Valve size

[r lane]

I am building a 3.6 track motor from a 964. The intake valves are 49mm in diameter. In the last 1-2 years of the 993, they went to a 51.5mm intake valve, and I am considering making that update. I measured the ID of the intake seat on my 964 head, and the ID of the seat on a late 993 head, and they are the same. My question is, what is the benefit of a larger valve if the hole remains the same. Love to hear thoughts. Bob

[faapgar]

Yes,the seats are the same but the lower inner dimension is enlarged which adds greater flow.Fred

[911 SLANT]

Would the same work with my 2.7 heads. I just bumped up to 2.8. I was wondering the same if I went from 46mm to 49mm on intakes.

[Bill Verburg]

Originally Posted by r lane

I am building a 3.6 track motor from a 964. The intake valves are 49mm in diameter. In the last 1-2 years of the 993, they went to a 51.5mm intake valve, and I am considering making that update. I measured the ID of the intake seat on my 964 head, and the ID of the seat on a late 993 head, and they are the same. My question is, what is the benefit of a larger valve if the hole remains the same. Love to hear thoughts. Bob

It's a complicated question that depends on the rest of the engine and design usage

964 3.8RS used the same port sizes as the rest of the 964 series but got the larger 51.5/43.5mm valves that were later used on 993RS.

here's a run down
964 all 41.5/38mm ports & 49/42.5mm valves w/ peak torque /hp @4800/6100, except 964 3.8RS & 964RSR which used 41.5/38 & 51.5/43.5 @5250/6500 and 46/42 & 51.5/43.5 @5500/6900 respectively

993nvr 43/39 & 49/42.5 @5000/6100

995vram 43/39 & 50/43.5 5250/6100

993RS & Cup & RSR 46/42 & 51.5/43.5 @5400/6500 @5500/6200 @5500/7000

The goal here as w/ the exhaust is to keep intake gas velocity high w/o blockage smaller ports keep gas velocity up but result in an unwanted obstruction to flow as revs build, so you want to use the smallest ports that still flow well at the highest rev levels that will be used, often for a street car some high rpm restriction is an acceptable compromise for more lower speed torque, this is less likely acceptable in a race motor that is almost always in the top third of it's rev range.

a bigger valve in a smaller port allows more gas to flow at a lower rpm so can be useful, the cost here is more weight that limits rpm, so again a compromise must be adopted.

Anecdotally, 9M heads use small ports and valves w/ excellent results

[stownsen914]

Originally Posted by 911 SLANT

Would the same work with my 2.7 heads. I just bumped up to 2.8. I was wondering the same if I went from 46mm to 49mm on intakes.

I've understood that with the smaller combustion chamber and steeper valve angles in the 2.2-2.7 heads that you can't fit much larger valves in there without hitting things (including the valves hitting each other at overlap with aggressive cams).

[targa72e]

993 valves also have 8mm stems vs 9mm for 964. The smaller stems flow a little better and weigh a little less.

john

[r lane]

Fred, your reference to the ''lower inner dimension''. Are you talking about the plain where the 51.5mm valve actually sits, which will be out on the perimeter, high on the very edge of seat, whereas the 49mm valve sits lower in the seat. My thought was that I would have to install bigger seats, which would then require removal of port material down in the bowl. It is dawning on me, that is probably ill advised. Thanks for every ones input. Bob

[Jeff Burger]

My 964 race motor used the the same valve seats with the larger 993 valves.