building 2.8 RSR engine

[raspy2point2]

Anyone have opinion in using 40's or 46 size carbs on a 2.8 High compression,twin plug engine What are the plus and minus of a 46 PMO for instance? Thanks Chris R- Gruppe #144

[Steve@Rennsport]

Chris:

A 2.8 RSR-spec motor really needs 46mm carbs to make the power its capable of. Without question, your choice of cams and heads define the operating RPM range of the engine but IMHO, 40mm carbs are too small unless mild cams are used and low-end torque is a big priority.

Assuming that you are using some kind of race cam and your heads are close to what RSR heads were capable of, I think you'll be pleased with 46mm PMO's over the smaller version. You can use any size of venturi to tune the engine to the cams you will use.

No matter which choice you ultimately decide on, you'll find that PMO's are far better than Webers in every respect. Not only do they run much better at throttle openings less that WOT, they also do not starve for fuel in the corners like a Weber can (unless a lot of mods are done). The other big advantage are the sealed ball-bearings on the throttle shafts that do not leak and pass air.

[Eric911S]

hello; Like Steve said above the PMO carbs are alot better. Firstly what are you going to use the enigine in? Is it for a race car or street/track car. If you are going to drive it on the street I would suggest the 40's The reason is throttle response and streetable power. The Weber 40's work very well with 38mm main venturis and you can also buy them everywhere. You may also consider EFI using TWM throttle bodies. This gives you the look of the carbs, kinda. You could also do a MFI setup as well. Hope this helps. Thanks Eric

[Bert]

Hello Steve and Eric,

Thanks for your valuable advice. I will build a similar 2.8 RSR spec engine to what Chris is doing. This engine would be for street/track use. Low end torque would be nice, but I have short gears and revving a little to exploit a higher power band is okay too.

Here is my planned configuration:

Mahle 92 mm P/C with 10.3:1 C.R.
70.4 mm crank and factory rods
2.7 twin plug heads
Weber 40 IDA's (but now I am thinking about the PMO 46's )
Header Exhaust
Sport Twin Pipe Muffler
S cams or modern profile

1) What port sizes should I use for the heads? I was thinking 36mm I/35mm E or would 38mm I/38mm E be too large?
2) What cam profile would you recommend?
3) How does question #1 & 2 change if I decide on PMO 46's?
4) What would be a safe red line for reliability?

Thanks,

[Eric911S]

Bert: The next question is how much street and track use. Would you say 90% street and 10% track? How many track events per month? Stock rods are fine if you use Racewear or ARP bolts and resize the rods. Bert you don't have to use Mahle P&C you could have the cylinders bored and recoated and use J&E or Cosworth,Ross or Wiseco pistons. 10.5:1 will work for twin plugged engine on premium pump gas. You need big cams to make the power I use either Elgin or Webcam. Good port sizes are 38mm intake and 36mm exhaust. I do these in my milling machine. I also use good valve springs and retainers. A safe rev limit would be 7500rpm. The engine should last a long time and still make great power with short gears. I would also decide on your twin plug setup-distributor or crank fire. I like crank fire, Steve will say Distributor. You may also consider a 3.0 engine? I would sit down and make a budget then decide what you would like on the engine. After you decide add more money to the budget right? Oh well take your time and I hope this helps. Thanks Eric

[911scfanatic]

I'm building a 2.8L as well. I'm not going to call it an RSR because all it shares is the displacement. So far, I have 92mm Mahle 10.3:1 p/c's, DE60 cams and racing spring. I'm using the SC oil pump and updated sump baffle. I'm working with Henry at Supertec. I sent him a 3.0L dizzy to do modify for Aaron's twin plug set-up. I'll be running two Bosch 6-pin CDI boxes. He also has my MFI and intake to rebuild. I'll be using '71 stacks opened and more open intake and exhaust ports on either my 2.4 or 2.7 heads, whichever are in better shape. All the exact specs escape me at the moment since I've been concentrating on the body work lately. I've budgeted about $15k for all of the machine work, parts, SSI's mufflers, etc. Let's just say I'm well into my budget. If you're very detailed about you budget, you'll be quite shocked at the number you come up with.

[Bert]

Hi Eric,

I plan to do 3 or more DE's per year and spirited weekend driving for the rest. I didn't think about the valve springs and retainers. I'll have to add that to the budget! I have a budget, but it's a moving target I am acquiring parts a little bit at a time to take the sting out of the wallet.

The port sizes you gave don't sound too large, so it sounds like there will be a good compromise of low end torque and high end power. I don't want all of the power to occur past 5500 rpm.

Bill - your 2.8 engine sounds fun too. The MFI should make a nice sound with better throttle response. I also plan to use the Supertec twin plug distributor. When will your engine be completed? I would love to get some feedback on your engine and Henry's twin plug set up.

Thanks,

[Eric911S]

Hello:Bert I would use drivable cams in your RSR engine. I have customers that do 1-2 drivers eds a month and drive the cars everyday. Needless to say they need drivable cars. If you do very large ports the low end will suffer greatly. I built an engine for myself years ago using factory 2.5 ST heads and tried alot of different cams as well. I then decided to port some heads myself using 2.2 T heads to start with. I ported those 38 and 36. These made more power at the bottom end up to 6500rpm. Not untill 7100-8200rpm did the big ported heads make more power. Bert the carbs when setup properly make some power as well even more than MFI. I know some historic racers that use 50mm carbs on 2.5 liter cars since they are lighter than mfi and have larger port sizes. These are light race cars. Lastly Bert try to find a early aluminum engine case to build your engine on. Thanks Eric

[Bert]

Hello:Bert I would use drivable cams in your RSR engine. I have customers that do 1-2 drivers eds a month and drive the cars everyday. Needless to say they need drivable cars. If you do very large ports the low end will suffer greatly. I built an engine for myself years ago using factory 2.5 ST heads and tried alot of different cams as well. I then decided to port some heads myself using 2.2 T heads to start with. I ported those 38 and 36. These made more power at the bottom end up to 6500rpm. Not untill 7100-8200rpm did the big ported heads make more power. Bert the carbs when setup properly make some power as well even more than MFI. I know some historic racers that use 50mm carbs on 2.5 liter cars since they are lighter than mfi and have larger port sizes. These are light race cars. Lastly Bert try to find a early aluminum engine case to build your engine on. Thanks Eric

Hi Eric,

I already have a 2.4 CIS 7R case. I have heard about the weak Mag case issues, but I am hoping that my engine won't leak when I perform all of the usual upgrades. Also I bought the 2.4 engine from Portland and I think it lead a cooler, less stressed life versus the thermal reactor smog 7R motors from 1975 - 77. I would rather use a lightweight Mag case over the heavier Alum case. Just like the lightweight historic racers you noted in your last post.

If I go with the port size you recommend (38/36) and "driveable" cams, would you recommend 46 PMO's versus my Weber 40 IDA's?

I appreciate your advice based on real experience.

Thanks,

[911scfanatic]

Bert, I hope to be done by spring, but that is a moving target as well.

I'm using a mag case as well. I have both a '73.5 2.4 7R case and a '74 2.7 7R case. I've decided to keep the '74 and sell the '73.5 to avoid the cost of opening up the spigots on the '73.5. What case mods have you decided on to address the mag case issues? Obviously, case savers, but what about shuffle pinning? I'm also using a center mount oil cooler in an RS bumper to keep temperatures down.