Blow Through vs. Draw Through

[Anonymous]

Ok I have been reading a turbocharging book, I will find the name for you later since it is out in my truck. Basically it reviewed all the basics and talked about carb set ups in the end. My basic understanding of it was like this.

Draw Through:
Turbo after carb, sucks air through. (+) No BOV, Intercooler, carb hat and only minor carb modifications needed. (-) harder cold starting and less throttle response.


Blow Through:
Turbo before carb, needs intake box/carb hat to pressurize entire carb to keep venturi low pressure. (+) Better cold start and throttle response. (-) lots of internal carb modifications, BOV required, possibly Intercooler needed as well.


Is there more to the equation I am missing? Is the added price really worth the difference in throttle response and starting?

 

[StrangeRanger]

The intercooler is not "needed" on a blow-thru but it adds a lot of performance potential. An intercooler is not possible on a draw thru; it adds a lot of explosion potential.

Also the draw-thru requires a much bigger than stock carb. A blow thru can often get by modifying the stock carb.

 

[Anonymous]

Also the draw-thru requires a much bigger than stock carb. A blow thru can often get by modifying the stock carb.

Would a Holley 7448 350 cfm 2 barrel be good enough for a draw through?

 

[wallaka]

The percieved lack of complication on the draw-through is mainly because they CAN'T use parts like blow-off valves and intercoolers. The draw-through also needs special seals in the turbo that most don't have. It would cost money to have those installed, so add that into the calculation.

Blow-through is the way to go IMO. The carb mods aren't that involved. Heck, Linc did them to the stock 1-bbl carb. Strictly speaking, the blow-off valve and intercooler aren't needed, the system will work without them. The BOV does help prevent damage to the turbo, but if draw-throughs work without them, so can a blow-thru. It's just cheap insurance.

 

[XFlow_Fairlane]

carb mods are pretty basic....solid float and remove the choke linkage. prob 20 mins worth of work on a 350cfm holley

don't forget you can convert to a 2V first before you go turbo and then things are much simpler. with a draw through you have all new linkage to setup too.

 

[hasa68mustang]

intercoolers are fun, after driving my car for awhile and not realy beating on it too much the piping before the intercooler would be pretty hot to the touch (you could touch it but not able to hold it there for more than a second) and after would be cooler than outside. plus it looks cool

 

[Bort62]

and after would be cooler than outside.

Well, that's not physically possible - but it feels cooler because it conducts heat out of your finger better.

 

[Anonymous]

Depends on pressure drop. That's how carb icing and A/C work.

 

[XFlow_Fairlane]

after a hard run it could be cooler for a short period. The heat flux out of the pipe could be great enough to allow a sub-ambient temp to be reached.

 

[Bort62]

Okay, well technically yes since you are expanding the compressed air through a restriction - the intercooler, it could get cooler, but in reality no.

Even the worlds worst intercooler is only going to drop the pressure 2-3 psi, that's not going to actually change the temperature measurably.

<-- Thermal Systems Engineer by day.

 

[XFlow_Fairlane]

what about the heat flux through the cooler surface itself? granted we are talking LOW temps (like 2-3 lower than ambient and for a short period of time hafter a full throttle run)

 

[Bort62]

http://en.wikipedia.org/wiki/Second_law ... modynamics

Here's the thing - In an Air conditioner or a refrigerator, You pump a gas up to a high pressure. You then use a heat exchanger to cool that high pressure gas to ambient (releasing heat energy into the outside air). Then, you expand that gas to a low pressure and as a result, it's temperature drops. You then heat the gas up to ambient (sucking heat energy out of the conditioned air) and start over.

The heating the gas back up cools down the air that you heat it with and as a result, your car or fridge gets colder.

So, in a turbo application - you are pumping up a gas (air) to a marginally higher pressure with the turbo. You are then cooling it close to ambient (or so you hope) in the intercooler. Now, since the intercooler IS a flow restriction just like any other peice of "real" hardware, there will be an associated expansion and pressure drop. This is the 2-3 PSI drop people see across the intercooler. Yes, technically, this drop in pressure corresponds with a drop in temperature. However, since the drop in pressure is SO low, and the upstream temperature is so much higher than ambient - the intercooler can never in reality get below ambient.

