Cam options for a modified 2.4E engine?

[blau911]

Wayne,
I had a 2.4E engine rebuilt a few months ago which is currently in my 67 911. It was rebuilt using the following:

- Weber IDA40's with ported 39mm intake manifolds
- 2.4e pistons and cylinders
- flycut 2.4S heads ported and polished (ports were opened up from 32i/32e to 39i/36e).
- Webcam mild race cams. The grind is 120/104. Lift in = .476
Lift out = .450
Duration in @ .050 = 264
Duration out @ .050 = 248

The car runs like the wind once the rev's go above 2,500 rpm's and pulls all the way up thru 7k rpm's. However, the car is very sluggish from a dead stop in 1st gear until I get above 2k rpm. As a result, I am wondering if the primary loss of my low end torque is due to over aggressive cams, or the large increase in porting - or a combo of both.
I had planned to use the original E cams. However, they needed to be re-ground. A local race shop I deal with had these fresh 120/104 cams sitting around and offered me a good deal for them. I went with them instead of having my E cams re-ground.
Webcam describes these cams as hot street/mild race cams. I have another set of Solex cams and a pair of stock S cams. I am wondering if the low end problem would be corrected simply by going with different profile cams, or if the problem has more to do with the larger porting and not the cams.
The question: What is the co-relation between port sizes and cam profiles, and is there some way I can calculate the amount of add'l low end torque I would get by switching out the current high profile cams with Solex or S cams before making the switch?

Thanks

P.S.
It was very interesting to read in your book how much of a decrease in engine temperature can be obtained simply by cutting off an inch from the inner air deflectors. The 10-15 degree reduction seems to be a much cheaper alternative than dropping big $$$ on a front mounted oil cooler. My engine temps hovers around 220-225 on a warm day. I was planning on going with a front oil cooler set-up, but am excited about trying this first to see if I can avoid having to spend the extra $$$. Have you actually done this modification yourself and seen a 10-15 degree reduction?

[jluetjen]

Blau911;
If you don't mind I'll share my $0.02 based on some calculations that I've done.

As a result, I am wondering if the primary loss of my low end torque is due to over aggressive cams, or the large increase in porting - or a combo of both.

* Your Cams: According to my numbers, your 120/104 cams are not a "Hot Street/Mild Race" cam -- at least if you don't consider more lift and duration then a 906 cam to be "Hot Street/Mild Race". Porsche didn't even bother plotting the torque on their 906 spec motors until 4500 RPM and generated peak torque at 5500 RPM and peak HP at 7800-8000 RPM which gives you a clue as to how radical they are. Do you know what the cam lobe separation number is on your cams? If the lobe angle is wide (thus reducing the overlap) it might help a little bit with the low speed performance.

* Your Porting: 38mm is what Porsche used in their full-race 2.5ST's which used the aforementioned 906 cams and rev'd to 8500 RPM. In general all of the carb'd or MFI'd 911 engines that I've checked (which is most of them) had a peak torque intake gas speed 60 to 80 meters per second. This includes the racing models. Your engine's intake ports are larger still at 39mm's. Assuming that your engine also develops its peak torque at about 5500 RPM, your peak intake gas speed is about 56 m/s. Too slow of an intake gas speed can result in the fuel actually dropping out of suspension in the air which will result in poor combustion.

So my answer to your question is YES. Your problem appears to be a result of too wild cams and too large ports. Putting a set of normal S heads (36 mm ports) or T/E heads (32 mm ports) will most likely move your torque peak down some and help the low rev. running. The engine will still most likely be pretty weak off of an idle, but you might be able to have it start to come to life at 2000 RPM rather then 2500 RPM. I'd recommend the S heads since in a 2.4 the 32mm T/E heads are pretty tight with the intake porting unless you are running a later E cam or a T cam.

Using your cam with S heads would be similar in philosophy to some of the early 2.0 rally cars that used 906 cams with stock T/E heads with 32 mm ports. The data on this configuration is pretty sparse, but I suspect that once you were moving these cars were pretty hot. There's a fellow pelican who lives a couple of miles from me who apparently has this configuration, it would be interesting to see how it runs.

