OK now, I've been digesting this good information for a bit. I understand that the Cam will determine the shape of the torque curve and that cam lift and port size/shape will mostly determine where the power peaks come in. (leaving out the bits about manifold height and Inj fuel plates).

However, I just reread the back issue of Excellence with the '69 Steelie. In it, the owner rebuilt a 7R mag case to 2.9 liters, 10.5 Compression, Elgin mod "S" cams and 911 S cylinder heads with trick springs and Ti retainers. Twin plug with MFI. The engine makes 247 HP at 6800 and 210 ft/lbs at 5300. My question is what makes the S heads flow better to raise the peak power band?

Joe;
I don't think that I have that article. Could you look up the bore and stroke? I'm guessing that the bore is 93 mm and the stroke the standard 70.4 mm. Also, was the engine twin plugged? I suspect that it was twin plugged at that CR, unless they found some way to increase the stroke rather then the bore to increase the capacity.

I also suspect that they ported the intake ports out to about 38 or 39 mm in order to generate those HP and torque numbers that the engine speeds shown. Keep in mind that from a combustion perspective, a 2.7 RS is the same as a 2.4S with a bigger bore. Everything (cam, valve sizes, port sizes, CR, etc) else is the same. When Porsche made the 2.7RS, the peak HP engine speed dropped 200 RPM (6500 to 6300 RPM), and the peak torque engine speed dropped 100 RPM (5200 to 5100 RPM). This is because they had the same intake ports and valve sizes. So each cycle the engine needed to draw more air through the same path -- which takes longer. As a result the engine got choked off at a lower engine speed.

Now the engine that you've referenced is larger then a 2.7, but generates it's peak HP and torque at even higher engine speeds then the 2.4S. I doubt that you can do that using the same sized ports, even with a cam with more lift, duration and overlap like the Mod-S cam.

I can get that info for you from home.

Correct.


This makes sense and explains why larger intakes are needed as displacement increases. You also need more lift to get more charge in at higher RPMs and continue to make good power. As I sit and think abouit it more, it takes some pretty serious lift and intake port work to get a larger engine to peak in the stock S engine RPM range.

Is it fair to say that a mod S cam in a 3.0 is kinda like an E cam in a 2.0?


BTW, I checked the mag @ home and the Crank is an NOS S crank and the heads are 911S heads.

Jamie,

John Camgrinder has steered me correctly in selecting the DC40 for my mostly street and occassional AutoX 3.0 liter application. I would love to see how the stock SC cam torque curve profile compares to John's mod "S".

DC40 specs Intake duration @ .040 266, @ .050 259. Lift =.474" , Centerline 102 "Mod-S" Exhaust dur @ .040 249, @.050 242 lift = .440"

Stock SC intake dur = 236 @.040, 228@.050, lift = .450 centerline = 113
exhaust dur = 218@ .040, .395 @.050 lift = .395

My current small port 3.0 pulls 214 flywheel foot lbs at 4600 and 203 HP at 5800. Above 6000, the engine falls off fast so I"m looking to extend my rev range to 6500 or so, w/o loosing low end grunt. I'm confident of John's DC40 but still undecided on porting, displacement, and compression. Limiting factor on this entire setup is the overall amount of torque my 901 will handle. I'm more inclined at this point to run 10.5 compression with twin plug and some mild port work on the intake side.

I have this graph at home. It was part of my initial study for a cam. The results are quite amazing on paper. I hope this will transfer to my right foot. :)

I will post it tonight after coverting to jpg.

Joetiii; Where in MA are you located? I'm up in Westford.

If you took your intake ports out to 39 mm, and went with GE40 cams, I don't think that your torque numbers will change much from where you are right now. On the other hand, I would expect your peak HP to jump up to about 242 HP up at 6500 RPM -- so the car would run pretty much like a 3.0 liter S from 5800-6700 RPM. The GE40s have more lift then an S cam, so the valve lift won't be limiting the port flow like you'd find with an S cam. I suspect that GE40's have less overlap then an S cam, so the engine should also run better at low revs.

It sounds like it should be a fun engine.

John, Last time I checked a map, Westford was down near Newport.;)

I'm way da hell over in Western Mass. near Springfield. I'm a member of Ct Valley Region. This is my second year with my 69 and I did some autoX in 07and got hooked, so I'm already planning to hit Ft Devon this year.

There's a Rennlist group in eastern Mass. that head out evry few weeks doing weekend coffee runs. You part of that group?

Actually, the Westford that I live in is UP near Lowell and the NH border. Actually only about 30 minutes east of Devons. I'd like to give that a try some time, but we'll have to see how the family schedule allows.

Nope, I'm not part of the Rennlist group that you refer to.

- John

Derr..... If I spelled westford instead of westport, I would have googled the right location. 119 into NH looks like to would be fun run.:)

Spent some time reading last night as this subject of cams and intake sizes is interesting. In particular, the 906 engine of the 911R fame and the 3.0 RSR engine rev right up to 8k and have rather large intakes and cams with 282 degrees of duration and .465 lift.

When I look back through the data in Bruce Anderson's book, it's obvious the 34 mm intake during the early 80's was an anamoly. All said and done, I believe opening up the intakes to 39mm will give the high rpm power I'm looking for.

Here are the numbers I have. These are for the 39/35 early SC ports. I would need to rerun some numbers for the smaller ports.

http://forums.pelicanparts.com/uploa...1203173796.jpg

that is 75 hp at 6500 rps!!!!!!!!!!!!!!!!!!!!!!!!!!!!! HOLLY SH^T !! Can you guys summize what all the engine specs are to get this as there were a lot of configurations discussed and a lot to weed through for the not so quick (me). WOW!!!
Bob

Keep in mind that 6500 RPM is a rev range that few cars spend much time in. In all likelyhood you'll pull that RPM for a second immediately prior to shifting to the next gear.

