Air Flow Requirement for the 6.5TD

[Spindrift]

O.K., so I think the exhaust mods I have recently completed made an improvement to engine performance. The more I look at the air flow design for my '98 Suburban, the more I think there might be an opportunity for improvement.

There have been many posts regarding the older trucks and folks have suggested two main fixes: going to the round filter and opening the LD intake extension casting as per JK's instructions. Since my '98 doesn't fit into either category, I'm looking to do something else to improve air flow to the engine. Currently, there is a rather small air inlet directly behind the passenger side headlight assembly. This plastic intake assembly takes a 90 degree bend, runs up and behind the grill, takes another 90 degree bend and then dumps into a cavity/space behind the fender well. Any air making it to this cavity then travels thru a 4" oblong/oval opening in the wall that separates the engine compartment from the cavity/space behind the fender well. A very short tube connects the 4" oblong/oval opening to a 3" opening in the K&N filter box, which contains the round filter. So, we have a passive air supply system to the turbo that appears to be restricted at several points along the way to the turbo. It just doesn't seem to be a good design, but I don't know. The system is clearly not a ram-air style. In fact, the inlet is obstructed by the headlight assembly. Whatever air is getting to the turbo seems to be getting there solely by the effort of the turbo itself. If air supply to the 6.5TD is in fact inadequate, as many people suggest, it seems that you would need to determine what CFM the engine "requires" and determine if that flow is being supplied by the design of the Suburban's '98 system.

I found earlier information that attempted to answer the first question:

A 6.5L = 397 c.i.

397 c.i. X 2500 rpm = 992,500 c.i. per minute (No boost and no loss)

992,500/1,728 = 574 cfm

Let's say we're reading 7 psi of boost (1/2 atmosphere?)

574 cfm + 287 (1/2 atmosphere) = 861 cfm at 2500 rpm

Assuming this calculation is correct, is there an instrument that can be used to measure air flow so that I can determine what flow is actually reaching the turbo?

If the 6.5TD isn't getting all of the air it could possibly receive, I'm thinking of a more radical modification. I could cut an opening in the fender opposite the 4" oblong opening that currently exists in the engine compartment wall and install something like the following:

http://www.westmarine.com/webapp/wcs...51&storeNum=24

I view this set-up as something of a ram air hybrid. More air would be directed to the air box because it would have a shorter distance to travel (less loss) than stock and it might even be delivered to the air box with some small increase in pressure. Water infiltration would be an issue but this could be corrected thru the use of a baffle in combination with the drilling of holes in the bottom of the air box as members have suggested in prior posts.

Anybody want to take a shot at this one?

[matt-max]

the factory design is definitely inadequate when you modify a 6.5td. the flaws you mention seem restrictive enough to me too and warrant improving.

my tahoe sucks in the indicator with a k&n in the stock box on hard pulls at 15lbs boost.

i have been considering an aftermarket intake/air filter like a friend has done on his dmax with good results.

also there is a member here who has modified a cummins intake to fit in place of the battery (moved it to the airbox location on top of the fender).

or i have a stock dmax airbox and unused amsoil filter sitting around too, i just haven't made the time to put it all together.

not sure how that vent setup in the link would work or look. could be pretty cool i guess.

feel like being the guinea pig?

matt

[gmctd]

Remember, guys, it's a four-stroke engine - divide by two!
Takes two full rotations to pump 400cuin.

And, a few corners in the 'ram-air' path are necessary to get rid of the heavier elements when its raining, snowing, or dusting.

Inertia and mass, right?

Wanna see how little effect those corners will have on the effective pressure available, try sticking your hand out the window, palm forward, at 30, 40, 50, 70 mph.

Even a flat plate stuck in front of the duct, as in the '70s - '80s trucks, does little to prevent necessary quantity of pressurized air from reaching the intake box.

You are correct - properly ducted, a cool source of intake air can be provided.
Even from the hole behind the headlight.

We're not feeding a tuned 454 with a 950cfm Holley, turning 7000rpm, right?

Later -

Fergot to mention: Ex-Tech, Fluke, and others make an air-flow transducer. Consists of a small pm dc motor with a four-bladed fan in a venturii arrangement. Gives a calibrated dc output for a DVM, cross-referenced to V = Flow map.

A Pressure transducer is available, and a Flow Rate transducer, in dedicated meter packages. Could be rented from Transcat, and such..

