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Let’s talk about the difference between single plane and dual plane manifolds. Most people opt for a dual plane manifold for drivability on the street. If you want to read about a comparison of single and dual plane manifolds, check out the article at the bottom from Super Chevy Magazine (2002). Here is a basic summary.
A single plane manifold has a central plenum that feeds all eight cylinders. This provides a more direct path for the airflow and allows each cylinder to draw from all four barrels of the carb at high speed. The problem is at slow speed the pulses of each cylinder disrupt the airflow and make for a very rough idle. This also causes vacuum of the engine to be low. For a comparison look at SBC single plane intakes (non-Vortec), check out the attached single plane manifold comparison from Car Craft Magazine (2013). You can find the original with pictures here: http://www.hotrod.com/how-to/engine/...-intakes-test/
A dual plane manifold divides the cylinders into two banks of four. This limits the tendency for the cylinder pulses to interfere with each other and makes for a much smoother idle, better low end response, and higher vacuum. These benefits disappear at higher rpm’s, however, since each cylinder can only draw from two barrels of the carburetor. Car Craft (2013) also did an extensive comparison of SBC dual plane intakes (also non-Vortec). It is also attached below. You can find the original with pictures here: http://www.hotrod.com/how-to/engine/...d-comparisons/
I will have to admit that I was sorely tempted to go with a single plane manifold, but in the end I decided to stick with dual plane since it made more sense for the street. Having decided to go with a taller manifold on my build, I traded the Weiand Street Warrior I had purchased to Joe (for some parts to be named in the future) and ordered a Weiand Steath Vortec (8151). I found one guy who was using it on a ’77 Corvette and said he had no problems. And I happened to score the polished version which looked really cool. More on the Stealth in a future post.
Weiand Stealth 8151 Vortec
I also toyed with the idea of going with a dual quad manifold (dual carburetors). Logic seemed to dictate that two carburetors spaced out over the top of the engine would be better than one in the middle. But Joe says that is not the case. In every test HVH did the single quad always produced more horsepower than the dual quad. If you want to know more about this issue, there is an article at the end from Super Chevy Magazine (2007) that confirms this phenomenon.
Weiand also makes an air gap version of the Stealth called the Air Strike (8502). Edelbrock has a similar option in the Performer RPM Air Gap. One difference between the two is that Edelbrock provides a plenum port in the Air Gap and Weiand does not in the Air Strike. Joe said that air gap manifolds only give a slight increase in power offset by a slight decrease in drivability due to less heat transfer from the engine. (If anyone has dyno evidence about this I would be happy to hear it). The Air Strike was significantly more expensive than the normal Stealth so I opted for the standard version. (Note: As of this date Weiand seems to have discontinued the Stealth line which probably means there is a new version in the works)
A popular solution to increase high end torque and power in dual plane manifolds is to add a plenum window/balancing port between the planes of the manifold. What this does is allow for cylinders in one plane to “borrow” from the other side in high demand situations. Many of the top manifolds in Car Crafts dual plane test had plenum windows. If you check out pictures of the different manifolds you will see slightly different shapes with a similar overall design.
Joe says that in general a manifold with a plenum window will outperform one without at the top end. While my Weiand Stealth did not come with a plenum window, we added one designed to mate up to an HVH Super Sucker carb spacer. This combination always produced the best results in HVH tests.
Plenum window to match HVH Super Sucker
Below is a head to head comparison of the Weiand Air Strike and the Edelbrock Performer EPS Air Gap. Notice that adding a plenum port to the Air Strike added 4-5 horsepower over the solid plenum. This was also on a stock crate engine. I imagine the increase would be higher on an engine with higher performance.
Back to the hood clearance issue. I am fairly confident that the Stealth will fit under the hood of my ’76. If not, I will probably go with an aftermarket hood with an increased rise and go ahead and add an HVH Super Sucker. Joe says the HVH carb spacer is good for an additional 10 hp generally, of course he was VP of sales!
The final attachment is a test on the HVH Super Sucker done by Super Chevy Magazine in 2002. If you are looking for a carb spacer it is best to talk to the people who make it about the specifics of your engine. All carb spacers are not equal and some carb spacer designs actually disrupt the flow rather than smoothing it costing you horsepower rather than adding it.
