Help me plan my 3.2SS

[jpnovak]

The time has come to really start putting the pen to paper in planning of my 3.2SS. I will start with what I have and comment on what I want from the motor. I have a few things in mind from my research but want to make sure everything is in order before I start collecting too many parts. This is just the planning phase and will likely not happen until next spring, or unless I find a core motor to start with.

I will be starting with a 78SC 3.0. This is a RoW motor (although I can't find any differences) with the big ports on the head - 39mm intake. It currently has Weber 40IDA's and SSIs/Dansk Muffler. The engine will be going into my ST replica that has all the necessary safety, suspension, and brake mods to support more power.

The car is mostly track but it is street legal and will enjoy the spirited run through the Hill Country of TX as well as being transportation to the track. Its not trailered... yet. I am looking for power from about 3.5-6.5K. I know that this is wider than the usual 2K rpm power band but this is the rev range that I have used on the track. I would prefer to not have a super high rpm motor but want to be able to spin it safely to 7.5K without worrying about valvetrain problems, just in case I get there.


I have a set of Mahle RSR 10.5:1, 98mm PC set. From my information, and from someone who built an engine like this, the final CR should be about 9.8:1 using the 90cc SC heads rather than the 78cc RSR heads. I have been considering a Mod_S cam to go with this compression ratio. I think I am on the edge of single-plug and have no issues finding 94 octane. I realize that the 98mm diameter almost requires twin-plug based on flame front travel time. I will have the heads redone and drilled for the twin plug. Right now my budget will likely not support twin plugs unless Aarons rotors come through for the Jag caps. This is an option I will leave open. I plan to run the stiffer valve springs and steel retainers. A previous post mentioned not needing Ti until above 7.5K.

Induction will likely stay with the weber 40s until I get the ITB project rolling and convert to EFI.

I am looking for suggestions here. I want to build this so it all works as a system for best results and reliability.

With the 98mm and 9.8:1 should I scrap the RSR pistons and get some JE's at 10.5:1 or higher to take full advantage of the twin plug?

Is the Mod_S cam a good choice for this motor? Would I be better off with something more agressive like a GE80 (or equiv.)?

Am I fooling myself into thinking that the 40IDAs will not run well in the 3.2? too small? This will not be a race motor that spends its life at 8K.

How possible would it be to run single plug? Am I just setting myself up for failure at 9.8:1?

[jluetjen]

Jamie;
I think that you're thinking of the right things. The constraining factor that I see in the plan is the Weber 40's. I'm guessing that they'll limit you to about 240 HP using 36 mm venturi. I'm not sure if you can get 38 mm venturi's for the carbs, but I suspect that the low RPM driveability would suffer pretty badly with that configuration. Basically with 40 mm carbs and 36 mm venturi, the engine will pull great until it starts to generate about 240 HP (about 6000 RPM) and then the torque will drop in proportion to the increase in rev's (thus holding the HP flat at around 240 HP until it started to drop). So it would kind of act like an early E. Certainly not a bad configuration for occasional street use, but choked off at higher rev's. Going up one size in carbs to a 46 mm carb with 38 or 40 mm venturis would get the carb gas speeds above 6000 RPM back into a zone which seems to be normal for 911's.

BTW, what do you have for specs for the "Mod-S" cam that you're talking about? My thought would be that something like a GE60 might be a good option. It has more lift then an S (which your larger ports most likely would support), and about the same seat-to-seat duration as the S. But with higher ramp speeds and valve accelerations should act like a bigger cam and pull strongly at higher RPMs. With a wider lobe angle (and as a result smaller overlap) compared to the S cam, it should still be pretty docile and pull well from low revs. I'm guessing peak torque around 5000 RPM and peak HP around 7000 RPM. Alternatively I know that Camgrinder was thinking of something like a 964 cam with a narrower lobe angle (964-102) which would might be nuts-on for your use given an anticipated rev range like an early E (4500 RPM peak torque, 6200 RPM peak HP), but have the lift needed to work with the larger ports and capacity of a larger engine.

