Bottom line up front: Porting will buy more hp than cams alone, and doing cams without porting will not be very effective. This goes even for K-jet. 40dcnf carbs are big enough for applications up to 330+ hp Spinning off of Mark's porting discussion some may have missed in the supercharged section, it looks like one of the best bang for the buck, either for 2v, 4v, K-jet or especially carbs is porting the heads. While the Kjetronic to Webers will gain almost 30 hp, many have found that the next large restriction is the headports. I have found in the qv case that the heads seem matched to the cams - when I increased the cam duration and lift, the rwhp was essetially unchanged, and increasing the carb venturi size had no effect on peak hp delivered. Ergo, the headport is the likely next thing that can be opened up. Mark calculated/computer simulated in the other thread that opening up the heads was anywhere from 30 - 70 hp depending on cams and induction. This seems to make sense. Talked to Tate Casey at Carobu who also feels there is hp hiding in the heads, but probably not much more than a total of 300 - 310 before losing a lot of bottom end. James Patterson at Norwoods, while more of an expert at forced induction, feels like there may only be 10 or so hp to be gained by porting the heads. Nick and Kermit have both claimed pretty impressive figures from head porting, with one of Kermits customers (RM) dynos approximating qv hp from a 2v k-jet from opening up the ports. Of course it gets down to cost - I've heard estimates from around $2 - 4K for just the porting/head re-freshening not counting the head removal/re-assembly. Tate also said that there is hp from just working the valves in cars of 40K miles or more. I am waiting on Mark's numbers on his qv before I commit, but it is promising. So just how much carb do you need? Well talking to Mike Pierce of Pierce Manifolds, the std 40 dcnfs can be opened up to 34 and 36mm venturis which well support into the 300 - 330 hp ranges. As far as a 3.2, he calculated that the venturi giving peak flow and hp for a 400cc cylinder is 34.5mm. So there is probably not the need to go higher (unless running 9K rpm) than a 40 dcnf as 'the governing factor is the choke size' - even larger carbs have found it neccesary to run 36mm and smaller venturis for adequate signal. I thought it was also interesting that he said it had long been proven that the Weber carbs have long been shown to produce just as much torque and hp as fuel injection (although perhaps not as emissions friendly). He also confirmed that Webers typically cruise a lot richer than injection, with AFRs in the 12-13 range as opposed to the FI 14+. So, the point of all this is a new appreciation for the possible benefits of porting the head. I'll post data here as I find out more.
I personally think you got some misinformation from Mike Pierce. I'm sure he's right when he says you can get all the air you need through a 34mm venturi...I had an engine that made 74rwhp though a 20mm venturi, so you can certainly flow 50 crank hp worth of air through a 34. BUT, the vernturi is not the hole story nor the actual restriction in the carb. The major restriction is the throttle plate. Some engines are really sensitive the plate, or the orientation of the plate. IMO, 40mm is about the biggest you want to go on the carb size for it to work on the street, but pretty borderline for flow. I think that on a stock engine with heads that only flow 94 cfm, a 40mm carb is plenty big. After you get the ports flowing properly, well, I’m not so sure. When I was messing with a 2.7 liter 911, the standard was that EFI gained 25-30 hp over the weber set-up. I see no reason to believe you won’t find about the same thing on a ferrari…so I don’t know when or where it’s ever been proven that webers make the same hp and torque as EFI…except maybe at the weber dealer. Seriously, you won’t find anyone putting them on anything except a vintage race car or as you did for the effect. They’re cool, but they are primitive, better than CIS, but…..
So, I got word from T.Rutland today that they don't have a practice head for me I need to start looking again...it's so much easier when you can grind without fear.
