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Is it worth it: port 34mm intakes to 36mm for DC40 cam

Long title explains my question.

I have new DC-40 cams for use with carburetors and small port (34mm) SC heads. John at DRC said this would be a good combination 3500-7000 rpm

Would it be worth it to port the intake side from 34mm diameter to the next size 36mm?

Will the effort improve the performance with the DC40 cams a noticeable amount??

36mm is the 911S port size and compatible with my gaskets and thermal spacers. I’m thinking that is a manageable amount of change to do by hand with a set of carbide burs.

39mm is the “big port” SC heads and I’m not even imagining that big of an effort. And I’m not buying another set of heads.

So before I pull the trigger I ported a scrapped head. A lot of work! The one on the left is the original SC head. The right is at 36mm. Doing this six more times, plus the intake manifolds is a large effort for a part time hobbyist like me. Am I kidding myself that there is horsepower waiting for me??


Last edited by VFR750; 12-29-2017 at 06:09 AM..
Old 12-29-2017, 06:06 AM
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There is probably more hp in fixing transitions in the port and seat, narrowing the guide boss etc than in enlarging the port from 34-36.
But if you gaskets, spacer and intake is 36 mm you should absolutely fix that transition! That is important! Air doesn't like to go round corners or meet sharp edges.
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Old 12-29-2017, 06:34 AM
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Current manifold is 34mm. So that would have to be opened up too

Already addressed the transition between the intake passage and the seats. Smoothed out the factory blends and removed any ridge or reversal around the prerimeter.

Cleaned up and polished the exhaust ports too.

Spacers are tapered to a 34mm opening. So they would have to be trimmed.

Standard gaskets are 36mm from PMO.
Old 12-29-2017, 07:00 AM
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I hope it pays benefits, I'm having the same thing done.

Had small port heads on my 2.7 and using a DC40 cam and opening them up and port matching to my larger intake manifold. Unfortunately I won't know for a long time if/how well it works.

I'll let the others that have actually done it chime in, but I'm very interested to read their thoughts.
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Old 12-29-2017, 07:22 AM
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I've driven a few MFI 2.7 carreras with 36 intake ports (and 35 exhaust ports) with S cams. The power and torque is excellent up to redline. This is the stock port size of the 73 RS Carreras from factory.

I think 36 is probably ideal for GE40 cams and would not go larger as midrange torque would suffer. If you stay at 34 you may not get as much at the top of the range. If you were going to use an E cam or something smaller than a GE or DC40 cam, then 34 would probably be better. JMHO.

Last edited by MST0118; 12-29-2017 at 09:25 AM.. Reason: Add
Old 12-29-2017, 09:24 AM
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I would definitely open them up to 36. I think that size is a good compromise for what you are doing. On my previous motor, a SS2,5 I opened the ports from 32 to 35mm with great results using either a T cam or E cams later. There was a great increase in power from 4 to 6K RPM with no loss in midrange.

My current 3,0L has the large port heads. Which I think are really too big at 39mm for a street motor. That said my motor pulls great from 3K RPM with GT2 102 cams. I did go a little conservative on my manifolds. I only ported them out to 37mm which may be helping my midrange. I'm running 40mm Webers with 36mm venturis.
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Old 12-29-2017, 11:16 AM
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Do it now or seriously regret it in the future.
Old 12-29-2017, 11:57 AM
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Its well worth the effort to open them up in order to use what that cam profile offers, BUT this really really needs to be done using a flow bench to confirm your work. I've seen a LOT of pretty nice ports that didn't flow as well as factory ones.
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Old 12-29-2017, 01:00 PM
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Good point on shape of the porting. I’ve documented the current 34mm porting and noticed the expansion from the inlet to the region of the valve guide. My trial part replicated this geometry with the added dimensions.