BTW, with the larger valves of a 3.0 (compared to a pre-SC engine) and an anticipated 7500 over-rev limit, I agree that stronger valve springs and lighter retainers would be a good investment.

As far as twin plugging is concerned, I don't have any opinion on that. But one thing that might work (I've never tried it) is to spec the motor as a 10.5:1 (in anticipation of twin plugging), and then add some extra washers under the cylinders to reduce the CR to the 9.5-9.9 level for use with single plugs. When you upgrade the ignition system, you don't need to do any machining or buy any parts, just remove the washers and adjust the cam timing accordingly. You could most likely cut the extra washers off without fully removing the cylinders. I'd be interested in what others think about this strategy.

[Jim Richards]

Jamie, the '70 911 race car Chris Streit is selling has a 3.2SS spec'd as follows:
3.2L Short Stroke motor.
- 76 3.0L case,
- 2.7L crank
- Max Moritz 3.2L Pistons and cylinders
- Raceware rod bolts
- Patrick MS lightened flywheel with 4 puck clutch.
- 98mm pistons
- twin plugged. Electromotive Crank Fire
- ported heads
- 46mm webers
- GE60 cam
- Titanum Retainers
- EBS valve springs

I hope this info is helpful.

[jpnovak]

John, I was thinking about the Elgin Mod S cam. From what I have seen they are very similar to a GE60 and slightly more agressive than an early S profile. I am looking for a similar feel to an early S engine and trying to compensate for the increase in engine bore. I admit I don't know exactly how lift, duration affect the movement of the peak torque value. I have seen your spreadsheet but have not seen the calculations. Can you send it to me or post the formulas you use?

Here are the specs according to Elgin.

Mod. S 296 I .470 lift; 276 E .440 lift; 102 lobe ; 4.5 mm lift at lap

I agree that the 40s would be a little small for optimum performance. I will be building my ITBs for EFI and these will be 39mm straight through which should flow enough air. they may choke it off a bit but I love the way my E spec 2.2 pulls at the track. I would rather have the low speed drivability.

Interesting idea about stacking washers to lower compression. Should be easy enough to remove if needed. I think that the 9.8:1 should be fine on pump gas.

Jim, I have seen Chris' engine specs. Sounds like a great motor. I think I want something similar.

[jluetjen]

Jamie;
Keep in mind that a full S cam's engine charactoristics (Peak torque at 5500 RPM and peak HP at 6800 RPM) isn't really consistant with your specified rev range. The range that you described in your original posting is similar to an E's.

I would prefer to not have a super high rpm motor

and

I am looking for power from about 3.5-6.5K.

As far as the calculations go, here's a couple of rules of thumb based on a regression analysis that I did on the factory cam information:

Peak Torque Engine Speed = -3151+(Intake Duration*32.53)

There is a somewhat weaker rule for the peak HP engine speed:

Peak HP Engine Speed = 5573+(Overlap in degrees*23.7674)

Of course this information excludes such factors as valve size, port size, valve accelerations and speeds, nor factors in assymetrical cams or cams with different intake and exhaust durations. Lift curiously didn't seem to coorelate strongly to these outputs nor to the residuals from the Duration or Overlap, but it does obviously have an affect on the total flow area available for loading the cylinder. For example, E cams seem to have insufficient lift to fully fill 2.7 liter cylinders with 36 mm intake ports and stock 2.7 liter valves at higher revs. The result is an engine that pulls great from low revs through the mid-range, but then tails off faster at high RPM's then a smaller engine would. But back to my point, those equations are certainly adequate "rules of thumb" for narrowing the list of cam options.