I think one of the more interesting applications of this porting thing is for Kjet guys (GTBSi, QV and Mondials) that wish to keep their K-jet but want more performance and retain a stock look. If RM's experience is any indicator, his 2v Mondial was laying down as much rwhp almost as a 3.2 with the ported engine and bigger tb. I would think a qv with 3.2 and a port job could possible pick up 30 hp even with the K-jet based on his experience. On this board, we have dang near quantified every alleged performance increase tweak save porting, although there are some impressive anecdotes and extrapolations from other cars. Bob Norwood is suppoed to be getting back to James with some of his data and sims, and combined with Mark's flow benching I think we are really going to get some insight. As far as the carb vs efi thing, it is difficult to totally cross compare. I think for total delivered hp carbs can be made to flow as much as efi, it's just the mid and lower range mixture control will not be as smooth or accurate. For a street application (without much emmisions concern), I think 40 mm carbs with venturi options up to 36mm are pretty durn cost effective and effective. While they might be described as primitive, I like to see them as mechanically elegant and more soulful. I believe that with the heads opened up, as your dynos predict, even with stock fi cams the 3.2 will be good for well over 300 hp. Tate seems to think 310 is a good conservative number. Again, I am constraining my development to keep a very strong lower end for street use - I ideally would like to see over 250 - 260 ft-lbs of torque. This is as opposed to Paul S.'s radical 3.0 qv with wild cams and 48 IDAs which will have an astronomically high output (>> 110hp/liter) I'm guessing up around 8000+ rpm. I'll still say that for street hp in the kjet engine, the experience will be like the K-jet 911 guys - the least expensive and most bang for buck is carbs. It may not be as flexible as FI, but it's sure a lot simpler to set up. I've copied Mark's dyno sim of a 3.2 with Webers, stock fi cam and a ported head. I sure hope he's right, as I may be about to find out. Image Unavailable, Please Login
Russ, I've never seem an engine that won't make the predicted number....some times it takes a bit to get it right, but when you get the air flow right, you make hp. I think you're point about top end vs dribablility is basically right. You can do a decent performace engine that makes good power across the rpm band with carb. You can also build a race engine that makes great hp with carbs. With slide-type carbs or EFI, you can build a raace engine that can be driven to church on sundays, it just lets you go a little more radical without losing drivablility....but that's not what everyone's after. I think Ferrari probably chose the redline for the 308s more based on what they could do with good drivablility than what the engien could stand. If they trying to spin it faster, it would want more air, so bigger carbs and causing poor bottom end. Good EFI allowed the 8500 redline in the 355 and the peak hp to jump from 7000 to 8200, brign almost 20% more hp with it. The same would be true on a 308 or 328. Flow and cam the engine to pull to 8500 and the 340 my sim predicted with stock cams at 7000 jumps 380-400...and hang on
I think the aura, or whatever, of the Weber, goes directly back to Formula 1, and road racing. Yes, you could probably achieve more power with a "good" and well designed fuel injection system, or perhaps a different style carburator, but nothing could really ever beat a Weber in either reliability, or trackside jetting changes. And you don't win races sitting on the side of the track with electrical or fuel system troubles. As far as I recall, Ferrari, Lotus, Jag, Alfa, and many others continued using Webers well into the 1960's in many forms of international motor racing. Not always because they made the most absolute power, but because they worked. The thing is, I dont have another $4000 (or more) laying around to pay for porting. I've gone so far as to look around and find plans to build a flow bench, and it can be done for less than $500. And I would probably do that if I had a good idea where to begin on the head. You can read so much sometimes you get glossy eyed, and forget where you started. But the reality is, I have heard as many negatives of porting as I have heard positive. I have seen port work where when they flow benched it, it actually showed a "decrease" in flow. So in this 308 thing, I wouldnt want to screw it up. But then again, if they flow so horribly, did they even try when they made it? They didnt bother to match the ports, on either side. On my engine, and I can only assume its representative, It seems like they just pulled parts off the shelf, ground off the flashing, and slapped motors together. I'm not knocking Ferrari, just stating that maybe what we get is akin to a 1/43 scale model kit of little roughly cast parts you have to tool on and massage to make it into something pretty. On my motor I see the carb base doesnt match the manifold. The manifolds dont match the heads. The heads exhaust ports have the header studs offset from the actual port, so the header dont line up. And the headers arent the most high tech things ive ever seen either. So, if all of these steps, which no matter how much I read tell me they mess up flow, if they were all removed, are there any guesses as to improvement? I mean, its totally obvious my car was never flow benched at Ferrari, nor even port matched, so it already would have flow rates that would vary wildly from cylinder to cylinder from day one. So the down and dirty question of the day is, are there some good baseline ideas for where we could start?????
