95mm to 98mm and one full point of compression H.P. Torque Increase questions

[joetiii]

Tweaks 911......

Originally Posted by jpnovak

Realistically, 98mm PCs will not like to be single plugged due to combustion chamber shape, piston dome and flame front propagation.

I know there was a Max Moritz short stroke w/ 98mm jugs that was done to 3.0s over in Europe where gas was ahigher octane. Possibly a 9.0:1 98mm would be safer than the 9.5:1??? There are those with more knowledge and experience than I.

Though I am weighing the same choice, twin plug is a bit more affordable since I already have an electromotive crank fire ignition that only needs 3 more coil packs and wires.

[jmz]

Interesting graphs. I would expect the h.p. increase when bumping the 3.0 to 3.2 high compression to be larger.

I'm hoping to add ~40 more h.p. and a nice bump in torque.

[jluetjen]

To be honest, to do that you'll most likely need to change the porting, and maybe the cams.

[jmz]

The heads have already had some port work done to them by Walt at C.E. He opened them up to a bit larger than the 78-79 heads and smoothed them to increase flow. I suppose that a step up from the Mod S. cam could be in order...

[jluetjen]

Could you measure the ID at the smallest part of the intake port? This will help me to quantity the impact a little bit better.

[jmz]

sure, I'll take a look at them sooner than later.

[shbop]

sign-up

[jmz]

Originally Posted by jluetjen

Could you measure the ID at the smallest part of the intake port? This will help me to quantity the impact a little bit better.

Intake ports are a tad over 38mm. They have been ported by Walt at CE, the bump around the valve was smoothed as well as the entire port to an extent.

[jluetjen]

Looking at my numbers, your engine with 98 mm cylinders with 38 mm ports -- I'd be surprised if your HP peaks above 6200 RPM, so 260 HP looks about right. I'd expect your torque to be peaking at around 4300-4400 RPM.

[Eagledriver]

For some reason the torque curve with carbs seems to be shifted quite abit higher than the port/valve/cam selection would indicate. I think it has to do with the intake runner length. We have found that the peak torque of a carbed SC motor with stock cams is around 5500 RPM and the peak power around 6500 RPM. There is alot of torque to be gained with the carb motors by running longer intake manifolds.

-Andy

[jmz]

I do run the PMO tall manifolds and have the booster venturis as well. For what that's worth.

[shbop]

I keep hearing that larger intake ports give up some power (torque) on the bottom end. How much is sacrificed if the intake port were to stay at 35mm? Does the motor give up performance, only on the top-end? To compare apples to apples; lets say both motors were equipped with 40mm carbs.

[jluetjen]

Yeah Jon; Using JMZ's short stroke 3.2 has an example, I would expect the peak HP with his 38 mm ports to be at about 6200 RPM. If you were to do nothing else but change the intake port diameter to 35 mm, I would expect the peak HP RPM to drop to about 5300 RPM. Above that point the HP will drop off. The flip side is that the engine should be stronger below 4000 RPM. Between about 4000 RPM and 5300 RPM, I would expect both engines to be pulling about the same.

To be honest, 35 mm intake ports would be too small for a 3.2, because a 911 motor should be capable of pulling way past 5300 RPM without giving away much in the 2000 to 4000 RPM range.

[shbop]

John, Thanks for the reply. I'm still trying to figure-out the direction for a 3.0 I've got on the floor. Increased displacement has been a consideration. I imagine the 3.2, being a larger pump, can certainly take advantage of more carb than the 3.0. I don't mean to divert the thread, but I was curious since some sharp minds were focused here. Thank you

[jmz]

One can build a pretty nice budget-wise engine out of a 3.0.

Port the heads a bit, add some J&E pistons with valve relief and higher compression matched to your current cylinders, cams, carbs exhaust and a distributor re-curve gives a fun engine with good power. ...much improved over stock.

or go big with the 3.2 and beyond!

Good Luck.

[joetiii]

Originally Posted by Eagledriver

For some reason the torque curve with carbs seems to be shifted quite abit higher than the port/valve/cam selection would indicate. I think it has to do with the intake runner length. We have found that the peak torque of a carbed SC motor with stock cams is around 5500 RPM and the peak power around 6500 RPM. There is alot of torque to be gained with the carb motors by running longer intake manifolds.

-Andy

Interesting... my carbed 3.0 w/ SC cams peak HP at 5800 while torque comes in at 4700. These peaks are 300 rpm more than the stock. This engine has the smaller intakes. Shouldn't I be making power lower w/ carbs?

This setup does make more torque (214 @ FW) than HP (203 @ FW).

