25 replies to this topic

[ferret]

Not to sure on these yet but I have heard some rumors and not sure what to believe.

1 if you put the carbie in the plenum all the pressure will even out in relation to the vacuum power valve and you wont need to mod the carbie.

-As far as I know the carbie would be before the plenum. Isn't the plenum the fancy term given to the intake manifold.

2 The plenum must be at least 1.5 times the capacity of the engine.

-dunno on this one.

3 does shape matter?

Can any 1 shed some light?

[aadamsvn19]

as long as u got flow it donesnt really matter

[ferret]

so if I can get my intake manifold to take the 900cfm from the 2 turbo's I should be right?

[dfry10]

Im assuming your car would be direct injection if you have an ECU so you wouldnt have a carby that thing that looks similar to a carby is the throttle body. The throttle body is befor the pleantum chamber, the pleantum chamber is not just a fancy name for the intake manifold it is what carries the air to the the intake manifold branches. Then well you should know how the intake minifold works. I dont know bout the sizes of anything but any shape should do it but multipoint gets air there faster.
Well hope that helps ya sum how.

[ferret]

^^^^helps a little but yeah the motor is carbied.

on one of those B/S pc dyno programs (dyno 2000) I seem to get more power with one T4 than I do with 2. Would this be more likely du to the 2 turbo's pushing too much air or just too much restriction blocking the flow?

Sorry forgot to mention it's for my 253

[NJA82]

Could be a lot of factors determining the loss of power with 2 turbo`s. Are you using an intercooler? If you dont you will lose power because the air charge is hot and using 2 turbo chargers is pushing a LOT of air and without cold air charge you wont be able to advance the timing much under boost so power will be sacrificed a lot.

Plenum chambers as mentioned earlier are of crucial design to get even flow to all the runners. If the design isnt perfect the last couple of cylinders or first couple might not receive equal amounts of air therefore running rich when it comes to fuel injection. With carby setups it is a bit different.

If you are turbocharging this engine and it is carby i would really suggest fuel injecting it. You will make a lot more power running EFI as you have a lot more control over your Air fuel ratios and it will allow you to run a proper intercooler setup which will give you a cold air charge which will make a lot more power. Carby turbo setups arent the most reliable and easy way of making power.

[VT-565]

in relation to air charge temperature, using either a twin or single turbo setup and considering the turbos are the same size either way, the air charge would be cooler with twins, for the simple fact that you have the same amount of air running past two compressors instead of one, thus they arent working as hard which equals a cooler charge. if a turbo is suitable to run in a single configuration then chances are it wont be suitable for a twin setup. as for plenums, manufacturers work on the same cubic volume as the engines capacity, common sense plays a big part, especially if you dont have access to decent airflow testing equipment and dont be scared to experiment with different shapes and sizes. (i presume) you are using a blow through setup, so plenum shape shouldnt be a drama, use a nice single plane manifold under your carb! make a nice blow through hat to bolt to the top and youre laughing

[NJA82]

doesnt matter if they are twins or not the same psi is put out at both turbos and both compressors work just as hard as each other. Only time they dont work hard is when they are quite large and airflow is massive on low boost. And the fact that it runs across 2 compressors means it gets twice the heatsoak in the air... hence why big singles are more popular to run big boost... common sense

[aadamsvn19]

big singles

e.g lllllllllaaaaaaaaaaaaaaaaggggggggg more llllllllaaaaaaaaaggg boost

just my opinion twin back back setup if ya know wat i mean

[VT-565]

S14 200SX said:

doesnt matter if they are twins or not the same psi is put out at both turbos and both compressors work just as hard as each other. Only time they dont work hard is when they are quite large and airflow is massive on low boost.  And the fact that it runs across 2 compressors means it gets twice the heatsoak in the air... hence why big singles are more popular to run big boost... common sense

i know common sense is king in places like this, but what i was referring to was using two turbos in a non-sequential set up ie. one not feeding the other but both feeding into the plenum directly. 'x' amount of air past one compressor with 'z' amount of heat, divide 'x' by two and you can reduce 'z' by a great deal also which equals a cooler charge at the plenum. common sense? you be the judge.

