383_Proof

my brain hurts

65Z01

Ummm, can you tell me how many cylinders are "pulling" at any given point in time, i.e. how many cylinders are on the intake stroke at any given instant??

To see what effects various mods had on a L98 check out the article data on my site.

CFI-EFI

VE DOESN'T always decrease with rpms. It INCREASES until maximum torque is reached. Do you know what VE is?



What is a "76cfm cylinder"? CFM is a measure of flow, cubic feet (volume) PER minute (time). Like miles PER hour. A 350 cid engine has cylinders that are (350 ÷ 8 =) 43.75 cubic inches in volume. WHAT is it you are describing as "a 76cfm cylinder"? A cylinder has no flow with no outside influence, it only has volume.



It would appear that the books, at least the first two mentioned are exactly what you need to get the thought process on the right track.



That is fine, but don't include the plenum as part of the hallway. The plenum is the rotunda that the 8 hallways branch off of. Porting the plenum merely rounds the corners making it easier to enter the hall. The hall entrance is where the tuning begins. The plenum is just a supply point for the ports (runners). The runner length is "tuned" as explained above, for a specific rpm range. That intake tuning (hopefully coupled with exhaust tuning) changes the point of greatest VE and therefore influences at what rpm the torque peak occurs.



You are flying blind. Pick up some books, as suggested, and learn. Your results, with the "I'm not sure I'm looking for more than the short answer" attitude, and half-assed approach, could get you a total failure and a horrendous waste of money, or at best, half-assed results.

RACE ON!!!

65Z01

Two good books for L98 are:
-"Insider Hints", by TPIS
-"Chevy TPI Fuel Injection Swapper's Guide, by John Baechtel

TPIS can supply the 1st book and Summit has the 2nd one. Here look on pp 39 for equations relating hp to injector size and air flow to injector size, hence hp to air flow.

BTW, I strongly recommend Dave Vizard's books for an in depth treatment of the SBC...

Yes this is true but there are several "dynamic" factors involved with cylinder filling too.
-exhaust gas scavenging, i.e during overlap exhaust gas exiting pressure wave actually tends to lower air pressure withing the cylinder at the start of the intake stoke
-since the intake runners are a finint length there is setup standing waves of air during engine operation; the wave length of said waves varies with RPM. The L98 "tuned" intake takes advantage of this "ram filling" effect to create its well know low end torque.

So, yea, an engine is an "air pump" but there are some rather complicated dynamics going on inside. For the L98 there has been so much work done that getting very good power is now basically a cook book deal.

GREGGPENN

Thanks for the link! The L98 mods page is definitely worth it!!!! (I understood everything except the TPis Cam lines... Why 2 lines, why ATCD vs BTDC??? Also these mods probably weren't done in one day, how do we know how wind/weather affected times/speeds?)

Regarding my math.... 607 was my direct replacement of a formula used to show the 48mm was big enough for a 502. I replaced the CID with 350 to get 607 cfm @ 6K RPM. There are 8 distinct pathways thru the intake to the heads. The cfm values for a single port are provided in links I've viewed. They avg 200cfm and up. 607 divided by 8 is about 76. 76cfm will be drawn thru each of the 8 individual pathways to feed the cylinders. The only airspace they have in common is the TB/plenum and I'm talking about the runners/intake manifold combo.

gp

GREGGPENN

Volumetric Efficiency: Without looking it up -- in my own words -- is the ability of the motor to expell and intake the necessary (volume) of air to supply the cylinders. From what I read, VE is near 100% at lower rpms but decreases due to air friction (etc) as rpms raise. It's the reason we mod exhaust/intakes!

Explained in my post above. It's the amount of air that has to flow in/out of a 350CID motor at 6000 RPM.

But the reading here is soooo much more fun!

Well, the rotunda is a supply of air too. I don't include it in the flows I've read/posted. I assume it's big enough to supply the 8 runners since it's bigger than the 48mm TB that supplies it. I do think the turns the air makes to enter the runners create loss. I understand that plenum porting can help that. I don't understand how much porting can be beneficial. (I've found a couple of links on how to port plenums and the corvetteplenum.com website. That latter seems to go a LOT further in his work).

Gregg

GREGGPENN

Thanks again! I did read Vizard's work on exhaust tuning last year. I what he said -- though it made my brain hurt.

Your "cookbook" conclusion is kinda what I'm getting at. I'm not opposed to learning more about this but I've got a car in the garage with failing injectors. (I'm pretty sure it's started cutting out on one cylinder after it gets hot ). Unless I want to leave it in the garage, it's time to ACT!!!!

gp

CFI-EFI

The way this started out, and from your PM, It seemed as though you were interested in learning. My mistake.

RACE ON!!!

GREGGPENN

OUCH!!!!!!!

65Z01

The key in his work on exhaust, IMO, is that 1chp requires 2.2cfm of exhaust flow to avoid power loss to back pressure. Combine that with the fact that a 2 1/2" pipe flows 560cfm and you can easily design a non-restrictive exhaust system.

GREGGPENN

Each side has a 2 1/2" pipe flowing at 560, right? Doesn't sound bad to me???? Plus I already increased ALL piping. I may still add LTubes and go to duals. I will say the increase in front Y pipe, addition of performance cats, plus increase of catback section can be felt. The motor really couldn't pull in 6th below 70mph before the change. Now, it can. (6spd with 3.07 gears).

I want to thank you again for the link to your webpage! The intake upgrades really seem meaningless in the 1994 test shown (i.e., 8HP). I suppose that supports the view that the stock intake ain't that bad!

