2.4T Cast Iron Cylinders and JE's

[Mark Sevin]

Am considering JE 9.5:1 85mm pistons in an early 2.4T. O.K. to bore out stock cast iron cylinders? What are best options?

[tcuuh]

Can't help with the answer but I am equally interested. I have a 73.5T (CIS) and just bought an E MFI to convert and want to use high compression pistons as well.

[jluetjen]

I've seen that the Byral (an iron sleeve cast into aluminum fins) cylinders can be bored out 1 mm to 85 mm without any issues, especially for street use. More then that and I've been told that the iron liner gets too thin and the cylinders are more likely to distort.

If you can go 1 mm over on Byral cylinders, I can't see why you can't go 1 mm over on cylinders which are completely iron. I'm planning on boring out the iron 2.4TK cylinders that I have to 85 mm.

[Ron,K]

I used this same combination of JE pistons (9.8:1 compression) and T cast iron cylinders. I bored mine to 86mm and have had no
problems after approx. 8,000 miles. I talked with numerous sources that indicated for street use, 86mm was acceptable. Racing might be a differnt story.

[tcuuh]

Thanks for the real world application Ron. The most I had heard of for an overbore was 85 but nice to know I can take it to 86. What cams and induction did you use? I just bought a 2.4E MFI system and was considering E cams.

Also, it doesn't look like you are using the special engine yoke (P201). It looks like you are doing this on the ground. That is what I was considering doing. At least for the teardown phase. Am I right that you don't have the yoke tool?

[Ron,K]

My engine was originally a '72 "T". I had the MFI pump rebuilt to
"S" spec, and had Elgin regrind the cams to their Modified S spec.
This cam has very close to the same duration of the Porsche S
cam but a little bit more lift. For now, I'm using the stock T throttle
bodies and plastic stacks. Long term plan is to find a set of S throttle bodies and stacks. I'm sure I'm losing some power on the upper end of the RPM range, but it still pulls very strong to
redline.

I didn't use the yoke when I was rebuilding the motor. If I were to do it again, I would invest in the yoke. As I approached the last stages of the engine reassembly, a standard engine stand with a 4 point mount stressed the bolts quite a bit. With a magnesium block, this is not good. I think the yoke would provide
more support and eliminate this concern.

I've been very pleased with this motor. Many have expressed concern over piston noise from the JE, but I've never had a problem. Oil consumption is moderate at 1qt/1200 miles.

[jchatfield]

I didn't know you had to bore out the T cylinders to accept JE's. Are not JE's also available 84mm?

I have a motormeister rebuild (previous owner) and it produces great power ( fly cut heads & Solex cams matched to Webers), but it has bad oil leaks on both cam housing to head mating surface. The fix is to reseal and since I thought this would be a good time to throw in new pistons. If I want a straightb swap, does that leave me with only used 2.2 E or S psitons to choose from?

[Tim Walsh]

jchatfield,
You can get JE's in just about any spec you can think of. I'm seriously consitering some 10.5:1 84mm JE's

[snowman]

TIp on cams for any T heads. The heads do not flow any more at higher lift, in other words, the lift adds to wear and nothing else.

The results of my own flow testing on these heads indicates that the biggest bang for the buck is duration, with nothing to be gained by lift, this is even true for ported heads.

By the way, except for minor porting, do not do anything else to the heads, eg go to S type ports, unless its a race engine. Only gains will be at high (6500 plus) rpms and you will loose low end torque. The stock heads will flow enough air to support much more power than the engine can otherwise produce.

Put another way you can do a whole lot of things to this engine to make more power and never butt up against the head flow as a limitation to the power. You can only make a whole lot more power with this engine by going to higher RPMs, much higher RPMs. The RPM limitation of this engine is the most significant limitation. I understand that about 8000 to 8500 RPM is max, any more and its history. A fundamental DESIGN limitation, ie rod and crank journal diameters. There is a maximum velocity that the oil can stay between the bearing surfaces and the journals, once that is exceeded you end up with metal to metal contact and sudden death. THe solution, change the stroke and rod and crank journal diameters, ie the basic engine design.

