Let's Talk WOT Vacuum!
 

Well I hit the dyno for the first time since I started working with HPTuners.

Started out with 380hp/380tq. Worked with some PE, scaled my MAF down, and took out the knock retard and ended up at 400hp/400tq. This is a TR224 cammed car with Absolute Speed Stage II heads, LS6 intake and exhaust. The intake is a Home Depot sewer pipe special, since the LS1 lids wont fit on LT1 cars.

Oh yeah, this is a 99 LS1 in my 1994 Camaro.

Anyway, I am getting around 1.5 PSI (3 inches) of vacuum in the intake manifold at wide open throttle once the revs come up.

I start out just below one atmosphere when the gas is nailed for a dyno run, or a run on the street for that matter--around 14.2 PSI on most runs--but once the RPM climbs up, I dip down to around 13.2 PSI. Sometimes its a little more and sometimes a little less of a pressure decrease.

A friend said on carbed engines, the rule is nothing over 1" Hg of vacuum, and if you have more than that, it's time for a bigger carb.

So going by this rule, my 3.x inches of vacuum is probably holding me back. My theory is the Home Depot Ghetto special cold air induction is not up to snuff.

To prove this, I wanted to see some logs from you guys. Here is one of mine below.

http://bervickbradstreetracing.com/c...pics/dyno1.jpg
http://bervickbradstreetracing.com/c...pics/dyno2.jpg

So can anyone post up some results they have?

Thanks as always.

-Kyle

Yeah, that tells ya you've got an intake restriction. Someone correct if I'm wrong.

If you are on the dyno again, you could test your theory by disconnecting your intake at the MAF and see if your MAP goes up.

I usually see 99+kpa at WOT through mid-range. I think it usually drops to a low of maybe 95kpa at the very top end. I will have to look over my logs again when i get home. Vacuum really sucks. Once you have a lid, the big bottleneck is the MAF, then a ported TB helps. Your intake system will only flow as well as the worst flowing component...something many people forget.

Indeed.

6900 RPM is higher than most people spin and will show
you things most don't see. Definitely an intake side
restriction in the pipe somewhere. A screened MAF will
be a bigger and bigger restriction, I believe quite nonlinear
as airflow goes up (all those channels will get highly
turbulent locally, lots of drag). But you could also have
something squirrely like a bellows / hose collapsing under
vacuum if it's too long / weak-walled, etc. 1.5PSI is a
huge drop, I see 0.1-0.2 tops (but only at 6000RPM).

Thanks for the info Jimmyblue. If you are only dropping 0.1-0.2 PSI then I am in trouble! Mine dips below 14 PSI at only 4,000 rpm and by 6,000 I have dropped 1 PSI or more.

My MAF is descreened/definned/sanded out. I removed my cone air filter and did a run and got the same results. I think this narrows it down to the small Home Depot elbow pipes that I used. Ah well, it's always something!

I have same setup as yours only with LPE components, in a 00vette I have been getting a SES light (P0107 low MAT vacumm) at high RPM's(6500+). It turned out to be a bad connection between the MAF and the Blackwing/took out the v/r velocity stack. I have sent the T/B to be ported and will get the car dyno tuned. Bottom line I hope this fixes my SES light, your info will help me with my tune.
v/r
THE YAV

I was wrong. I still see 98+kpa at 6000rpms. Sweet.
http://users.adelphia.net/~someoneelse/kpa.JPG

i get 96-97kPa at 4500rpm-6100rpm and 80%+ throttle
i can get up to 98-99kPa at 2700-4300rpm and 75%+ throttle

97 kilopascal = 14.0686606 pound-force/square inch (PSI)
99 kilopascal = 14.3587361 pound-force/square inch (PSI)

does that mean I have a problem somewhere? i have a slp lid+filter+granatelli maf+LS6 intake+smooth bellows. any comments?

There doesn't sound like there is anything wrong. You could probably get a bit more airflow with a ported TB. Airflow through the stock lid, MAF and TB is nowhere near as good.

Also is you maf screened or unscreened

question about your home depot special.... did you use 4" pipeing or 3"??



im just wondering if i could expect the same restriction. the angled pipe is a 4" - 45* lowes PCV pipe.... the elbow is a 4" to 3.5" (for the MAF) but i cant do anything about it.....

http://66.83.134.202/travis/LS1%20pics/Engine2.JPG
http://66.83.134.202/travis/LS1%20pics/CAI.JPG

holy big filter!! lol, that intake looks pretty decent if you ask me.

Man...that intake is nasty. It looks like a horse's dingaling.
:jest:

i've done some experiments on this topic on 3 different cars, i put it up on my blog:
http://redhardsupra.blogspot.com/

lemme know what you think