It is, however they also exaggerate the duration split on the lobes, hanging the exhaust open much longer. While it has been proven these cams can actually make power, I believe their design in large is to make a more tame engine sound much more nasty than it really is. These can make a low compression engine a little more fun. For a purpose built engine, such as yours, the cam parameters should be designed more for engine output than for sound. I've noticed a lot of well built performance engines using single pattern cams, or ones with smaller duration split numbers.
340 street build help from experienced input
- Thread starter Cmanhattan
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Single pattern cams allow as good as no timing adjustability, and unless it is installed straight up, any timing change subtracts Effective overlap.
For this combo, the single most important thing is to close the intake as early as possible, to trap as much pressure as possible,.......
and still make some power, cuz nobody wants to brag about a sucked out 340 that doesn't wake up until 3800 or more rpm, right.
As far as the cam is concerned, the next most important thing is to extract as much power out of the hot exhaust gasses as can be done without sacrificing exhaust duration.So then you want a very late exhaust opening. This will also help with fuel-economy.
After that is overlap. You want as much as possible with this combo, to make up for the other crappy events. And THAT is the purpose of tightening up the LSA.
At the extreme, a cam like this will not make a lot of average power. So you gotta put a lil spike on the curve, using overlap and must-have headers, with must-have free-flowing exhaust. Nothing mus hamper the exhaust when it is time to go, because that will kill the overlap, the very thing we are trying to promote.
Now, this spike, created by the overlap, comes over a very narrow rpm band right at the top of the power curve. Si if you had hiway gears you will likely never get to feel it. Usually the tires are spinning right off the get -go and they spin all the way thru first. So the overlap powerspike is there, but you go thru it so fast that you never feel it; even if the tires quit spinning. So then, Lets say the spike comes at 4200 to 5000. In Second gear, with say 2.94s , this is from 59 to 70 mph., centered at at ~65mph...... which is usually were I shut down. So if the tires are NOT spinning you will feel it. It's sorta like a 2-stroke dirtbike when it comes up "on the the pipe", it comes, flashes, and then time to shift!
So Ideally, you might want to gear the car so that the engine runs thru the spike, and down the back of the power curve but the 2-3 shift comes just after, say 60 mph. And then you deal with the traction issue. In this way not only will you feel the rush, it will show up on your zero to 60 ET. right.
Ok so first you, OP, gotta know that the spike I'm talking about is not huge, and depends on the size of the cam; the bigger it is the taller the spike. But, we already know, that to keep the pressure up in your combo, we are seriously handicapped, so the cam, of necessity, has to be fairly modest. So in your case, the spike may be I'm guessing, 8 to 10 hp at the top. This sounds like not very much, but hang on; this is nearly a full cam size, in the sizes we are (OK; I am) looking at. So altho the number "looks" small, it actually is not, and it's a bonus. But you only get it if you have headers and a free-flowing exhaust. Oh and, your thin hi-altitude air, has to have a straight shot at the carb, and remain as cold as possible. If you feed it hot underhood air, that air will be even less dense by the time it gets past the carb.
So this is why I recommend a solid-lifter for your combo, because it MIGHT be one size bigger at .050, yet the same or slightly smaller than a similarly sized hydro on the "advertised" Just know that as to advertised these two types of cams are measured differently, on account of the lash . This is also why I said to call a grinder and let him sort it out. This is also why you need to select the ICA as soon as you have an exact Scr, because the Wallace Dcr calculator bases all it's parameters on knowing that and the working elevation.
I guess it would also be prudent to mention, that after this engine is built to operate at 6000 ft, with an Scr of 10.83 or higher, you have a bit of a headache. If you drive it downhill, the air will get more dense, and the cylinder pressure will rise. Now, at 10.83Scr and 160psi IIRC, you have a good amount of a detonation safety factor. But the lower you go, the higher will be the pressure. By 1000 ft, the Wallace is predicting 187 psi, still ok for pump gas, but at sealevel the pressure is predicted to be 193, so you better have 91 or better gas in the tank, if you intend to nail it. You have to pay attention to this, whenever you descend from 6000ft.
Another thing you have to pay attention to, is the tune. With more dense air, the engine will run leaner and hotter and will want less timing. You have to stop about every 2000 feet and retune it. By the time you get to sealeve, having the right tune, your V/P has risen to 163, and that 340 of yours will be making a ton of low-rpm power.
