Alfa Romeo Forums banner

1 - 16 of 16 Posts

·
Registered
Joined
·
328 Posts
Discussion Starter #1 (Edited)
Is there any difference between the plenum of the 2500 and that of the 3000 V6 engine?
Have they the same volume?
Many people say that the 2500 plenum (the one without "6C") has less internal volume than the one with "6C" stock on the 3000 engine and also that the 2500 one is better for torque while the pleunum of 3000 is better for power... are only legends?
Seeing the pictures of my engines bay I have noticed that before rebuilding the engine I had the original "6C" plenum...



...while after having it rebuilt, the Milano 2.5/GTV6 one



Looking at other pictures I have, it seems to me that the runners matched with the 3L plenum were straighter and longer than the other ones matching with the 2.5L plenum which are shorter and more curved... is it only an impression?
May be there some reason why the mechanic replaced a plenum in favor of another?
I mean... eventually connected with some (theorical) improvement??
 

·
Registered
Joined
·
2,892 Posts
Fact is that the plenums of GTV6 2,5, Alfa 75 2,5L and Alfa 75 3L all have different part numbers! That could be different hose attachement, striping and marking of 6C etc. My personal wiew has always been that they all have the same internal volume as we have used 2,5L plenum on 3L engine and later changed to another from 3L without seing any difference. For what its worth. If you are in doubt you can just test it for yourself by taping all the outlets and fill the plenums with water and compare the contents! I have not bothered as I am pretty sure the contents are the same, or so close it doesn´t matter!

GK.
 

·
Registered
Joined
·
2,269 Posts
Alfatakesitall, why don't you just tell us what your goal is an stop nibbling around the edges of the cracker?

With all the cam combo's you've tried, what have you learned?

I think you're trying to make the power of a 4.0 with a 3.0. Not possible.
There is no replacement for displacement.

The only replacement is going blown (supercharge or turbo-charged).

4-valve technology can certainly get you closer than 2-valve but with short stroke, you will not be too impressive at low rpms. Why do you think the 3.2 GTA was derived from a longer stroke?

Examples:

Alfa Romeo GTA
3.2 L GTA (3,179 cc (194.0 cu in) 247 HP (184 kW ) at 6200 rpm and 220 lb·ft (298 N·m) of torque @4800 rpm. 93 mm (3.7 in) bore and a 78 mm (3.1 in) stroke

Infinity I30/Nissan Maxima
3.0 L VQ30DE (2,987 cc 182.2 cu in) 230 HP (170 kW) @ 6400 rpm and 217 lb·ft (294 N·m) of torque at @4400 rpm 93 mm (3.7 in) bore and a 73.3 mm (2.88 in) stroke

Cadillac CTS
3.2 L LA3 (3,175 cc 193.8 cu in) 220 HP (164 kW) at 6000 rpm and 220 lb·ft (298 N·m) of torque at 3400 rpm
87.5 mm (3.445 in) bore and a 88 mm (3.465 in) stroke

Which one of these do you think feels the strongest?
 

·
Registered
Joined
·
4,075 Posts
There are a number of different plenums for these cars, at least 3, but as Gabor said, most of the differences are things like hose connections, holes for brackets, and cosmetics. There are some internal differences, especially relating to the inlet to the 6 throats that connect to the runners.

Do they have different internal volumes? I don't know, I have never heard that, but I have never measured it either.

Greg,
Silicone Hose Kits
OKINJECTORS--Gasoline and Diesel Injector Cleaning, Rebuild, Overhaul, and Testing Services
HPSI Motorsports - Performance Products for Street and Race
 

·
Registered
Joined
·
328 Posts
Discussion Starter #5 (Edited)
Toonerboy relax... this is a friendly forum!
People says... people asks... I do not teach anything to anybody.
Stop to consider yourself the only wise guy around.
Your aggressive way to answer betrays the typical fake strenght of many internet users... not all of them thank God.
It ends here for all I care.
 

·
Registered
Joined
·
2,892 Posts
Stefano,

There is a fine line between informing and lecturing, its friendlier to inform!

Anyway here my Super(or twin-) plenum for the 3,2 GTA engine with big runners underneath. I dont know if it would improve power on the 3L 12V, but on the standard GTA it has resulted in 300Hp. So a comparison.