[jpnovak]

John Thanks for the info. I have a question for you. From my reading and in Wayne's book it reads so that a cam that is agressive in nature (high rpm power and strong "on-cam" feeling) will become much more mild when the displacement increases. I would think that the nearly 30% increase in displacement from a 2.4 to a 3.2 would change the drivability characteristics quite a bit. This is why I thought the 6.5K would be a good number for peak with that cam and a 3.2. That being said, engine displacement is not part of your emperical equations. Just looking for your thoughts and comments.

Its interesting that the E cam will not fill a 2.7 cylinder at higher revs. I suppose I need to check the volumetric requirements of my engine and make sure there is enough lift to fill the cylinders at redline.

I guess I need to sort this out as I build my ITBs. I want to tune the length to the rev range of the cam. I found some good information on this.

[jluetjen]

Jamie;
Personally I put that belief about big engines mellowing cams in the Urban Myth department, or at least a gross simplification. In checking the BMEP curves for different sized Porsche racing engines (all using a variant of the 906 cam), I saw no change in the peak torque or peak HP engine speeds, nor in the shape of the torque curve. I think that what would be more accurate to say would be the observation that an off-cam big engine makes a lot more HP then an of-cam small engine. Assuming that the car which it is pushing weighs the same, makes the big engine feel more flexible. It's not -- it's just bigger. (Standard disclaimer about assuming adequate valve size, port dimensions, car venturis, etc. etc. etc.)

So if an S cam has peaks at 5500 and 6800 RPM in a 2.0, it will in a 3.2 also. As far as the lift limiting an engine's performance, 69911E built a 2.7 engine for autocrossing with E cams. The engine seems to generate it's peak torque at about 4300 RPM (versus 4500 RPM for the E's), but rather then developing peak HP at 6200 RPM, it peaks at 5700 RPM. Keep in mind that it's not due to carbs (69911E's engine has MFI), nor is it port or valve restricted since an RS engine (same spec but with S cams) developed peak HP at 6300 RPM. That pretty much ruled out everything but valve lift.

[jpnovak]

Interesting and great information. I guess a regular S cam would work great for my engine wrt overlap and duration. Do I need more lift to help cylinder filling for the increased capacity? It sounds like I want to build a 3.2SSS engine.

I really need to get Paul Friere's book and some other sources for these torque curves.

[BURN-BROS]

Jamie, how about sprint cams or similar spec? Too much for the street?

[jpnovak]

Aaron, I think those will be too much for the street. I don't really want to have a high idle and I will not need maximum HP. I would rather have a nice strong torque curve at a reasonable redline.

how is the rotor project coming along? This might greatly influence my decision about twin plug.

[BURN-BROS]

I'll be machining the copper pieces this weekend. Last weekend I was replacing parts on the machining center. Mold is done(my part anyway). When the parts are done I will make a trip to the injection molder......Soon!


Ask camgrinder about your application. I will be hitting him up for suggestions when I am ready.

[jluetjen]

Jamie;
Also keep in mind that if you are running carbs, but still want to use the car on the street, you may not want to run much more overlap then an S cam. This is because all of the Race Cams' overlap can cause reversion just before the engine comes on cam. Using tall secondaries can help, but don't address the fundimental issue of a high-pressure wave arriving at exhaust valve while both valves are open. Careful selection of the exhaust system can move this point on the rev range to a potentially less annoying zone.

[KobaltBlau]

My $.02:

I don't really think that the RSR pistons are OK with single plug even if your compression is only 9.8:1. I say this because the RSR piston shape is very different from the late euro 3.0 piston that is also 9.8:1. Specifically, I don't think your burn will be as good with the RSR pistons on single plug.

I would use the RSR pistons and do a twin plug. 7 tenths increase in compression just doesn't buy that much horsepower, and if you sell the RSR pistons with no cylinders they aren't going to be worth all that much. The Mahle pistons are also potentially a more long-term item than the JEs. But I understand this challenges the budget. I would use Megasquirt and Spark (MSnS) to run the EFI and the plugs. As I understand it you use GM coil packs or something. I think you could also use this setup to just to spark until you do your EFI conversion. This reduces the cost of twin plug to the machine work plus a couple hundred bucks for the ECU setup and the junkyard coil packs.