A 24mm socket with a few layers of vinyl tape is 33.5mm in diameter. A 27mm socket is just under 36mm OD. So I can insure I get the comparable taper. And next is to makes sure the transition region is similarly smooth and gentle slopes.
Old 12-29-2017, 03:49 PM
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Make sure that the resulting port is centered between the studs and your intake manifold's ports match up and you will be good to go.
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Last edited by Trackrash; 12-29-2017 at 05:11 PM..
Old 12-29-2017, 05:08 PM
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Where Do You Want Power

Steve W's the expert / pro - so I would do as he recommends…

Meanwhile, based on the research I did on my rebuild w/upgrades - it seemed the gain you achieve with larger ports primarliy influences / improves upper RPM performance.

For my build, I was more interested in lower end torque - and having relatively smaller ports (and venturi in the carbs) appeared to improves the engine's ability to rapidly draw in a full fuel/air charge at lower RPM's.

All in what you want to achieve - if you want high RPM / redline HP, open them up. If you are more interested in lower RPM grunt - leave them as is and apply your time elsewhere. Besides - you can always open them up later / next winter and have a solid comparison (although it would likely also involve carb main & main AC jetting adjustments).

My 2 cents…

Good luck,

Gordo
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Last edited by Gordo2; 12-30-2017 at 02:04 AM..
Old 12-29-2017, 09:17 PM
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Gordo

I agree 100% with knowing your goals and the influence of porting and venturis.

I “need” more power above 5000 rpm. On the track, windows down, fat tires, the car just about stops at 120-123 mph. Stuck in the gulf of screaming in 4th or loping along in 5th. Hence, I really care about the high end.

I will be bolting the heads to the cam towers and the manifold to match the ports in the true assembled condition.

I also can see that the region where the flow splits around the valve guide is an area where you need to make sure there isn’t a flow restriction. Metal needs to be removed to insure the flow area is not constricted. A critical region.
Old 12-30-2017, 05:33 AM
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Gordo

I ran the last rebuild for two seasons.

This rebuild is 100% not necessary. Doing it because I wanted a hotter cam.

And it escalated into head work. At least this power increase doesn’t cost more than the burs and time.

Long, very cold winters make for big projects. 😀
Old 12-30-2017, 05:45 AM
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I know you can see CFM losses from home porting, but I'd think you'd have to break every rule in the book to actually make that happen. I ported some heads back when I didn't understand much about flow at like 18 years old, and the engine picked up a TON of power.

But anyways, have you thought about a stock Carrera 3.2 head swap?

They flow nicely.
Old 12-30-2017, 05:59 AM
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I decided on this being a simple cam swap. I have a “bang for the buck” budget. The head work is a free adder.
Old 12-30-2017, 06:19 AM
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What size carbs and chokes are you running? If 40s then I’d say 36mm ports max, bit if 46 carbs I’d go to 37 or even 38
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Old 12-30-2017, 09:17 AM
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I have my 2.7 set up with S cams, 36mm ports, five angle valve job ~ topped by Italian Weber 40's. It feels good down low, and it makes great power from 4000-7250 without falling off for a second.

If I were to do it again, I would repeat it.
Old 12-30-2017, 09:52 AM
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I have 40mm Zeniths which have been heavily modified.

alfa1750 sells Zenith parts and has venturis of all sizes with the max of 35mm; I'll be moving up from 34mm to 35mm

so a DC40 cam, 36mm intake ports seems like a winner.

so far, this is my spare head ported to 36mm. Working on the transition from the port into the valve chamber. Surface is still a little rough. Once I am happy with this, I have to repeat 6 more times. Hopefully it will be faster....
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Old 12-30-2017, 11:31 AM
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About head porting.
Has anyone tried a 30 degree valve seat cut rather than the stanard 45?
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Old 12-30-2017, 03:21 PM
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Hey Mike

Think about using a shop vac with a synchrometer flow meter to give a quantitative measure of the flow improvements (changes) of your porting efforts.
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Old 12-30-2017, 05:37 PM
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