Whats the real world difference btw 3.0SC small port heads vs big port heads with ITB

[Nine-one-one]

Hi guys,

im in the middle of putting the engine together for my 911 SC.

Car will be a backdated " Singer inspired" funcar with no real powergoal...just a blast to drive...

Enginespec is:
3Liter 930/10 baseengine
10,5:1 forged Wössner pistons with forged conrods
Jenvey 45mm ITBs
small port heads
Cams Grad: : 296/284 Spreizung: : 110 Ventilhub: : 12,4/12,1
Hub im OT: : 2,4 Steuerzeiten: : 38/78 - 72/32
Free programmable ECU from Rasant Products
headers without heat,freeflowing muffler (need to decide which one)
Gearbox is a 915/62 with normal 1/2 and short 3/4/5 gearing,so it will max out at about 230km/h with the 17" tyres im running....
car will be pretty light...no AC, no powerwindows, no heater...

As said, i dont have a real powergoal for the car, but i would not like to make big mistakes while building the engine. So now there comes the question, that even after reading of all threads i could find, i cant really answer myself....

Is it worth to upgrade the heads for a engine like this to the big port factory heads?? What the actual benefit powerwise, how much torque is to be expected missing in lower revs with the bigger ports vs the smaller heads... my small port heads are 100% finished, like new,so i would just need to throw them on the cylinders and call it a day....

If the difference is about 10HP plus with the same or lesser torque, i guess it keep the small port heads like they are....if its more like gaining 25HP with the same torque i would decide to build big port heads.

Anybody did something like this and can give me some advice, please

Thanks i advance, Alex

[Joe Bob]

Flow and/or matching to to ITBs to maximixe.....engine performance increases with a smooth transition.

[tirwin]

This is a great question. I would love to hear quantitative numbers. I have pondered this many times myself. You're doing what I've always wanted to do so I'm super jealous.

The conventional wisdom I have always heard is that the small ports were done for emissions reasons and are the real bottleneck in late 3.0 motors. One way to look at it is that if you are going to the trouble of hotter cams and ITBs then it seems silly to leave this on the table. (the "while you're in there" curse)

But you are asking a fair question in so far as cost vs benefit.

Maybe you will get a quantitative answer but I suspect you'll get a squishy answer. Hope I'm wrong!

Is there an apples/apples comparison for early vs late 3.0? Seems like the thing that complicates answering this question is the fact that other changes were being made from the late 70s to early 80s because of all the emissions requirements. That makes answering the question more difficult but maybe not impossible. Or it at least gets you a 'good enough' answer.

Good luck with the project and keep us posted.

And we need pics.

[jpnovak]

If I am reading this right that cam is HUGE. You will need to be turning that engine to almost 8K rpm to take advantage of it based on the seat duration (294/284). NO way that stock small ports will support this kind of airflow. I don't think this would be much fun to drive.

The engine will probably make good power if built correctly but not at any reasonable street rpm.

Also, don't forget the case and oil modifications to support that kind of rpm.

My order of planning that usually covers all bases and works really well. .

Port size is less than 75-85% of valve diameter. depending on rpm range. Smaller ports for lower rpm range.
Pick valve lift to match flow of heads. Size lift to meet flow values between knee of curve and plateau.
Spec exhaust side of cam duration to match desired rpm band.
Double check that port size meets rpm flow at less than 105 m/s intake gas speed (Thanks John Leutjin for calculations)
Match intake to meet airflow requirements
match exhaust to meet airflow requirements
Build engine and enjoy.

Edit: My calculations show you need a port size of 42-43mm to support the airflow for that cam. That exceeds the usual flow for the size valve. Additionally I think you will have such large ports the intake gas speed with stall at mid-rpm giving you a huge flat spot. I would seriously revisit that cam spec.

[tirwin]

And just to echo what Joe Bob said, my understanding is a decision you need to make is the size of the ITBs relative to the size of the ports. In other words you don't want a mis-match. So answering your port sizing question is important for properly sizing the ITBs.

[Nine-one-one]

Originally Posted by jpnovak

If I am reading this right that cam is HUGE.

