Ducati 750 head prep
Posted: March 28th, 2008, 1:15 am
Hi all
I just wanted to drop my 2 cents into this forum for any new members wanting to to learn as i have over the last few years.
The head in question is from a modern 750 and it is a very good piece from standard.
The head is capable of more flow than i have achived but there are practical limits (costs). Removing all the valve shrouding will give much better flow but in a 750 engine compression will suffer , also the port choke sizing is important as we need velocity. When the 750 is big bored to 900cc (wider bore and much higher compression ) we can overcome these issues but as this is a mere 750 i have to accept the results as adequete.
Anyhow moving onto what i did.
On initial flow testing bare port flow was reached by 12mm at 170cfm.
The cylinderhead has 29mm ports feeding two 38mm valves.
My first modification was to open out the throats from 82 percent of the valve size to 85 percent with a 70 degree cut down into the port (this made the bowl 1mm larger in diameter. I then smoothed the entire port with 50 grit using ATA bands making a nice venturi shape.
I flow tested again and a gain of 3cfm followed but high lifts stayed the same so did the bare port flow. My second modification was laying back the short side radius by 2mm and again blending it in with 50 grit ATA bands (Sometimes i find the short side needs to be wider ) . I flow tested again and the bare port jumped from 170 to 180cfm and at this point i was quite exited but when tested with valve in there was no gain at all anyware in the range.
At this point it was obvious from the order i test in and previous experience that valve shrouding was the problem . In order to release this new found bare port flow I deshrouded the inlet valves to the tune of 1.5mm around them (practical limit at 750cc as compression will suffer) . I polished it up with 120grit /preformed a clean up of the existing 45 and 30 angles to 1mm wide with my neway cutters and tested again.
Here are the results from a days work.
If i had longer on the job i would have used the peg multi angle cutter to try some different seat combinations but the budget was tight to warrent. The cross section of the port is still 29mm keeping the velocity the same as standard and with the little flow we have gained i know from experience it will reward me with approx 6hp from 5-10000 rpm. There is more flow to be had (quite a bit) from opening up the controlling section to 30mm but as this is going on a 750 velocity will suffer and so will power.
Cris
I just wanted to drop my 2 cents into this forum for any new members wanting to to learn as i have over the last few years.
The head in question is from a modern 750 and it is a very good piece from standard.
The head is capable of more flow than i have achived but there are practical limits (costs). Removing all the valve shrouding will give much better flow but in a 750 engine compression will suffer , also the port choke sizing is important as we need velocity. When the 750 is big bored to 900cc (wider bore and much higher compression ) we can overcome these issues but as this is a mere 750 i have to accept the results as adequete.
Anyhow moving onto what i did.
On initial flow testing bare port flow was reached by 12mm at 170cfm.
The cylinderhead has 29mm ports feeding two 38mm valves.
My first modification was to open out the throats from 82 percent of the valve size to 85 percent with a 70 degree cut down into the port (this made the bowl 1mm larger in diameter. I then smoothed the entire port with 50 grit using ATA bands making a nice venturi shape.
I flow tested again and a gain of 3cfm followed but high lifts stayed the same so did the bare port flow. My second modification was laying back the short side radius by 2mm and again blending it in with 50 grit ATA bands (Sometimes i find the short side needs to be wider ) . I flow tested again and the bare port jumped from 170 to 180cfm and at this point i was quite exited but when tested with valve in there was no gain at all anyware in the range.
At this point it was obvious from the order i test in and previous experience that valve shrouding was the problem . In order to release this new found bare port flow I deshrouded the inlet valves to the tune of 1.5mm around them (practical limit at 750cc as compression will suffer) . I polished it up with 120grit /preformed a clean up of the existing 45 and 30 angles to 1mm wide with my neway cutters and tested again.
Here are the results from a days work.
- 749s head prep (600 x 450).jpg (45.27 KiB) Viewed 3521 times
If i had longer on the job i would have used the peg multi angle cutter to try some different seat combinations but the budget was tight to warrent. The cross section of the port is still 29mm keeping the velocity the same as standard and with the little flow we have gained i know from experience it will reward me with approx 6hp from 5-10000 rpm. There is more flow to be had (quite a bit) from opening up the controlling section to 30mm but as this is going on a 750 velocity will suffer and so will power.
Cris