Future engine rebuilt and Slidevalve.

[Patrice911]

I'm new to this forum, I search previous tread and found some interesting one, I think I'm pretty much set as to what to do when the time to rebuilt my 3.2 will come up. Bump it to 3.5 with all the necessary hardware ( con rod, twin plug, porting etc.) with an aftermarket EFI. What I'm not sure is the intake.
Any info on Slidevalve? pro/con? Who as some? etc.

Thanks

The following link will give you some info on my car.

My 911

[David]

Slide valves have been discussed a few times before. You might have to search back a year or two though.

[jpnovak]

HI Patrice, I think I spoke to you at a Tarheel Autocross back in 2000. I remember your car being quite quick. I moved from Raleigh in 2001.

I think you might want to provide a bit more information regarding your build spec. I think your usage will be mostly track. What cams will you use? What is your desired rpm range? These things will make the difference in deciding an intake.

If you use a cam with a mild lobe separation you will have less intake reversion. This would allow you to use your stock 3.2 manifold. A more agressive cam would prefer to have individual runners. Examples of such could be carbs (weber or PMO), modified MFI stacks, Slide valves, or aftermarket EFI runners (TWM, Jenvey, etc). Since you mention EFI you also need to decide on an ECU system. Motec, Autronic, Emerald, Megasquirt, Electromotove are all examples that vary in price, ease of use, expandability and the number of functions.

Slide valves are an option and this has been discussed many times. Some say they can stick. Others say it is no problem. I think the biggest issue is price and availability of a slide valve setup.

Try the "search" button in the upper right corner of the screen. Each of these subjects have been covered. Once you have done more background reading you will probably have more direct questions.

[Quicksilver]

A slide valve assembly is hard to do right which is why they have a reputation for sticking. Think about it. You have a large plate loaded by engine vacuum and it has to have tight enough clearances to avoid leaking any air. Even with this and any dimensional changes from changes in operating temperature, it still has to slide easily back and forth. Not an easy engineering task.

Butterflies have it much easier. The only thing the slides is the shaft and it is much easier to seal. The only other thing that could cause sticking is if the butterflies are misaligned on the shaft so they bind. That is easy to fix though.

The reason that slide valves were invented was to get the slight improvement in airflow that you get from removing the butterflies from the bore. This is pretty marginal.

That being said, a good slide valve arrangement is way way cool. If you do it right I will stand there and drool along with everyone else!

[David]

I've built a slide valve for a 3 rotor Mazda and if I built a radical NA 911, I'd build a set for it. So I do think they work great, but I wouldn't put them on a car that gets a lot of street duty. I also wouldn't buy or build any that didn't have tight tolerances and good bearings.

[Patrice911]

Hi, jpnovak, yes, it's me, still the same car but it's even quicker now. Using the search button on the top of the page give me better search results than the one on the bottom on the page. I’m still going thru these pass posts and trying to digest them.
You are right the car is mainly used for track and AX, it is still road legal but not used much on it, but it's practical when you want to check that everything work fine or to bed in new brake pads before a track weekend. Since the car do track an AX I need something with a nice power band, I will need some suggestion on, port size, throttle size, cam profile, etc. for that 3.5 with ITB that will either be butterfly or sildevalve.

Thanks

[pu911]

If you are going to a 3.5 I would suggest some larger rear tires.
Phil

[jpnovak]

Patrice, you might want to contact Paul Thomas on the board. He is in Greensboro (thomasville) and has a 3.5 in his early hot-rod. I know he just completed a Motec install and could answer some questions locally.

Jamie