OTOH, I'm shocked, stunned, and flabbergasted to see cut lid, Stage II chip, and air foil gains -- especially so big! Everyone else says they're B.S.!!!!!

If I'm reading right, I can get some decent gains with the frisbee, plug grind, air foil, K/N + cut lid, chip, and MAF screens. Goes against the grain for sure!

I might still do the port/polish of the heads and maybe port existing plenum/runners.

The siamese work on manifold and runners worries me a little because it looks like it could interupt SMOOTHED air flow! I get why it work though. Looks like each port could pull air from the adjacent tube. If rear tubes get more air (personal guess), it might even out air flow/engine performance between cylinders. (This is a reference to the earlier post about uneven runner/port flow).

Note: The link to the first article on MANIFOLD "siamesing" (from your website) is no longer there.

Great help!!!!!!!!

GREGGPENN

Since there are 8 independant air paths to the heads, why doesn't 607/8 work? No one has answered that....

I suppose this is one of my fundamental questions. Again, flow number for the (intake) manifolds and runners start at 200cfm (stock) and go up. In a similar fashion to the math used to show a 650cfm TB (48mm) is big enough, I'm curious to see what -- if anything in the manifold is undersized.

There are endless posts that start with "What's the biggest restriction in my TPI?" Well..... is there one?

The lack of porting and polishing are about the only things I see. (Note: this is in regard to realizing the EXISTING 350 longblock's potential -- without changing head/cam).

gp

95PoloVert

For ~40 seconds of that minute, the intake valve is closed...which puts a real damper on flow. (Review the IVO/IVC event timing for your cam for accurate figures.)

Your 200cfm runner doesn't get a full m to do its work...like it did on the flowbench when it racked up that number.

Using the 20 "working" seconds assumption above, your runner would need a flow rate of 76 * (60/20) = 228 cfm to move 76cfm of gas in the amount of "working" time that it has available.

This is a *vast* oversimplication, of course...but directly or indirectly, it accounts for the bulk of the "missing flow" that you're seeing...and delving into more esoteric considerations (ie. fluid dynamics) is going to be counterproductive here.

65Z01

Good explanation. Looking at flow bench data is steady state or max flow while looking at engine dynamics one must consider the actual flow over the intake valve open/close cycle per cylinder which goes from zero to full head flow to zero.

Yes, so the total flow on a true dual system (assuming cats & mufflers also flow >560cfm) would be 1,120 cfm, enough to support 510chp.

Indeed the frisbee, cut-back plugs, K&N and 160deg T-stat gave nice cumulative gain, though 52mm TB w/ gutted MAF didn't bring any gains on current motor.

From what I've read port & polish & work of the #113 heads can net up to 30chp gain. I think porting the stock base would also net some top end gains, especially if you siamese the base to remove the small stock runners from the "equation".

The idea of the siamese base is to increase the effective runner length while still maintaining runner length. Actually it creates two separate resonant chambers, one in the intake base the other in the two runners. Net result is a little low end loss but a real gain in top end power; the latter can be seen by my dyno pulls.
There is no longer siamese base data on my site but, if you drop me an email (w/ reference to siamese base) at jgkov@msn.com, I'll return some pics of my siamese base with matching work on the lower runner flanges.

GREGGPENN

Damper? Damper? I'm having a personal crisis and you're making jokes? Pfffffatttt.... Seriously. Good one.

Dang... Believe it not, I get it. Good thing you chose the oversimplification route!!!!!!!!!!


Gregg

GREGGPENN

What about the stageII chip? I've read they're marketing mumbo jumbo with actually values at stock with minor mods to insignificant things like fan control temps. FYI: Even if it had value, I'd still work toward a custom tune.

Also, I've heard a K&N flows the same as a new paper filter. From displays in the stores, they flow more. From the results, it seems more likely that they do help.

And, that 160 degree stat.... Well, I'm not going there.

Thanks. I did see a pic in your links -- so I at least understand what the modification is. Seeing what's being done caused my concern about cross-flow and turbulence (creating a negative effect on the overall flow).

I'll send an email as directed, but others might want you to post data on the results -- if you have it!

Gregg

htrdbmr

Increased fuel pressure doesn't mean greater flow in lbs/hr, because as stated above, the computer will limit the pulse width in accordance with the O2 sensor. But-- this is only in closed loop operation.(???) Is it possible that the higher pressure gives a better spray pattern, or better atomization in all modes of operation. I'm as curious as you are, and I don't know the same things that you don't know.

GREGGPENN

That was certainly my first guess, but then I learned that the fuel rails fire in banks. In other words, each bank (of 4 cylinders) unloads fuel for it's cylinders at the same time. At higher RPMs, the fuel is probably sucked into the cylinder so fast, that benefit may not be lost. At lower RPMs, I wonder if it is.

Maybe better HP but not better mpg?

gp

GREGGPENN

I indicated in the OP that I was getting frustrated. Guess you didn't catch that either....

(In case you hadn't noticed, there's a lot of conflicting info running around as well).

CFI-EFI

I thought it best to ignore your first reference to this post of mine, but since you insist, it was long afterward that you posted.Of course you are getting frustrated. You are asking for pat answers to complicated questions. You want magic answers where no simple answer exists. There are people of differing amounts of knowledge on this forum. Most all are well meaning, but some are better informed and have more experience than others. You are not going get a firm grasp of all you wish to know without going to the effort of studying and learning. It is going to take a little independent effort and study. This is called paying your dues, doing your homework. It is going to take some effort if it is going to be worthwhile. You can't ride on the back if others indefinitely.

RACE ON!!!