Engine Update and Exhaust Question!!

[dtw]

OK guys – I just got off the phone with David Brown of Carolina Speedwerks. Haven’t talked to them in several months and was starting to feel guilty. Turns out he was just looking for me a couple weeks ago, so he hasn’t been left hanging too long. The case, head, and crank work is all done, and he’s finishing up with the rods. We discussed the new plan for my engine, he believes the setup should be a heck of a lot of fun. To refresh, we’re looking at 9.5:1 compression 2.4L JE pistons with Webers, GE-40s. In fact, he suggested talking about GE-60s instead of the 40s, and said WebCam would be able to provide some insight. Right now I’m planning on picking up my engine parts at the end of September – I’ll have plenty of pics then!

I wanted y’all to know that I’m taking the advice proferred here. I believe the biggest proponents were Warren and Wayne. The oil bypass modification will be done, with a used valve and new OEM spring. David has also agreed to Timecert the case studs. The machine work will now include:

-Timecert transmission stud bosses
-Timecert all head studs
-Install new valve guides
-Increase port size & flow heads
-Perform oil bypass modification
-Re-round main bearing bore
-Inspect, clean, polish crankshaft
-Inspect, clean, recondition connecting rods
-Match damaged cylinder from used stock

The tab for this right now is hovering right around $1,000. I’m pleased! He cuts DIYers a pretty serious break.

Question: This setup will be using Webers to start but it’s likely I’ll eventually go back to MFI. What are my exhaust options with the MFI? I've read that systems with insufficient backpressure make MFI unhappy.

-d

[HawgRyder]

Don't forget...if you are planning on returning to MFI...make sure the cam you choose has the drive option for the left bank!!
It would be expensive to change that cam again.
I assume that you have taken into account the compression ratio when you build the engine...so that the octane requirements are OK with you for an everyday driver.
Bob

[911pcars]

-d,
It sounds like you'll have a very sweet engine. If you're planning MFI in the future, make sure your new cam will be able to accept a pump drive pulley (driver's side) with associated engine sheet metal.

For your exhaust system, you can go with the factory HE's or SSI replacements and factory dual in, single or dual out (sport or otherwise). I think MFI should be fine with this setup - can't speak for aftermarket mufflers (e.g. Dansk, B&B, Bursch, custom, etc.).

Sherwood Lee
http://members.rennlist.org/911pcars

[jluetjen]

-Increase port size & flow heads

To port or not to port? That's a question that I've done a lot research on and my suggestion would be "it depends? What it depends on is:

What cam are you using?
What are the sizes of your existing ports?

According to my estimates, both the GE40 and the GE60 are slightly milder then the early "S" cam. So they will be developing their peak torque and HP at lower engine speeds. If you already have "S" spec heads then you'll be fine. If you have T or E heads you >>> might <<< need to port them. I have seen conflicting data regarding the stock dimensions of the 2.4 T and E heads, so I don't have a clear recommendation. But ideally you want to keep the intake gas speed (and to a lesser degree the exhaust gas speed) in a happy zone. If you are in the zone, then larger ports will hurt rather then help performance.

[dtw]

jluetjen- he's doing a very mild port- I don't have the specs in front of me but they are definitely not going to be very wild. I'm sorry to speak in such subjective terms right now - I'll try to get some harder data in the weeks to come. The base port size is whatever a 2.4E head is - I am in a hotel room for the next several weeks and don't have my BA in front of me.

I'll be getting my existing cams reground, which has the MFI drive already.

Sherwood- as far as the dual outlet- is that the style with the two tips coming out near the center of the muffler, or one tip on each side of the banana? I'm thinking Dansk stainless muffler- unless anyone has data that rules this out w/r/t the MFI.

Bob - Yep, 9.5:1 should be right around the maximum streetable compression on pump gas. I can't wait to fire it up for the first time.