[ferret]

the turbo's are not sequential. I am putting one on each side as it is a V8. I think this will ballance the engine better. Both turbo's will happily push 450 cfm up to 35Psi each. My original theory as you would expect was to run a low boost (5 Psi) Then jack it up and see how the times change down the 1/4. If they don't I will take one turbo off then do the lot again. In theory 1 of these turbo's should be heaps for the little 4.2 litre. But the pc dyno program I used (dyno 2000) works everything under ideal conditons therefor the losses in the system might be taken up by the second turbo.

Strangely enough it's the fabrication and problem solving I enjoy rather than actualy dragging. Mind you bragging rights are alwayse a pleasant reward.

[Robbo]

The plenum design you mention in your first post is a simple, crude, agricultural way of making a turbo carby setup..

I think its probably a better idea to work out how its actually going to work before you concern yourself with how to make it work great. there are more important issues to address before you concern yourself with efficiency and heat soak.

The reason you would put your carby into a plenum is so that your fuel bowl is at the same pressure as your intake. Problems faced by this is that your floats in your fuel bowl will be crushed as soon as they see any significant pressure. Apart from that, there are a few other issues like, the fact that flow would be horrendous!

As far as a crude turbo charger setup built to a budget might work, your probably best off going with a draw through and no intercooler.
the problems they face;

[mr-squiggle]

is this going to be fuel injected?? if so wh dont you buy a fuel injected kit that comes with plenum throttle body, ecu... well the lot, i got a book here with all different setups ill find it tomorrow and give you the company name

if i was you i would be going efi..

[david kriedeman]

