HKS Plenum

[24OZ]

I guess TonyD would be one of the men to answer that one.

 

BTW - If anybody is selling a cartech turbo plenum I have put up a wanted ad in the classifieds. (Sorry for my shameless plug).

[MazterDizazter]

OK, so are there no other turbo plenums available? I just put up a thread asking about doing a carb'd turbo setup; now I feel like an ass.... Apparently it's doable!

[Shuyun]

I am currently running a Tom’s Turbo Draw though setup and have lots of fun tuning and running the simple system. I am looking at a new engine setup in my car and am planning to build a new motor setup completely before swapping the running system out of the car.

 

I am planning on starting a bench racing project for information on a Blow through triple carburetor setup. Before I start I am reading through all the information I can find in the L6, carburetor, turbo, and 240Z sub forums.

 

I have found that a lot of the information about intercooler routing a plumbing will be appropriate for the setup weather carburetor or fuel injectors.

 

I have found several good ideas on jetting and simple O2 volt meters to help with tuning the setup once it is complete

 

Next I have not found a lot of info on sizing the carburetors for L24 I have seen a lot of 40 to 45 mm mentioned, so with a L29-30 and turbos 45 to 50mm would be my guess I am currently looking for a couple good books on the Dellorto/Weber carburetors that should help me size the carburetor correctly

 

My plan is to use all the great information about Stroking a L28 on this sight to build up a street driven L29-30 with an idle of 800 RPM and a red line of 6500 RPM. I am hoping for some help on deciding weather I will need twin turbos to get a spool up by 2800-2900 RPM ( or earlier if possible ) and have boost all the way to the red line. Boost goal is 20 to 25lb at max boost. The Dellorto carburetor is rated for up to 30lb but I am looking at a new set form OER as I have seen several turbo setups using them. I am still searching for more definite info in this area.

 

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[savageskaterkid]

Would tuning on this setup be somewhat universal, seeing as how its under boost, or would I tune it for weather conditions, and then do the boost thing?

[Shuyun]

I would think that once the jetting was approprate for an average day only small tuning for large weather changes would be needed. I am not looking for all out preformance only high street preformance. I have watched my uncle tune his bracket race cars with carbs and he would rejet and tune the carb on every pass with an eye on the weather station pressure, temp, and humidty. But he was tuning an engine for 2.5hp per cubic inch for drag racing we also adjusted the valves every pass.

 

This being said I have no experince with triple carbs on an L series engine but I have been reading all the post i can search there is a good weber sticky start there for more info.

[savageskaterkid]

I wouldn't mind just a base line idea of what people in similar conditions are running, that way I can go buy some jets.

[SHO-Z]

Shuyun

 

GT-240 was looking at changing out his Blow Trough SK racing set up to FI. He is located in Richland WA. SKs are an earlier version of OERs.

 

Do a search for him.

[Tony D]

But he was tuning an engine for 2.5hp per cubic inch for drag racing we also adjusted the valves every pass.

 

That's only 420 HP on a stock bore/stroke L28.

 

Which is why, other than some nostaligic reason, or period restoration using ITB's and Fuel Injection is a better option at these horsepower levels.

[Shuyun]

Tony D sorry for the confusion the engines I watched my uncle tune carborators on were Chevy big block 454 and 502 for example the car I learned to bracket race in was an avanti with a chevy 454.

 

He currently runs a Merlin 632 block with a poinac head and massive FI setups in a tube chasie 1968 corvet. My Cousin races in NOS Top Dragster with a simlar setup on a rail car.

 

My uncle helped me rebuild an rejet the holly 395 on my tom's setup I had strugled with a couple of holly books and had it to what i thought close but it took him about half an hour to completly change the way the car responded it is now fun to drive.

 

SHO-Z thanks for the heads up.

[proxlamus©]

Tony would you recommend a 2x4 plenum?!

