I started reading some other threads on here, and it got me thinking again about the performance of my L-head straight-eight. Yeah, sure- spend the most money to go slow, I know. Not a great photo (only one I have to hand right now that shows it), but it gives a basic idea of how funky the manifold is on this engine. That's a little two barrel Carter WCD sitting on top of the heat riser, dumping both barrels into the common space for all 8 cylinders. Two very long runners and two short ones. Couple into that the fact the intakes are siamesed into pairs in the block, and all the ports are round (and it's a flat head) all I can think of there is "sub-optimal" in terms of carburetion. The exhaust ports are at least individual. Looking at the firing order, the cylinders can be split up into evenly spaced pulses by grouping 1&2 with 7&8, then 2&3 with 5&6. That leaves two long intakes and two short again. Sideways space is limited but i could always go upward. So would that be the most optimal intake collection? This is a slow, slow engine and I am mostly interested in generating torque, not masses of power. My thought is a pair of carburetors (probably just 2 WCD's) each feeding 4 cylinders with a balance tube between the two, evenly spaced in time in terms of intake draw around the cycle of the crank (no the circles on the left don't represent crank position, they represent cycle completion). Exhaust would be next up- for best scavenge at low speed I'm thinking 8-4-2-1 on the collectors, again with the opposite pulses grouped in the same tubes. Am I thinking right, or should the pipes be grouped differently? I'm not after a warble to the exhaust, I'm after better engine characteristics. Phil
I would think that, just to keep it simple, a pair of small two barrels with one carb at the front four cylinders, and one for the rear four. The runners could be kept more or less equal length and fairly short. It seems inline engines with a center mounted carb tend to run rich in the center and lean at the ends. My own inline, a 300 Ford, which isn't as unique as yours, is still becoming difficult to get aftermarket stuff for. If I were to do a custom manifold, I would probably run three singles. Balance tube would be optional. If the runners were short enough, perhaps three Mikuni CV carbs off a Harley. That would be a hoot to look at! I may try it one day, since off the shelf intake manifolds seem to be on permanent backorder with the price rising monthly. For example, the Offenhauser manifold has topped $600 in some outlets. That right there puts it in the category of building rather than buying. So I would say go for it. It's just tinkerin' time and some material. You might come up with something great!
Phil, Hamber @Stooge has made a Custom manifold for his straight 8 in his project...Might be worth a look... https://www.jalopyjournal.com/forum/threads/slow-and-poor-37-buick.1100567/page-11#post-14212626
Very much worth looking at. That's along the lines of what I was thinking doing. What's the benefit to such a large plenum?
I am not up on such things but it's really a nod to past and present experimentation with such things... Two others running homebuilt Custom Intakes are @dana barlow and @GasGas300 perhaps they may better understand these principles your referencing...perhaps a larger plenum means cooler gas to the cylinders...
Gonna be watch'in your progress! About that plenum ..think of it as a holding container for vaporized fuel. As an intake opens and draws on the intake there is a plenum full of vaporized gas waiting! Of course 'more is better' does NOT apply here. too much fuel and you get puddling as the vapor falls out of suspension and goes back to liquid form. I doubt I'm telling you something 'new' here.... "Common sense is not a tree that grows in everyones's garden" Think tunnel ram! 6sally6
Yeah, I was looking at that- I want for the intake charge to be above the point at which the fuel condenses- hence my moan that the ports are round (there's some physics there, similar to the reason storm drains are that funny D shape). I was thinking a minimal plenum but what came to mind then was seeing a hazy cloud of fuel vapor spit out of the intake upon snap opening the throttle wide- that there's negative and positive forces at work in the chamber. I would like longer runners to try and keep the resonant frequency low and boost torque. There's also the issue with siamesed intakes charge robbing which was why I was thinking about keeping the intake pulses even in each plenum group. Or, if I'm overthinking it and it's such a low-flowing, slow engine that these factors drop off in effect enough to not really worry...?
Your last sentence is golden ! Keep the plenum large enough to have enough volume, but small enough runners to keep velocity up as well. Possibly a log connecting individual equal length runners to each port with a pair of 2bbls feeding the log? Make it "nice" & enjoy what you've done!! Think of a pair of "E"s end to end for the plenum & the runners?
@Stogy 's right, it's very much an experiment, and if it doesnt work, i'll try something else. It is largely an amalgamation of studying an array of period factory, aftermarket and homemade intakes for inline engines as well as some new ones being built that use a log style plenum. I think it might look a little bigger than it is, the tubing and plate i used was 1/8" wall, making the the interior height just a hair over an inch. The pair of Big 97 Strombergs should be about 500cfm, so not too crazy for a 320ci ohv engine. If i could have found one that i could have afforded, i would have tried to get an old Edmunds Custom one but it would have been an expensive experiment to see if it fit around the homemade exhaust manifold.
I might be off base here, but I'm thinking tuning two separate manifold/carb combinations might be touchy... how about a single log manifold with two carbs? Since this isn't a high speed, high performance engine (not a slouch, but not a bracket racer), it might make tuning easier, and might be add to drivability.
