Daveinet

What is typical manifold pressure at WOT. Mine will go as high as 98 @ 2K rpm and gradually decreases to 92.5 @ 5200 RPM. Just wondering if that means I'm loosing potential HP in the top end, or is what I'm seeing typical.

92Z

iTrader: (1)

Daveinet,

See the following thread as i had the same question.

https://www.thirdgen.org/forums/diy-...tml?highlight=

Maybe a little insight.

Ronny

iTrader: (1)

Depends on your elevation. I am around 1800 feet. Key on I am 98 and I see 95-96 WOT for a drop of 3. If you are higher elevation that may be OK. If you are at sealevel then you have a restriction.

Daveinet

Thanks for the link and the advice. I'm running ScannerPro. Key on, engine stopped, reads 98.84. When I graph manifold pressure, it stays fairly steady - 96.5 until I hit 3200 RPM. At that point it starts dropping off. It dips as low as 91.5 @ 4300 RPM, but then starts coming back up and is around 93 @ 4825 rpm. I would guess that were it stays steady state (96.5KPa) at lower RPMs is my point of reference, regardless of were it reads at key on. Is there any why to approximate the loss, based on a percentage? I created a funky intake tube to bring in fresh cold air. It consists of two small long tubes. When I orginally set the engine up, it was running way rich. I added the tubes, it made a significant diffference in the low end, so my assumtion was that it increased flow in the low end, that compensated for how rich it was. Based on that assumption, I tunned with the tubes on. Now that it runs good, I'm paying attention to more subtle details, which is why I noticed the drop in manifold pressure in the top end.
Since this did seem to improve low end, I'm not sure what to do with it. If there was some way to estimate how much loss I'm getting in the top end, to know if its worth redesigning the intake. This vehicle is not for competition, so if I'm loosing only a few percent, its not critical. If I was loosing 15 or 20 %, then I would certainly care. Is there a way to gestimate the % loss?

Ronny

iTrader: (1)

key on 99. normal. WOT you say is 91.5 and climbs to 93. So that suggests a restriction in intake tract. Is the restriction serious enough to invest in a diffent intake manifold and air inlet system?. i am not sure if stock head intake intlet/valve is a restriction. I dont know. It depends on what you want on performance. Is this a performance settup or stock?. I recently removed my standard 14 inch x 2.25 inch KN A/C with a 3 inch cold air inlet to carb bonnet with a large KN canister at about 5 x 9 inches. I thought the flat AC was a restriction and witnessed that with MAP readings. cost be about $240. Worth it? I guess

Daveinet

So when you did your mod, was enough difference to tell with your butt meter, or is it something you could only tell with a stop watch? Its not really a race vehicle, it's actually in a motorhome. The engine is a 502HT with Howell/Edelbrock MPFI, using an ECM from a '95 GM truck. I'm running Thorleys with Magnaflows and an H-pipe. Sure, performance is important, but if its not something I could feel, its probably not worth it. Trying to get an educated guess before diving into the cold air ducting.

Fast355

iTrader: (2)

That engine is going to want fresh cool air and LOTS of it. What type of throttle body is on it, is it the OEM 2bbl TBI without injectors or is it a 4bbl after market unit? If the MAP is dropping at WOT, you are losing power. With a good cone filter and a single 3" sewer pipe intake, my 340 RWHP 350 TPI is pulling about 2-3 kpa @ 5,000+ rpm and is 100.4 KPA @ lower rpm.

Daveinet

The throttle plate is the 2 barrel with the injectors removed. It is a Holley and rated for 745 CFM. Since the throttle plate bore was 2", I used 2 - 2" PVC pipes to feed fresh cold air - 2 cone filters on the end of each PVC pipe. The length of the 2 PVC pipes is 32". Don't know if that long of pipe is creating the restriction or if its just the limit of the Holley 2 barrel.

Ronny

iTrader: (1)

Honestly I cant remember any diff. But I suspect it had to help performance. I make too many changes to have any way to determine. car never ran better than this past season.

Daveinet

OK, so now I have some really strange info. I swapped my funky intake tubes for the OEM stock round air cleaner and ran some new tests. The end result is not what I expected, nor logical. I gained a bunch of low end, and lost a ton on the top end. The engine will barely pull 4800 RPM. This seems so backwards to what I expected. In theory, my custom intake tubes should have helped the low end, but possibly choking off the top end, but instead, they do much better than the stock round air cleaner in the top end, and are hurting the low end.