[911pcars]

"...as far as the dual outlet- is that the style with the two tips coming out near the center of the muffler, or one tip on each side of the banana? I'm thinking Dansk stainless muffler- unless anyone has data that rules this out w/r/t the MFI. "

D,
From owner comments and from the archives, the stock muffler is a pretty good match for the factory MFI. The RS used the sport muffler which seemed to work pretty well. I don't think the twin pipes at the back or the "bull-horn" mirror image end tailpipes are that much different. It's the modifications to the internal baffles that determine ultimate flow (or backpressure).

Don't know about the performance advantage a Dansk has over the factory muffler other than many report it sounds pretty good (looks great too). Whether "just the sound" is worth ~$500 or so is up to you. If the stock muffler is not rusted through it can be made to sound and flow pretty good as well.

Sherwood Lee
http://members.rennlist.org/911pcars

[jluetjen]

What I would recommend is use the following formula to estimate your porting requirements:

Intake Gas Speed (M/s) = (Cylinder Stroke * Peak Power RPM)/30000 * (Cylinder Bore/Intake Port Diameter in mm))^2.

The intake port diameter should be taken at the tightest cross-section of the intake port, which may not necessarily be at the manifold face. Ideally (based on what I've found in reviewing 36 different 911 engine configurations) you want the intake gas speed at peak power RPM to be between 70 m/s and 100 m/s. If the number is slower then 70 m/s, your engine will most likely lose torque at lower RPM's. If the gas speed is greater then 100 m/s, chances are your engine will be a little choked off which will limit the engine's performance at higher rev's. If you stock ports result in the gas speed being in the desired range, porting the heads will most likely gain you little except for a lighter wallet.

As far as the cams go; here is what I've been able to determine:

..........In Dur.....In Lift.....Ex Dur.....Ex Lift.....Overlap.....Peak HP/Torque Speed.

GE40:.....256........470.......248........440..... ..48 .....6722/5175 (est.)
GE60:.....266.......490.......248.......455....... 54.....6800/5500 (est.)
S.........267.......459.......235.......396.....57 .....6800/5500 (Actual)

[dtw]

OK John,
I'm gathering data now so I can call the machine shop and discuss port sizes. Questions for the forum:

-does anyone know the bore of a 2.4l motor? I don't have my BA handy (it's 300 miles south of me right now)

-in your equation above, is the result of the arguments on the right side of the equation squared? Or just one of the arguments? I don't think all your parentheses agree.

Thanks!

[skinnerd]

One thing to consider when going back to MFI is that there are certain space cams available, and you almost need to build the engine to the available space cam, not visa versa. If you are really interested in ultimately using MFI in your car, you need to find out if the cams, displacement, c.r., etc are all compatible with an an available space cam. Otherwise you will never get the car to run quite right. Please correct me if I am wrong on this account. To be sure, I'd check with one of the experts on MFI pumps like Pacific Fuel Injection.

[jluetjen]

-does anyone know the bore of a 2.4l motor? I don't have my BA handy (it's 300 miles south of me right now)

>>> 84mm

-in your equation above, is the result of the arguments on the right side of the equation squared? Or just one of the arguments? I don't think all your parentheses agree.

>>> Actually the formula is right. When I put "mm" in parenthesis, it looks confusing. You should be squaring the following term: (Cylinder Bore/Intake Port Diameter). Note that the intake port diameter should be measured in millimeters.

I hope that this helps.

- John

[dtw]

John,
Ok, I set up an Excel worksheet based on my understanding of your formula. I plugged in the following:

Peak Power: 6800 rpm
Intake gas speed: 85 cubic meters/second (average of your "happy" gas speeds)
Stroke: 70.4mm
Bore: 84mm
Port diameter: X mm (solve)

This yielded a result of 36.4mm. This seemed a bit large for port size so I tried 70 m/s gas speed. That gives me 40mm! Also contrary to my expectations, 100 m/s yields a less radical 33.55mm. Help me out here- I would expect intake gas speed to increase with port size (and vice versa)? Where am I going wrong? I can send you the excel file if you like. Another related question: How does exhaust port size tie into the intake port size?

