Help with port size.

[777joee]

Hi guys,

I'm building a 3.0 for the street and want low end torque and power v top end. I was thinking on having the 34mm intake ports enlarged to 39mm on the stock 80 3.0 heads and using 46MM PMO's . If I enlarge the intake ports will I loose the low end torque? I have a set of Weber 40's already and could just use a proven combo with the 34mm stock ports and 34mm vents.

This thread has some very interesting data points and is about what I'm looking for out of my 3.0.

3.0 dyno day - a pleasant surprise

Thoughts?

[jpnovak]

Size the ports based upon the cam that you are using and where the peak torque occurs.

[jpnovak]

Sorry, I couldn't finish the thought.
The smaller ports will provide increased intake gas velocity that will aid cylinder filling at lower rpms. Of course, the problem will arise that they will not flow as much air at higher rpms due to the smaller ports. This will choke the engine down and reduce HP even more.

If you are only driving on the street I would suggest keeping the smaller ports and maximizing the cam around the port size. the only reason to make the ports bigger (or use the early size) is to support aggressive cams designed for high rpm torque and HP.

Size the ports depending on the cam you use. Please post your expected build. Once you have the cam and port size then pick appropriate carbs. They all have to work together as a system. choosing the power band will define the cam that will define the port size that will define the carb requirements. The carb jetting will also depend on the desired rpm band (peak HP).

[777joee]

Jamie,

I have two things going on. First is the budget and second is drivability. I'm putting this engine in a Intermeccanica Speedster with a full tube frame and all 911 suspension. I want reliability and I'm not so much out for all out performance. I'm looking to build low end torque stump puller and peak results around 5,000 to 5,500 RPM. 200+ HP will be plenty.

So far I have this to work with:

Stock bottom end 1980 3.0 including the 9.3:1 original CIS pistons and Nikasil cylinders.
I already have a set of weber 40 IDA's to use.
New thick flange SSI heat exchangers and M&K Sport muffler are on the way.

The cam is not selected yet and the heads are at the machine shop now. I was originaly thinking of going to 39mm intakes but Wayne's top picks for the performance 3.0 says 36mm with carbs. The link in my original post has a sample engine building about my perfect HP/Torque needs running the stock 34mm intake and 34mm vents and points out using smaller ports for better low end which is what I'm after. I have built many a VW, 356, or Detroit iron motor but I need help from you guys with the last piece of the puzzle on my first 911 engine.

I guess sometimes bigger 39mm intake ports is not better........I'm just trying to confirm.

Thanks,

[jpnovak]

Keep the current pistons. Keep the stock port sizes. Use a stock CIS 3.0SC cam and bolt on the weber 40s (jetted for the 3.0). It will be a very good engine to match your description of a stump puller. In such a lightweight speedster replica this should be quite fun.

This brings the question of gearing. Low range torque favors taller, widely spaced gears. High rpm HP favors shorter, close ratio gears. Will you use a vw transaxle? If so, make sure it is not geared too short for your newfound torque. It might rev quite high cruising on the freeway, especially depending on tire diameter.

[MrPerkles]

Dave speaking from experience I think 46s are far to big for a street 3.0l bteer off keeping the 40s as Jamies says

[777joee]

Jamie,

Trans is the stock 80 915. Take a look at what they build. Here is a copy of the link Excellence did in 2003:

http://www.intermeccanica.com/site_assets/pdf/ARTICLES/EXCELLENCE.pdf

Also here is a link to the tour of the shop. INTERMECCANICA - FACTORY TOUR There are some very cool pictures of the jig welded 911 tube frame used. What I did was buy a ratty 1980 Targa with 115K miles for all the running gear.

Not for the faint of heart price wise but I can't afford a real Speedster and this one will have electric windows, heated seats, upgraded suspension and a 3.0L for fun.

If you think I'll be on the right track keeping the stock CIS cams then that will save me some money. The original ones from the motor are like brand new. I split the cases and the bearings even the intermediate shaft bearings looked like brand new.

Any other thoughts from anyone?

Thanks!