Some would say that new stock valve springs and stock retainers are OK for that over-rev limit, even with the 3.0 valves. But I agree with John that they might be a good investment. You could always choose either the springs or the retainers since you don't need 8k+. You will want fresh springs of some type, though.

I think the 40s will be OK (but not perfect, john's analysis above is good) since you have other plans down the road anyway. I personally would build the motor with an ideal induction system in mind, and switch to that when possible. It's a lot easier to change an induction system than pistons!

I have talked to John D. (camgrinder) some and you should really talk to him about your application. I think the GE40, GE60, and Elgin Mod S are neat grinds, but I am convinced that he can offer a much better cam recommendation for your purposes than I can. He can very likely help with things like port and induction sizes as well (in case he thinks they should be changed).

Best Regards,

[cstreit]

JP,

I'm going to second Andy's comment about the RSR pistons (though I'm not nearly technical enough to support my belief). THey have a very high dome that bisects the combustion chamber, I would be worried about the flame-front's ability to propogate across the chamber given the highly peaked nature of these pistons. This is one of the reasons I did a twin-plug mod with my RSR 95mm's

[KobaltBlau]

Originally posted by jpnovak
From my information, and from someone who built an engine like this, the final CR should be about 9.8:1 using the 90cc SC heads rather than the 78cc RSR heads.

Let's clarify a little:

Do you have 98mm P+Cs that are intended for use with a 70.4mm crank (3.0 base)?

Did a 3.0 RSR have the 78cc heads?

Were these P+Cs originally developed to convert a 3.0 RSR to a 3.2? I'm not familiar with the motorsport rules that would have been applicable here.

Are you certain that the person you got your information from used the same P+C set?

[jpnovak]

Yes, I have 98mm RSR pistons setup for 10.5 (3?):1 on the 76-77cc heads according to BA's book. They have the correct 22mm wrist pin for the 70.4mm crank (3.0). BA lists the SC head measuring about 90cc. I think emcon5 built his engine with these pistons and measured 9.8:1 compression. Here are some pics.



Chris, you are correct that they have a steep wedge that will split the combustion chamber. I agree that twin plug would be best. I am planning to go the 12plug dist route with the Jag cap and rotor that Aaron (Burn-Bros) is starting to manufacture. Of course, Now that I have another suggestion (Spark MSnS?) This may change. Andy can you provide a link or more information on this spark controller to go with the Megasquirt?

[KobaltBlau]

Jamie, here's a link:

http://www.megasquirt.info/index.html

scroll down to:

"What if I need ignition control as well?"

I'll try to contribute more information in the morning, but I haven't built a system myself.

[Jeff Alton]

Chris' take on why the RSR pisstons might not be the best is spot on. But they will easily take an S cam which might not be true of the standard piston dome shape. Tough call. 10.5 or less with a moderated to big cam (maybe "s or a little more) and you should not have any detonation issues with single plug though. When you start to put this motor together we are going to need to see some pics...? Sounds like a great motor project. Talk to camgrinder as he can expand on the compression/cam/detonation issue a lot more than I can. He is a one of the great resources here!

Keep us posted, Jeff

[gumba]

In regards to the carbs I had a local Weber guy recommend I use the 36mm chockes in my 46's vs. the 38mm. Since I have the redline set at 7500 he felt the 36's would offer a more usable power band. With this set up on our 2.8 twin plug I reach max torque at around 5800, and max h.p. at around 6400.
Someone told me that Henry at Supertech can modify the 40mm Webers to 46mm.

[Mike Bonkalski]

Here is some discussion over on the Gruppe B board about MegaSquirt and EDIS-6 Distributorless ignition:

http://www.gruppeb.org/forum2/viewtopic.php?t=860