The second law of thermodynamics basically states that a cold soda sitting in a warm room can't get colder. My statement about the intercooler not being able to drop below ambient is the same. It's the cold soda in this case.

Now, the reason that it feels cool to the touch is because the conduction of heat between your finger and the aluminum of the intercooler is much higher than the conduction/convection between your skin and the air. As a result - it feels cooler. What you "feel" is actually heat flux, or heat flow in or out of your body.

That's why a 70* swimming pool feels cold, but a 70* day feels nice. Air is a poor heat transfer mechanism. This is also why there is such a thing as "wind chill" the movement of the air due to wind increases the heat transer out of your body via convection.

Or a peice of metal or glass will feel cold (as long as it's not in the sun) On a warm day.

It is certainly not at a lower temperature.

 

[Anonymous]

Even the worlds worst intercooler is only going to drop the pressure 2-3 psi, that's not going to actually change the temperature measurably.

Well, there are some restrictive IC's out there (you CAN get too small) but in reality, even with a 3 psi pressure drop, your intercooler would have to be 100% efficient (NONE are) to have the outlet temps drop below ambient.

<- I work on air-pacs on BIG airplanes!

 

[XFlow_Fairlane]

hmmm I guess me driving my turbo fairlane 275 miles home from college in 20 degree weather means it runs rich gets poor milage and makes no power.

if yo uhave even driven a turbo car with a boost/vac gauge on it you would know there there isn't alot of low vac no boost conditions.....AT WOT

if you are at WOT and not building boost you have more things to worry about than black plugs or poor milage.

I was getting fine milage in my car when I was driving it and still having issues (had a clogged fuel line)

if you have low vac and no boost you are prob in a transition period on the way to making boost (IE spooling)

 

[XPC66]

Ok I have been reading a turbocharging book, I will find the name for you later since it is out in my truck. Basically it reviewed all the basics and talked about carb set ups in the end. My basic understanding of it was like this.

Draw Through:
Turbo after carb, sucks air through. (+) No BOV, Intercooler, carb hat and only minor carb modifications needed. (-) harder cold starting and less throttle response.


Blow Through:
Turbo before carb, needs intake box/carb hat to pressurize entire carb to keep venturi low pressure. (+) Better cold start and throttle response. (-) lots of internal carb modifications, BOV required, possibly Intercooler needed as well.


Is there more to the equation I am missing? Is the added price really worth the difference in throttle response and starting?

I wouldn't try the draw through unless you intend engineering it properly Carby hat, pilot operated fuel pressure regulator, sealed carby, poppit valve and blow the air in.

Forget BOVs, although you might get some joy from a recirculating bypass valve. Remember if you are running positive crankcase ventilation you will have oil vapour in the air stream and a BOV will release that pollution into the atmosphere. BOVs aren't generally required, and contrary to popular opinion they don't stop damage to the impellor, they merely stop air stall (that is air stall, not impellor stall) so that the boundary layer on the blade contors doesn't readily sheer causing pumping to stop. With the throttle shut the turbine is slowing, the impellor has negligable flow and flow seperation occurs at the blade contours as it rides the pressure curve, giving the flutter sound

With blow through you get some minor cooling from latent heat of vapourisation as the fuel evaporates at the venturi and venturi amplifier. You can also fit an intercooler, which should be at least 85% and maxiumum 2 psi drop. This will allow a larger mol count per litre of air, but of course it will introduce a minor lag as the air becomes more dense.

When you select a turbo you need to be cognisant of the efficiency, not only for stall and choke, but for raised heat of compression resulting in higher than desirable air temperature. This is a common problem that people don't seem to able to work out. More often than not that 25 psi boost that Joe Bloggs Streetracer is running would yeild less than a properly sized turbo/huffer on 18 psi, merely because the sensible heat is so much lower.