Btw, 163 is what my 223* cam made at 900ft so I know 163, lol; it is brutal.
For this combo, the single most important thing is to close the intake as early as possible, to trap as much pressure as possible,.......
and still make some power, cuz nobody wants to brag about a sucked out 340 that doesn't wake up until 3800 or more rpm, right.
As far as the cam is concerned, the next most important thing is to extract as much power out of the hot exhaust gasses as can be done without sacrificing exhaust duration.So then you want a very late exhaust opening. This will also help with fuel-economy.
After that is overlap. You want as much as possible with this combo, to make up for the other crappy events. And THAT is the purpose of tightening up the LSA.
At the extreme, a cam like this will not make a lot of average power. So you gotta put a lil spike on the curve, using overlap and must-have headers, with must-have free-flowing exhaust. Nothing mus hamper the exhaust when it is time to go, because that will kill the overlap, the very thing we are trying to promote.
Now, this spike, created by the overlap, comes over a very narrow rpm band right at the top of the power curve. Si if you had hiway gears you will likely never get to feel it. Usually the tires are spinning right off the get -go and they spin all the way thru first. So the overlap powerspike is there, but you go thru it so fast that you never feel it; even if the tires quit spinning. So then, Lets say the spike comes at 4200 to 5000. In Second gear, with say 2.94s , this is from 59 to 70 mph., centered at at ~65mph...... which is usually were I shut down. So if the tires are NOT spinning you will feel it. It's sorta like a 2-stroke dirtbike when it comes up "on the the pipe", it comes, flashes, and then time to shift!
So Ideally, you might want to gear the car so that the engine runs thru the spike, and down the back of the power curve but the 2-3 shift comes just after, say 60 mph. And then you deal with the traction issue. In this way not only will you feel the rush, it will show up on your zero to 60 ET. right.
Ok so first you, OP, gotta know that the spike I'm talking about is not huge, and depends on the size of the cam; the bigger it is the taller the spike. But, we already know, that to keep the pressure up in your combo, we are seriously handicapped, so the cam, of necessity, has to be fairly modest. So in your case, the spike may be I'm guessing, 8 to 10 hp at the top. This sounds like not very much, but hang on; this is nearly a full cam size, in the sizes we are (OK; I am) looking at. So altho the number "looks" small, it actually is not, and it's a bonus. But you only get it if you have headers and a free-flowing exhaust. Oh and, your thin hi-altitude air, has to have a straight shot at the carb, and remain as cold as possible. If you feed it hot underhood air, that air will be even less dense by the time it gets past the carb.
So this is why I recommend a solid-lifter for your combo, because it MIGHT be one size bigger at .050, yet the same or slightly smaller than a similarly sized hydro on the "advertised" Just know that as to advertised these two types of cams are measured differently, on account of the lash . This is also why I said to call a grinder and let him sort it out. This is also why you need to select the ICA as soon as you have an exact Scr, because the Wallace Dcr calculator bases all it's parameters on knowing that and the working elevation.
I guess it would also be prudent to mention, that after this engine is built to operate at 6000 ft, with an Scr of 10.83 or higher, you have a bit of a headache. If you drive it downhill, the air will get more dense, and the cylinder pressure will rise. Now, at 10.83Scr and 160psi IIRC, you have a good amount of a detonation safety factor. But the lower you go, the higher will be the pressure. By 1000 ft, the Wallace is predicting 187 psi, still ok for pump gas, but at sealevel the pressure is predicted to be 193, so you better have 91 or better gas in the tank, if you intend to nail it. You have to pay attention to this, whenever you descend from 6000ft.
Another thing you have to pay attention to, is the tune. With more dense air, the engine will run leaner and hotter and will want less timing. You have to stop about every 2000 feet and retune it. By the time you get to sealeve, having the right tune, your V/P has risen to 163, and that 340 of yours will be making a ton of low-rpm power.
Btw, 163 is what my 223* cam made at 900ft so I know 163, lol; it is brutal.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
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Ok that is worded wrong I think; You can time the cam anywhere you want to lol. But here's the deal;
If this theoretical single-pattern cam had a mathematical overlap of say 46* at split overlap AKA straight up. And say you advance it the typical 4*. By doing that you loose double that in Effective overlap, which cannot be more than double the smallest half of the overlap, so in this case 2 x 19=38*Effective .