Changing to a normal S or an E/Solex cam will also help, but the large ports will still most likely make the engine a bit rough at low rev's. The porting and the cams need to work together to deliver a certain level of performance. Having one seriously un-optomised will certainly be an improvement from having them both be out of whack, it will never be as good as a properly designed configuration.

The last variable that might make a difference is your carb settings. What sort of venturi are you running in your carbs?

If you ask me it sounds like the engine shop sold you an engine that wasn't appropriate to your application.

[blau911]

Hi John,
I am using 34mm venturis.
I was reading Wayne's 911 rebuild book, and in the section on cylinder heads on page 89 he writes:

"So why not just enlarge the port diameter for improved airflow? Well, you can. This is exactly what the Porsche factory did on their 911S model and racing engines. For example, the intake port diameter of a 2.0 liter 1969 911T is 32mm. The 1969 911S has a 36mm intake port, and the racing 906 engine (also a 2.0 liter) has a 388mm port. Power ratings for those engines are 110, 170, and 210DIN hp respectively. The problem here is that the large ports slow down the incoming air-fuel charge, so the cylinders fill less efficiently at low rpm's, resulting in poor torque at low engine speeds. At high rpm, the intake has greater momentum, and the open ports offer an advantage in reduced resriction. The result is higher horse power at high engine speeds. The 906 engine produces maximum torque at 6,200, a full 2,000 rpm above the 911 T's torque peak."

Despite the above comments, I must emphasize that the engine has very good mid range power as well as top end power once I get rolling in 1st gear. In fact, I have had the pleasure of owning and driving my share of early 911S's, and this engine would easily run with the best of them. Furthermore, since it is not my grocery getter, the low end torque problem only becomes and issue when I get caught in heavy city driving or highway gridlock, which is rarely. I am merely hoping to make more of a minor adjustment, rather than rebuilding my top end with smaller ports. Of course switching to milder cams would be my prefered option if it helped out on the low end without giving away much of the power up top. Another possibilty might be to go with smaller intake manifolds. If I reduced air flow at the intake manifold to 37mm, wouldn't it effectively reduce airflow into the head intake ports in the same way as closing up the intake ports themselves? Just a thought.

[Chas911]

Blau911: I think John's assesment is right on. But, before you go changing parts, I would try improving your low rpms by advancing the timing if you havn't already. Hot cams tend to like more initial advance. Be careful though not to get to much total advance. Total advance should probably not be any more than 34 degrees. Your initial advance could probably be around 10-14 degrees. You may need to mess with the curve to get these numbers, and if it starts detonating retard it. With the right advance you may find the soft spot is tolerable.

Chuck

[jluetjen]

Chuck;
My concern with what you describe is that when the engine comes on cam Blau will have too much advance and melt a piston. BTDT. I think that in order to make your idea work that Blau will need to recurve the distributor as well as advance it.

Blau;
As far as reducing the manifold diameter rather then the ports, it's worth a try. It would certainly increase the gas speed through a portion of your intake. It might work or it might just choke off your high RPM HP. I don't have the data to say.

[dtw]

Hijack:
John, I'm looking at a dilemma on an engine I'm building right now. This is not the 10.5:1 monster I've previously discussed, but a warmed-over "small budget" T motor. I've got Crane 288 cams, 2.2T pistons which have been relieved to deal with the wild cam and shaved heads. Compression is calculated at 9.5:1.

Issue: If the T heads really are 32mm on the intake, then I calculate I am going to just squeak by on intake speed at 100 m/s. However, if they are actually 29mm as some have reported, it looks as though I may be outside the recommended speed range at ~120 m/s. What will the potential impact of this situation be? I'm loath to spend additional money modifying this engine, since this was supposed to be my budget rebuild (yeahhh right).

[TimT]

You could change the cam timing to move the power down the rpms a bit.

However your talking about a small displacement engine. There isnt going to be alot of "umph" below 2500rpm. I dont know the application you intend to use this engine but if its a street engine you may have to live with a flat spot till the car comes on cam.

If its a race motor, once you get rolling just dont let the revs drop... shift at redline, dont let the revs drop below 4000 and have your gearbox set up to accomodate..

That isnt a really wild aggresive cam that you have.. .450 lift isnt extreme.