Simpler, a combination 20psi\30"vac gage can be had from JCWHITNEY.com for around 20 bucks.
Just stick a pitot tube in the duct.
Have one on my truck for boost - vacuum gage indicated a plugged air filter, from driving thru heavy construction zone for several months, at idle rpms.

27" = 1psi, would be better if you could get a 5psi\27" gage, but the JCW gage is easily obtainable.

Keep thinking, guys - good ole American ingenuity improves our rides.

[ 01-03-2004, 06:15 AM: Message edited by: gmctd ]

[Spindrift]

jd,

Are you saying the correct number is 574 cfm ? If so, do you know that the current air intake design of the '98 model is actually supplying the 574 cfm? Can we test for that specific flow?

Thanks.

[gmctd]

A four-stroke engine displaces its designed volume in two rotations - intake\compression on first, combustion\exhaust on the second.

A 400cuin eight-cylinder engine will displace 4 x 50cuin per revolution (400/8 = 50cuin per cylinder) for 200cuin total.

(I round off for ease of calculation)


Takes two crank rotations to displace 400cuin air.

At any rpm, divide the total displacement by two to get actual flow rate, minus some percentage for efficiency in naturally aspirated engines.

Remembering that a turbocharger overcomes many inefficiencies, and that a Diesel is more efficient than a gasoline engine, you can roughly base flow rate on displacement and the compressor pressure ratio.

Near's I can remember, max 6.5L flow rate is around 400cfm at 3500rpm, naturally aspirated.

I addended my previous post just before you posted, so you might reread that, also.

I am unfamiliar with the newer trucks intake configuration - mine is '87 style, but the pressure front at that front bulkhead is easily tapped with the proper flexible ducting, obtainable from '70s - '80s TransAms and etc.

[Bobbie Martin]

You can get a free engine calculator HERE that will, among other things, let you calculate the CFM requirements of your engine. Given your example of 2500 RPM and 397 CI figuring 150% VE, the CFMs are 430. Its fun to play around with and, hey, its free so why not get it. Remember, if you boost your engine to approximately 7.5 PSI (at sea level) you have really increased the displacement by 50%, not increased the VE to 150%. So it would probably yield more accurate results to input figures something like this: Displacement would be cubic inches [397] X boost factor (PSI of boost / 14.7+1). Then you can figure the VE at something like 80-90%, I really don't know what a 6.5 would be, but I doubt its really that high - could be even lower. That should give you a pretty good idea of the CFM requirements you need.

Then you need a flow bench to see what you really have. You would have to test the entire assembly together and separately to see where the restriction is and what needs to be changed. My guess - and that is all this is - is your current air box is not the restriction. If you really wanted a quick and dirty method, you could probably find a rolling road dyno and get a baseline HP figure on what you have, then remove the tube from the turbo inlet and retest. It wouldn't hurt to make up some sort of radiused air entry to the turbo for the test. It you get much more HP, you know the air box is a problem - if not then you know its not.

[gmctd]

Bobbie Martin, you done good! Wish I'd had that Calculator, back when.
Thanks.

[Spindrift]

Thanks for all the good info. I've downloaded the program and I'm beginning to get my feet wet. It's quite cool.

Not to beat a dead horse, but I want to be sure that I'm making myself clear. Bobbie Martin says, "My guess - and that is all this is - is your current air box is not the restriction." I don't believe the air box is the restriction either. I believe it has a lot to do with the air inlet which is not only very small, but it is directly behind the headlight assembly...as in almost smack dab behind the headlight.

I'll keep thinkin', and hope you all do the same.

[gmctd]

Here are some thoughts for consideration, open for debate.

Power relates to flow rate, n\a or turbo'ed.
Restriction reduces flow rate - goes without saying.

Boost is directly related to inlet flow rate and turbine flow rate.

When compressor flow rate exceeds engine flow rate, it stacks up as boost pressure in the intake manifold.

When engine flow rate exceeds designed turbine flow rate, exhaust pressure (EBP) stacks up at the exhaust port(s).

When exhaust back pressure stacks up higher than boost pressure, adjusting for increased boost pressure becomes increasingly ineffective.

