300 Dollar Supercharging ..
05-12-2004, 10:03 AM
#21
Senior Member
Join Date: Nov 2003
Posts: 480
Okay, so looking through the thread, zbrown has his upstream of the throttle - see my earlier post regarding massive blow off vale and lots of wasted crank HP at part throttle... (or just a little supercharger not moving much air).
05-12-2004, 03:09 PM
#23
Senior Member
Join Date: Nov 2003
Posts: 480
No, not really. In the first pic, the guy had relocated his TB to where it really belongs for a roots SC. Looking through the thread, I see that the other guy was actually blowing through his TB, with a blow off valve to handle the excess air. Not real efficient, but apparently it worked.
So, assuming you aren't moving huge amounts of air (ie keeping the boost sort of low), and don't mind the inefficiency, you should be able to blow through your carb.
Note that the inefficiency I am referring to deals with compressing air and then throwing it away. If the TB was upstream of the SC (where it belongs), you would be compressing less dense air at part throttle, thus not wasting power.
If you want to try this, you will need a boost compensated fuel pressure regulator like a Malpassi (this is a "return type") or a Mallory 3 port (also return type), and a fuel pump capable of supplying extra pressure, as fuel pressure required will increase with pressure in the carb hat.
Now if you could trick the blow off valve into opening at a lower pressure during part throttle, you could increase efficiency (by not making as much pressure when you don't need it).
So, assuming you aren't moving huge amounts of air (ie keeping the boost sort of low), and don't mind the inefficiency, you should be able to blow through your carb.
Note that the inefficiency I am referring to deals with compressing air and then throwing it away. If the TB was upstream of the SC (where it belongs), you would be compressing less dense air at part throttle, thus not wasting power.
If you want to try this, you will need a boost compensated fuel pressure regulator like a Malpassi (this is a "return type") or a Mallory 3 port (also return type), and a fuel pump capable of supplying extra pressure, as fuel pressure required will increase with pressure in the carb hat.
Now if you could trick the blow off valve into opening at a lower pressure during part throttle, you could increase efficiency (by not making as much pressure when you don't need it).
05-12-2004, 03:50 PM
#25
Senior Member
Join Date: Nov 2003
Posts: 480
Not exactly a project I'm working on (my FB's turbo'd), but I can give some hints:
1 - fuel pump and regulator (already mentioned). Fuel pressure needs to be ~3.5 psi+supercharge pressure. Note that this isn't that same as boost (manifold) pressure. You need to reference it to the pressure in the carb hat.
2 - pressure gauge to measure SC outlet pressure.
3 - serious BOV. Remember that the SC is moving max volume all the time, and you're blowing off the rest. At part throttle, pressure could get pretty severe even with the BOV screaming away. Lots of heat, engine load (read: lowered mpg because the engine is working to drive this compressor)... Also look back to #1 - fuel pressure must compensate. If the 2nd gen installation builds up 20 psi, it doesn't really matter. With a carb, you're looking at 23.5 psi fuel pressure. Besides with that much blowing into the carb, there's no telling what's going to screw up.
4 - couldn't hurt to add an oxygen sensor
5 - carb may need modification for the secondaries to work properly. Mechanical secondaries maybe?
6 - carb may need rejetting
7 - you can add an intercooler to this setup!
Also it would be useful to be able to dump the supercharged air when cruising, say by using a throttle body at the SC outlet you can open. If you're not making any pressure, there won't be alot of parasitic drag. This goes for an FC installation as well.
1 - fuel pump and regulator (already mentioned). Fuel pressure needs to be ~3.5 psi+supercharge pressure. Note that this isn't that same as boost (manifold) pressure. You need to reference it to the pressure in the carb hat.
2 - pressure gauge to measure SC outlet pressure.