Dual quads do work if you have a correctly designed intake. My old vette came with dual quads and the intake had no thought as to tuned length ports creating a ram effect to increase cylinder filling. The modern Edl dual quad is the same way. Tunnel rams actually work.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Originally Posted by mbhnm
A single plane manifold has a central plenum that feeds all eight cylinders. This provides a more direct path for the airflow and allows each cylinder to draw from all four barrels of the carb at high speed. The problem is at slow speed the pulses of each cylinder disrupt the airflow and make for a very rough idle. This also causes vacuum of the engine to be low.
This is probably a generalized statement but it is not the case for my Team G. My TG idles very smooth and does not affect my vacuum. I was a bit leary about using a single plane but the TG has worked out well and room for a 4" air cleaner using a drop base.
My use of HVH material is not an endorsement or advertisement for HVH. And it is freely available on the Internet. I am just using what I have learned and the HVH people a where a lot of my information comes from. If someone feels like I am crossing a line here just let me know.
From my perspective I think its great that you have a relationship with a shop as close as you do with the guys at HVH and can trickle down some of their experience with us.
When you find someone you trust and gives you results you'll be their biggest advocate; be it the guy at the paint desk at home depot, or product/brand, etc.
I don't blame you for referencing them and I hope you continue to do so. Like I said before I never heard of HVH and I am glad you brought attention to them. The more people that know about the american "little guys" the quicker we can take back our country back!
Originally Posted by mbhnm
Back to the hood clearance issue. I am fairly confident that the Stealth will fit under the hood of my ’76. If not, I will probably go with an aftermarket hood with an increased rise and go ahead and add an HVH Super Sucker. Joe says the HVH carb spacer is good for an additional 10 hp generally, of course he was VP of sales!
My general feeling on carb spacers is this: They are tools used for tuning. They effectively increase runner length and plenum volume. You can't say that in every application a 1" spacer will add 10hp and a 2" will be 20 hp or what have you. It's all about what the engine wanted in the first place. So if you run an intake that is mismatched for the rest of your combo (too "tame", ie edelbrock performer, the old school ones) and you put a 1" spacer on and get 10hp, that gain would have been noted anyways if you went with something like a performer air gap, and so on down the line until you find that gives you the best effective runner length / plenum volume / airflow transition. If you care to extrapolate, in that sense it allows you to experiment with different intakes to a new factor. So take that list you have of intakes (40 or so) and multiply that by 5 (however many popular configurations of carb spacers are out there). There are essentially 200+ different intakes to choose from when considering spacers in the mix
The one thing I do use spacers for that you can only get from the use of a spacer is to eliminate heat soak into the carburetor (depending on material of spacer). From a personal preference, I always factor in using at least a 1/2" wooden 4bbl spacer on my engines just to keep the carb a couple dozen degrees cooler than the intake.
Last edited by AW IR C3; Dec 23, 2015 at 09:37 PM.
This is probably a generalized statement but it is not the case for my Team G. My TG idles very smooth and does not affect my vacuum. I was a bit leary about using a single plane but the TG has worked out well and room for a 4" air cleaner using a drop base.
I agree. If an engine is designed with the intake pulses in mind the larger stroked SB will idle better with a single plane. Large CID with dual planes will "hunt" for air. With singles they are much happier.
One thing I forgot to add is that my Weiand was ported to match the HVH Street Sweeper spacer (#ST4150-4AL).
I've had single planes for 30 some years on my 79 Vette. I had some motor way back when that when you shut it off on a 100 degree summer day that you could actually hear the gas boiling in the bowls. Then it had hot start problems.
My research led me to 1/2 wood spacers as being the best thermal block. It cured the carb boiling. In actual engine dyno testing on multiple platforms the 4 hole wood was the best.
Being young and dumber I actually tried a weiand stealth dual plane from a weiand X-celerator single to regain some bottom end on an over cammed 355. I did not see a vacuum change at all that i can remember and it didn't change any seat of the pants feel. So I took it back off.
My elderly advice to you is: I subscribed to all the car and motorcycle rags for many years. It became obvious that testing can be slanted to produce the desired results favoring the product that paid for the article. So always be skeptical and don't quote them as facts
My elderly advice to you is: I subscribed to all the car and motorcycle rags for many years. It became obvious that testing can be slanted to produce the desired results favoring the product that paid for the article. So always be skeptical and don't quote them as facts
Hence the caveat, “…of course he was VP of sales!”
My original plan was to make my new engine look as much as possible like a stock L79. That meant going with exhaust manifolds rather than headers. Joe was against headers from the beginning, "They crack and they leak!" was his philosophy. Corvette rams horn seemed the best choice. According to some sources they flow as well as shorty headers and are pretty dependable.
