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Intake manifold CSA

General engine tech -- Drag Racing to Circle Track

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Knight rider
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Intake manifold CSA

Post by Knight rider » Sun Oct 13, 2019 5:36 pm

I'm after some opinions on an intake I made for my 393 clevo, I'm in the middle of freshing up the engine.

I made this intake more out of interest of just doing it.
Now I'm looking at weather the CSA is to small for what I need. Car runs and drives great with the intake


Some basic specs on the intake

Runner length 136mm
Diameter of runner taper is 44mm-41mm
Plenum is around 4.5lts

Camshaft is compcam HYD roller 245@50 612 lift
Heads flow around 290cfm @600


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Re: Intake manifold CSA

Post by Carnut1 » Sun Oct 13, 2019 7:31 pm

Resized952018102895191809954382 (1).jpg
Did you use a radius or bellmouth inside the plenum?
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Knight rider
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Re: Intake manifold CSA

Post by Knight rider » Sun Oct 13, 2019 7:49 pm

Carnut1 wrote:
Sun Oct 13, 2019 7:31 pm
Resized952018102895191809954382 (1).jpgDid you use a radius or bellmouth inside the plenum?
Yes they there all radius but nothing like the big bellmount you have there.

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Re: Intake manifold CSA

Post by treyrags » Mon Oct 14, 2019 2:22 pm

PipeMax is your friend on this and will get you close.

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Re: Intake manifold CSA

Post by F-BIRD'88 » Mon Oct 14, 2019 4:34 pm

Single 4bbl carb flange → big mistake on that intake.

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Re: Intake manifold CSA

Post by Knight rider » Mon Oct 14, 2019 4:52 pm

F-BIRD'88 wrote:
Mon Oct 14, 2019 4:34 pm
Single 4bbl carb flange → big mistake on that intake.
I don't know about BIG mistake but I do hear what your saying, I'm running Holley's terminator TBI and did look at trying to run twin TBI but there wasn't room.
Even had thoughts of maybe running two extra injectors one at each end high in the plenum.

It does run pretty good and I enjoyed making, just trying to figure out if I should stick with it or move on to something like TFC 4v cheers

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Re: Intake manifold CSA

Post by F-BIRD'88 » Mon Oct 14, 2019 5:11 pm

Got to appreciate the effort, reguardless.

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Re: Intake manifold CSA

Post by enigma57 » Tue Oct 15, 2019 1:08 am

:D Nice looking intake you've fashioned there! CSA really depends on how high you intend to rev it. I would expect a 393 cu. in. engine with those head and cam specs to rev to around 6,500 RPMs. That would require (in theory) a round port runner cross section of 48.75mm diameter.

41mm round port cross section should begin to restrict flow at 4,500 RPMs. But that's all theory. How does it perform in actual practice? Should have really good mid range with 41mm runners.

You mentioned runner length. Is that inclusive of runner from where it joins the plenum (or extends into, if raised above plenum floor) down through center of flow path to back side of intake valve when resting on its seat?

Best regards,

Harry

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Re: Intake manifold CSA

Post by Knight rider » Tue Oct 15, 2019 1:58 am

enigma57 wrote:
Tue Oct 15, 2019 1:08 am
:D Nice looking intake you've fashioned there! CSA really depends on how high you intend to rev it. I would expect a 393 cu. in. engine with those head and cam specs to rev to around 6,500 RPMs. That would require (in theory) a round port runner cross section of 48.75mm diameter.

41mm round port cross section should begin to restrict flow at 4,500 RPMs. But that's all theory. How does it perform in actual practice? Should have really good mid range with 41mm runners.

You mentioned runner length. Is that inclusive of runner from where it joins the plenum (or extends into, if raised above plenum floor) down through center of flow path to back side of intake valve when resting on its seat?

Best regards,

Harry
That was just the runner length on the manifold only down the centre line, the port length to the back of the valve is
4.840" or 122.94mm plus the manifold runner length

Seems to pull well to 6500 I've been running to 6800 sometimes when it goes to the drags but I don't really know if it's wanting more manifold or not.
When I first fitted this intake I had to completely redo my tune on the TBI, if you went WOT it wouldn't pull if you backed off just slightly it all picked up again. New tune fixed that. Cheers

I've only just ported the heads so I'm starting to think maybe it would be a good time to change the manifold too. Cheers

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Re: Intake manifold CSA

Post by enigma57 » Thu Oct 17, 2019 10:17 pm

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Good luck!

HB

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Re: Intake manifold CSA

Post by cardo0 » Fri Oct 18, 2019 9:33 pm

I don't like it even though I've never built a intake before. I'm not looking to flame you but I've worked in power plants for decades and your manifold isn't going to turn air very well.

It tapers at the ends where it should open up to increase volume there. Run the plenum past the runners as much as you can. The plenum floor should be dropped for greater volume and the runners should have the ends radius'd/bellmouthed like Carnut1/b] mentioned. Now a wet flow intake with a plenum floor below the runner bellmouths will create some dangerous fuel puddling so you would need to terminate the runners onto the plenum floor but bellmouth or at least radius the runner entries. And using a single throttle body is making air turn more than with 2 throttle bodies which loses vacuum signal and velocity.