John has analyzed tons of cam and port combos... but try as you might your not going to get serious low end grunt out of a small displacement engine...

[cstreit]

JOhn,

I'll second your notion on those cams....

WebCam DOES sell them as a mild street/club race cam, but I also run them in my car and they do NOT make power untill 4500 RPM. Perhaps their idea of a mild cam is different, but I think an agressive street cam should make power at 3k RPM at least. THere's plenty left at 7000 RPM...

[jluetjen]

Blau911 -- Did they also by chance sell you a lightened flywheel? If they did, going back to a stock flywheel may help to keep the motor from fluffing when you release the clutch from a dead stop.

DTW -What year are your heads? They will generally have a month/year cast inside an oval on the bottom (cylinder) face of the head, on the intake side. BK911 has posted some head flow data and it looks like the bigger ports will (among other things) flow more at maximum lift, but don't have as much affect below that. He checked a 2.4TK head that I had with 30mm intake ports and they really seemed to max out at about .35 - .4 inches of lift. We don't have any 32 mm T/E heads to test so I don't have any data on that version. (If anyone wants to share a head to be flowed, BK911 has been posting the results.) Based on the data that we've gotten so far, I suspect that smaller port heads may not respond to the extra lift, but will still respond to the longer duration. I'm not familiar with the Crane 288 cams, what are the lift and duration?

As far as your ports, it's a small matter to get a metric ruler and measure your ports (assuming manifolds have been removed. You can also measure it in inches and multiply by 25.4.

[dtw]

John,
They are '72 heads. I'm not sure of the exact casting date; they are at the machine shop right now. I was scheduled to pick them up Saturday, but that'll likely be delayed if it is determined they should be ported. I left a message at the shop earlier this morning asking them to measure the ports and call me back - haven't heard anything.

Here's a link to the Crane cams data: http://www.cranecams.com/master/apps/porsche.htm
The lift is .462; duration is 288 degrees (!).

I checked my macho-man heads and they are ported out to 40mm. That is more than I want to run on this motor so I will save those for the hotrod. Basically, if 32mm or even 29mm will allow me to realize most of the benefits of the motor's setup, I'll probably run them as is. I just don't have enough data or experience to know...

[jluetjen]

DTW;
I've updated my data based on your link (I didn't have all of that data previously) and it looks like the Crane 288's are somewhere between an early E cam (aka Solex cam) and a GE40. I suspect that they will be fine with 32 mm ports assuming that your engine is a 2.2. Even with a 2.4 they will most likely be OK but you won't have all of the peak RPM HP that you might have with larger ports, but I doubt anything drastic. The peak HP will be about the same in either case.

If you have a couple of weeks between when you get them back and when you're going to be putting them on your motor, maybe we can have one flowed and add to our database.

[dtw]

John: The engine is in fact a 2.4. My calculations bore out that 32mm would work, but I'm more concerned if they end up being 29s. Your thoughts on 29?

Flowing the heads: I assume you want to do it on your bench so you can replicate testing conditions? I would be willing to do that if you could return it to me within a couple weeks, no problem. The motor is indeed going together soon though - I've got everything ready to go.

There are two heads I could send you: the head currently in question, stock 2.4T head freshly rebuilt with race valve grind (not sure what he means by that, I assume it is just a good triple-angle cut), factory port size but "cleaned-up" ports. You're also welcome to flow the massaged heads awaiting my hotrod motor. Formerly '72 2.4E heads, now ported to 40mm intake and very clean ports.

[jluetjen]

I'll get back to you...

[blau911]

Hi John,
Thanks to you and the other PP posters for their comments.
To answer your flywheel question, I stayed with the stock flywheel.
As for confirming that the peak hp comes at above 7k rev's using my current cams, I thought it was just my giddy imagination which made me feel like the engine had more to give at 7k. Turns out that it does!

I will probably end up switching out to a lower profile cam and go with the s grind so the car is a little more manageable around town - if I can bring myself to open up the top end again. The fact is the engine idles well. It's just that it's sluggish until I get to 2k rpms. Otherwise the car is a blast. I guess I want my cake and eat it too...
Just in curiousity, I was wondering if having the engine currently mated to the early 901 gearbox in my 67 911 actually helps deal with the low end torque problem. Eventually I was planning to put the engine back into my 2.4E body with the stock 915. That will happen when I am done rebuilding my 2.0S engine. Doesn't the 915 have longer gears? If so, perhaps that will aggravate the low end torque problem further, and thereby necessitae the change to a lower profile cam - maybe even Solex instead of the S cams. Any thoughts?