 

 

The intake I am building I decided against a 2x4 plenum because i assumed it was way way way too small. For a boosted engine the intake volume should be 1.5 times the total engine displacement.

 

Maybe i'll build a spare 2x4 plenum during my welding class tomarrow for kicks, I could always sell it =)

[savageskaterkid]

Tony would you recommend a 2x4 plenum?!

 

 

The intake I am building I decided against a 2x4 plenum because i assumed it was way way way too small. For a boosted engine the intake volume should be 1.5 times the total engine displacement.

 

Maybe i'll build a spare 2x4 plenum during my welding class tomarrow for kicks, I could always sell it =)

 

 

If you decide to sell it, let me know, I've been considering building a plenum for a base setup, but my welding class just ended.

[Tony D]

IT IS TOO SMALL!

 

I made one, and it was horrid on carburettors. MUCH worse than the knockoff HKS I build in Japan.

 

It should be more like 4X4, which is about Cartech size, but it's still just a box and doesn't pressurize the float bowls to enrich on-boost like the HKS and SK Plenums do. With the proper entry strategy, and plenum that size could probably be baffled to work correctly, but inside an extrusion it would be a job. You may as well fabe the whole thing and not take shortcuts.

[MONZTER]

IT IS TOO SMALL!

 

I made one, and it was horrid on carburettors. MUCH worse than the knockoff HKS I build in Japan.

 

It should be more like 4X4, which is about Cartech size, but it's still just a box and doesn't pressurize the float bowls to enrich on-boost like the HKS and SK Plenums do. With the proper entry strategy, and plenum that size could probably be baffled to work correctly, but inside an extrusion it would be a job. You may as well fabe the whole thing and not take shortcuts.

 

Tony I was just thinking about you comments on the old HKS manifolds. It sounds like the pressure drop in the large volume of the main plenum box isn't enough to balance out the floats. I wonder if you could tap into the system anywhere before the air goes into the large plenum and run this to the floats to get the boost like the sub plenum is providing. So a simple 4x4 box for the plenum with lines running to the float area to somewhere before the 4x4 box. Maybe before the intercooler?? Does this make any sense??

 

Just a guess

[SHO-Z]

Here is a sketch of what I am planning on running.

 

 

There is a tube that runs from the supercharger discharge to the float bowls. There will be a needle valve for tuning between this line and the plenum box. I will use the pressure drop through the intercooler for the differential pressure. My Carbs are SKs and have a small cover plate that I fabed new ones and installed a 1/4" fitting for the tubing.

 

To answer the question of 2 air cleaners, the supercharger has an electric clutch so I can run it when I want extra power.

[Tony D]

Jeff,

That is what many of the higher horsepower cars started doing with OER's since they have a separate vent for the float bowl. Like SHOZ's diagram, taking the pressure from say the Torus of the Turbo (or S/C) and ducting it directly to the float bowl is an option. You will then tune on-boost enrichment via the bleeder or fixed orifice size.

 

What ended up happening was on twin turbo cars with really high HP, the two holes in the stock HKS plenum was over-richening the mix. Rather than enlarging the plenum holes and rebolting it, they took out the divider wall, or made the holes quite a bit bigger so it was not creating as much of a pressure differential, and then vented externally.

 

The two holes in the plenum will be a flow restriction at higher HP levels, so they eliminated them as it was easier to change the venting externally.

 

As I mention, though, neither the Cartech, nor many of the 4X4 boxes I see incorporate that enrichment feature. And on 2X4 box with the A.R I was running, I went immediately to boost on slight throttle application at highway cruise, and was at 0 manifold vacuum cruising. On the HKS plenum I was slightly negative, and slight throttle applications didn't make be go positive.

 

On an EFI car, things would be different as tuning is usually MAP based. Then the only thing you notice is your fuel mileage goes to hell compared to a larger plenum....

[Forrest]

On an EFI car, things would be different as tuning is usually MAP based. Then the only thing you notice is your fuel mileage goes to hell compared to a larger plenum....

