Don Terrill’s Speed-Talk

I had to get a new head for my Fiat twincam 1800 project due to suspect looking ex seat w some corrosion in the original head.

This is a good used 2L head that has never been worked on.

My plan is to do port work per Stage II of the Guy Croft manual. A die grinder with 60 grit drums cuts aluminum like butter at low speed setting.

As of now, I'm done with dimensional work and port matching. Waiting on some flapper wheels to finish.

Manifold to port match is just about perfect now with the aid of a bore snake cam.

Those Exh port outlets at the flange look really big for that throat size I see in those Exh port bowls!

I would bet that welding up the floors of those Exh runners to make them a tad smaller would pick up bunch of mid range Torque and help the motor to rev faster and not drop off any top end power to boot!

They do look huge. What are the dimmensions you are working with?

Those heads look like an Alfa head with quench pads.

Port dimensions at flange are pretty much OEM, unchanged.

What I did was open up the smallest dimension of the port, mostly at the sides leaving the roof and the floor rather untouched. The average port min diameter was increased by about 1.5mm IN, 1mm EX.

Seat inserts are original.

Port dims:

Intake at flange: 35.8mm
Intake seat insert ID: 37.6mm
Port minimum diam: 33.0mm avg (34horiz, 32vert)

Exhaust at flange: 35mm
Ex seat inser ID: 32.7mm
Port min: 28mm avg (~33 wide, ~23high at the guide bump)

If I where you I would really want to get the head flow tested now so you know what the Exh to intake ratio has changed to!

On the few of these motors I have worked on ( flow wise at a shop I used to help out at) even with the head in stock form then intake Manifold was the main choke point in the whole flow system, so doing any work to the head is a waste of time.

mag2555 wrote: ↑Wed Jun 24, 2020 5:35 am If I where you I would really want to get the head flow tested now so you know what the Exh to intake ratio has changed to!

On the few of these motors I have worked on ( flow wise at a shop I used to help out at) even with the head in stock form then intake Manifold was the main choke point in the whole flow system, so doing any work to the head is a waste of time.

I would have no baseline, nothing to compare flow bench data with. I'm following the instruction in the book by G Croft who built these engines for racing in England for decades.

For manifold, I have a nice IR with 44 IDFs.

Did Guy share flowbench results for each of his porting Stages?

Nut124 wrote: ↑Wed Jun 24, 2020 8:50 am For manifold, I have a nice IR with 44 IDFs.

Many Twincam IR-manifolds have an abrupt 90deg bend right at the carb flange...

Calypso wrote: ↑Wed Jun 24, 2020 1:36 pm

Nut124 wrote: ↑Wed Jun 24, 2020 8:50 am For manifold, I have a nice IR with 44 IDFs.

Many Twincam IR-manifolds have an abrupt 90deg bend right at the carb flange...

That is correct. Mine came from Pierce Manifolds and is as good as the space allows.

Nut124 wrote: ↑Wed Jun 24, 2020 2:54 pm

Calypso wrote: ↑Wed Jun 24, 2020 1:36 pm

Nut124 wrote: ↑Wed Jun 24, 2020 8:50 am For manifold, I have a nice IR with 44 IDFs.

Many Twincam IR-manifolds have an abrupt 90deg bend right at the carb flange...

That is correct. Mine came from Pierce Manifolds and is as good as the space allows.

Would bike carbs fit in?