Researching Corvette manifolds I found a lot of guys were using the ones off the early C2 327s. Seems for a few years Chevy made them with 2 1/2" outlets and then switched in the mid sixties to 2". Nearly all of them are 2". So I set out on a search for the 2 1/2" version.
Original 2 ½” manifolds are extremely pricey and they are nearly 40 years old. So I decided to look for a replica version. From what I read the Dorman copies are the best available, they corrected some minor flaws in the original casting, and are made in China like all the clones but are a higher quality than most.
The correct Dorman numbers for the right and left are 674-503 and 674-504. They have brackets cast on the front end for the accessories but a lot of guys cut these off. (There are also part numbers 674-500 and 674-501 which I think are the 2” versions, you probably don’t want these).
Dorman 2.5" manifolds
Having my part numbers on hand, I started the search. First found the right ones for $168, then $148, then another site for $134. Was about ready to order them when I found a Dorman part number that led me to Amazon. $109 for the pair with free shipping. (Note: as of this date – Dec. 2015 – Amazon has them for $96 plus shipping and Speedway for $114). The manifolds looked good when they came in and had exactly the same ports as a 2” set of originals Joe had in his parts bin.
The build progressed for a while and the manifolds went on the back burner until some of the other parts came together. The issue came to a head (no pun intended) when we did a test fit of the HVH heads on my block with the head studs I was planning to use. What we found was that the two studs in the lower center of the head interfered with the exhaust manifold.
Head stud and manifold clearance issues
Manifold bolt recesses
The manifolds had recesses cast into them to clear bolts but they are not deep enough to clear studs. I didn’t want to do a lot of grinding on the manifolds since you can create a weak spot that can cause a crack. I also did not want to cut off the head studs. And given I was using a 35 year old block I didn’t want to change out the head studs for bolts either. Now I realize this is not really a big deal, but I decided that is not what I wanted to do. That is the beauty of building your own engine, you can do what you want.
We tried Joe’s original 2” manifold and it just barely hit the studs and could be made to work, but I didn’t really want to use 2” manifolds. I found the clearance problem quite interesting since I have never heard anyone on any message board comment on the problem of using head studs with stock manifolds.
So I went back to the drawing board and re-thought the whole header equation. One reason I was planning a stock looking engine was Tennessee law concerning antique cars. Tennessee code says you can run antique plates if your car is 25 years old and has an “unmodified” body and engine. Antique plates would bring the cost of insurance and registration way down but had some restrictions on when and where you could drive the car. But for a guy on a budget it seemed like a good choice.
While I was pondering this dilemma, Joe and I went to a cruise-in at a local restaurant in Greenback, Tennessee not far from my house. There was a guy there running a drag Ford Anglia on the street with a huge supercharger sticking through the hood. And he had antique plates. I surmised from this that the local authorities are probably not very diligent about enforcing the “stock” clause of the Tennessee statute and decided to go with headers. The car would still look stock as long as no one opened the hood. (And yes, I realize you can disguise headers with heat shields which I am considering)
So I began the header search. I spent a lot of time pondering a header combo with side pipes. Two I considered were Doug Thorley and Hooker. Both use 1 7/8” primaries and 4” collectors but Doug’s are heavier (16 ga versus 18), have thicker flanges (3/8”) and are a tad cheaper. You can get the combo in chrome, ceramic or black. And they make three mufflers: glasspack, loud (wonder how much louder can you be than glasspacks?) and quieter reverse flow.
I liked the look of the side pipes but ultimately decided that side pipes would be an advertisement that things under the hood were not stock. If I kept everything under the hood there would be no obvious clues that the engine was not stock and I wouldn’t be constantly looking over my shoulder. (Yes, I know some Vette’s in the 60’s came with side pipes but I never really liked the look of the stock versions and they didn’t come on ’76 Vettes anyway). So back to pondering the header selection, there were plenty to choose from.
Ultimately I settled on Hedman’s Elite Series #68308. Summit had a deal on them at the time ($293.36 when I bought them, $429.97 as of December 2015) and also had free shipping. Several people in the forum said they had worked for them although a few people mentioned problems with specific Corvettes. The most common problem was an issue clearing the idler arm. The second was a few people that mentioned clearance issues with the power steering. I looked at both these problems and neither seemed insurmountable.
