Rebuilding 2.7 engine

[911s]

Thanks, John. Ive been out of the car arena for some time, but what you've said is slowly starting to sink in. Now, if the 2.7 heads are similar to the 2.4, with an increase to displacement only, then the 2.7 should have better air velocity entering the engine (air has to move faster through the same port size in order to fill a larger volume). When I used to race, we were always after air velocity when porting out heads. The increase in velocity seemed to affect our low rpm operation, with total flow affecting high rpm operation. Now if this velocity is already built into a 2.7, then that would explain why an S cam seems to have more torque in it, then it would in a smaller engine (plus the torque that comes from the increase in displacement).

What I'm really after is strong pull from 3-6k. Here in Hawaii, we're on an island, and we don't have the long stretches of road that is available on the continental US. That and the fact that I am a proponent of the saying that speed kills, and one can begin to understand why I'm trying to set up my motor in this way. What I'm after is hard acceleration in the lower rpm range (so that I can keep up with the Acuras and Nissans that abundant this part of the States').

So, I'm going to ask one more time. What rpm range can I expect the E cam to operate in? If it tops out at 6K, that's fine. On most of the roads I frequent, you'd look like a maniac if you were winding your motor that high, anyway. As long as it has the 'pull' to compete at 3,4,& 5 k, that is fine for my requirements. If I need more at some point in time, then I'll consider the S cams then. This may sound crazy, but I'll tell you, I am originally from Kauai (one of the smaller island's in the chain), and compared to there, this place (Oahu - Honolulu) is miles ahead in the way of roads. I saw a clip on another guy that has a 930 on Kauai, and there's only one place on the whole Island where he can open that thing up (al be it, it's one neat stretch which goes from Anahola out to Kilauea). Maybe, he can find a few other places out towards Princeville, but the roads there get kind of hairy, and you never know when some idiot in the other lane will fall asleep and cross the centerline (it's all two-way roads there). Must get kind of frustrating for him sometimes, especially since the roads there aren't in the best shape. Hey guy, I sympathize with you, man. I have family there, but there's little chance that I'll ever move back there, just because there's nowhere for me to run my car.

Anyway, this thread is beginning to wear itself out. I'll check back to see what kind of responses I get, but I think I've asked enough questions already, that I kind of get the picture on what my options are for this engine. Thanks for all the input, and I'll reply to any specifics that haven't already been addressed.

Thanks, Pelicanites!

Bob.

[jluetjen]

Bob;
Maybe the following will help. I've plotted out the BMEP for 2.0 engines using different cams and induction systems. I've used the 2.0's because I could find published information for the widest variaty of cams.



Here is the data. Note that I've highlighted in red where each cam tends to be strongest compared to the others. In the case of the E/'67 and the MFI/Non-MFI'd engines, I've grouped them. You can see the improvement that the MFI provides at peak rev's as well as at lower rev's too (Take THAT SCWDP! )



BTW: BMEP (Break Mean Effective Pressure) is directly proportional to torque, but it is independant of engine size. So it makes comparisons between different sized engines easier. As long as you maintain adequate porting and maintain the stock timing, you could expect these cams to have similar rev ranges in the larger engines.

PS: For some reason it didn't capture that the 906's had 38 mm intake ports. I also highlighted the S's at 4500 RPM when in fact the E cam is better at that point. Oops.

[J P Stein]

I've driven 3 2.7L cars.

73T with CIS, A 68 (?) SWB with (stock 2.7L but with Webers), and my 914.

The 68 had the same lineal power delivery, but the throtttle response was much improved over the 73. The 67 was quicker. It may have been due to the lighter weight. My WAG at power improvement is....yes.

In any case, it is no contest twixt the 68 and the 914. The weights are prolly close to the same...with the 914 being maybe 50-100 lbs lighter . The hot rod motor has 40-50 hp (butt dyno)
more urge.

I agree with John's assesment of "peakyness". The trick is to get the bigger cams to run smoothly at low rpms in any size motor.
The cams overlap is still there, just disguised by more torque in the larger ones. Mine was prretty herkey/jerkey when I started out.