To put it differently -- from 1000 to 4000 RPM the SC cammed engine will outperform the mod-S or GE40 engine. From 4000 RPM on up it's all Mod-S or GE40. If you're racing on a track, it's pretty common for an engine to spend 90% of the time between 4000 and 6500 RPM. That's much harder to do when driving on the street without drawing a lot of attention due to the noise, getting a ticket or having an accident.

For comparison here's a chart that compares the different torque curves for the different 2.0 liter configurations.

http://forums.pelicanparts.com/uploa...1203184373.jpg

I like your graphs, however, if both are pretty close to one another, I agree that the Sc is a bit better below 4k, HOWEVER, the mod s screams soo much more than the SC. Even if the mods is noticably, though it looks like minimal, most drivers of P cars go to red line 50+% of all gear shifts so tell me if I am missing something here as to why not go with the mod s set up?
***Also, please tell all the engine specs are of the engine... are we only talking cam here possibly?? I would bet there is a lot more different on these engines than a stk engine comparing diffreent cams... thus my question earlier.

Thanks! You have my interest way peaked here... now you all are going to tell me this is only on paper and that the engines never come close to performing this way in reality, right??
Bob

Bob,
I would have to say that the absolute numbers of peak HP and TRQ are suspect to accuracy but the most important thing is the shape of the torque curve itself.

From your posts I've read so far, you are building this engine for a guy who is a street warrior and will be revving the snot out of his engine. With trans gearing set up correctly, the mod S will be the way to go.

My torque curve looks a bit different than the stock SC that Jamie has posted. His example shows the engine making at least 235 ft/lbs from 3000k to 4800 RPM. With my PMO 40s and SSIs, 90% of maximum torque is available from 3k to 5.5k. My range is extended over stock SC, but it drops off just as fast after that. Compounding my issue is the small intakes. I reach peak HP at 5800 but after that I'm all done. I get more thrust by shifting up at 6k. With a 7.31 ring gear and 50 series tires, I spin up real fast and having some usable power from 6-7k is sometimes useful. ;)

Jamie,

I spent a few hours reading up on John's & Snowman's port testing. From your graph and what I've read, I'm thinking I might want to keep the ports small to keep low rpm intake velocity up. :confused:

The DC40 itself can keep power up longer than the SC profile and since I'm mostly street then AutoX, this should be a broad responsive power band without a big rush at the top.

I first ruled out a 3.2 SS as I thought this would be too much torque for a 901's first gear. With the small intakes and the DC40, this should be true. With the bigger ports, I'm not so sure. True?

I'm more intrigued with a 3.0 twinplug and higher compression pistons. As John suggests, the power band moves up vertically across the entire RPM range and with mild porting, perhaps this can offer similar low end performance to what I have now with more power in high RPM's.

As I think about it now, I want my cake and I want to eat it too. This SC cammed small port 3.0 I have now makes 90% of peak power from 3k+. More compression and twinplug add back a little low end PLUS better throttle response across the RPM range. No?

Be curious to see a torque curve of a small port 3.0 SC compared to a twin plug DC40 mild port.:D

Joe; To be honest we could be here for ages comparing different configurations. Especially if the problem isn't bounded in some way -- either by some set of rules, or capacity or something.

My chart pretty wells runs the gamut from mild to wild. You could make an analogy of a small port SC to the T cammed engined on my chart, and the GE40 to the S cam, and you wouldn't be far off from the differences in shape of the torque curves. In fact, Jamie's chart comparing the SC configuration to a Mod-S is most likely very close comparison to the 3.0SC vs. DC40 comparison. If you want to twin-plug and bump the CR of the DC40, you could just bump the torque line up vertically by a couple of % and there's your chart.

Bob, I don't disagree, but if they did do this on every up-shift, that still is maybe only 2% of the engine's running time that they're above 4000 RPM. How many dollars would they be spending to improve the performance of the engine maybe 2% of the time. The rest of the time the engine would be weaker then if they left it as it was since they'd be running the engine where's it's off cam.

Not to mention if you're going to build an engine that pulls from 5500 up to 7000 RPM, you'll need to look at your bottom end (rod bolts for example) to make sure that they're up to the abuse. That extra 500 RPM will also have a disproportionate negative impact on the engine's expected lifetime between rebuilds.

spent a few hours reading up on John's & Snowman's port testing. From your graph and what I've read, I'm thinking I might want to keep the ports small to keep low rpm intake velocity up.


This is the direction I was going with my earlier question. In trying to decide how to build my 3.0. The usable power on the bottom-end seemed most important, to me, as there is no track out here. Hard to find a reason, other than kicks, to build a twin-plug 3 or 3.2ss for the street. Jamie's graph looks pretty good to me, and beyond that, the opportunity costs seem to say 3.6 time.

What I thought :D

thanks John for your input.

Maybe I should just have a large printed clause at the bottom of my final build saying, "RACING AND OR EXCESSIVE HI REVING OF ENGINE will SHORTEN LIFE OF ENGINE SIGNIFICANTLY AND VOIDS ANY AND ALL WARRANTY!" I will also glue on those temp discs to be able to monitor if engine is abused thus overheated.

I have to build in some product obsolescense here or I will run out of customers, right? ;-)

Thanks guys! I will do some external stuff and let him learn from a "lightly tweaked stk SC" set up. And I will keep this in mind as the st step up to consider! Great thread!!

Bob