GK.
 

Attachments

·
Registered
Joined
·
328 Posts
Discussion Starter #7
Thanks Gabor,
very good work.
I like much more than others hand manufactured I have seen, it is more similiar to the stock one.
Do you have any to sell?
 

·
Registered
Joined
·
2,892 Posts
No, sorry this is the one I have, and I got it made.

However if you get two plenums and have a good welder, you are not so far away from the end Product! Its a lot of work!

GK.
 

·
Registered
Joined
·
328 Posts
Discussion Starter #9
Yeah, but I have two different plenum: one with "6C" and one without... I'll look for another one to make something similar! ;-)
 

·
Richard Jemison
Joined
·
7,381 Posts
Plenums

Volumns are the same runner diameters are the same and all have too little volumn at the front for cyl 1 and 4.

What it needs is sectioning and increasing volumn by raising the front.

Its easy to do.

Leave the rear alone, but cut down the sides of the plenum and front. Pry the front up 1 inch and then weld in a taperd piece of aluminum on each side and a retangular piece to close the front.

Then from the opening in the plenum where the TB mounts, cut away the obstructions that prevent air moving to the front. You will create a hole that has to be welded up. Then you have a proper plenum with short runners that will flow.

Don`t open up the runners.
 

·
Registered
Joined
·
4,075 Posts
Wait, I though we wanted to open up the runners. Alfa used a smaller plenum (I admit I haven't measure it, but it looks smaller) on the 164 and bigger runners, about 38mm id vs about 34mm id. So I though the runner issue was a factor. I know Jim K says it is in his book.
 

·
Registered
Joined
·
2,269 Posts
If I'm not mistaken Greg, Alfatakesitall had the 164 intake installed on previous posts where he was asking which cams were better. On this post it appears he has gone back to the original set-up. Unless these are from another time.

According to this nifty calculator from Wallace Racing 34mm runners are good to 6200rpm and start choking at 6400. Everything is decimal so you have to convert mm. Interestingly, the shorter stroke (and higher rod ratio) of the 2.5 allows the same 34mm to stay under choke point a few hundred more rpms.
 

·
Registered
Joined
·
4,075 Posts
If I'm not mistaken Greg, Alfatakesitall had the 164 intake installed on previous posts where he was asking which cams were better. On this post it appears he has gone back to the original set-up. Unless these are from another time.

According to this nifty calculator from Wallace Racing 34mm runners are good to 6200rpm and start choking at 6400. Everything is decimal so you have to convert mm. Interestingly, the shorter stroke (and higher rod ratio) of the 2.5 allows the same 34mm to stay under choke point a few hundred more rpms.
Hmm, well I don't know. It seems JimK and RJR have very different thoughts on runners. I guess you have to chose which expert to believe.
 

·
Richard Jemison
Joined
·
7,381 Posts
Runners

Wait, I though we wanted to open up the runners. Alfa used a smaller plenum (I admit I haven't measure it, but it looks smaller) on the 164 and bigger runners, about 38mm id vs about 34mm id. So I though the runner issue was a factor. I know Jim K says it is in his book.
Diameter of runners is dependant on length and valve/seat flow, and engine capacity.

Large diameter short runners will kill all but very high RPM power. The Long runners on 164 motors have to have larger runners.

The power builder in V6 12V heads (FI type) comes from proper port work, straighting out the curves and most importantly matching the runners to the head. The factory did a terrible job with that.
 

·
Registered
Joined
·
2,892 Posts
I agree with most of what its said here: big runners slow down the airflow and filling, so it will decrease power at medium rpm, however big runner will increase power at high rpm. So what rpm range does one intend to use, is the basis of choice.

JimK s testing shows clearly that the huge runners(no diam. given) gave modest Power output. His best result was with 39mm standard enlarged runners. That is very close to the 38 mm runners used in the 164 3L engine. I have personally tried 38mm runners on a 3L engine and it did not feel down on power or torque at low revs and also the engine was more free running than my other car with original runners. So without more Scientifics I can say nothing bad happened using 38mm GTV style runners on that engine!

GK.
 
1 - 16 of 16 Posts
Top