How does the taller manifolds raise torque? Same way that the smaller intake ports flow faster charge at lower rpms?

How to tell if my PMOs have them and where to get please

[jluetjen]

Originally Posted by joetiii

Interesting... my carbed 3.0 w/ SC cams peaked hp at 5800 while torque came in at 4700. This HP peak is right around factory while the torque is 300 rpm more. This engine has the smaller intakes which has something to do with it for sure.

Personally I think that the removal of the air flow sensor plate from the intake track has a lot to do with the shift in the torque curve. As long as there is a pressure differential to move the sensor plate, the cylinders won't be getting a full charge. If the cylinders don't get a full charge, you won't get as much torque.

How does the taller manifolds raise torque? Same way that the smaller intake ports flow faster charge at lower rpms?

How to tell if my PMOs have them and where to get please

There are two different things that I think that you are referring to:
1) The taller intake manifolds that connect the carbs to the heads.
2) The tall secondary venturis which are inside the throats of the carbs.

1a) Taller intake manifolds generally help high-speed running by moving the point where the fuel is introduced to the air flow further up-stream, thus giving it a little more time to atomize prior to being sucked into the cylinders. The downside of tall intake manifolds is that at low RPM, the airflow may be so slow that the fuels drops out of suspension, which hurts low speed running in cold engines. The largely vertical intake tracks of a 911 I suspect are a little less sensitive to this as opposed to engines with horizontal intakes.

1b) Taller intake manifolds also change the acoustical properties of the intake path, just like a trombone player can lower the pitch of the instrument by pushing the slide out and making the length of the pipe longer. This is tied in with the shape (preferably conical) of the intake path. If your car has cams with a lot of overlap, it's possible at certain points in the rev range (generally just before the engine "comes on cam" with the throttles partly closed) to have higher pressures in the exhaust port and a partial vacuum down stream of the throttles, resulting in the intake charge being pushed back up intake -- aka: Reversion. By making the the intake an inch or two longer, it's possible to put that whole series a little out-of-tune, which can then suppress the reversion and improve part throttle running in the 4000-4500 RPM range. Cars with MFI or other independent throttle-body injection systems are less sensitive to the reversion BTW.

2) Tall secondary venturis inside of the carb. I'm pretty sure that these provide a stronger vacuum signal to the carb at high air flows, so that you get a more even fuel metering across the rev range. The standard secondaries in a weber carb are about 1 inch tall, the "Tall secondaries" are more like 2 1/2 inches tall if I remember correctly.

[joetiii]

Originally Posted by jluetjen

Personally I think that the removal of the air flow sensor plate from the intake track has a lot to do with the shift in the torque curve. As long as there is a pressure differential to move the sensor plate, the cylinders won't be getting a full charge. If the cylinders don't get a full charge, you won't get as much torque.

Being a 20 watt bulb in a package of 100 watts, please forgive my ignorance here but I have to ask....... I thought airflow sensors were only for electronic injected cars. Do AFS exist in carbs too?

If so, what other effect does the AFS plate have on driveability or fuel milage or what, I don't know. Again, I'm a 20 watter trying to understand.

[jluetjen]

Yes, CIS systems do have a metering plate. EFI systems have a flap that moves a rheostat, or else a heated wire that is cooled by the air flowing past it. I believe that in general both of the EFI metering systems constitute a smaller restriction on air-flow compared to metering plate in a CIS system. Here's a diagram of the CIS system from a BMW 320i, essentially the same as the CIS system in a 911.



The metering plate is item 9, which is pulled up by the air-flow past it. It's connected by an arm/crank arrangement to a metering device, that increases the fuel flow in relation to how far the plate is displaced. So the throttle plate in a CIS system is always in the air-flow. If it is being displaced by the air flow, it must be taking energy out of the system and creating some sort of a partial vacuum down stream of it. I wish I had access to a car with CIS on it so that I could measure how much vacuum is being generated. I suspect that it's only a couple of %, so maybe .5 PSI, but that 3.4% would be equal to about 7HP at in a 200 HP engine at wide open throttle, or 6 lb-ft of torque!

The metering flap on an EFI system such as Bosch's L-Jetronic or LH-Jetronic on the other hand is pretty well pegged to the side of the intake track after about 4000 RPM at wide open throttle, at which point the EFI system switches over to a map of some sort. So past 4000 RPM, I doubt that a good EFI system is losing much HP over an open loop mapped system (ie - a system with no metering loop, but which runs only off of a map) assuming that both engines have the same level of tune.

[TibetanT]

Subscribing for information purposes. Hope I am not too late. This is a great thread.