So I went with the Hedman’s. They had a lifetime guarantee, thicker flanges (3/8), and heavier tubes (14 GA) than most others I found. They were also HTC ceramic coated which should improve the life and reduce under hood temps a bit. The Hedman’s also had 1 5/8” tubes versus the 1 7/8” of the Doug Thorley and Hooker side pipe units. For a discussion of header function and design issues, see the article from Corvette Online at the bottom also available at the original site here: http://www.corvetteonline.com/tech-s...n-performance/
Hedman Elite 68308 outside view
Hedman Elite 68308 inside view
Why concern yourself with the exhaust when you haven’t even gotten the engine assembled? One of the things you want to do is make sure all your ports line up before you get to the point where you can’t do anything about it. With the intake and exhaust settled on I was in position to port the heads so that everything matched. More on that in the next post.
From: Las Vegas - Just stop perpetuating myths please.
Wow, thats more than your fair share of exh manifold problems. Who would think stock manifolds wouldnt clear the studs? Well im glad to see u installed headers anyways and i think headmen headers have some great features/products. And your set looks great in ceramic coated. Im surprised to read they are 14ga though as most of their street headers are thinner like 18ga. Thats a pretty awesome price for quality brand headers - the welds look great.
Something i can share with u is when u need to fit them up to the exh pipes an easy way is to measure the offset from header collector to exh pipe. I use the centerline of each as it made sense to me though u may find a different reference line that works better. U can take something like a piece of welding rod and tape it to centerline of header collector then another to centerline of exh pipe. This distance between them will be the offset. Any exh shop can bend up an offset piece for u - local Midas did mine. U will need to estimate the length also but go long so u can trim it at home for best fit.
This should save u from driving w/opem headers to the exh shop.
(And yes, I realize you can disguise headers with heat shields which I am considering)
Headers disguised with heat shields look like headers with heat shields on them. Not much of a disguise, IMO. Some folks consider them a "dress-up" item, but I don't. I wouldn't install them. Your spark plug wires will thank you.
Good luck keeping the flange gaskets sealed with those headers, the only happy 3/8 thick flanges are stainless. Youll be using the oem iron manifolds before long. Not to mention the extra noise under the hood.
If one is stuck on manifolds test fit a set from 92-96 LT1 they flow pretty well for...manifolds. Might lose your acc mounts not sure about exit on a C3...but would laugh if they worked and they are dirt cheap used
Something i can share with u is when u need to fit them up to the exh pipes an easy way is to measure the offset from header collector to exh pipe. I use the centerline of each as it made sense to me though u may find a different reference line that works better. U can take something like a piece of welding rod and tape it to centerline of header collector then another to centerline of exh pipe. This distance between them will be the offset. Any exh shop can bend up an offset piece for u - local Midas did mine. U will need to estimate the length also but go long so u can trim it at home for best fit. This should save u from driving w/opem headers to the exh shop.
Thanks Cardo0! This should help some people down the line. I have the advantage of having a guy who runs a custom exhaust shop behind his house half a mile from my house. How can you go wrong with a shop named "Duck's Muffler" and a big yellow duck on the sign. Must be a story there. Have to try to catch him one day to find out.
A funny for you. Joe lives across the street from a big cow pasture. Primarly Angus beef cows. First time we fired up an engine (open headers on a run stand) the cows all perked up and came across the field to see what that big bull looked like that was bellowing. I will never forget the sight of twenty cows lined up along the fence watching us run in the engine!
Thanks Cardo0! This should help some people down the line. I have the advantage of having a guy who runs a custom exhaust shop behind his house half a mile from my house. How can you go wrong with a shop named "Duck's Muffler" and a big yellow duck on the sign. Must be a story there. Have to try to catch him one day to find out.
A funny for you. Joe lives across the street from a big cow pasture. Primarly Angus beef cows. First time we fired up an engine (open headers on a run stand) the cows all perked up and came across the field to see what that big bull looked like that was bellowing. I will never forget the sight of twenty cows lined up along the fence watching us run in the engine!
That's cattle, not cows.
Sorry, just the Nebraskan in me comin' out.
An interesting experience for sure.
If one is stuck on manifolds test fit a set from 92-96 LT1 they flow pretty well for...manifolds. Might lose your acc mounts not sure about exit on a C3...but would laugh if they worked and they are dirt cheap used
First time I have heard that suggestion. There is a nice write-up on the LT1 and L98 manifolds in the C4 forum. Here is the location:
Something to watch for is that there seem to be variations of the port types in those manifolds. Many of them have "D" shaped ports, I will talk about that more when I get to exhaust porting.
If you check HVH's website, most of their Brodix heads have "D" shaped ports.