That cam is a medium aggressive cam, could be the US cambuilders use different specs/data for their cams, as i know where your coming from,comparing them 1:1 makes it look very aggressive

Id say its rougly around a Dougerty Racing DC60 cam,maybe even more a DC44, but as im getting no reply from him on my questions, im decided choosing a german cambuilder, to saves time and hassle for me.

[Nine-one-one]

Originally Posted by tirwin

And just to echo what Joe Bob said, my understanding is a decision you need to make is the size of the ITBs relative to the size of the ports. In other words you don't want a mis-match. So answering your port sizing question is important for properly sizing the ITBs.

As the ITB´s are allready bought, i need to match the intakes after them....but whats the best match, small or big....and whats the gains/losses of each....i hope someone allready made all the faults i could do now and say whats right and whats wrong....at least help a bit

If the powergain is low,lets say 10 hp for the bigger ports but i loose 10 NM in the low down revs...im not even interested in thinking about changing the heads as my small ports are allready 100% finished.

But if i gain 25hp from the big port heads,get a better midrange and maybe just loose 10NM low down, i would tend to go big port heads....as said,if they are a real bottleneck....they need to go or the other option, i port them to something inbetween big and smallports....

[tirwin]

Jamie has given some very sound advice as far as approach.

I'm not sure the question about ports is as important given that you have already purchased some components. You've fixed some variables in the equation now. Not sure how that affects your desired result.

[Nine-one-one]

Originally Posted by tirwin

Jamie has given some very sound advice as far as approach.

I'm not sure the question about ports is as important given that you have already purchased some components. You've fixed some variables in the equation now. Not sure how that affects your desired result.

Sure, he did, but i guess his post would have been more informative for me if i had included the camspec better, we germans use different cam data then US companys....as said, the company who does the cam sells this as their highest steet spec cam, so its def not overly aggressive.

[kent olsen]

1978 3.0L with small port heads. Dougherty GT2-102 cams (special grind for lower peak torque), 9,5:1 pistons, 40mm webers, twin plug heads. Engine dyno numbers: 246 HP @ 6600 rpm and 226 ft/lbs torque @ 4700 rpm.

1972 911T weight is 2200 lbs, suspension upgraded, stock rear brakes, SC front brakes. Engine was built to withstand 8000 rpm. I now have 17,000 on engine, 237,000 on chassis. I can drive the daylights out of her then take her home and adjust the valves and change the oil once a year.

[Nine-one-one]

Thats the real world results i was hoping for.....great answer Kent Olsen

How does it feel from peak HP around 6600 rpm to revlimiter, still feeling free and fun or is it more like reving the **** out of it for nothing and shifting at 6600rpm is way faster ??

[ClickClickBoom]

Originally Posted by kent olsen

1978 3.0L with small port heads. Dougherty GT2-102 cams (special grind for lower peak torque), 9,5:1 pistons, 40mm webers, twin plug heads. Engine dyno numbers: 246 HP @ 6600 rpm and 226 ft/lbs torque @ 4700 rpm.

1972 911T weight is 2200 lbs, suspension upgraded, stock rear brakes, SC front brakes. Engine was built to withstand 8000 rpm. I now have 17,000 on engine, 237,000 on chassis. I can drive the daylights out of her then take her home and adjust the valves and change the oil once a year.

Nice looking dog(s), perhaps more tennis ball, less car that they cant ride shotgun in!

[kent olsen]

My rev limiter is set at 7200 rpm. The power only drops about 5-10 hp to the limiter so I shift about 7000 rpm, but it's still pulling hard. The lower torque is what I really like. When I shift at 7000 it drops to about 4200 rpm right in the torque peak,(915 transmission).

[Nine-one-one]

Sounds like a real fun ride....my 915 will be pretty short in gear 3/4/5....so i would like to have a rev happy engine, torque should not be the biggest problem with such short gears.

[Trackrash]

Originally Posted by kent olsen

My rev limiter is set at 7200 rpm. The power only drops about 5-10 hp to the limiter so I shift about 7000 rpm, but it's still pulling hard. The lower torque is what I really like. When I shift at 7000 it drops to about 4200 rpm right in the torque peak,(915 transmission).

