Intake piping/ bottleneck ?'s

[GN138]

Posting this for a buddy of mine. He's got an 87 GN with a CPT-65 turbo, stock MAF, stock intercooler, etc...

He just recently got the CPT-65 and just got some Racetronix 72-75# injectors and is looking to get an Extender chip/ Translator plus combo with the 3.5" LS1 MAF. He's also looking at the Thunder Fabrications IC/ or the Mease big neck 24 row stock mount.

Question is: With the turbo having a 4" inlet and the MAF being a bottleneck @ 3.5", should he spend the extra money to get the 4" MAF piping, or, will the 3" piping with a reducer to the turbo and another @ the MAF do well enough? Do the bottlenecks make a huge difference or is the 4" piping worth it?

Thanks!

 

[GNSCOTT]

It is a waste with the 3.5 maf. It might also be a waste if he had no maf and ran a 4" pipe depending on how good the rest of his combo flowed.

 

[JDEstill]

I'll take the opposite stance: I think that any improvement you can make here is worthwhile. Compressors don't like to suck, and anything you can do to keep the pre-compressor restriction to a minimum will pay dividends. Big short pipes, like good flowing air filters, are the way to go if you can get it to fit. That being said, the more air he's going to be flowing the more important this is. If he's going to be tooling around with his boost set at 15 psi, then don't bother. But if he'll be cranking it up to 25 psi, then I think it'll be worthwhile.

John

 

[GNSCOTT]

John, I would agree that it doesn't hurt, but the 3.5 maf 18 inches away from the turbo inlet is where the sucking will be.

 

[azgn]

a stock gutted maf will flow 1000cfm...I don't think a 65 turbo flows that much......

maybe the most important thing is a straight shot into the turbo.....cold air kits and the like tend to reduce flow a bunch (although the cooler air may help some in a non race track situation.....at the track, you want a straight shot....)

 

[325DR]

Try and find a set of 4" MAF ends for the MAF sensor. Thats what I did, although when I ordered them, I was told they were discontinued and I got the last set.

 

[GNSCOTT]

I still don't see how the 4" maf pipe, after the Maf will help any. Like you said, the stock maf can outflow the turbo, so a 3.5 maf with a 3.5 maf pipe is more than enough. The maf pipe is almost a straight shot.

 

[GN138]

Thanks for the info guys!

I'll have my friend come on line and read these replies later.

Thanks again....

 

[JDEstill]

GNSCOTT, it kinda depends on what you mean by "outflow the turbo". That whole "it flows xxx cfm @ 28" " thing can be rather misleading. Look at it this way instead:
1. Say you are running WOT at 25 psi boost.
2. The amount of work the compressor is doing depends on the air flow rate and the pressure difference between compressor suction and discharge. More air flow, the harder the compressor is working. More pressure difference, the harder the compressor is working.
3. The pressure difference factor is outlet pressure divided by inlet pressure.
4. You can make that pressure difference smaller by decreasing the boost pressure, or by increasing the suction pressure.
5. Or, any increase in suction pressure means the turbo doesn't have to work as hard, even at the same boost level and air flow rate.
6. With a stock MAF, stock air filter, stock MAF pipe the compressor has to suck really hard. If you put a vacuum gauge at the compressor inlet you might see something like 5 or 6" Hg.
7. Replace the air filter with a K&N and the suction pressure might drop to 4 or 5" Hg.
8. Bigger MAF, and it drops some more.
9. Bigger, shorter piping, and it drops some more.
10. The closer you can get the suction pressure to 0" Hg, the better off you are.
11. A stock air filter/MAF/MAF pipe *can* flow the amount of air required to feed that big turbo at 25 psi, just like a K&N/LS1 MAF/4" MAF pipe can. But the resulting vacuum drawn at the compressor inlet will be different. And the resulting performance will be different as well.

John

 

[GNSCOTT]

John, I agree on the filter, and bigger maf, but there is absolutly no need to go to a MAF pipe (turbo to maf) that is larger than the maf. He has a 3.5" Maf, that is where the turbo will be "sucking" from. There is no advantage to a 4" pipe AFTER a 3.5" Maf.

 

[JDEstill]

OK, then why not?

In a car running a big turbo and big boost, lets say is moving 450 gm/s of air (25 psi or so boost at 5000+ rpm) the air velocity in a 3" ID pipe is about 267 ft/s (182 mph) while in a 4" ID pipe the velocity is about 150 ft/s (102 mph). IMO, the lower velocity in the bigger pipe means less pressure is lost getting from the MAF to the turbo, and a higher pressure at the compressor inlet does nothing but good.

Or, to take another tack, why would you want a MAF pipe any smaller than the compressor inlet? With a 4" inlet, I would want an inlet pipe AT LEAST that big. No matter what MAF size I was stuck with.

John

 

[GNSCOTT]

Ok , re read the original post and he stated that the Maf was a bottleneck when in fact the 3" Maf pipe is the bottleneck. I thought he already had a 3.5. The 3.5 will help, the 4" won't help, won't hurt unless it creates clearance problems with the upper rad hose, Iac, ic up pipe, etc. Have him call Cottons and ask Jack. Jack doesn't sell people parts they don't need unless the customer insists. Some people want it there for looks too.

 

[Roger]

Another train of thought - I like to think outside the box.

Use the LT1 3" MAF in front of the throttle body, in the up pipe. It'l now have pressurized air blowing through it & you can then use a 4" pipe all the way from the filter to the turbo inlet.