[Keven]

Anyone know the bore size on the intake on Spica cars?

Thanks

 

[papajam]

I understand from a fellow bb-er that it's 37mm.

 

[turbolarespider]

If you are considering modifying your SPICA, I was given a tip once by Wes Ingram...

Due to the casting method used on the intake, there is a significant taper at the end of the manifold (not the throttle body part), which on my GTV was about 2-3 mm smaller. Its an easy thing to port match this to the head, and does increase flow quite a bit.

Plus its legal in pretty much all modifed classes (SP, IT... one inch from the interface may be matched).

Eric

 

[JoeCab]

Wes also sells intake manifolds that are already ported out. I have one on my GTV and it's noticeably bigger than stock.

 

[GianniB]

Wes ports them to 40 mm. I seem to recall them being 37 mm stock with the taper turbolarespider mentions below.

Ciao,

 

[papajam]

Due to the casting method used on the intake, there is a significant taper at the end of the manifold (not the throttle body part), which on my GTV was about 2-3 mm smaller.

I read somewhere years ago that there's a reason for the taper. Seems that Autodelta, after qualifying a 1600 GT, tore down the engine to freshen it for the race. After blowing every car into the weeds and winning the race with new found horsepower, the team discovered that the engine assembler installed the wrong intake manifold. Instead of using the straight bore 1600 manifold, the mechanic installed a taper bore 1300 manifold. Seems velocity, and not volume, is the key. Production intakes have been tapered ever since.
True? Beats hell out of me. But the '67 Veloce version of the Giulia Sprint GT gets more engine power with just a slight cam timing change and a SMALLER intake manifold!
Things that make you go hmmmm....

 

[turbolarespider]

If the taper was gradual, I would think that were true, but its quite sudden, and does not match the head ports.

What's cool is that port matching 1" is legal, and takes care of it...

Eric

 

[papajam]

Seems that Autodelta, after qualifying a 1600 GT, tore down the engine to freshen it for the race. After blowing every car into the weeds and winning the race with new found horsepower, the team discovered that the engine assembler installed the wrong intake manifold. Instead of using the straight bore 1600 manifold, the mechanic installed a taper bore 1300 manifold. Seems velocity, and not volume, is the key.

Came across this passage on page 44 of John Tipler's book 'Giulia Coupe GT & GTA'.

"The GTA's twin-plug head endowed it with 115bhp at 6000rpm, which may seem only a modest gain over the standard 1600cc Sprint GT's 109bhp, but an Autodelta-tuned car with twin 45mm DCOE Webers and 10.5 compression ratio would yield at least an extra 50bhp at 7500rpm. A significant increase in performance came when the inlet port sizes were reduced; the only difference between the Sprint GT and GT Veloce is the port sizes, and the cars were found to run much better with smaller inlet ports. Racer John Dooley, who was editor of the Alfa Club magazine from '67 to '74, suggests that the discovery may have been made by accident when someone fitted a 1300 head by mistake; the 1300 port sizes were at the time the same as that adapted by the GTA and Sprint GT Veloce. It wasn't long before the 1300 GT also got smaller inlet porting".

This account by Mr. Tipler, and the one I read about (at least the way I remember it), both indicate that Alfa discovered, at least in this case, that smaller is better. While the account I remember refers to the intake manifold, Mr. Tipler's research (and/or personal knowledge) points to the intake ports in the head.
So it would appear that porting the head on an L4 Alfa engine actually reduces performance.
So the question is - what happens if you straight-bore and port match the manifold?

 

[turbolarespider]

Based on the suggestion of Wes Ingram, I did it, and it's worked quite well.

The CRH has the head inlet port for a 1750 to be 36mm, tapering doen to 31mm as it approaches the valve.

So having a 2.0l engine be 38mm does not seem unreasonable.

Also, bear in mind, a lot of the performance is based on cam timing, if you have really long events, the faster the velocity, the more will be packed in after bottom dead center.

The cam's Wes sold me are a little shorter in duration, and are tuned to give performance in the mid-range (my car is for autocrossing).

I'll have to dig up "How to Power Tune Your Alfa Romeo Twin Cam" book tonight. I think he was suggesting 42mm at one section.

