2.7 rebuild with 8.5rs piston S cams...

faverymi · 12-22-2010, 08:37 AM

Hi everybody.

After being a member for a short period of time, reading and learning from everybody I decided to rebuild my first aircooled engine. I just disassembled the engine. I'm inspecting everything and deciding what way to go.

I have to thank Tom Butler for kindly answering my rookie questions and suppling plenty of parts. Thank you Tom.

Anyway, I'm not super familiar with the Porsche engines other than the internet and all the manuals and books I could get my honds on. I want to build something fun and reliable to learn and take over bigger future projects. Fun street car.

Old engine was running, smoking at high rpm and gear changes. I'm going to do all the upgrades for reliability. Tensioners, Exhaust, Fan, Case save. valve covers...

I wanted to go fully stock but Tom got me the bug and I'm thinking that I want to go with 8.5cr RS pistons, hot cam(possibly S) and carburetors.

Do you see a problem rebuilding the engine with the new pistons and cams and leaving the CIS during break-in period?

My only experience with carbs is on my old chevy 4barrel edelbrock. For this reason I want to troubleshoot the engine first and them move to fuel supply. I'm not comfortable doing both and I feel having the engine ready will allow me to work on the carbs and focus on their own problems.

Any thoughts? Advice. Words of wisdom?

Flieger · Flieger's Garage

The S cams will not work with CIS. It is very sensitive to intake reversion. This is where the upgoing piston on the compression stroke forces some air back out the intake due to the intake valve being open. The intake valve is open because at higher rpms, there is enough momentum in the incoming air to cram more into the cylinder and compress it a little as the piston starts coming up.

The reversion causes the CIS throttle flap to oscillate and provide funky fueling and bad mixture.

You would need to have the CIS cam in there with CIS. Carbs can only be tuned to the cam you are going to use, otherwise tuning to the CIS cam is worthless.

faverymi · 12-24-2010, 03:07 AM

I guess my engine rebuild crush course is going to include an appendix on individual body carburetors also. Go figure. I will post pictures soon. Right now heating up head studs cherry red in this lovely 20 degree weather. Yes and they are coming out on at time. Happy Holidays.

red 2.7 · 12-29-2010, 02:56 PM

Just a word of encouragement I have a 2.7 with Mahle rs pistons and E cams with PMO carbs and it's wonderful. Pulls strong all the way till the limiter says woah!! Good luck.

Eagledriver · 12-29-2010, 03:31 PM

I'd recommend the following cam choices: Solex, E, GE-40, Mod-S. The "S" cams are just to weak below 5000 RPM. The Solex and "E" cams can use stock valve springs to 7000 RPM and will provide a much better driving experience (even on the track) than the "S" cams. I'd done both on my car and when I switched from the Solex cams to the "S" cams I was dissappointed. GE-40/Mod-S cams will give every bit as much power as the "S" cams and still have the low end torque of the Solex cams but will require stiffer valve springs due to increased ramp rates.

-Andy

irl · irl's Garage

Good luck and keep us up to date, looking forward to the progress.

Henry Schmidt · 12-31-2010, 09:52 AM

I whole heartedly endorse the DC40 / mod "S" cams. In our last project (2.45 lt, 9.5 :1, 36 mm ports, SSI exhaust, 40 mm Webers) we made 180 RWHP and it pulled from the ground up.

If you have a good set of 90mm Nikasil barrels, I have a new set of RS pistons for sale.
The set comes with 6 matched pistons, rings, pins and clips.
Email me if you're interested.

faverymi · 12-31-2010, 12:16 PM

Thanks for the answers.

Considering the E cams or Solex. Trying to build a fun street machine not a track car exclusively.

I like the idea of better power among the whole range not just top over 5k.

I will post pics after I finish tearing the engine apart.

Happy New Year, Cheers!!!

faverymi · 01-01-2011, 02:42 PM

I saw a nice post about duration and valve lift.

I like Solex Cams on paper. I think would get best drivebility and good punch when pushed.

Question...

Solex cams will work with the stock 911s heads, right?
Assuming Im using 8.5rs pistons. Do I have to worry about other problems? (valve clearence, valve springs stiffnes, others)

thank you.

Eagledriver · 01-01-2011, 03:37 PM

The will work with stock heads and the RS pistons. You should always check clearances however, because they are often close to mins.

-Andy

faverymi · 01-03-2011, 12:43 PM

Just spoke with John Dougherty.

We are going with Solex, based on car use and 8.5CR

He sugested if I was going to track the car I should go with DC40 moving the Hp and rpm curve to the right. I want more longevity out the rebuilt not planning to go over 7K rpms. That is also why S cams are out.

Staying in the 3k to 6.5k for a nice street rod.

He is doing my rocker arms too.

Pictures coming, thanks

RoninLB · 01-07-2011, 09:46 AM

Quote:

Originally Posted by red 2.7

2.7 with Mahle rs pistons and E cams with PMO carbs and it's wonderful.

Pulls strong all the way

works great for me

it's very street drivable with plenty of 3-4k torque and doesn't load plugs doing so. It makes a stock 3.2/G-50 look weak imho

changing cams at this engine state is no big deal so the E is a personal choice

different strokes

tobluforu · tobluforu's Garage

Quote:

Originally Posted by faverymi

Just spoke with John Dougherty.

We are going with Solex, based on car use and 8.5CR

He sugested if I was going to track the car I should go with DC40 moving the Hp and rpm curve to the right. I want more longevity out the rebuilt not planning to go over 7K rpms. That is also why S cams are out.

Staying in the 3k to 6.5k for a nice street rod.

He is doing my rocker arms too.

Pictures coming, thanks

I just put solex cams in my 2.7 motor (same pistons you want) with mfi and I love it.

Henry Schmidt · 01-07-2011, 11:49 AM

RS spec 2.7 with Solex cams is what I have in my 914-6 and it is a very nice combination.
Far better than the E cams for an every day driver.

cherylfoster · 01-07-2011, 01:18 PM

S cams will not work with the CIS. It is very sensitive to the contribution of reversion. That stamp is rising on the compression stroke forces the air again taken as a result of the intake valve open. The intake valve is open, because at high speed, there is enough momentum in the incoming air to cram more into the cylinder and slightly compress the piston begins to rise. Reversion causes CIS throttle flap swing and funky and make food a bad mixture. You must have the Cam IEC within the CIS. Carbohydrates can be attached to the cam you need, if not the adjustment of cam CIS is worthless.

RoninLB · 01-07-2011, 01:33 PM

Quote:

Originally Posted by Henry Schmidt

RS spec 2.7 with Solex cams is what I have in my 914-6 and it is a very nice combination.
Far better than the E cams for an every day driver.

i was told head work should be done if using Solex

what say you

faverymi · 01-09-2011, 03:38 PM

After talking to John and other people I'm just redoing the heads stock. Call it a refresh.

They told me I would need stiffer springs for really high RPMs (and S cams) . over 7k. I'm going to limit the then engine to 6.5k so stock should be OK. Porting and polishing would be an unnecessary expense for the street use of the car.

We shall see.....

Walt Fricke · 01-09-2011, 04:23 PM

Stock 911 valve springs are really pretty capable. I ran them on my 2.7 race motor for a while, and I wasn't getting float at 8,000 rpm. The factory used the same springs from the beginning until at least the end of the 3.2 era, even on the high RPM early S motors. So you should be fine.