Bigger Inlet Valves

[Snowy Smith]

BIGGER INLET VALVES
I have a 1983 Porsche 944
Normally aspirated
Is it possible to fit bigger inlet valves?
Where can I buy them?

Best Regards
Snowy

[fast924S]

Yes it is possible, but it will require heavy head work. Also you will loose low end power. Also unless running a MAF or MAP setup the valve change wont help much

[drew1]

I think one of the later, bigger than 2.5, motors has heads with bigger intakes. Might could be a swap.

[DannoXYZ]

No, you can't put bigger valves into the 944 without boring out the cylinders. This is because the edge of the intake-valve is about 1mm away from the cylinder walls. The only way you're gonna get larger valves to fit is to bore out the cylinders. Like on the '89 2.7L 944NA with a 104mm bore. This extra bore allows the use of a 3mm larger valve.

Besides, you can do a lot to improve flow with headwork. I've got a Stage-5 head with the works:

- backcut narrow-stem valves (much lighter & stronger than stock)
- fully radiused contoured valve seats (flows better than 7-angle valve job)
- full D-porting of intake ports, extends into intake-manifold
- milled for 0.5 increase in compression

Flow-bench testing shows 50% increase in flow over stock head.

[fast924S]

Danno, How much was all that work??? also what is the air speed, is it all high end flow??

[drew1]

Danno

Thanks for info. 1 mm is about .040" so .080" would hit the cylinder wall. Is this right?

I guess even with 2mm or .080" bigger you would have to notch the cylinder wall like on big block Chevys, in order to take care of valve guide clearence. With the valves coming straight down on a 944 valve lift would have to be considered to make sure this notch did not get into the top piston ring area at TDC.

Smith & 924S,

These blocks are silicon aluminum like the old Vega blocks. Be prepared to eat up tools notching them & since the cylinders are sort of out there with no deck be careful & assemble, check & reassemble.

drew1

[AznDrgn]

Danno what kind of gain in power do you see with your stage 5 head and a good matched cam? Got any dyno runs yet?

[drew1]

Azn,

Ought to be interesting. Probably runs good with a stock cam with narrowed stem valves& D port. I'm assuming the stems are cut below the valve guide. More flow can be picked up with smaller, like 5/16 valve stems all the way at high lifts, but this would need spefcial guides.

i don't have a whole lot of experience in flow work. I have a kind of crude flow bench, I made myself & no dyno. All I have to judge things by is "seat of the pants" & what folks smiling.

I haven't ported & flowed a 944 head. From what I have seen about the heads, they have a better angle or path than a lot of heads. The port goes straighter into the valve. From what I rember the guide is kind of streamlined instead of just sticking down in the port. This is important because the guide blocks flow from the far edge of the valve where it seems that a lot of air goes. A bad thing I have seen is an intake port or two that has the seat lip kind of above the casting.

From what I've seen on my crude bench Danno's cut stems & D port, are a good idea. The 50% increase in flow is impressive, especially with stock size valves. You might want to ask him where, at what lift he gets these.

If it's abig increase at low lift, from what I'veseen, you have a loss at overlap with back flow, lower low RPM . At about .200 or 5mm on up is great. Any increase there doesn't hurt the bottom & helps the high RPM.
Flow increases above 1/4 of valve lift are hard to come by & are not that nuch helpful in a street motor.

The full raduis seat is interesting. It takes a lot of work to get a 25% to 30% flow increase with a 3 angle seat, so Danno is onto something . I like the idea he flows the intake with the head. Now this is a lot of work, either valving all cylinders or blocking off & then opening.


drew