[iolguy]

Thanks Snowman I didn't realize that. I had purchased an E MFI unit and was looking at modifying slightly. As I understand it my "T" heads have 32 mm intake/exhaust ports, while the valves on all 911s of this vintage are the same (~41/46 I think). I noticed that the "E" heads of this time (1972) use the same ports as the "T" and only the "S" uses 36 mm ports. I was thinking of going to the middle ground and porting the heads to 34 mm.

As far as the MFI unit is concerned I measured the plastic velocity stacks and throttle bodies last night. The throttle body is ~31 mm on the engine side and 34 mm on the velocity stack side (if my memory serves me well). The velocity stack is 31 mm on the throttle body side and 38 mm at the top. Therefore, it seems like some mis-match between these different flow areas. I know that the bore on both the stack and TB is not straight to create a venturi effect. I was considering opening up the TB on the head side to 34 mm (to match my head porting) and port matching the mating surface of the body and stacks.

Any thoughts?

[neilca]

Snowman,
I agree with you on the lift theory. At what lift have you found in your flow testing does the benefit drop off? I am trying to decide how much lift is required.

Thanks,
neilca

[Tim Walsh]

I'm having the same thing done to my throttle bodies right now. I have a set of T throttle bodies opened up to match my E heads. You can have them sent to eurometrix and they'll come back looking new. I'm just having them bored to 32mm since my butterflies are tight.

[iolguy]

I talked to Eurometrix about boring the throttle bodies but it seems pretty expensive to me, I am pretty cheap though. I have access to a machine shop at work and was thinking of having them help me (they work on medical optics therefore are used to sub-micron precision). I could probably get custom throttle plates, etc. Are the "E" stacks and TB different size than T? My measurements for the TB seems to match a T but I know the MFI unit came from an E (p/n matched Tech Article in PP).

[Tim Walsh]

I've done some comparison and the only difference between the two is the size of the manifold side. The T's are smaller than the E's but I don't remember by how much

[snowman]

Originally posted by neilca
Snowman,
I agree with you on the lift theory. At what lift have you found in your flow testing does the benefit drop off? I am trying to decide how much lift is required.

Thanks,
neilca

If I remember correctly its about 0.375" lift where it flattens out, pretty dramatically.

This dosen't sound much like a performance engine, unless you see how much the flow is at very low lift, its tremendous! and thats where it counts. The more air you can flow at low lifts adds more to the overall flow, much much more than the very brief peak flow at the highest lifts. For you math types, its the TOTAL area under the curve that counts, not the peak.

The numbers I have measured are as follows:

Stock 911T w 46MM intakes 42MM exhaust
intake
lift flow (cfm at 28" )
0.1 " 60
0.2 " 121
0.3 " 174
0.35" 192
0.40" 200
0.45" 200

exhaust
0.1 " 52
0.2 " 99
0.3 " 129
0.35" 142
0.40" 149

[neilca]

Thanks Jack,

That is some good data. For those who haven't given this much thought, a lower lift on the cam means less problems with piston to valve clearence, less spring pressure on the nose of the cam (less wiped out cams) , fewer broken valve springs and higher revs with less spring pressure. High lift cams working are an old wives tale. Another on is long intake duration cams. Nowadays it has been proven that the exhaust valve should have the longest duration. Basically you can't have flow without an exit.

Today is a good day.. I learned something!
neilca

[jluetjen]

Another on is long intake duration cams. Nowadays it has been proven that the exhaust valve should have the longest duration. Basically you can't have flow without an exit.

Hi neilca;
That's an interesting conclusion and contrary to all of the information that I've found for normally asperated motors. Basically my understanding is that exhaust gasses have so much energy that they push themselves out of the cylinder, sometimes even bypassing head gaskets to do it. Let's keep in mind that exhaust gasses even have enough energy to spin a turbo and compress the intake charge. In many cases designers have gone with smaller exhaust valves in order to fit larger intake valves.

Could you say some more about how you came to that conclusion?

