ULTM8Z

I was reviewing some running data after doing some tuning with the new 1.6 rocker arms...

I was shocked to find that at WOT and above 3500 rpm, the manifold pressure starts dropping significantly. After about 4200 rpm, it's down to ~92 kPa. Key on, engine off, I'm getting 97 kPa. So I'm down about 5 kPa.

Something is restricting the intake...

Considering where my air filter is (under the battery tray area, with a 4" tube going to the throttle body) I thought maybe I was getting some sort of negative pressure behind the air dam. But the vehicle speed seems to have no effect. I get the same low manifold pressure condition at 35 mph as well as all the way up to 90 mph... no relationship between manifold pressure and vehicle speed.

I'm thinking the 4" tube is plenty big, and except for the turn at the TB, it has one turn at the other end to link up to the air filter. So it has to be either the air filter itself or the throttle body. The throttle body is an old SLP I bought in the 90's advertised as a 52mm, but I recently measured it out to be 50mm.

But, an 8 kPa drop in manifold pressure has to be worth a significant amount of power, no?

I recently put in much hotter ignition coil, hotter HEI module and 1.6 rockers as well as replaced the igntion wires which were showing their age after ~15 years. The engine must be wanting a lot more air...before all this, I was getting around 98 kPa.

QwkTrip

iTrader: (22)

As a reference point, my 4" intake was good to almost 500 RWHP with an 11" long conical air filter above battery tray. It was a bit of a restriction above that and I switched to a 5" intake. My method of sizing filter is explained in post #1305,

https://www.thirdgen.org/forums/ltx-...ml#post6330021

What shape filter do you have? I've learned that air DOES NOT like to turn the corner into a cylinder filter. Use conical or panel filters. Gotta have that straight shot through the filter into the tube.

ULTM8Z

This is the K&N filter I'm running.... 4" inlet, 6" tall.

ULTM8Z

You'd think K&N would at least post the effective area of this filter in the specifications section! Grrrr...

Running the calculations using their formulas...

Assuming a 6000 rpm power peak, gives me a nice round 100 in^2. I'm probably below that, but I'll use that number for conservatism.

Based on their formula, and using a 5" average diameter, it's saying I need a 7" tall filter. But that doesn't take into account that my filter has additional filter element on the top. Also, I'm experiencing the restriction at WAY below 6000 rpm.

So based on this, it has to be the throttle body?

The data shows 100% TPS and I just verified I'm physically getting WOT when I put my foot to the floor. So the restriction appears to be very real, whatever it is.

QwkTrip

iTrader: (22)

It's on your car, you can measure the top and bottom diameters and length.

QwkTrip

iTrader: (22)

I'm not really in tune with what throttle bodies work with TPI, but dual 50mm bores is 6 in^2. Now compare that to a 400 Hp LS2 with a 90mm bore (3.5") at almost 10 in^2. I'd venture to guess your throttle body is restrictive.

ULTM8Z

A 58mm would get me to about 8 in^2... hmmm... I always thought 58s were too big for a basic 350 build...

QwkTrip

iTrader: (22)

I don't think port injected engines are sensitive to big throttle bodies as long as it can be tuned at idle. Carburetors need close attention to air flow because pressure drop is required for the fuel to be delivered.

ULTM8Z

Is a 5kPa drop necessarily a loss of 5% power?

QwkTrip

iTrader: (22)

Never thought about it before but that might be a reasonable way to look at it.

https://wahiduddin.net/calc/calc_hp_abs.htm

QwkTrip

iTrader: (22)

Instead of putting money into the TPI, have you considered going to a multiport Performer RPM Air Gap with an elbow and LS style monoblade? That would be sweet on a H/C/I ZZ4.

ULTM8Z

I'm actually running a Miniram.

ULTM8Z

So this is saying if I was down 5kPa (had to convert to inHg in the calculator, since I don't know what they mean by hPa), yeah, I'd be down by about 5% power. So if the engine is supposed to be making 400 hp, then I'm choking off roughly ~20 hp. Wow...

Orr89RocZ

iTrader: (20)

Did you try running without filter? Or open throttlebody?

97 kpa key on means you likely only have potential to reach 97 kpa under throttle unless you have ram air

5 kpa drop is .72 psi loss. Seems like alot for a filter. Pulling it should show a diff. I wouldnt say 5kpa is 5% power but in this case a motor typically gains twice its power in twice the atmosphere pressure. The ratio of .72 psi would be approximately 5%. 20 hp on a 400 hp motor. But to me that seems optimistic. 10 may be more realistic.

theres also some potential it could be exhaust restriction limiting air intake as well.

QwkTrip

iTrader: (22)

Yeah, it's not necessarily one thing. Could be a combination of MAF and throttle body.

If manifold vacuum is created from the suck of the cylinders, then wouldn't a lack of exhaust flow cause higher MAP values?

ULTM8Z

Figured out what happened. The hose I had going to the map sensor developed a small crack in it due to age. I replaced it and now I'm getting ~96 kpA for ~97 kpA ambient pressure at WOT.

Well that sucks... I was actually hoping for some sort of restriction where I could gain some more power... lol...

Kevin91Z

iTrader: (1)

For reference, we tried a 58mm throttle body on the white Trans Am, and at it made no difference in peak horsepower or torque over the 52mm.

I'm glad you figured out the issue. Those rubber hoses are tricky.

hpm400

A stock 48mms TB will support 500 plus HP. GM used it on the Ram Jet 502. NHRA super stock run deep in to the 9s at over 3100# on a stock throttle body.

z 28 jari

iTrader: (1)

See post #16 https://www.offshoreonly.com/forums/...-bodies-2.html