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On just about every forum this topic comes up and usually gets peoples bowls moving. Let me start of by saying I don't claim to be an expert but like to follow along on this topic and enjoy reading people's opinion. What me feeling on the topic is this. I'm not convinced to spend the money on a hyd roller. As I understand, the big problem with them is their weight and somewhat limiting RPM to slightly above 6,000 RPM. It would seem to me that despite the advantage of a roller tappet to follow more agressive profiles, it's usable profile is going to be determined by the RPM limit due to the roller tappets increased weight. Sure there are 5.0 guys spinning a lot faster with hyd rollers but that's a whole different story IMO.
One can argue that you can install stronger springs but again from my understanding this has a negative effect by the spring's pressure trying to collapse the tappet, reducing net duration and lift. According to an article David Vizard publishe in PHR a few years ago, a flat tappet actually has the advantage in duration VS lift over a roller up to about 280* advertized duration at which point the roller has it. As I see it the roller can make more power then the flat but up to a point. With modern valve springs, a fla hydraulic tappet can really be buzzed without valve float or harmonics effecting the springs and valve seal. Since the flat tappet either solid or hydraulic has lighter mass, it has a much higher RPM ceiling, hence making more power then a hydraulic roller since more RPM equals more power.
I think the difference in cost between a hydraulic roller and a hydraulic flat tappet is substantial, the savings could be used elsewhere to offset the hydraulic usuable range advantage, such as going towards a set of heads. My feelings are this, if I were to go use a roller cam, I'd go for a solid roller where the added spring pressure needed to follow the lobe is some what of a non issue since it's not going to collaps a tappet.
The OEM's such as Ford started using hyd. rollers only because they offered less friction. But in my opinion this is true when comparing net lift cam timing at the valve where both the flat and hyd roller would have exactly the same numbers. The main purpose of a roller is to follow more aggressive profiles that a flat tappet can't, not for reduced friction as I see it. I recall reading on another forum or someplace where Ford tested the 82-84 Mustang cam agasinst the 85 & up roller and both were almost exactly the same.
While harking on friction, just how much less friction does a hyd roller offer compared to a flat tappet of similar specs? I recall a rebutal from a Pontiac engineer once wrote about the friction difference between a 400 & 455 main bearings since the 455 usess a 3" journal and the 400 uses a 2.5" IIRC. He said the difference was negliable. He went on to state if I remember correctly that the piston and ring pack of a typical engine accounts for about 50% of friction, bearings were about 25% and the cam, valvetrain was about another 25%. If I were looking to reduce friction, I'd be focusing on the pistons by using metric style rings and a coated piston.
I'm not trying to come off sounding like an internet expert but rather my own personal reasoning why I would or would not use a hydraulic cam. Sure, I believe that a hydraulic roller will make more power over a hydraulic flat tappet but I believe that is with in a limited range. I think it would be interesting to see a back to back test with all things otherwise being equal which make more where. Could a hydraulic flat tappet with a right set of springs that allowed it to spin to 7,000 make more power then a hydraulic roller? Especially with Chrysler, Ford or AMC's that use a larger .904" diameter tappet to that of a GM's .874".
One can argue that you can install stronger springs but again from my understanding this has a negative effect by the spring's pressure trying to collapse the tappet, reducing net duration and lift. According to an article David Vizard publishe in PHR a few years ago, a flat tappet actually has the advantage in duration VS lift over a roller up to about 280* advertized duration at which point the roller has it. As I see it the roller can make more power then the flat but up to a point. With modern valve springs, a fla hydraulic tappet can really be buzzed without valve float or harmonics effecting the springs and valve seal. Since the flat tappet either solid or hydraulic has lighter mass, it has a much higher RPM ceiling, hence making more power then a hydraulic roller since more RPM equals more power.
I think the difference in cost between a hydraulic roller and a hydraulic flat tappet is substantial, the savings could be used elsewhere to offset the hydraulic usuable range advantage, such as going towards a set of heads. My feelings are this, if I were to go use a roller cam, I'd go for a solid roller where the added spring pressure needed to follow the lobe is some what of a non issue since it's not going to collaps a tappet.
The OEM's such as Ford started using hyd. rollers only because they offered less friction. But in my opinion this is true when comparing net lift cam timing at the valve where both the flat and hyd roller would have exactly the same numbers. The main purpose of a roller is to follow more aggressive profiles that a flat tappet can't, not for reduced friction as I see it. I recall reading on another forum or someplace where Ford tested the 82-84 Mustang cam agasinst the 85 & up roller and both were almost exactly the same.
While harking on friction, just how much less friction does a hyd roller offer compared to a flat tappet of similar specs? I recall a rebutal from a Pontiac engineer once wrote about the friction difference between a 400 & 455 main bearings since the 455 usess a 3" journal and the 400 uses a 2.5" IIRC. He said the difference was negliable. He went on to state if I remember correctly that the piston and ring pack of a typical engine accounts for about 50% of friction, bearings were about 25% and the cam, valvetrain was about another 25%. If I were looking to reduce friction, I'd be focusing on the pistons by using metric style rings and a coated piston.
I'm not trying to come off sounding like an internet expert but rather my own personal reasoning why I would or would not use a hydraulic cam. Sure, I believe that a hydraulic roller will make more power over a hydraulic flat tappet but I believe that is with in a limited range. I think it would be interesting to see a back to back test with all things otherwise being equal which make more where. Could a hydraulic flat tappet with a right set of springs that allowed it to spin to 7,000 make more power then a hydraulic roller? Especially with Chrysler, Ford or AMC's that use a larger .904" diameter tappet to that of a GM's .874".
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