[neilca]

John,
Though the exhaust gases are expanding it's the piston that is pushing them out particularly at the end of the exhaust stroke. The long duration exhaust helps to improve the volumetric efficency of the engine. That is the complete evacuation of the exhaust gases after each cycle. This leaves a clear path for a fresh charge of intake. The header plays a role here too. If properly designed the header will also help evacuate the cylinder of spent gasses.

I was also taught that the exhaust valve is smaller due the idea that there is less mass after combustion than before. If you think of the internal combustion engine as an air pump, where the intake has a portion of solid mass (gasoline) that is converted into a gas during combustion then there is some validity to the smaller valve. But it still important to evacuate that gas once converted.

The turbo motor is a comletely different case from a naturally aspirated motor because you are blowing the gases through the motor. It is not unusual to see turbo motors with a volumetric efficiency in excess 110%.This is because the turbo not only blows out all the exhaust but also some of the new intake. A good example of this is a top fuel car. At night you can see spectacular flames from the exhaust. You will also notice that turbo headers are smaller than naturally aspirated motors in order to keep the exhaust velocities high enough to spin the turbo.

Hope that helps,
neilca

[iolguy]

Thanks for the data Snowman. I have a question about the measurement technique though. A flow bench as I understand it is a large vacuum pulling air through a chamber. You can adjust the lift of the valve to see how it's travel impedes air flow. But to me that seems to be a static measurement. What I mean is that at low RPM these numbers are valid but as the RPMs increase I am not sure. Although most consider that air does not have a mass it really does on a microscopic level. To start it flowing in a direction (into intake or out of exhaust) there is a time lag (although maybe on the order of microseconds it could be valid). Therefore, higher lift may be important at higher RPMs to allow flow dynamics to occur. I will see if I can consult a friend who develops fluid dynamic software. That is really what we are talking about here fluid dynamics. At least in my mind. I guess it also depends upon where you want your engine to run best low-mid RPM or mid-high RPM. I would think the guys at Web-cam could help answer this question.

[jluetjen]

neilca;
Thanks. I understand your words but your discription of the ways and whys things are happening is different then my understanding of the situation. I'm not sure that I necessarily agree with your conclusions. For example how can the piston be the primary cause of the exhaust gasses exiting at the end of the piston stroke. At the end of the piston strone the piston has comparatively little speed.

Wouldn't it be more accurate to say that the inertia of the exhaust gasses exiting through the exhaust valves pull the remaining exhaust gasses out of the cylinder, and will even begin to suck some of the intake charge into the cylinder during valve overlap. Ideally the intake stroke will pull most of this intake charge back into the cylinder, but if your exhaust valve stays open too long you will also pull in exhaust gasses and reduce the exhaust velocity in the exhaust ports?

Also you describe that the mass inside the cylinder is less because the gasoline's solid mass is converted into a gas. Mass is mass. Admittedly the exhaust should have less mass then the intake because some of the mass has been converted into thermal energy which is used to drive the car. Unfortunately a lot of that thermal energy also goes out the exhaust pipe as heat. But the remaining mass is just that mass. It has inertia and it doesn't matter if the mass is gas or solid, the inertia depends on the mass and the velocity.

I guess my point is that you shouldn't have to evacuate the exhaust gasses. For the most part they will evacuate themselves (given the opportunity of an open exhaust valve) until the pressure inside they cylinder reaches ambiant. The amount of time that it will take this to happen will depend on the valve size and the flow potential of the exhaust ports and system. But in general it will take less time then

As far as the exhaust gasses which are remaining at ambiant, you will need to both pull them out with the aforementioned exhaust inertia and push them out with the incoming intake charge.

Here's a very interesting link to users manual for Engine Analyser Pro. If you look at page 132; there is a chart that graphs cylinder pressures, intake and exhaust pressures. Note that as long as the cylinder pressure is above the exhaust port pressure (and the exhaust valve is open), that the cylinder contents will flow into the exhaust. This is even happening when the piston is at TDC according to this graph. By the same token, as long as the intake port's pressures are above the cylinder pressure, mixture will flow into the cylinder.