what is the bore size in a Spica intake?

[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]

Quote:

Originally posted by turbolarespider 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.

[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]

Quote:

Originally posted by 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.

[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]

Quote:

Originally posted by JoeCab 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......

[Roadtrip]

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.