PLENUM, storage vessel or RESERVOIR. When we refer to a PLENUM CHAMBER we are refering to a RESERVOIR of air. An on demand supply of, in a BOOSTED engine, PRESSURISED air ready for the engine to ingest.Theoretical optimum plenum volume for a performance boosted engine is 2 - 2.5 times engine capacity. 253 cu/in is approx. 4.2 ltrs. therefore requiring approx. 8.4 ltrs of plenum volume. Not always a carved in stone value, PLENUM VOLUME is also dependant on type of engine eg: ROTORY or RECIPRICATING PISTON, also the modifications done to the engine which are governed by it,s end use. WHAT THE ENGINE WAS BUILT FOR OR TO DO. The LARGER the PLENUM VOLUME the more dislacement you need to fill before boost pressure is achieved. EG, when u fit a LARGE front mount intercooler and LARGE pipe work you WILL get an increase in LAG TIME. More capacity to pressurise, slower air speeds due to plumbing I.D. increase. I would recommend you start with a plenum volume of 1.5 - 2.0 times your engine capacity to start with. This should give you enough on demand air reserve and help keep air speed up and lag minimised for your combo, ALL RELAVENT TO PIPE WORK, ENGINE MOD'S,TURBOCHARGER A/R's ETC. As far as THEORETICAL CFM required, without knowing anything about mod's or condition of your engine the FOLLOWING IS ONLY A GUIDE LINE FOR YOU.Your engine is 253 cu/in's total CFM required for this displacement at a theoretical max. rpm of 6000rpm,boosted with a thermal efficeincy of 80% would be around 702.77 cfm. If your engine was naturally assperated you would only require approx. 351.38 cfm. I am using a THERMAL EFFICIENCY VALUE OF 80% as 80 - 85% therm. effic. is typical of a standard to lightly modified engine in GOOD condition. THERMAL EFFICIENCY VALUES OF 100 - 120% ARE ONLY SEEN IN EXTREME RACE ENGINES. "NORMALLY" You will need to enclose the carby completely in the blow through set up you wish to run. The carby requires to have equal pressure applied through out. If you dont enclose completely, not only your distribution circuits,EG, idle,progression and main circuits be incorrect but you run the risk of 'FIRE'., FUEL leakage past throttle shaft bushes etc. As stated in an earlier response to your questions the FLOATS WILL COLLAPES "UNLESS" the are the white plastic style or FREFERRABLY the BLACK NEO TYPE FLOATS. POWER VALVES ARE PART of the HOLLEY's MAIN circuit. They are activated by a predetermined ENGINE VACUUM VALUE. EG: a 65 power valve is actually refering to an operational opening vacuum value of 6.5"HG. [ "HG is inches of MERCURY ] Therefore a 65 power valve will open and enrichen the main circuit at 6.5"HG and below. You can remove the power valve or power valves [depending on carby type ] and replace with proper powervalve blank off plugs, but in doing so you need to go at least 10 sizes up in your main jets relavant to the metering block you removed the valve from. I STRONGLY ADVISE AGAINST THIS UNLESS YOU HAVE A GOOD UNDERSTANDING OF HOLLEY CARBS AND THE OPERATION AND FUNCTION OF ALL IT'S METERING CIRCUITS. Check engine vacuum at idle, making sure the rest of the engine tune is correct, sparkplugs,valve lash if applicable,ignition timing is correct & most important the engine is in good health. All of these variables effect engine vacuum,also if running auto trans must put into gear as the load of the auto will make engine vacuum drop. Check idle vacuum after the above is checked and ok, read idle vacuum value,EG: your 253 has a mild camshaft and is running an auto with standard stall convertor in "D" it has a vacuum value of 10"HG, a starting power valve value of 8.5"HG would be used, i personally would use a 6.5"HG valve but only because i can modify the metering block circuits to make possible. In saying this you can still run a 6.5 but you need to mke sure your progression enrichment circuit is rich enough. REMEMBERING THE LOWER THE POWER VALVE VALUE THE LATER IT WILL OPEN AND ENRICHEN THE MAIN CIRCUIT. A TOTALLY STOCK ENGINE IN GOOD CONDITION AT RUNNING TEMP,CORRECT TIMING,IDLE SPEED,IN NEAUTRAL,NO LOAD AIR/CON ETC WILL HAVE AN IDLE VACUUM VALUE OF AROUND 20"HG. ALWAYS A GOOD IDEA TO FIT A BOOST GUAGE THAT READS BOTH VACUUM AS WELL AS BOOST,GIVES YOU ADDED DIAGNOSIS TOOL WHEN CHASING TUNE FAULTS. VACUUM VALUES WILL BE LOWER THAN 20"HG IF RUNNING BIGGER CAMSHAFT WITH TIGHTER LOBE SEPERATION & HIGHER DURATION @0.050 VALUES or if engine has mechanical problems, EG: VACUUM LEAK or LOW COMPRESSION etc. REMEMBER ALL VALUES ARE THEORETICAL START POINT VALUES FOR YOU AND ARE ALL DEPENDANT ON ENGINE CONDITION,ENGINE MODS,TYPE OF FUEL ETC. HOPE SOME OF THIS HELPS YOU. "GOOD LUCK" INJECTION IS THE GO, HOWEVER I FULLY UNDERSTAND THE CUBIC DOLLAR VALUE.

[mr-squiggle]

^^^ good write up!

[ferret]

thats a great help.
I have looked at a few setups and in the states they dont bother running an intercooler or a plenum so I may try that 1st for experimental reasons. This should give me an idea on spool up rpm, boost levels and a few other things.

Thanks david, your a wealth of knowledge.

Mr squiggle I'd love a read of that book if we could work something out.

[mr-squiggle]

yeh i shall look for it today.. just one of dad's old ones when he had the shop

btw how much boost are you looking at running??

[ferret]

hopefully 30 psi

[sinister]

You will no way be able to run 30psi without some sort of cooling, big boost means big heat which means big bang

[aadamsvn19]

least it will go hard for a few hours lol