Longer Duration Cams?

[willxs 0]

having only a basic understanding about how most bits work im interested in knowing the difference in which bit of the cam affects what. i.e. longer duration, effects higher up the rpm? higher lift does.... and etc etc

 

reason i ask becuase in the future i may be looking to upgrade my cams, BUT, due to the nature of the build of my engine it hasnt left alot of space for a 'wild' cam with having 1.6mm at TDC for the inlet and 1.75 10° BTDC. Is it a case of you generally dont get within 'x' amount of distance from valve to piston for obvious reason. but generally what is the factor of safety?

 

Is it worth looking into not so much a higer lift cam, but a longer duration, would it still be as beneficial etc? and i guess it'll have to be more a custom grind, sort of thing PeterT is able to do?

 

 

Cheers, Will

[DrSarty 94 1 Cars]

Several people like Sandy, PeterT, SuperJosh, Mattsav (QEP), James R, Anthony, Jackherer and BazGTMi to name a few, will be able to give you a very good explanation.

 

I'm interested too, not only in how higher lift differs from longer duration (because my mind tells me they'll do the same job, i.e. leave the port open longer for more fuel/air mixture), but how a reground cam can appear to summon up material from nowhere?

 

This is quite an interesting read, at least for starters.

 

Good topic chief.

 

EDIT: One thing I know from my chats with Mattsav about my engine, is that I may wish to retard my inlet timing to effectively reposition my max torque output further up the rev range, so I expect cam timing is also going to be a factor in answering your question, i.e. how the cam is used, rather than what profile it has. Just a thought.

Edited January 30, 2008 by DrSarty

[taylorspug 7]

Reground cams pretty much gain material from removing it. The grinding will take place on the base circle and the ramp, not the tip of the lobe. By doing this the difference between the base circle and the tip of the lobe becomes bigger (ie more lift), effectively making the lumpy bit even more lumpy.

[petert 601]

due to the nature of the build of my engine it hasnt left alot of space for a 'wild' cam with having 1.6mm at TDC for the inlet and 1.75 10° BTDC. Is it a case of you generally dont get within 'x' amount of distance from valve to piston for obvious reason. but generally what is the factor of safety?

 

for hydraulic lifters, a generally acceptable safety margin is 0.080" (2.03mm) for inlet and 0.100" for exhaust. On the inlet, this needs to be measured 6 degrees ATDC, not before. Perhaps you should verify your measurements? I'll answer the other questions later.

[petert 601]

What you've asked in relation to duration, lift etc. is a very open ended question, as it varies from engine to engine, and from application to application, but I'll do my best to explain, using the Mi16/S16 head as an example.

 

The reason my Stage I and II regrind cams work so well is that they have a lot of area under the lift vrs. duration curve. They open quickly but don't necessarily have a lot of lift. They don't need to because the Mi16 intake port flows so well. Use the same grind in another engine and it may not work as well. Theoretically, it's desirable to open the intake valve to 28% of it's diameter. So around 0.400" would be ideal, but the cost of steel billets makes this a more expensive process. I've seen other manufacturer's Mi16 regrinds and they're nothing more than pointy replications of the original grind, just a few degrees bigger. The effect of port flow at low lift, rod ratio and runner/port volume all have an effect on the cam timing and placement.

 

The exhaust cam doesn't need to be as big because the Mi16 port already outflows or equals the intake port up to approx. 0.300" of lift. The inlet/exhaust flow relationship needs to be approx. 80%. So less duration is an effective method of achieving this relationaship. Less exhaust timing will make more torque. More exhaust timing will make more power at the expense of low end torque. The longer you keep the exhaust valve closed, the more the piston will be exposed to the explosive force of combustion. But the port needs to flow enough to expel all the burnt gases.

 

There's an excellent website somewhere showing the effects of moving the exhaust and inlet cams independently of each other.

 

Duration has by far the biggest effect on an engine's power band. The most important event being the closing point of the intake valve, as this is when the compression stroke starts.

 

I hope that helps a bit.

[jackherer 543]

Several people like Sandy, PeterT, SuperJosh, Mattsav (QEP), James R, Anthony, Jackherer and BazGTMi to name a few, will be able to give you a very good explanation.

 

I don't know about that, I just buy them off Matt @ QEP

[willxs 0]

for hydraulic lifters, a generally acceptable safety margin is 0.080" (2.03mm) for inlet and 0.100" for exhaust. On the inlet, this needs to be measured 6 degrees ATDC, not before. Perhaps you should verify your measurements? I'll answer the other questions later.

This is the specification clearances i got from the engine builder:

 

Inlet valve

30° BTDC + 5mm

20° BTDC +4mm

10°BTDC + 2.4mm

TDC = +1.6mm

10° ATDC = +1.95mm

20° ATDC = +3.5mm

30° ATDC +3.5mm

 

Exhaust Valve

30° BTDC +5mm

20° BTDC +2.5mm

10° BTDC + 1.75mm

TDC + 2.3mm

10° ATDC +4.1mm

20° ATDC +5mm

30° ATDC + 5mm

 

so what does that tell me/you? if anything useful. the bore is 86.6 and the stroke is 86. im curious as im wanting to know how far i'd be able to go with this particular engine and becuase the valves are already quite close, i feel quite limited as to what i can do with it.