Jacko has two carbies and the spacer for that style inlet- really! Sponsored by OPEC, it is called the 'Thirst Monster'.
The youtube site Hybrid has mentioned a few times made an interesting discovery on dual versus single plane manifolds when testing throttle body efi set-ups. Put simply, the dual plane setup led to different vacuum pulses across the throttle body thereby messing with the efi brain that expected consistent vacuum across the four throats. Poor atomisation at a certain rpm led to a stumble in hp and torque before it evened out as they revved the guts out of it.
'68 J-code GT Fastback
'67 S-code GT coupe, 'Pink Bitz' formerly known as 'Hookin' up a brother'
'69 M_____ GTS Fastback 'Blasted'
Thats interesting boofhead. I currently have a Weiand X-Celerator on my cleveland with 2V heads. I have to take it off to fix oil leak at the back of the manifold and was thinking about replacing it with a RPM Airgap as I was of the belief that a dual plane was the better option .
Do you think that the RPM Airgap would be a waste of money compared to what I currently have ??
It is a hard question to answer as I do not know what is done to your engine and how you use it. So answering in general;
You need to understand the purpose of the dual plane manifold and go from there. So the reason they divide the plenum in the manifold is to reduce the affect of the intake pulse on other ports. So the design tries to force the intake event to occur on each side of the manifold thus isolating the previous event from the current event. Because the design means the port cross over there fore there are two layers -a high level and low level halves of the plenum. This is suitable for stock to mild engines. The issue is the runners have lots of bends in them, the angle into the head is low (especially in the lower level), and the carbi gets non-uniform signal as one side then the next gets a pulse, while the port lengths vary greatly and the plenum area is also different. Its design results in it being restricted (even with good sized ports because of the bends) with uneven fuel delivery. Having said that the result is that the manifold design will produce peak torque at lower revs which suits a milder better. A single plane can have better port angles, closer to same length ports (so they can be tuned length which promoted improve VE), large plenum area etc. All of the features works best at higher RPM rev ranges. The issue is the middle ground. What most do is get a high dual plane manifold (improves port angles), and add a (1 inch) open spacer to add plenum volume both of which raises the peak TQ RPM. To really improve it you need to port the manifold especially the plenums areas to allow the air to flow smoothly by rounding the edges to direct the flow. The last point to raise is for those they stroke their engines - in these cases they still use manifolds for the old engine size (say in the case of a 347 you use a designed for 289/302 manifold) which often a single will be best simply because of the added volume it is designed for is provide at a lower rev range simple because of the larger capacity engine. So what do you do; no idea. The X-cellerator is a good manifold - smallish ports so designed for lower RPM than say a VicSn type manifold. If you do go RPM Air gap then do run a spacer. I am not sure you will see to much difference - if your happy with the low rpm torque no need to change it.
I had a air gap knockoff in a worked 302 it had Patriot alloy heads and a 3/4 cam with a 1" open spacer. The combo made great torque just under 400Nm at 800 rpm. It was carbed and made smooth power peaking at 174 KW and 423Nm. I have read manifold comparison articles where they cut the plenum top an inch to replicate the spacer and get improved power though these were using a carb rather than EFI. The vacuum signal issue explains why EFI setups use single plenum intakes. What's the YouTube site you've mentioned as I'd like to have a look. I have old school performer which I'm planning to use with a throttle body EFI.
1965 Fastback A code
I am Borg. Resistance is futile. You will be assimilated.
at 800RPM had 400NM of TQ? Really!
3/4 Cam? (I hate that description...sorry).
EFI - I would use a single even for a TBI though otherwise make sure you use the spacer.
Most tests show lower rpm peak torque for the dual and higher rpm peak torque for a single. In many cases the raw torque figure is also higher for the dual at that lower rpm peak point. But it does not mean it is faster.
If you have one engine with peak 500 ftlb torque at 3000 RPM and another with peak 400 ftlb torque at 5000 RPM which is going to be faster?
I can assure you it will be the 400 ftlb @ 5000 RPM engine.
I will someday think of something clever to say.
Last edited by boofhead on Sun Jul 10, 2016 11:26 pm; edited 1 time in total
Thanks fro the reply Boofhead.
Might just stay with the current manifold for now at least as the bottom end torque is quite good as the engine is now.
Was only thinking of changing as I have to take the current manifold off to fix an oil leak and the Airgap seems to be the rage at the moment.
Currently the motor is quite mild..bored 20thou, balanced, 600 vac sec Holly and nothing to big as far as the cam goes. see link below.
It's just that reading some of the posts here on some of the engine builds it's starting to make me envious and want more so somewhere down the track I'd like to go a bit bigger in the cam, do something with the heads (CHI, AFD or just some port work on the standard heads, not sure yet) and was thinking if the airgap was a better manfold I could do it now and would be 1 less expense to sneak past the wife later down the track
A street Cleveland is a nice engine. Lots of potential while the CHI heads are excellent.
A crow cam - brings back memories. Good choice - very mild but a great choice for a regular street engine. At the end of the day, you spend most time on the street. You can if you decided to put your hair on fire and make the Cleveland scream - well you can. We are here to light the fire ... lol. In the mean time enjoy your car.
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