IMO, loosing this much overlap is bad for this combo. If OP had log-manifolds, the overlap has no meaning and you could almost ignore it entirely.
Take that same cam with 46* overlap, but with an 8* split, and install it 2* advanced, the Effective overlap becomes the same as the mathematical overlap. The penalty for this phenomenon, when keeping the LSA the same, is 4* less intake duration, and 4* more exhaust duration. But
keeping the same intake duration and same Effective overlap, the LSA changes just 1 degree and the exhaust grows 4 degrees. with 4* split, whoot-whoot. So now this cam can be moved in either direction and still loses double thee change, but it is already 1*advanced which is also 1* advanced from the single pattern, but the single pattern, for the same intake duration, had only 38Effective overlap compared to 46 with the 4* split. For this combo, IMO, we have to play this cam business pretty tight.
How I get my numbers is;
I play the numbers off the Ica, and put a reasonably sized amount of overlap on the cam, then calculate the other numbers from those, being ever mindful NOT to drive the LSA to tight. And I try to work with known camlobe sizes to make it easier on the cataloging, Cuz you know, if I come up with what I think is the perfect cam, but nobody ever heard of the lobes, what good does that do anybody.......
Last edited:
yellow rose
Overnight Sensation
It’s 2021. We should all know by now that “tight” lobe centers giving a bad idle, poor drivability and poor fuel mileage is long since been debunked.
You can dick off with spilt pattern this and jacking that around, but if you buy the correct cam the first time, it won’t matter.
If you want a SFT I’d be 234 @ .050 on a 106 LSA with as much lift as I could get and install it straight up.
If you want a HR I’d be 230 @ .050 on a 106 LSA with as much lift as you can get and install it straight up.
Split pattern cams don’t make sense to me. By the end of the year I hope to be testing this with data I own and can post.
I still feel it’s a gimmick and I haven’t had a single cam grinder convince me otherwise.
As for compression ratio, I’ll post it again...what they cylinder head is made of doesn’t make a pinch of **** difference regarding compression ratio. Not one. Where you are you can run easily over 11:1 on pump gas and not have an issue. No one should ever use low grade pump gas, even in a lawn mower. That fuel is cheap garbage, with a ton of fillers and junk. Premium pump gas is much more regulated and it much closer from brand to brand.
Slow down and think about how all this fits together. The myths of the 70’s and 80’s should be dead, but they live on, crippling builds every day.
And yes I’m tired and in a pissy mood. Doesn’t change the facts.
You can dick off with spilt pattern this and jacking that around, but if you buy the correct cam the first time, it won’t matter.
If you want a SFT I’d be 234 @ .050 on a 106 LSA with as much lift as I could get and install it straight up.
If you want a HR I’d be 230 @ .050 on a 106 LSA with as much lift as you can get and install it straight up.
Split pattern cams don’t make sense to me. By the end of the year I hope to be testing this with data I own and can post.
I still feel it’s a gimmick and I haven’t had a single cam grinder convince me otherwise.
As for compression ratio, I’ll post it again...what they cylinder head is made of doesn’t make a pinch of **** difference regarding compression ratio. Not one. Where you are you can run easily over 11:1 on pump gas and not have an issue. No one should ever use low grade pump gas, even in a lawn mower. That fuel is cheap garbage, with a ton of fillers and junk. Premium pump gas is much more regulated and it much closer from brand to brand.
Slow down and think about how all this fits together. The myths of the 70’s and 80’s should be dead, but they live on, crippling builds every day.
And yes I’m tired and in a pissy mood. Doesn’t change the facts.
Quote from YR;
Been saying that for a long *** time. You got the head for it? ........ take advantage of it!
So you run a .028 head gasket with flat top pistons and closed chamber heads?
Yes. (Make sure your parts are good and your clearances are checked.)
harrisonm
Well-Known Member
I have a Comp 268XE in my 69 Fastback, 340 4 Speed, 3.55. It is an awesome cam. It runs great, it will scald the back tires, it has a great lopey idle, and it is great on the street.
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