What retainers are you using with those cams Kent?

[Trackrash]

Originally Posted by Nine-one-one

Hi guys,

im in the middle of putting the engine together for my 911 SC.

Car will be a backdated " Singer inspired" funcar with no real powergoal...just a blast to drive...

Enginespec is:
3Liter 930/10 baseengine
10,5:1 forged Wössner pistons with forged conrods
Jenvey 45mm ITBs
small port heads
Cams Grad: : 296/284 Spreizung: : 110 Ventilhub: : 12,4/12,1
Hub im OT: : 2,4 Steuerzeiten: : 38/78 - 72/32
Free programmable ECU from Rasant Products
headers without heat,freeflowing muffler (need to decide which one)
Gearbox is a 915/62 with normal 1/2 and short 3/4/5 gearing,so it will max out at about 230km/h with the 17" tyres im running....
car will be pretty light...no AC, no powerwindows, no heater...

As said, i dont have a real powergoal for the car, but i would not like to make big mistakes while building the engine. So now there comes the question, that even after reading of all threads i could find, i cant really answer myself....

Is it worth to upgrade the heads for a engine like this to the big port factory heads?? What the actual benefit powerwise, how much torque is to be expected missing in lower revs with the bigger ports vs the smaller heads... my small port heads are 100% finished, like new,so i would just need to throw them on the cylinders and call it a day....

If the difference is about 10HP plus with the same or lesser torque, i guess it keep the small port heads like they are....if its more like gaining 25HP with the same torque i would decide to build big port heads.

Anybody did something like this and can give me some advice, please

Thanks i advance, Alex

For the best running motor you need to have all the components matched. The cams Kent is using are midrange cams that are matched to the small ports. Sounds like a good match. BTW, I am using the same cams on my new build, with large port SC heads. But that is a story for another thread.

What an engine builder needs to decide is what RPM range you want the power to be usable.

You then match the rest of the motor to that goal. If the cam you want to use will give best performance at 5 to 7K rpm, then 34mm ports will really cut down on your power output. You need to find out what RPM range the cam you mentioned is designed for.

You can decide what RPM range you want your motor to work best in then go form there. Match the cams, ports, and induction.

There are calculations that can be done to estimate port velocity, which will give you a idea of what you need to achieve your performance goals.

I doubt you will be utilizing the potential of your 45mm ITBs with 34mm ports.

It is not hard to open those ports up. The valve size is the same on all of the SC heads, it is just a matter of opening up the top area to match your manifolds.

That said, I don't think you will be happy with 45mm ports on anything but a full race motor either.

[Nine-one-one]

The cams im using are designed for a street driven engine, from 4000-7000rpm. This should pretty much suit what i want from it. I know i did choose the 45mm ITBs a bit too big, but so i have the chance to go a bit higher without the need of buying expensive stuff twice....😉

Opening them 34mm ports to 38-39 should not be a real problem...,,but i dont want to do if i only gain 10hp....not worth the trouble😉

[kent olsen]

Gordon
Don't remember what retainers we used. Mike Bruns @ JB Racing in Taveras, Fl is the one who helped me build the engine and who's engine dyno we ran the engine on.

[Trackrash]

Originally Posted by Nine-one-one

The cams im using are designed for a street driven engine, from 4000-7000rpm. This should pretty much suit what i want from it. I know i did choose the 45mm ITBs a bit too big, but so i have the chance to go a bit higher without the need of buying expensive stuff twice....😉

Opening them 34mm ports to 38-39 should not be a real problem...,,but i dont want to do if i only gain 10hp....not worth the trouble😉

From my limited experience, if it was me, I would open them up to around 36-38mm depending on doing some more research on flow calculations.

You should open a thread in the engine rebuilding forum. You might get some more experienced opinions there.

[angelny911]

lurking ...LOL have a 1979 3.0 row large port motor looking for idea"s also a 3.0 paxton supercharger setup and a 74 915 tranny with 7:31 gears waiting to install in the right ......