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General Buick Tech / Camshaft shift points
« on: July 22 2021, 08:40:41 PM »
My friend, Ed Baker, made a statement the other day that shocked me. He advocated shifting at 5000 rpm on a 206 deg cam because it was a small cam with a relatively low power band. This did not jibe at all with my memories Yeah, yeah, I know my memories are best of the early 1950's, but...
The factory cam was around 194-196 degs. This means that the 206 cam has about ten more degrees of duration and according to Harvey Cranes notes from his day, ten degrees at 0.050" moves the powerband up about 500 rpm. I looked up a few 206 deg cams and the rated powerband was 4800-5200 rpm depending upon which cam which makes sense, I think, as Buick rated the hp on the intercooled factory Buicks was around 4400 rpm as I recall. Not sure what good factory specs are but I don't have any dyno runs from those days on stock cars.
On NA cars we always went past the top of the powerband so when shifted, the cars stayed in the peak area. Personally, I don't think that means a lot on a FA engine as the situation is far more complicated by the addition of boost.
I remember that we always tried to shift well above 5k with a stock cam and improved valve springs. I asked Steve Y what he shifted his Black car at as it has the factory cam. He shifted at 5600 rpm and it ran a 10.4 the week after he sold it with a new, bigger turbo on it. I think he had a Craig 62 on it when he sold it and ran a couple of tenths slower. He did not recall the precise springs that he put on it but said it was one of the usual sets that people used back in the day which probably meant the old Chevy LT1 springs off the 350. That was the go to spring in the early days.
Brad always mentioned that he ran a factory cam with some Pioneer springs that was even stronger than the LT1 spring. I would like to say it was 125 psi closed. He mentioned it here and provided the part numbers. He was shifting in the same range.
Now, why do we want to go higher than 5000 on the shift points? The most obvious reason is that we have a better gearing advantage in first and second and the car accelerates faster in those gears due to this advantage. If one takes a piece of graph paper and projects the acceleration rate from 5000 up to say 5500 in low gear, and then picks up the acceleration rate in second and takes off from the 5500 extension point, one will be up several mph when it goes into third which can be very beneficial on a 11 sec car to enable it to dip further down in the time band.
If the car has an improved cam duration of only ten degrees in it, and the powerband moves up 500 rpm, then it seems logical to take use of this and extend the the shift point a bit more assuming one has good valve springs that match the cam.
Now, as I said, FA cars are not quite the same as naturally aspirated. The turbo keeps pushing air thru the port and into the chamber where a na car might be falling off. Of course, this assumes the turbo can supply the volume at the higher rpm without falling out of it's efficiency zone and heating up the air so it becomes less dense than before and it becomes a no win situation. I suspect that when we see a car that starts showing less boost at higher rpm, this is what we see. A turbo that cannot keep up any more.
This is probably why the 6262 or 6265 is so popular on fast street cars. It handles higher volumes without moving out of the efficiency zone at 5500-6000.
Another associated problem with FA engines is that boost tries to keep the intake valve from closing and this seems to be more obvious in higher gears where the acceleration curve is flatter than would have been guessed. This was really bad with factory springs and even the Comp 980's seem to get weaker pretty fast.
Now, if one wants to go say mid elevens on a 28" tire, he will need to be pulling somewhere around 5500 rpm which is about 118-119 mph in third gear. That is simple math. Moving the shift point up in first and second helps get a running start on the situation.
My GN has a 210 deg roller cam. I think those were rated around 5500 as to powerband. My experience that it is accelerating as fast at 6000 rpm as it was at 5500 as per the acceleration curve. It is still accelerating as quickly at 6200 when things go bang when the valves lose control. I used to shift at 5800 trying to avoid that problem. I try harder now after installing a new set of heads.
The factory cam was around 194-196 degs. This means that the 206 cam has about ten more degrees of duration and according to Harvey Cranes notes from his day, ten degrees at 0.050" moves the powerband up about 500 rpm. I looked up a few 206 deg cams and the rated powerband was 4800-5200 rpm depending upon which cam which makes sense, I think, as Buick rated the hp on the intercooled factory Buicks was around 4400 rpm as I recall. Not sure what good factory specs are but I don't have any dyno runs from those days on stock cars.
On NA cars we always went past the top of the powerband so when shifted, the cars stayed in the peak area. Personally, I don't think that means a lot on a FA engine as the situation is far more complicated by the addition of boost.
I remember that we always tried to shift well above 5k with a stock cam and improved valve springs. I asked Steve Y what he shifted his Black car at as it has the factory cam. He shifted at 5600 rpm and it ran a 10.4 the week after he sold it with a new, bigger turbo on it. I think he had a Craig 62 on it when he sold it and ran a couple of tenths slower. He did not recall the precise springs that he put on it but said it was one of the usual sets that people used back in the day which probably meant the old Chevy LT1 springs off the 350. That was the go to spring in the early days.
Brad always mentioned that he ran a factory cam with some Pioneer springs that was even stronger than the LT1 spring. I would like to say it was 125 psi closed. He mentioned it here and provided the part numbers. He was shifting in the same range.
Now, why do we want to go higher than 5000 on the shift points? The most obvious reason is that we have a better gearing advantage in first and second and the car accelerates faster in those gears due to this advantage. If one takes a piece of graph paper and projects the acceleration rate from 5000 up to say 5500 in low gear, and then picks up the acceleration rate in second and takes off from the 5500 extension point, one will be up several mph when it goes into third which can be very beneficial on a 11 sec car to enable it to dip further down in the time band.
If the car has an improved cam duration of only ten degrees in it, and the powerband moves up 500 rpm, then it seems logical to take use of this and extend the the shift point a bit more assuming one has good valve springs that match the cam.
Now, as I said, FA cars are not quite the same as naturally aspirated. The turbo keeps pushing air thru the port and into the chamber where a na car might be falling off. Of course, this assumes the turbo can supply the volume at the higher rpm without falling out of it's efficiency zone and heating up the air so it becomes less dense than before and it becomes a no win situation. I suspect that when we see a car that starts showing less boost at higher rpm, this is what we see. A turbo that cannot keep up any more.
This is probably why the 6262 or 6265 is so popular on fast street cars. It handles higher volumes without moving out of the efficiency zone at 5500-6000.
Another associated problem with FA engines is that boost tries to keep the intake valve from closing and this seems to be more obvious in higher gears where the acceleration curve is flatter than would have been guessed. This was really bad with factory springs and even the Comp 980's seem to get weaker pretty fast.
Now, if one wants to go say mid elevens on a 28" tire, he will need to be pulling somewhere around 5500 rpm which is about 118-119 mph in third gear. That is simple math. Moving the shift point up in first and second helps get a running start on the situation.
My GN has a 210 deg roller cam. I think those were rated around 5500 as to powerband. My experience that it is accelerating as fast at 6000 rpm as it was at 5500 as per the acceleration curve. It is still accelerating as quickly at 6200 when things go bang when the valves lose control. I used to shift at 5800 trying to avoid that problem. I try harder now after installing a new set of heads.