boost control valve possibly a bypass valve. Now it's a matter of having a larger intake and how well you can control the bypass system.

 

[bentwings2]

oops typo. I have 2 750 cfm carbs not 3.

 

[ejm211]

I apologize, it seems I wasn't getting notifications on this topic.

Hemi, I don't think the throttle response would be affected... the throttle body is before the air is being compressed and it's not like there isn't air waiting when the throttle blade is closed. The throttle blade would essentially just allow flow to resume. Why would the throttle body need to be larger for the same result? If anything, I feel that air delivery would be smoother as the throttle body is moved further and further away from the supercharger. Since the throttle body is smaller than the inlet, the air is slowing down just before the supercharger regardless of the size of the throttle body. To eliminate this, the throttle body, inlet, and supercharger would need to be cylindrical to one another, which is not feasible.

I will need to investigate vacuum though, and I'm not sure how this would change with a longer tube post TB. The mechanics related to the throttle body change would be minimal, as Chrysler and MB use the same logic in their drive by wire


Bentwings, the stock blower is a clutched screw type, Eaton form. There is a stock bypass valve in the inlet after (beneath) the throttle body, obviously not visible in my initial picture. I hope to retain the bypass system in it's stock position, no reason to modify. For fun, I found a picture online and added the path to better show, the yellow is bypass.

Stock flow system


While I have the topic open, the stock MB intercooler is of a three-pass design (left to right). I find this to be terrible inefficient and would prefer a single-pass design with larger liquid transfer lines. There are two reasons I can think of that MB would build the intercooler in this way.

1) MB was experiencing a large variability in individual runner IATs from front to back, so they slowed the flow down to provide more consistency while still allowing some thermal transfer.

2) MB found out at what temperature the blower is most efficient (given internal tolerances) and they designed the intercooler to match those temperatures. The issue with the latter is that this was calculated on a stock pulley so if this same temperature range can be found with a more efficient intercooler and a larger crank pulley (more boost), then the function is retained.

Any input?

 

[Lou Scannon]

Hemi, I don't think the throttle response would be affected...

You should think again. I recommend you read up on the subject of throttle response and how it is affected by plenum volume between the throttle and intake valves.

Why would the throttle body need to be larger for the same result?

Do you understand how pressure affects density, how density affects velocity, and how velocity affects pressure drop?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[bentwings2]

Here is a link to the LS7 Corvette supercharged build.

http://www.gmhightechperformance.com/camaro_now/sucs_1130_supercharged_ls7_engine_build/viewall.html

Note that they make close to 800 hp and over 800 lbs/ft torque.

This is a really big screw blower on a big cu in. motor but only has a 90mm intake. They talk about adding a 100mm or slightly larger intake and gaining a few more hp.

From this I would think a 90 mm intake would be more than enough for the MB you are working with.

Based on what I see with o,ur carbs, the larger the throttle bore or as we measure CFM, the better the throttle response. The top and bottom of a 6-71 blower is far larger than the carbs are so the restriction is at the carb. This is why you see massive injector housing or by various terms, Bug catcher, Bird catcher etc. I can't remember wat the exact numbers are but 40 sq in area seems likely and even more on the larger ones.

Now on the street, with 2 750 cfm carbs throttle response is far faster than NA motors. I don't have numbers but attempting to watch the tach and boost gage then jump the throttle I see boost about as fast as the needles can move. Meantime the car is likely sliding around with tires smoking. things happen very fast so I have to watch the road.

The difference between the 750 cfm carbs and say 600 cfm carbs is that the throttle movement per degree of acceleration is slightly greater in favor of the larger carbs. Now the becomes more critical on the larger carbs as you use less opening to hold road speed. This means that the idle and transition circuits are very critical. I like to get reasonable mpg so I have worked hard to get these adjust just right. There is a limit to the accuracy of the carbs themselves.

EFI would be the ultimate since you can use direct port injection and only have to flow air thru the blower.

I think you are probable correct in your analysis of MB's engineering. I'm sure a roomfull of engineers worked everything out for their product so that it is street leagal everywhere. This puts some limitations on what they could achieve in the space design gave them.

Since the blower is a compressor (screw blower) it will try to compress any air that gets into it. It needs a bypass valve which it has. In order to reduce "pumping" loss and hp consumed they added a clutch. (for mpg) I suspect that the bypass valve will also allow air to flow the other way when the motor is under little load or non boost conditions. You are operating NA at this point and the EFI takes care of the fuel requirement.
I'm sure there is vacuum at this point in the ports or what ever plenum there is. This will go away very quickly as soon as the throttle is opened and the bypass valve close.

So now you are going to hop this system up with a larger crank pulley and larger intake. Surely the boost will go up, air flow will increase. The computer may not be able to handle this change so you may be into having to reprogram the computer. Just like I had to change a handfull of carb jets and feed restrictions. Assumint you can get into the program with a lap top it should not be too difficult as at least you will have some information to start in the stock program.

It will be fun, I can assure you, but a lot easier than messing with these relic carbs. haha

 

[bentwings2]

Once again I make a mistake and not to note horrid spelling. Also I don't know how to do the quote thing so I have to make up my own.
Sorry.

"The difference between the 750 cfm carbs and say 600 cfm carbs is that the throttle movement per degree of acceleration is slightly greater in favor of the larger carbs."

This should read "slightly less". In other words it takes more throttle opening with smaller carbs to achieve the same level of performance of the larger carbs.