3 - serious BOV. Remember that the SC is moving max volume all the time, and you're blowing off the rest. At part throttle, pressure could get pretty severe even with the BOV screaming away. Lots of heat, engine load (read: lowered mpg because the engine is working to drive this compressor)... Also look back to #1 - fuel pressure must compensate. If the 2nd gen installation builds up 20 psi, it doesn't really matter. With a carb, you're looking at 23.5 psi fuel pressure. Besides with that much blowing into the carb, there's no telling what's going to screw up.
4 - couldn't hurt to add an oxygen sensor
5 - carb may need modification for the secondaries to work properly. Mechanical secondaries maybe?
6 - carb may need rejetting
7 - you can add an intercooler to this setup!
Also it would be useful to be able to dump the supercharged air when cruising, say by using a throttle body at the SC outlet you can open. If you're not making any pressure, there won't be alot of parasitic drag. This goes for an FC installation as well.
05-12-2004, 06:08 PM
#27
Senior Member
Join Date: Nov 2003
Posts: 480
Let me explain the difference between a dynamic compressor and a positive displacement one.
Take a hair dryer set to cold, and put your hand over the outlet. The fan continues
to spin. This is (sort of) a dynamic compressor. Now take a bicycle pump and cap off the hose.
Try to pump it. This is a positive displacement compressor.
If you install a roots blower (positive displacement) upstream of the throttle, it will
always be gulping in air at atmospheric pressure. With the throttle closed, pressure will
build, regardless of what boost level you have designed for. This is where the blow off valve
comes in and limit the pressure. This is not the normal function of a blow off valve, which normally
acts to vent excess pressure resulting from the compressor wheel spinning too fast
(like when the throttle is closed and the turbo is cranked up to 100,000 RPM).
With a roots SC blowing through the throttle, the BOV will be singing away at virtually all
times short of full throttle. Not exactly an ideal case.
I don't know whether the guy who has this setup on his 2nd gen doesn't realize this or
simply isn't concerned.
The correct way to install a roots blower is as a draw through. Next best thing
is to have a throttle body at the blower inlet which is controlled somehow in
tandem with the throttle in the carb (this is difficult).
Take a hair dryer set to cold, and put your hand over the outlet. The fan continues
to spin. This is (sort of) a dynamic compressor. Now take a bicycle pump and cap off the hose.
Try to pump it. This is a positive displacement compressor.
If you install a roots blower (positive displacement) upstream of the throttle, it will
always be gulping in air at atmospheric pressure. With the throttle closed, pressure will
build, regardless of what boost level you have designed for. This is where the blow off valve
comes in and limit the pressure. This is not the normal function of a blow off valve, which normally
acts to vent excess pressure resulting from the compressor wheel spinning too fast
(like when the throttle is closed and the turbo is cranked up to 100,000 RPM).
With a roots SC blowing through the throttle, the BOV will be singing away at virtually all
times short of full throttle. Not exactly an ideal case.
I don't know whether the guy who has this setup on his 2nd gen doesn't realize this or
simply isn't concerned.
The correct way to install a roots blower is as a draw through. Next best thing
is to have a throttle body at the blower inlet which is controlled somehow in
tandem with the throttle in the carb (this is difficult).
05-12-2004, 08:40 PM
#29
Senior Member
Join Date: Nov 2003
Posts: 480
Not sure how much I spent - it wasn't cheap. If you want to turbocharge
on a budget, get a TII turbo and manifold, and modify the manifold to fit
a 12A. You will probably need to space it out from the engine to clear the
intake - you can use header flanges, or make a steel spacer to space the
turbo off the exhaust manifold.
You will still need a bigger fuel pump and a boost referenced regulator.
An intercooler is recommended, although a turbo will heat the intake charge
less than a roots blower will.
(and if you have an 83-85, get a real oil cooler first)
on a budget, get a TII turbo and manifold, and modify the manifold to fit
a 12A. You will probably need to space it out from the engine to clear the
intake - you can use header flanges, or make a steel spacer to space the
turbo off the exhaust manifold.
You will still need a bigger fuel pump and a boost referenced regulator.
An intercooler is recommended, although a turbo will heat the intake charge
less than a roots blower will.
(and if you have an 83-85, get a real oil cooler first)