(I normally limit boost to 8 - 10psi, GM-8, non 'intercooled', factory 195hp, no 'chip', where intake air temps run up around 180 - 210deg.
On these cool Autumn\Winter days, where IAT stays around 100deg, I like to play a little, at around 18 - 20psi. It will hold 20psi long as I keep my foot in it. EGT's climb rapidly, telling me when to pull out.
BUT, and here's a mighty big BUT, I know from experience that EBP increases way over my 20psi gage limits at around 8 - 10psi boost.
20psi back pressure against 10psi boost ain't good.
And, it increases seemingly logarithmically from there with increasing rpm (increasing flow rate).
That's thru a factory FORD 7.3L PS 3.5" stainless exhaust system, connected to the 3" GM stainless downpipe via industrial 3.5" stainless flex, intended for pressure\vacuum service).

I get 20psi max, not from intake restriction, but from exhaust turbine restriction.
Standard 6.5L air box, short section of the 'snorkle' installed for routing, standard AC\FRAM\STP air filter. Removing the 'snorkle' section does not affect boost.

If you can easily make boost, limiting it to 15psi with a controller, then you don't have much intake restriction. EBP blows the wastegate open at that setpoint.
The Restricted Air Filter Indicator is spring-loaded (calibrated) to factory specs, requirements which were limited to factory PCM programming, probably 8 - 10psi boost, with some spikes to 12psi.
Try one off a n\a 350 or 454 for comparison - see how it 'sets', or try one off a 7.3L Power Stroke.

Remembering the 6.5L limit of 3500rpm (Bill Heath ran his twin-turbo drag 6.2L to 4400rpm, with one rather exciting event to 8500rpm, but I don't think he would care to repeat that, often!), and remembering the turbine restriction in the exhaust, improving the intake path will mainly involve providing ducting from the air box to the high pressure area at the front bulkhead.
That pressure is free at road speeds, and the cooler air temps will lower the compressed air (boost) temps.

My headlights and escutcheon may not be as restrictive as the newer trucks, and pressure thru those openings will blow a plastic-wrapped bi-folded paper towel stack, lodged between the battery and the fender, right out of the eng bay, repeatedly.
(Yeah, I keep it at the back of the bay, now, wedged in the fender-mounted plastic molding for the jack)


What size duct?
Make it same size as air box inlet, with no restriction less than 1.5 times diameter of compressor inlet. Watch those corner angles.
Mainly, ducting it to an opening in that front bulkhead will greatly improve efficiency.
Running it inside the fender panel provides some shielding from radiated engine bay heat.

[ 01-05-2004, 07:31 PM: Message edited by: gmctd ]

[matt-max]

wow.

excellent info and viewpoints.

i will try my dmax airflow indicator tomorrow and see if it gets sucked in too.

thanks
matt

[Kollin Syverson]

If air supply to the 6.5TD is in fact inadequate, as many people suggest, it seems that you would need to determine what CFM the engine "requires" and determine if that flow is being supplied by the design of the Suburban's '98 system.

Think of it this way, if you are not getting enough airflow, because of a restrictive air intake, how do you build boost? It takes air to build pressure. In my truck, I have no problem at all getting 15 psi of boost pressure, depending where I have my Tubo-Master set. This tells me I am pumping more air into the intake than the engine is capable of consuming.

If the engine were capable of consuming more air than the turbo could push, then I would not be able to build boost pressure. Make sense?

[ 01-09-2004, 07:19 PM: Message edited by: Kollin Syverson ]

[Spindrift]

I just got this truck and it's completely stock except for the recent exhaust mods. Now I know why it's so important to get the boost and pyro gauges installed. Fortunately, Santa left both under our tree.

So...what I hear being said is that there is a direct relationship between air flow and boost and there is no ancillary benefit to having more air? Right now, without my boost gauge installed, I'm walking in the dark.

I don't understand the effect of the Turbo Master. My understanding is that with the Turbo Master installed you can reach, and sustain, boost levels greater than the factory set point of 10-12. If it's relatively easy (economically speaking) to increase boost with the addition of the Turbo Master, should this addition be my next modification? I'm also assuming that the engine will have an increased air flow requirement with the Turbo Master installed. If I can't achieve an increase in boost, this should tell me that modifications to the air intake and ducting are warranted. Everyone agree?

jd...how are you reading or determining exhaust pressure (EBP)?

[whatnot]

The computer won't let you add much boost with a turbomaster without setting a code.
You are better off getting an electronic controller. Then with the electronic controller, it just fools the computer so you can add a turbo master later if you don't like how the electronic one works.