That's pretty much what I was thinking, and something like this would be my first design choice. Yeah, the firing order may be a bit wonky. But so what? If ultimate power were the goal, this discussion would not be taking place. Y'see, I'd take this manifold design, and put a couple Mikuni CVs (side-draft) on it. It would have a high "Whiz-Bang" factor! Those carbs have a lot of flexibility and even have an accelerator shot.
Yeah, I was thinking two U channels joined at the hip, with two carbs. I would love side-drafts but there's not enough room. Plus, I do have to factor in the exhaust design, which needs to give enough room to be able to set the lifters. Phil
Those are pretty. Yes, that's definitely an option- I have a pair of Carter WCD's which are about 270CFM each, which is a little much for the motor but still just about in the correct ballpark. Phil
That was my first choice as well, 4 SU's (HS4 or HS6s i think i was looking at) but steering and exhaust and the angle of the front end/ hood sides, all would have made it kind of clumsy. Looking forward to seeing what you come up with!
I'd try for a dual-plane design with two carbs. You could reduce the separate plenum sizes promoting velocity/torque, and having 1/2 of each carb feeding 4 cylinders would eliminate the need for a balance tube and any pesky carb syncing. Pair 1/2 with 5/6 and 3/4 with 7/8 to keep runner length roughly equal. Jigger the carb location on the plenums to give the same equal length as much as possible, although anything you do will be a compromise. V8s use dual planes for these reasons. If using side-drafts like Mikunis, I'd go IR (individual runners), one carb per one intake port and install some vacuum ports to make carb syncing easy. Overkill on the flathead, as their main advantage is fast throttle response.
Response isn't what I'm after- it's primarily to try and provide a better mixture balance across all the cylinders. The single carb with uneven runners is fine for low engine speed (200-1000 RPM) which is putzing-about-town speeds. Peak torque at 2200 RPM relates to about 45 mph which is fine for back roads but once I hit the highway I'm in the 2500-2800 RPM range and pulling the plugs shows a visually different set of deposits across the cylinders. If I can reduce that uneven burn then 55 mph cruise should be a little better with improved torque. Economy is a secondary concern because it's already pretty bad. A bit more torque in the upper rev range to get up a short onramp safely or pass up a slowpoke is the main goal. Re dual plane, I was thinking of doing that principle. The WCD's I have are twin barrel so one tube facing one barrel, one the other with a common shared space between them just below the barrel for balance. Basically each intake port would have its own barrel, meaning two cylinders per carb barrel. That would be a sacrifice in firing order optimization but ultimately should be better as a twin-carb setup to vend mixture more evenly. (Not a picture of two sets of scrotæ) Basically that set-up, duplicated for the other 4 cylinders and a balance tube between. (Mimicking a 1950's Austin design). Phil
I think I'd keep the plenums separated. I believe the cutaways you see on some dual-plane intakes is for higher-RPM operation, not something that particularly applies here. I'd build the intake with full separation, then maybe experiment with an 'open' spacer under the carbs to see if anything would be gained.
Fair enough, either way they'll have a bit of a gap, due to the footprint of the WCD requiring it. There'll be a phenolic/rubber spacer too, and a heat shield. Phil
That would put a constant draw for half the cycle and nothing for the rest. Pairing 1/2 with 3/4 and then 5/6 with 7/8 would give a more even draw pattern. Also simplifies the intake. (Refer to my bad drawing of firing pattern towards the start of the thread). That would be OXOXXOXO on each carb. The only way to achieve OXOXOXOX is to pair the two outer ports and the two inner ports. Phil
There's always compromises involved.... LOL. What proves to be more important? Firing order or runner length? I'm not that smart....
I believe you may be over-engineering for a low RPM engine (but MUCH better than under-engineering). I do not think the pulse configuration at these RPM's is nearly so critical as plenum size (if plenum style manifold), and carburetor size regardless of manifold configuration. Remember that the ancient "formula" CFM = (RPM x CID) / 3456 is an approximation for a four stroke multi-cylinder engine of A MINIMUM OF FOUR CYLINDERS at 100 percent VE. If you have separate plenums and too small a pulse equalizing passage, the two carbs would need to be each almost as large as a single to not lose torque and HP. . If you decide to go IR, then basically you are dealing with four 2-cylinder engines. The approximate "international fudge factor" to amend the well-known equation above is to modify the result obtained above by (4/N) where N is the number of cylinders. Looking forward to seeing your finished product. Jon.
That statement really caught my eye. Now, Detroit built literally millions of dual-plane intakes without any 'pulse equalizing passage' with seemingly no ill effects, although I'll admit that later HiPo offerings did take this into account, but I put that off to better high-RPM potential. I'll also note that these are all V-configured motors. Inline motors on the other hand rarely (if ever) use dual-planes but I assumed that was due to the crudity of the design and/or an effort to save money as most of these were lower-cost power plants built more to a price. Most HiPo inlines tended to use IR intakes when they were purpose-built, not 'adapted' from a more plebian design. So I guess my question here is, just what is the benefit of a 'pulse equalizing passage' in this case?
The Jalopy Journal