You can easily build a good flow bench for under $500, there's nothing hard about that part of the job. The hard part is getting the heads to flow. It is very very easy to make them worse than they were when you started or make a head that has good flow number ion the buch, but on the engine make a nice hp peak and no hp anywhere else. There is a lot of power in the shap and the height of the valve seat, the size and shape of the valves, watching your cross sectional areas, and making things smooth - no ripples, no bumps, no divots. As a general rule, grindding is never required on the bottom of the port, it's too low already on every head. I like to use oil-based modeling clay, I put a little clay on the area I think needs grinding, if the flow goes down, I grind a bit. But if the flow goes up...make a note to yourself not to get anywhere near that spot with the grinder. As for port matching, it should help, 10 hp maybe? The exhaust side I'm not sure about...if the exhaust port on the 2V is too big, the offset heads maybe actually be helping. You can test it on your new flow bech by removing the studs and clamping on the header to see what happens. The exhasut flow through the header should be about 80% of the intake flow through the carbs. The last thing to note about your post is that you talk about webers being top-shelf race parts into the 60s...I would have even given you the 70s, but that was 30-40 years ago. They're still good parts and pretty cool...but they haven't been race parts for a long long time.
Well, the little bit of time I have taken to start matching the intake ports, and the time I have taken to read deeper into porting than ever before in my life, I can certainly see how fast you could burn up a lot of money having someone else do it. Just the exhaust alone, trying to not only improve the flow in each port, but matching it, and with and without the header, back and forth, back and forth, a guy could spend days. As to the header being offset from the port, does anyone else have this, or seen it, or did I get the wednesday heads? And the headers? The back header, when I look inside the section that attaches to the muffler? It goes a good two or three inches inside past the weld where the collector cones down. I bet if a lot of air was blown through it it would whistle. No clue what the front one is like as the pipe it to long. I think Hedman could have done as well. Here is my plan, as I really have neither the time right now to either build a flow bench, or play around with a lot of porting. I intend to match the intake manifold ports to the head ports, by taking off just enough on both sides to smooth them out. I think I should close up the air injection port as well so it dont whistle. I want to re-align the header studs/bolt holes to match the exhaust ports, then finish up both ports and polish the combustion chambers. I also will carefully look at the valve seat area after I get that work done, try to keep everything as smooth as I can and remove as very little metal as possible. The last thing, is what to do with the manifolds where the carbs attach, as they are much larger than the carb bores and not all matched either. I am thinking of having them welded up some and grinding them back to match. The last thing I want to do, is cut open the headers and see if I can clean them up. Mark, you mentioned mandrel bends on the headers, and mine arent, they have that wrinkle in the inside radiuses. Any idea what kind of difference that would make?
I have used porting epoxy on the intrake tracts of my race bikes, (right next to the valve seat) It held up perfectly for as long as I had the bike. I am sure that it would be fine to use it to match the carbs to the manifolds.
It seems like just yesterday to some of us...[/QUOTE] Now don't get started on "we must be getting old.." LOL On the exhaust side, and intake too, for that matter, port matching is much more important than most realize. It is perfectly fine to have the port/header be a bit larger than the port that it is flowing from. To put it another way. To "fall off", or go from one size into another will not effect flow. The area left will usually become dead air. To run into a step, however will create turbulance and will always cost power. An excellent example it an anti reversion step in the exhaust port. The flow goes out very nicely, a it is opening into a larger area. On reversion however, the tumbling caused by the flow striking the step will dramatically reduce flow, thus cutting down on the reversion effect. The stock headers are better than a cast manifold.... From a standpoint of power, they leave much to be desired. The ripples on the tubes arent too much of a problem, compared to the collectors. The collectors are not good for much more than getting the flow into a single pipe. Had they been longer, and larger, there would have been performance gains. They are however from a production standpoint comparitively good. The effects of backpressure from a restrictive muffler can easily make it all a moot point, as the bst ports and headers in the world will gt choked by a bad mufler. To sum it all up. The system must flow smoothly, and evenly from air cleaner to muffler tip.
The porting epoxy that I used was more like clay than JB weld. You would cut off two chunks, and mix them together. When pressed into place, and smoothed with a wet finger, the job was done.