I don't know if this is correct but wouldn't a turbo still be able to build 15 psi of boost even if there is a vacuum before it because of restriction but make more heat from compressing more? (15psi + -5psi = 20psi total compression?)

[gmctd]

Pcm controls boost by pulsing vacuum to the wastegate - limiting value was decided upon by the programmers.

Some electrical add-ons fool the PCM by offsetting the Boost Sensor output - PCM sees 5psi where 10psi exists, and maintains vacuum control for the lesser amount. Net result = greater boost pressure.

Mechanical add-ons substitute adjustable spring\arm arrangement for vacuum servo at wastegate.

Both types, vacuum and mech spring, hold the wastegate closed against exhaust back-pressure developed in the turbine housing.

When playing with PCM control overrides, a boost gage is very important, but - an EGT gage is critically important.
I cannot stress that enough. At low intake air temps (IAT) I can hold 20psi boost till the cows come home, even tho I know anything above 12psi is not efficient. But - I never hold it past 1200deg EGT measured in the driver's side exhaust manifold.
That's my 'programmed' safe limit.

When I first built my truck, I plumbed a 304 stainless 1/8" line to the exhaust, to determine EBP levels in the GM-X series turbines. I had no other data available at that time. Had one tap in the EGR blockoff, one tap in the GM-4 turbine housing.
Had to remove it to pass state inspection, but I had seen enough. Then I installed a GM-8 and non-EGR manifold, and lost the taps. May install one again, since I've resumed 'playing'.
Did a post on that and some ideas, called "Pepto-Bismol" in a humorous vein.

Remember, EGT climbs as exhaust pressure EBP climbs - exhaust heat cannot get out of the manifold, and is accumulative.
Another reason why 12psi is generally accepted as upper boost limit with the GM-X series turbochargers.


Addendum for Whatnot's post, affirming Kollin Syverson's post -

If a blocking plate is installed over the inlet of a squirrel-cage blower, it will develop zero pressure (and over-speed). Flow can be controlled with an adjustable plate, as with the throttle-plate in a carburetor.
Same thing happens here - restrict compressor intake, reduce boost. It cannot compress what it cannot in-take.
Put your hand over the end of your vacuum cleaner hose and check pressure on the output end.
'Course, I'm talking gross restriction, here.

But, if the 6.5 can build and maintain 15psi boost throughout rpm and load range, any oem intake restriction is not enough to impede the cfm flow requirements.

If it builds 15psi at 1000 - 2500 rpm, but pressure always falls off above that, a restrictive condition exists.
Could be intake, could be exhaust, could be fuel, could be control.

[ 01-05-2004, 01:49 PM: Message edited by: gmctd ]

[Spindrift]

Thanks for all your time on this one. Learned a lot.

[gmctd]

When your gages are installed and functional, it will all be very enlitening.

Good luck with your project

[Arlie]

I had this notion to get a ram air effect. The support structure for the hood is tubular and one arm passes near the air box. It would be fairly easy and cosmetically nice to put two small hood scoops (stick-on types don't cost much) with holes through the hood entering the tubular support, a duct mounted to the air box, braced to the fender could have a foam seal mating to a hole in the tubular support arm. The other arms could easily be sealed off.
Funny what strange ideas pop up sometimes. Funnier yet, if I were convinced there was really a significant intake restriction I might actually do this project.
Arlie

[Spindrift]

The fun in this project will be exactly that...determine whether or not there is a restriction and if so, where.

As jd said before, it could be intake, could be exhaust, could be fuel, could be control. There's a lot of work yet to do.

...but darn, a fender scoop would still look cool.

[gmctd]

Intent was not to discourage innovation - just to offer some factual info to possibly assist with direction.
Simple as I suggested, innovative as you choose - ideas and ingenuity improves the 'ride'.

Just don't forget to post it, here.

[vernon morgan]

Speaking of fender scoops, I have installed the late 70's hood scoops from a Pontiac TrasAm. One in each fender just above the flare. They look really wild and have a great ram air effect. The one on the drivers side is just a dummy, I never removed the backing plate. You do have to cut the fenders for the install but well worth it. There cheap, I paid $10 for the pair. You also have to cut the mounting brackets from the original hood. I cut the fenders welded in the brackets and used the factory T/A scoop that just bolts to the bracket. Works great with more than enough air flow.