Eric

 

[turbolarespider]

I looked at my updated CRH, and it was interesting- for the 1900 GTAm engine, it had port diamters of 40mm, tapering to 38mm in the head.

Plus, in the "Power Tune" book, Jim K has 40 mm manifold and 38mm head port as well.

1600 is quite a bit smaller than 2000...

AND the manifold taper does not match the head, so there is a nice little step at the transistion, which is really bad for flow...

Eric

 

[JoeCab]

Papajam,

In a sense, what Autodelta found regarding the smaller intake manifold and intake ports can be extrapolated to porting a head. It is important to maintain velocity, that's why good port jobs involve a flow bench and don't come cheap. So, your statement that "porting the head on an L4 Alfa engine actually reduces performance" is correct if you insert the word "poorly" - porting the head on an L4 Alfa engine poorly actually reduces performance.

In porting, you want to continue removing material until the flow starts to fall off. Then, you've found the optimum porting for that head - one step ago, just before you killed the flow. You've also just scrapped a head.

Some of this stuff is counter intuitive. Huge, hogged out bowls don't necessarily flow better. A mirror shined finish ('Polished") does not allow maximum air velocity. Only the flow bench knows. This is why good head shops are worth every penny.

 

[papajam]

In a sense, what Autodelta found regarding the smaller intake manifold and intake ports can be extrapolated to porting a head. It is important to maintain velocity......

Correct me if I'm wrong but isn't this a contradiction? I though porting meant to make larger, not smaller. If porting does mean to make larger, then by applying the venturi principle, which states (roughly) that by decreasing the area thru which a given quantity of air flows, it's velocity will increase and it's pressure will decrease, then porting, with no other mods to change the volumetric effieciency of the engine, will result in a lower velocity of the incoming charge. Since atmospheric pressure pushes the intake charge into the lower pressure area created by the engine, it stands to reason that by lowering the pressure even more by increasing velocity will result in, what Eric said, that 'more will be packed in' resulting in more power.
So I'm still (so far) standing by my original statement but with a few extra words in brackets;

So it would appear that porting the head [or manifold] on an L4 Alfa engine [with no other engine mods] actually reduces performance.

Or am I missing something?

 

[144]

Like many modifications, it depends highly on what rpm band that you want to optimize. Fitting large throat carbs will allow more power at high rpms but are lousy for low speed response. I think porting is similar. There's that intersection of the optimum velocity curve and optimum flow quantity that you have to decide upon, depending on the power range you're going to be using.

 

[JoeCab]

Papajam,

I think what you *might* be missing is this: while an Alfa head is a *far* more efficient design than most, there are still some restrictions to optimum flow in the head. A "mild street" port job mainly focuses on removing these restrictions, and I do think this can benefit even an otherwise stock engine. In other words, Alfa left a little bit on the table as these are after all mass produced, cast heads.

If you have cams, oversized valves, headers, etc. then, as you state, a more aggressive port job that removes more material and concentrates on reshaping the combustion chamber to a higher degree. No argument that an aggressive port job on an otherwise stock engine would not be a good thing.

 

[papajam]

I've understood porting, port matching flow benching, multiple valve angles, etc. for ages. Most of my personal engines have some sort of intake tract modification. What I would like to fully understand is why the GTA and GT Veloce engines get more power using smaller ports.

 

[Sniady]

One Dyno guy I met, was saying that sometimees there is so much intake noise (or something), that is was creating a small supercharge effect.

So maybe when this supercharging effect kicks in, with the tighter ports, there might be more pressure build up, and the air is getting forced in faster. And maybe the bigger ports don't build up good pressure, when the 'supercharge' effect kicks in.

Hmmm... so yeah, this might be why eh?

I'm just repeating what I heard.

p.s. and I wanted to join in on the cool topic

 

[turbolarespider]

The supercharging effect is correct.

With the intake, the air gets some mometum built up in it, so when the actual intake ends, and the piston starts back up again, the more the momentum, the more air gets pushed in when the intake is still open and the piston goes up.

To everything, there are limits. Which is why the larger ports work better on the 2.0l engine...

From what I understand, in a 3rd person reporting, the 1600 head was never very good for flow or ultimate power. Still need to find out why, but that is what my Alfa sources say.

Eric