Sorry but that's my 2 cents.

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Re: Intake manifold CSA

Post by Knight rider » Fri Oct 18, 2019 10:11 pm

cardo0 wrote:
Fri Oct 18, 2019 9:33 pm
I don't like it even though I've never built a intake before. I'm not looking to flame you but I've worked in power plants for decades and your manifold isn't going to turn air very well.

It tapers at the ends where it should open up to increase volume there. Run the plenum past the runners as much as you can. The plenum floor should be dropped for greater volume and the runners should have the ends radius'd/bellmouthed like Carnut1/b] mentioned. Now a wet flow intake with a plenum floor below the runner bellmouths will create some dangerous fuel puddling so you would need to terminate the runners onto the plenum floor but bellmouth or at least radius the runner entries. And using a single throttle body is making air turn more than with 2 throttle bodies which loses vacuum signal and velocity.

Sorry but that's my 2 cents.


Thanks for not flaming me lol

The taper in the runner is for a reason to create air speed, The intake has been on and running fine I have done a mod or two along the way, not to fix anything but more to try stuff, one mod was to give the rear of the intake some room to breath, at the front the plenum is already cut in around the disy so no more room there. The runners don't poke through the plenum and are radius, maybe there room to increase that. Any testing I do in regards to performance is tested at the track. So far this intake has run the same times as the last on which was just a low rise single plan, but I have picked up some time changing retainers and springs so the intake has gone faster than last but just not a direct result from it.
My only concern is maybe the CSA is on the small size but i guess without trying another intake I probably won't get my answer, cheers and thanks for the input.

also I'm running holley's TBI so the I think it's a little more forgiven with the signal to the carby-TB


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Re: Intake manifold CSA

Post by cardo0 » Sat Oct 19, 2019 2:23 am

Okay that pic of the intake as its in use shows a much better plenum end on the firewall side which is something of what I was trying to describe. I can see what you say about the distributor end and I don't have cure for that either though you could raise the roof of the plenum there to help turn the air. I don't like tapers where you need to turn air.
So I do like how you blended the runners into the plenum floor. To late now since it's closed up but some build up around those blended runner entries would have helped turn the air and resist reversion an undefinable amount. But on second thought any build up might create dams for fuel to puddle and makes that intake a experiment - my bad on that.

Only you would know if you could turn a pair of throttle bodies sideways to get them to fit as I've seen that done frequently on carbureted intakes.

So to answer your question with a question what is the CSA of the head runner port? If your 41mm runner CSA matches the head port CSA (or is greater) I can't see how your manifold runners can be a restriction - any more than the cylinder heads themselves.

Hope this helps.

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Re: Intake manifold CSA

Post by Knight rider » Sat Oct 19, 2019 3:18 am

cardo0 wrote:
Sat Oct 19, 2019 2:23 am
Okay that pic of the intake as its in use shows a much better plenum end on the firewall side which is something of what I was trying to describe. I can see what you say about the distributor end and I don't have cure for that either though you could raise the roof of the plenum there to help turn the air. I don't like tapers where you need to turn air.
So I do like how you blended the runners into the plenum floor. To late now since it's closed up but some build up around those blended runner entries would have helped turn the air and resist reversion an undefinable amount. But on second thought any build up might create dams for fuel to puddle and makes that intake a experiment - my bad on that.

Only you would know if you could turn a pair of throttle bodies sideways to get them to fit as I've seen that done frequently on carbureted intakes.

So to answer your question with a question what is the CSA of the head runner port? If your 41mm runner CSA matches the head port CSA (or is greater) I can't see how your manifold runners can be a restriction - any more than the cylinder heads themselves.

Hope this helps.
Thanks

The taper in the runners are only tapered one way so there a little oval in shape, I'm not sure how to work out actually the CSA in a oval shape but it's probably closer to 42.5mm which is 2.19" CSA, the heads at the pinch point is 2.69" CSA. I did purposely make the intake smaller as what was the idea at the time. It does open up to meet the head but its still smaller.
The runners are a direct shoot at the port no turn to make other the a slight one on number 5 to miss the disy.
The other thing I've been wondering what effect does it have when you have fast moving air going into a larger port runner in the head? Cheers

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Re: Intake manifold CSA

Post by cjperformance » Sat Oct 19, 2019 6:30 pm

Cool XC and intake, its great to see people make stuff and experiment rather than just buy catalogue parts.
With the small runners opening into a larger port, you get a velocity loss as the charge exits the runner. In an extreme case it essentially makes the runner/port overall length "look" shorter the the engine below peak HP so you loose some TQ potential, at peak HP you are then hampered by the smaller runner.
But for street use where partial throttle settings are used more often thus scenario "can" improve driveability especially if the port is on the big side for the combo.
Theres of course a ton more variables as with all things engine!
Craig.

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