 

Why's that?

[Tony D]

You go onto boost mapping with the slightest touch of the pedal due to insufficient plenum volume. The Centrifugal compressor will overfeed the reciprocating compressor (engine) really quickly. It's why they put such large receivers on screw and centrifugal compressors (suction bottles is another name for them) when feeding a reciprocating booster compressor.

 

As a result of the instant boosting in the manifold, you end up running in a 12:1 mapping, instead of something more conservative. Your mileage goes to hell... Now, if you put a much larger turbine section on, so it's slightly laggier you may restore the balance of NA cruising, but it kind of defeats the idea of having a turbine optimized for response. With a larger plenum, small throttle excursions will not result in the increased flow causing a bump to say 3psi instead of '0'.... In a megasquirt the difference in sized can be seen as a jump almost instantly from a 54kpa cruise to 108 or 110kpa instead of from 54 to 80kpa with the larger plenum. The only thing changed in the test was the plenum. I haven't had much time to play with the new setups and datalog everything to see what kind of numbers I get and relate them to what was seen on the same setups with carbs instead of ITBs.

 

Also I note the BOV tuning has to be retweaked due to the plenum change. The blowoff action is much harsher on the smaller plenum from what I can hear.

[Forrest]

Are you saying the MAP reading on a smaller plenum is inaccurate or that the larger plenum creates lag?

[Tony D]

The MAP reading on an ITB setup is not from the plenum...

 

Lag is a function of the turbo flow, changing the plenum does nothing to change the flow, so 'lag' may be a way to describe it, but 'drivability' is also a prime consideration. If you have 150HP at 3000 rpms, and 600 at 3400, driving that car may be slightly difficult in normal day-to-day situations.

 

Similarly, if your MAP goes positive EVERY time your foot caresses the throttle pedal, your gas mileage goes to sh*t. In that case, a larger plenum will work as the appropriate accumulator allowing some 'lag' between partial throttle applications and manifold boosting.

 

I hesitate STRONGLY to say it's 'lag' simply because of the negative connotations related to it, and because of generally the 'lag' everybody is talking about is from a WOT application, and NOT from a partial-throttle application. Generally lag is the time it takes to build FULL BOOST after WOT is initiated ABOVE 'boost threshold'----'lag' is not something associated to partial throttle applications.

 

In partial throttle applications, you most definately DO NOT want boost happening instantaneously (which is an indication of improper plenum sizing in both ITB and single throttle body applications). There has to be some deadband in the throttle pedal application where boost does not occur, or you will have fuel economy that is terrible.

 

The question you asked was two different things, and unrelated, so on the face of it, "NO and NO"----it's not an 'either or' situation.

 

The key on carburetted applications is to have MORE deadband than you do with an EFI setup as the carburettor just doesn't function well at '0' manifold vacuum, while EFI can operate there easily. The problem with Carbs is transitional leans, and if you undersized the plenum, you hit those points all the time, and get lean pop on cruise. As a result a larger plenum gives you more useable throttle play without boosting. You could always go with a larger turbine and have the same effect...but at the expense of spooling at higher rpms, meaning your boost threshold is higher, and you have less of a useable rpm range.

 

Which do you want more? Driveability and the ability to make 17 psi at 1700 rpms, and pull to 6000, or Drivability and the ability to make 17psi at 3400rpms, and pull to 6500? I know which setup I chose, and it was FAR better with a smaller turbo spooling at a lower rpm than a larger turbo spooling at a higher rpm.

 

The discussions should really be separate: Carburettors and EFI. They are so totally different when tuning plenum sizing and other variables if you try to apply one to another you will end up seriously confused.

[Forrest]

Fine, the map sensor reading taken WITH a smaller plenum. I think you knew what I meant.

 

Improved throttle response makes it harder to drive conservatively, certainly. But I think that's why most of us went with ITB's in the first place. I think I'll hold off chucking my cartech plenum in the bin.