BTW, my set for the carbs is right out of BA's book:
34mm Venturies, f3 tubes, 135 mains, 145 air correction....I also use 60 idle jets.

[sleeping_beast]

Originally posted by 911pcars
Compared with E cams, S cams are more "peaky". A "peaky" cam is usually described as one having longer duration, increased overlap cam timing (both valves open at TDC exhaust/intake) . This results in the intake and exhaust gases mixing at idle/low speed and an engine that has a characteristic loping idle, relatively little torque at low-to-mid engine speeds and poor fuel mileage.

Hum... that is interesting.

I have no idea of what I have in my engine. However, I did notice relatively little torque at low-to-mid engine speeds (it really kicks in at 3500 rpm and above) and poor fuel mileage (I just measured 12.3 miles/gallon in city driving, between 16 and 17 miles/gallon in highway driving). Don't know if my idle is the characteristic loping idle of an engine with S cams.

Is there an easy way to figure out the cams I have? What do you think about my fuel mileage?

Thanks,

Alain

[jluetjen]

Alain;
Some of the charactoristics mentioned (lack of torque at low rev's, loping idle, poor milage) could also just be the affect of an engine out of tune. If your cam's come on strong at 3500, you don't have S cams. Especially if you still have the stock CIS system. Based on that number alone I would suspect that you have the run-of-the-mill 2.7 CIS cams. The other things that can pull down your milage is to convert from the CIS to carbs, especially with a low CR engine.

[sleeping_beast]

jluetjen,

Thanks for your input. So... if we assume I have the stock CIS system on my '75 Targa, what should be my fuel mileage?

Thanks,

Alain

[JTO]

Alain,

I have a recently rebuild 75 S. It is slightly modified but runs CIS and 911 S cams. i get 22 mpg combined, with quite a bit of spirited driving. I think the mileage is really good considering the performance of the car.

Troy

[911s]

John,
From the charts you've provided (primarily the 2nd one, which provides actual BMEP numbers at specific RPMs), it appears that the E cam has the flattest torque between 2-6k. As long as I can reasonably assume that those numbers will translate in a 2.7, that is the cam that I would desire. I would think that, all else being equal, the cam should make any engine perform similarly (ie: operate in a specific RPM range), although with more power at every point, given it will be in an engine with a larger displacement. Does that sound reasonable?

Troy, does your engine have the CIS cams that came with the engine, or is it an early S cam, like those found in the MFI engines? In my current ride (77' 2.7S, unmodified - except for early heat exchangers and pop-off valve), I get a little more low end kick out of the engine by bumping the timing up a few degrees (idles at 1200 with a/c off). Haven't noticed any increase in operating temp, so I leave it there.

[911s]

Hey John,
Do you have that BMEP info for the CIS engine? Interested in seeing what it does in the 3-6k range. CIS all I have driven, to this point, so at least I'll have something to compare it with when deciding what direction to go (afterall, no sense in bothering if the performance of the CIS is comprable). I have to believe that the 74-77 CIS has to be inferior, as the advertised hp numbers for those years are less than the earlier MFI and/or carbureted engines.

[jluetjen]

Bob (911S);
It's a small matter to calculate the BMEP of an engine if you have the HP and or torque graphs. Use this formula:

BMEP =(Power * 13000)/((Capacity in liters) * RPM)

To be honest I haven't spent a lot of time analysing the CIS engines since I don't have one. Generally I've seen the following ranges...

T Cam: 127 - 144 PSI
E Cam: 140 - 158 PSI
S Cam: 160 - 165 PSI (Note this is early S, not 2.7S)
TK Cam: 136 - 154 PSI
2.7 CIS: 127 - 156 PSI
SC: 152 - 162 PSI

Generally the higher number occurs at the peak torque engine speed while the lower number occurs at the peak HP engine speed.

Generally the CIS injection is fairly capable of metering fuel to the engine, and so the peak torque generated by those engines isn't too bad. There are two big handicaps with the CIS system.

1) It is a fairly substantial obstruction to the intake system, much like a butterfly that's partly closed. As a result they seem to be generating a noticable vacuum at higher rev's which pulls down the cylinder pressures and thus drops the BMEP's and the resulting HP. (BTW - anyone with a CIS engine and a vacuum guage want to test this?)

