Inlet manifold

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Inlet manifold

Postby RichDixon » Sat Mar 21, 2020 6:51 pm

We're getting to the bottom of our power woes, after three sessions on the rolling road. If anyone is feeling like assisting we could do with a few ideas for an inlet manifold as we've narrowed it down to that. Can yo let me have photos of yours. We're now running a 45 dcoe, could also do with an idea of a start point with jetting. Something to do while you're self isolating in the workshop. Thanks in advance. RichDixon.
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Re: Inlet manifold

Postby F1300Tony » Sat Mar 21, 2020 9:01 pm

I have wasted a lot of time trying and failing to apply logic to the design of an inlet manifold for 750F. I have made five of them and two have been used to win the championship but I am sure what I have made is less effective than at least one of Jon Lee's three-pipe manifolds. One showed a lot of promise on the rolling road but blew the head gasket on the track because fuel collected and was sucked into the head when the throttle was shut. I suspect the best of mine is no better Sue's standard manifold with a carb flange welded on. That is the one with three D shapes, which at least ensures equal mixture distribution which is the first aim for a inlet manifold.
My first effort was based on what worked best in F1300, and that was about 7bhp down on the best and required a large jet. We could not get it to work with a Tillotson in the two hours available two days before the car's first race, so a 40IDF was hastily rebuilt. This worked better with a spool piece to lengthen it by about 40mm and this reduced the jet size required. So I learnt if the manifold length is wrong the wave form coming back from the valves kills the suction at the choke.
The correct choke size is about 34mm. The bore of the manifold is one of those things that opinions differ, but theory would have it at about 32mm for the four branches. The idea is to keep the gas speed up, partly to avoid using energy to accelerate the gas and partly to keep the fuel gaseous or at least in small droplets. It is obvious a good design does not ask the gas to change direction too sharply, and there needs to be no dead areas where fuel can hide.
My method to make a manifold is to use a steam pipe bend as a knocking block to make two halves from 3mm ali sheet and fit and weld them together for each bend.
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inlet manifold.jpg
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Re: Inlet manifold

Postby F1300Tony » Sat Mar 21, 2020 9:08 pm

The photo shows my first effort. Each Y joint has equal angles so as to keep the mixture equal.
This joins 1 to 2 and 3 to 4. Having thought about the cam timing I now tend to join 1 to 4 and 2 to 3. I see Emlyn is joining 1 to 3 and 2 to 4, something I am not sure has been tried yet, and he has clever people to consult.
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Re: Inlet manifold

Postby pbove » Sat Mar 21, 2020 10:08 pm

I have the previous Jon Lee one to the one I’m now using which you’re welcome to try. It was pretty good, just not quite as good as the three branch one that Jon made. Let me know if you’d like to try it.
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Re: Inlet manifold

Postby RichDixon » Sun Mar 22, 2020 7:33 pm

Thanks guys, Thats great.Thanks for the advice Tony, Peter, I'll drop you a message when i can get over to borrow the manifold. Obviously we'll be on full social distancing measures and will resemble a spy exchange at checkpoint charlie.
Now I'm going to self isolate in the workshop with half a chassis of the 002. Keep safe peeps. Rich
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Re: Inlet manifold

Postby pbove » Sun Mar 22, 2020 8:29 pm

Hi Rich,

Happy to post it and, if you use Jon’s rolling road, happy for him to share my engine performance figures and carb settings. The settings between the two manifolds are very different and I can’t recall exactly what they were, but I think Jon has a record of them.
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Re: Inlet manifold

Postby RichardJ » Mon Mar 23, 2020 11:20 am

Peter, before arranging post or collection of the manifold it would be worth checking it would fit our configuration. Currently the manifold has a swan neck to clear the top chassis member (not easy to see in the photo but the red, green & blue wires from the coil run on the top of it)
Attachments
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Re: Inlet manifold

Postby pbove » Mon Mar 23, 2020 1:24 pm

Rich,

It also depends on the engine's angle of lean in the chassis. My engine is at 11 deg off vertical - is this the same for yours? Also, I think my manifold might be longer, so how much space have you got behind the carb at the moment?
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Re: Inlet manifold

Postby RichardJ » Mon Mar 23, 2020 2:06 pm

Ours is 11deg off vertical when measured from the side but the rocker cover is almost horizontal

We can cut a hole / make a scoop to fit the carb if necessary, so the length shouldn’t be an issue
Attachments
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B9110221-08AD-4A50-AC5A-7C7653411050.jpeg
B9110221-08AD-4A50-AC5A-7C7653411050.jpeg (115.19 KiB) Viewed 1982 times
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Re: Inlet manifold

Postby dopdog » Mon Mar 23, 2020 2:14 pm

should you not measure off the inlet mounting face to get the angle? I sure someone will be along to put us right 8-)
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