Doing some porting soon

[vetteboy86]

I am planning on porting the plenum very soon. Does anybody know where the procedure is for taking the plenum and the runners off in the service manual. I am sure it isn't too hard a job, but I would like to look at something to keep my mind busy while there is snow on the ground.


Thanks

Craig

 

[Edmond]

Craig,

Unfortunately, I know how to do it by heart!

The bolts that are holding the runners onto the plenum are T-40 torx. The bolts holding the throttle cables are 10mm's. Don't forget to take the one where the throttle cables are clipped onto.

If I recall correctly, the TB bolts are 14mm's. It's real easy, 10 minutes and you've got them all off. Before I put them back on, I used a little anti-seize.

 

[Edmond]

I forgot some things. Don't forget to unplug the two vacuum lines on the passenger side of the plenum. They could be brittle so be careful.

There is also a little plug that you must unplug on the bottom of the plenum, it's roughly by the EGR.

At the rear of the plenum, on the bottom is the power booster hose. I think it's a 3/8's.

 

[vetteboy86]

Thanks Edmond I have looked at it before. I figured it was somewhat simple. Did you take the throttle body off as well. I figure once I get it off, I am going to clean the TB and do the bypass. I also thought about the MAT relocating kit. Just something to tinker with. What is the best way to get the brake booster line off? Is it better to disconnect it from the booster then take the plenum off?


thanks again

Craig

 

[Edmond]

Craig,

Just take the line off from the plenum, the line from the booster that is. Do not mess with the bigger nut that is on that line. I think that line is a 21 or 19mm, can't exactly remember. But I made the mistake of thinking the bigger nut was the one to take off and ended up kinking the fitting. I had to order that kinked nut because the threads wouldn't go back together.

I only took the 4 bolts off the TB, I didn't actually remove the unit. I kept all the hoses and other stuff on there.

 

[SSTibet]

Back to the basics again... What do you mean by "porting of the plenum". Now I am guessing this is the intake manifold, right? If so, what do you do to it when you say "porting"? Changing the port geometry???

Cheers

 

[Edmond]

Back to the basics again... What do you mean by "porting of the plenum". Now I am guessing this is the intake manifold, right? If so, what do you do to it when you say "porting"? Changing the port geometry???

Cheers

SS,

Our L98's are more of a PITA to work on than your LT style intakes. You guys go from TB to intake manifold. We go from TB to intake plenum, which divides the air into 8 runners which then feed the air into the intake manifold. You guys have it good with a one piece unit.

When Craig says porting, he will increase the opening where air will flow through and therefore allowing more air to flow, which should increase the power that the engine is capable of producing.

 

[SSTibet]

Edmond,

Thx for the information. I am reading literature that leads me thinking that once the intake is modified (typically made larger / smoother for more air) the exhaust system (runners, cats, etc) also needs some modification so that the increased air flow is discharged with no restrictions.

And literature also says that more air requires more fuel, which results with denser mixture. This then leads to improved v.e. for a specific rpm range, and to take benefit of that ignition timing & fuel adjustment needs to get done.

Can I conclude that such mods will not end at the plenum but extend into the exhaust, fuel, ignition system as well???

Cheers,

Selim

 

[Edmond]

Edmond,

Thx for the information. I am reading literature that leads me thinking that once the intake is modified (typically made larger / smoother for more air) the exhaust system (runners, cats, etc) also needs some modification so that the increased air flow is discharged with no restrictions.

And literature also says that more air requires more fuel, which results with denser mixture. This then leads to improved v.e. for a specific rpm range, and to take benefit of that ignition timing & fuel adjustment needs to get done.

Can I conclude that such mods will not end at the plenum but extend into the exhaust, fuel, ignition system as well???

Cheers,

Selim

Selim,

The mods never end! :L You are correct, those modifications have to match to a given application. You just can't throw any parts together and assume that they will work. It also depends on your gears, the transmission you have, your suspension, torque convertor, etc... It's almost a never ending project.

The big weakness for our L98 motors is the intake manifold because the L98 intake manifold was originally engineered to be used on the 305CID motor. Chevy actually considered dropping the 350 and had the 305 intake all in place. They didn't have enough time to redesign the intake manifold after deciding to stick with the 350 so they stuck the 305 intake on there.

One of the reasons why people usually modify the exhaust first is because it provides 2 gains: it provides the horsepower gain due to less restriction and it also gives you an audible gain. The stock exhaust systems are quite wimpy on C4's.

I believe that the stock ignition system is quite capable of handling more horsepower unless you're really getting radical. Many guys go with MSD ignition systems but from what I've read, those are more for the cars who need to make power above 6,000 rpm.

As for fuel injectors, there is a formula for determing what lbs./hour you need the injectors to be in order to meet your horsepower goals.

And after you have all the physical tools in place, you'll need the brains, a chip custom burned to all those mods to fully take advantage of them.

 

[SSTibet]

L98 is a 350cid engine with an intake designed for 305cid??? That is a shame if my understanding is right. But then, so many many many mods...