2) The meter doesn't like to see pulses in the intake system, which means that you can't use cams with alot if any overlap, which also handicaps the generation of peak HP numbers.

[69911e]

Has anyone compared E vs S cams with the same induction system and heads. In B Andersons book the 2.7 E spec motor with carbs put out almost the same HP as the RS spec(if I remember right less by 10 HP). But it had carbs; some people say MFI puts out about 10HP more than carbs because the airflow is better. It would be interesting to find somone who has the same motor dynoed with E and S cams.

[jluetjen]

Ed; Didn't I plot that out above? The MFI'd E has the x points and the MFI'd S the brown dots. Also the '67 2.0 had pretty much the same cam and configuration as the 2.0 E except for the MFI, so that's a pretty good comparison of the value of MFI. You can also compare the Carb'd S to the MFI'd S. The only glitch there is that I only have partial information available for the 2.0S.

If anyone has the factory HP chart for the 2.0S with MFI, I'd love to update to the data.

BTW - I've also plotted out the 901/20 (906 engine) and the 901/21 (906 with MFI) and the benefits of the MFI were pretty much the same, right down to the incremental 10 HP.

[69911e]

John: This thread started with a 2.7 motor and suggested modifications. I was trying to make a camshaft comparison with all else being equal. It looks like the 'S' has 36mm ports, and the 'E' has 32mm ports. This port difference would probably lower the highRPM torque with any cam. I don't think Porsche ever put an E cam in a 36mm intake port motor, therefore it would need to be an aftermarket test. If you have Andersons book(page131old book; I can email if you don't) there is The "RS" vs 2.7 rs motor with E cams. It looks like 5hp peek less for the E cams(@ 500rpm lower peek), but it is using carbs. Looks like both use 8.5/1CR and 36mm ports. The other variable I see is the sport muffler on the E motor vs an unknown muffler for the RS motor.
PS: I respect your analytical approach

[steve911sc]

I am a new guy...just regestered. Been following the discussion.
One thing not mentioned....before machining the CIS pistons, make sure there isn't too much side clearance in the ring grooves. I have had to discard a couple of sets of used p&c because of excessive side clearance. Just my $0.02 worth
Lots of great information by all

[jluetjen]

OK Ed, Now I understand your question. The only examples that I know of that might help are these, and the data is pretty sketchy.

901/02 (Carb'd 2.0S), Ports: 36/35, CR 9.8:1, 160 HP @ 6600 RPM, 132 lb/ft at 5200 RPM.

901/10 (MFI'd 2.0S), Ports: 36/35, CR 9.9:1, 170 HP @ 6500 RPM, 134 lb/ft at 5500 RPM.

901/26 ('70 914 Rally 2.0 w/ S cams, 40 IDA's), Ports: 32/32, CR 9.9:1, 180 HP @ 6800 RPM, 132 lb/ft @ 5200 RPM. I can't find any other data about this engine.

901/30 ('67 911 Rally 2.0 w/ S cams, 46 IDA's), Ports: 32/32, CR 9.8:1, 150 HP @ ???? RPM, ??? lb/ft @ ???? RPM. According to Boschen and Barth, this engine hand polished intake ports, platinum plugs, a different exhaust muffler, free venting of the oil tanke, a lightened flywheel, and a different alternator. They report 170 HP at 7300 RPM and 134 lb/ft at 5200 RPM.

If you map out all the different engine configurations and the intake gas speeds through the ports, you see that there is a very clear range of 73 m/s to 100 m/s at peak HP that all of the factory cars fell into, including the race cars. The 2.4 E pretty well defined the high limit at almost 100 m/s, the the 2.7RS was not far behind at 92 m/s. The S's all were in the 73-83 m/s range for comparison. Looking at the data closely, it also seems like smaller engines can tolerate slightly higher intake speeds then the larger engines. I've posted graphs of the results, do a search under my username and "porting" and you'll find them.

It's not an answer to your question, but it is as far as I've been able to figure out so far. BTW - Thanks for the offer, but I already have a copy of BA's book. That's where I pulled out most of the data I listed above.

Welcome aboard Steve911SC!