So is this maybe a mixture issue? Could the intake tubes be so much more efficient that its too lean on the bottom?

rocko350

remember that you tuned for the tubes. That means you fattened up the low end and then probably leaned out the top end. Big blocks love timing, in some cases lots of it. I'll bet you find that your somewhat rich down low now and getting very lean up top.

Actually i just reread the thread and noted that is a howell mpfi intake. Dual Plane? If it is, it might be the intake itself. Mot enough plenum/runner volume. with the long tubes attatched, the probably acted like velocity stacks(shape and attatchment dependant). Basically its acting like the manifold is no longer in tune with the motor.

Ronny

iTrader: (1)

Why not just drop on a 14.4 standard air cleaner and test the result? somewhere on this site is a calc to determine air requirement-required flow for a TB and/or AC? Possibility yours might be too small? K & N rep once looked up that info on a chart they have for AC. they stated my 14.2 was too small for 350cid at 5800 rpms yet WOT I see like 96-98 or so in logs. Since then I replaced the 14.2 with a can(KN) at 6 inch dia and 9 inch long with 3 inch inlet to a carb blower bonnet. I have not yet studied logs the MAP but WOT A/F went a bit lean on WB logs.

va454ss

iTrader: (1)

I'm guessing that it has oval port heads. All of the Edel MPFI intakes I've seen have ports for the peanut port heads and need to be enlarged to match the ports in the heads.

I'm using the same intake but using a bored TB (2.2" bores) with the injectors in the stock location. On my 496, my MAP is 98-100 at WOT @5500 rpm.

Daveinet

Yes, it is the peanut port single plane MPFI.


The 502 HT has peanut port heads, so it matches. Its just so weird that the round air cleaner did better in the low end. It is interesting that most favor a flow issue over a mixture issue in the low end. Mmm.

Daveinet

Ok, so here is a thought. If it was mixture and since I don't currently have access to reading and burning my own chips, can I fudge the fuel pump voltage higher temporarily just to do a test run? How much can I get away with? At what pressure do the injectors have trouble working? I can drop a DC supply on the pump and crank up any voltage I want.

The weather up here has just gotten extremely warm, so I can do another test run today. (it was below zero 6 days ago, which I don't think that would tell me much)

Ronny

iTrader: (1)

Upping the voltage wont enrichen the A/F if that is what you are thinking of accomplishing. I believe the FPR will just bleed off the excess fuel.

Daveinet

I have a feeder pump at the tank that supplies low pressure to the high pressure pump. When I turn the pump to bypass, the pressure drops two lbs across the range of pressure operation. I made the assumption based on what I saw, that the regulator was suedo proportional.

Daveinet

SO, the weather has finally broke, and I had a chance to do a test run. I removed everything and ran open throttle body - no air cleaner. I get nearly the same drop at 3200 RPM, dropped from 97 down to 94 KPA. Looks like I need a bigger throttle valve. Anyone ever tried to build their own? Seems like it would be that hard to machine out of a piece of aluminum. Seems there is a lot of loss opening a 2 round bores into a single large oval.

Ronny

iTrader: (1)

Are you using as the TB one from a 350 CID TBI truck? flows maybe 520-550 cfm. Are you are prob aware next step up is the GM 7.4L TBI at 670. Holley will provide you with a 4 barrell 700 cfm and a 900 cfm. That was a TBI application but can remove the inj pods for use MPFI. Also available is the 1000-1550 cfm aftermarket 4 barrell units for MPFI with IAC support.

Daveinet

Its a Holly and I assume it is for a 7.4L With the injectors removed, it is supposed to be rated at 745 CFM. At only 3200 RPM, it should be no where near the rating. It is odd to see the KPA drop, but of course that is where the power band is. The problem with going to a 4 barrel, is that means a new manifold and $$$ and starting over.

Ronny

iTrader: (1)

What is the part #? Is it inscribed on the TB?

Daveinet

I could not find a part number on the TB. Its supposed to be 670 CFM but with the injectors removed, 745 CFM. It looks like they are 2 inch bores. The bore matches the inlet opening, on the sides. I figured the only way I could increase flow is to make it a large oval. Although I'd be a little afraid to try it, I think there is enough meet to cut the center out of the Holly and just come up with a single plate. I have to look where the vacuum holes are, although drilling new ones should be no big deal.