I agree with you on the stock exhaust, such as mine. It does not get me excited... Once I took the car to a drag race (I only watch the races, do not get any further than that) and people loved looking at my Vette; they are very few here after all. But one commented that the exhaust did not sound alright, and he was right

Selim

 

[JEFNLSA]

I agree with you on the stock exhaust, such as mine. It does not get me excited

Selim

Change it...you won't be dissappointed. It'll sound better, run better/stronger, look better and even get ya better gas mileage. Some systems even use the original style tips....if that's what you want. I got my Magnaflow Stainless Steel for 320.00 including shipping from performancepeddler.com. Easy install and sound KILLER!

 

[SSTibet]

Jeff,

What is the Magnaflow product designation for your exhaust? And what type of resonance / noise are you getting inside the car at low - medium - high rpm's / speeds???

Selim

 

[Edmond]

Jeff,

What is the Magnaflow product designation for your exhaust? And what type of resonance / noise are you getting inside the car at low - medium - high rpm's / speeds???

Selim

Selim,

If you want to get a system is is loud, obnoxious and that will scare the neighborhood kids and intimidate some would be challengers, get a Flowmaster. :L

 

[WhalePirot]

I am planning on porting the plenum very soon.

Do a lot of homework on this before you start messing with this. Does your 350 have trouble reaching redline with the intake? Are you willing to sacrifice low end torque for slightly (perhaps) more top end? Your time and money might be better spent on a good valve job and perhaps some cleanup of the heads, buy a good shop who knows what your use and goals are. I had my original heads worked and modified to accept larger valves, on my (former) L83 engine. The difference was slight and coupled with a more aggressive cam, which demanded ECM work.

There is quite a lot of discussion on various threads, discussing more airflow vs. intake air velocity, desired turbulence for better mixing of the intake charge; change in the power curve with an altered intake, etc.

FWIW, my LPE intake (damn thing) does not have very well matched plenum to runner ports. My talk with LPE about matching and porting revealed that for my enigne, it was not worth the expenditure. I was miffed but grateful, and I followed their advice. The car pulls very hard from 2000 to 6300 (fuel shutoff), with stock HEI.

Good headers, with smooth, properly sized tubes will deliver more bang/buck on than about anything. Again, too-large tubes decrease the velocity and scavenge effect of the headers. One tradeoff is increased underhood heat, which is hard on many components located there. Man, do I know this one.

The stock HEI is perfectly fine and avoids the sometimes-troublesome MSD, for normally aspirated engines runing below 7000RPM, I was advised. I sold my MSD before finalizing the install and am glad I did.

 

[JEFNLSA]

Jeff,

What is the Magnaflow product designation for your exhaust? And what type of resonance / noise are you getting inside the car at low - medium - high rpm's / speeds???

Selim

Selim,
They (Magnaflow) only make one system for our year 92-96 C4's. It is item# 15623.

As far as resonance, I only get a bit inside the car if I cruise at 60mph in 6th. Above that it goes away. No doubt about it, they are definitely louder than stock but good loud like the way it should sound!

The lower the rpm the more resonance you get. You can definitely still hold a conversation or listen to the radio etc...

Makes your Vette sound Mean!

Jeff

 

[Edmond]

Good headers, with smooth, properly sized tubes will deliver more bang/buck on than about anything. Again, too-large tubes decrease the velocity and scavenge effect of the headers. One tradeoff is increased underhood heat, which is hard on many components located there. Man, do I know this one.

Would coated headers such as the TPIS model alleviate some of that underhood heat?

 

[vetteboy86]

WhalePirot,

I read a corvette fever magazine, that talked about ten steps to more horsepower. I can try to retrieve that article. Well on the second or third step, porting the plenum was listed. There is a large ridge right behind the throttle body. It actually inhibits incoming air. What I plan on doing is making the plenum openings equal to the TB openings. Also Cleaning the TB and plenum. Wont polishing the inside of the plenum increase air velocity into the intake. When I am done I plan on port matching the gaskets. Does this sound right?

Craig

 

[vetteboy86]

Here is the link for that article. You may or may not have seen it.


http://www.corvettefever.com/techarticles/9156/

Craig

 

[Edmond]

Craig,

If it were possible, I would also replace the intake or at least port the stock intake because that is a very big restriction. Like it has been said many times, the intake was originally designed to be used with the 305.

And then you'd probably want bigger runners with the bigger/ported intake. Summit Racing sells an Edelbrock intake/runner set for about $740 or so. That with a ported plenum and some 1.6 roller rockers would get you a pretty nice gain.

And if you would've gone that far already, may as well do a 160 thermostat and get a chip burned for the mods.

 

[vetteboy86]

Yeah that is all in my future. As for right now, I have somewhat of limited funds. The car still runs strong, so for now the cheap stuff will do. I think I will wait until the motor starts running poorly and do a complete rebuild. Then I am thinking perhaps a stroker motor. That is all in the future.


Craig