[zuffenhausen]

dtw,

Your spreadsheet is working fine. To move a particular volume of gas through a smaller port in a fixed amount of time, the gas must move faster than through a larger port.

BTW, many of these engine parameter calculators are readily available on the web:

http://www.mtv411.com/auto/Stroke-Bore-Cylinders-Intake-Piston-Speed-HP-RPM.htm

-zuff

[jluetjen]

Yup - I agree with Zuff'.

[dtw]

Ahhhh good deal. I was thinking in terms of cubic meters but left out the per second when I was thinking through it. Thanks for clarifying, Zuff. Sounds like 35-36mm would be a reasonable intake port size, in that case. Now I am almost ready to call David.

One last piece of the puzzle: what relationship will the exhaust port size have to the intake port size? I assume I don't want to flow 90cm/s into the combustion chamber, and then cause a bottleneck in the exhaust. Would the exhaust port size simply be the same as the intake? Or do I need to change that equation around a bit...thanks for all the help so far, btw.

[zuffenhausen]

For the majority of naturally-aspirated 911 engines, Porsche has kept the size of the exhaust port the same as the intake port. There are a few exceptions where the exhaust port size is one millimeter smaller, presumably to increase exhaust gas velocity for better scavenging.

Just as a point of interest, 35mm ports were used on most of the 2.7L engines, while 36mm is the port size of the 2.4L 911S.

-zuff

[dtw]

Figures I'd get a response on that 8 minutes before a meeting - now I have to wait until after lunch to talk to David...crap. Well he's probably at the track until Monday anyway. I'll let you guys know what I find out. Thanks Zuff!! I'll be in town to stir the soup this weekend (my engine bits are in Zuff's parts washer ).

[zuffenhausen]

You're welcome, dtw. Oh, I swapped out the varsol for a stronger solution--sulfuric acid. Hmmm. I hope your parts are still there.

BTW, the 'z' isn't capitalized in my nickname. Unlike that once magical German suburb, my nickname isn't a proper noun.

-zuff

[jpnovak]

>Oh, I swapped out the varsol for a stronger solution--sulfuric acid.<

That would do it!

Jamie

[jluetjen]

For the majority of naturally-aspirated 911 engines, Porsche has kept the size of the exhaust port the same as the intake port. There are a few exceptions where the exhaust port size is one millimeter smaller, presumably to increase exhaust gas velocity for better scavenging.

I've seen some convincing cases made for keeping the exhaust ports smaller then the intakes for velocity reasons. I know that Chris Bennet posted an interested post to this BBS to that affect. I'd do a search on his name and "exhaust" and see what turns up. I've also seen it suggested that while the intake air only has the help 14.7 lbs/sq-in behind it (the delta between ambiant air pressure and a perfect vacuam), the exhaust has considerably higher pressures trying to push it out. So generally the exhaust will get out of the cylinder as long as the exhaust ports and valves are not grossly undersized. Some engine designers have even made the exhaust valves "undersized" compared to conventional wisdom just so that they could fit larger intake valves.

(BTW all you Turbo owners, the rules are different for Turbo engines!)

[dtw]

OK guys- I called David this morning. Here's the lowdown:

-Intake ports have been taken out to 38. We use John's equation to back into the intake gas speed and end with around 78 m/s - inside the window specified.
-Exhaust ports were taken out to 36. David felt this relationship of exhaust to intake port size optimal for exhaust scavenging, and would be a good setup if I wished to return to MFI down the road.
-I'm looking at a nice setup of Webers, manifolds, linkages, etc. for $1100. There may also be a set of PMOs for sale for 900, but I don't know yet what that includes. Both are good deals. I'm thinking though- if I'm looking at $1000 for carbs, maybe I should just call Pacific FI and see what sort of cash Gus wants to rebuild my throttle bodies, pump, and do a space cam recurve for my new hot motor.

Opinions?