Also, I was still hoping for Wayne to offer his thoughts/insights.

[blau911]

By the way, while looking at the engine/model data on page 201 of Wayne's 911 engine rebuild book, I noticed that all of the heads are the same across the board for 2.4 T,E and S engines (46i/40e valves). exept for the port sizes. Better breathing thru large intake/exhaust ports and higher compression pistons appear to be the difference between the 140 hp 911T and 190 hp 911S (putting aside cam profiles and fuel injection set-ups). Is that right? If so, is it possible to mathematically calculate exactly how much additional hp can be specifically attributed to larger intake ports?

[jluetjen]

Is that right? If so, is it possible to mathematically calculate exactly how much additional hp can be specifically attributed to larger intake ports?

Not really, since my belief is that it is the cam (and engine capacity) that really determines the HP as long as the head(s) can flow enough to support it. As long as the heads' flow is within an acceptable range, the cam will do its job and develop the desired torque at the desired RPM.

As far as the transaxle is concerned, it really all depends on the ratios. True the 915 box is beefier, and also has more drag, but it is most likely pretty trivial compared to other factors.

[dtw]

Just got off the phone with the machine shop. The ports are indeed 32mm. This is excellent news. The shop also agrees that this should be enough port to run the aforementioned pistons and cams. Sooo big sigh of relief here. My "cheap" engine is still somewhat cheap. Somehow I've gotten to nearly $1,600 in machining and custom work on this "bone stock" 2.4T rebuild. Oh what the hell...I'll be counting the days now till I can actually crank this engine.

John, tell us more about your database? What format is it in, what've you got in it, etc. Have you got it linked to any spreadsheets/calculations? Using your gas speed formula I have created my own little spreadsheet that has proven very useful for determining intake gas speed or backing into a required port size.

[Tyson Schmidt]

Dave, sounds like you have 2.4 T heads from an ROW engine. The U.S. Mechanically injected 2.4T motors had the 29mm intake ports. My guess is that they needed to detune the MFI T motors for a lower, fuller torque curve and to better match the T cams. The ROW cars had carbs, and 27mm venturis to choke them down, so they had the 32mm ports like the E.

But even if you had the 29mm ports, it is actually really easy to open them up to 32mm yourself. It takes a little skill, but all you are doing is removing a bottleneck, so it's not like a real port job. The bottleneck only goes in an inch or so, and all that is needed is to port match with the intake gaskets that have a 32mm opening. I used a socket that had a 32mm O.D. to help guide me in finding any high spots and to help keep the port round. Any bigger than 32mm, and you'd really need to know what you're doing, since the porting would need to go deeper into the head.

[Tyson Schmidt]

Oh, and Blau911, those low compression 2.4 E pistons aren't exactly helping either. Even if you flycut 1mm from the heads, and didn't space the cylinders to compensate, that's still only 9/1 at best. Big ports and wild cams like high compression to help fill in the bottom of the torque curve.

You can get away with higher compression on an angine with high overlap cams, since the overlap acts like built-in EGR. Some of the inert exhaust gas gets left in the cylinder due to the overlap, and this results in lower combustion temps since a portion of the mix is incombustible. This helps fight detonation.

[Bobboloo]

Dave,

Also wanted to point out that the 2.4T MFI heads being 29mm intake as Tyson pointed out are also 32mm on the exhaust side. Just incase your machinist picked one up and only measured one side.

Blau,

It's to bad your mechanic led you into a race motor for a street car because a stock 2.4E is a great street motor.


Have you considered going with a set of 90mm RS P+C's? Increasing displacement will bring the torque curve down without sacrificing the HP. Better yet get a used set of 90mm cylinders bore them out to 92mm and Nikasil plate them so you could put in a set of JE 9.8:1 pistons. This will give you a 2.8 motor. This should bring back the low end torque and give you even more top end.