pajsh
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| posted on 20/4/08 at 01:12 PM |
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Camshaft Rules Of Thumb
I'm pretty sure I've gone too wild with a Kent RL21 for my pinto and so I'm looking to go for something a bit milder.
I have my rolling road session booked on 03 May but I'm thinking I may change the cam so I'm not wasting my money.
Just wondering are there any Camshaft rules of thumb.
Thinking logically can you say
High valve lift=More power
Long duration=Top end power
Late exhaust valve=High torque
I guess it's probably not as simple as this but I just wondered if there were any rules that might help me select a replacement cam.
The following diagram shows my RL21 against a Piper 285 and A8. I think I not going to get the benefit of this cam as a mostly road based car.
 
rl 21
I used to be apathetic but now I just don't care.
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bracey
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| posted on 20/4/08 at 01:20 PM |
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i put in piper 270 cam
no other mods
made big difference on kick and pulling power also runs to a higher redline
this was in 1800 ohc (vauxhall)
cheers
richard
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mark chandler
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| posted on 20/4/08 at 01:24 PM |
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Try advancing the cam that you have before swapping out and see how it feels, around 7 degrees advance should see a 10% gain in bottom end with 5%
loss at the other end, thats assuming its timed dead centre now.
Regards Mark
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snapper
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| posted on 20/4/08 at 01:29 PM |
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RL 21 is ceratainly a lumpy cam and the power band is 3000 to 7750, pushing the standard rods and piston ring lands to the limit, however some friends
have a complete Lightning Rod engine in there car and whilst it takes some starting once moving it is very tractable in the lower revs and just takes
off once you hit the rev band.
I have a 285 cam and feel that it could do with a bit more cam as i get peak power at 5900.
Try the cam you have got you may be pleasently surprised.
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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madteg
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| posted on 20/4/08 at 01:49 PM |
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Know where you are coming from i had rl21 and comp clutch in my escort, could not pull of without wheel spinning, good fun though
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Jon Ison
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| posted on 20/4/08 at 02:01 PM |
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The 285 was always my fave cam in my pinto days, better power all the way through with just a hint of lumpy tick over.
One thing that did make a big difference to was fitting "roller" followers, the std pinto ones are about as agricultural as it gets, the
roller ones release a fair bit of power simply by removing loads of friction, cam wear is eliminated to along with the spray bar if you go for a x
drilled cam.
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bilbo
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| posted on 20/4/08 at 02:28 PM |
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Another vote for the 285 in a Pinto. Made a big difference without any other mods, yet still very driveable.
---------------------------------------
Build Diary: http://bills-locost.blogspot.com/
Web Site: http://locost.atspace.com
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jacko
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| posted on 20/4/08 at 05:38 PM |
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I have a piper 285 but Bogg brothers say a piper A8 is good with bike carbs and big valves
Jacko
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mookaloid
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| posted on 20/4/08 at 05:49 PM |
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I found the FR33 was very good 
"That thing you're thinking - it wont be that."
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pajsh
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| posted on 20/4/08 at 06:16 PM |
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Whilst replies are much welcome I was really only asking if there are any rules of thumb on which to base a judgement.
Like in "long duration is only important in the higher rev range" or "go for a later opening exhaust valve for more low down
torque"
Maybe no such rules exist I was just curious.
It's not so much the fact it's lumpy, it idles surprisingly well but it is blowing back through the carbs and I'm not sure I'm
really going to be able to drive everywhere at 4000-5000 rpm.
Having said that I will probably try what Mark says and set the cam timing later or go with Mookie's FR33.
[Edited on 20/4/08 by pajsh]
I used to be apathetic but now I just don't care.
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MikeRJ
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| posted on 20/4/08 at 07:16 PM |
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quote:
Originally posted by pajsh
Whilst replies are much welcome I was really only asking if there are any rules of thumb on which to base a judgement.
Like in "long duration is only important in the higher rev range" or "go for a later opening exhaust valve for more low down
torque"
Maybe no such rules exist I was just curious.
Camshaft rules of thumb:
1) There are no other rules of thumb, use whatever has been proven to work rather than guessing from the specs.
Older cam grinds tended to rely more on long durations to increase mid-peak power.
More modern designs tend use shorter durations but higher lifts combined with faster opening and closing ramps to get a similar improvements, which
gives better idle quality, emissions and bottom end power.
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DarrenW
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| posted on 21/4/08 at 09:39 AM |
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Ive used FR32 - feel it is too mild.
Boggs and Dave Walker both suggested Piper A8 so might be worth comparing yours to that.
Mookys car went well on the FR33.
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pajsh
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| posted on 21/4/08 at 07:05 PM |
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Ah ah ! 2 rules of thumb. LOL
1) long durations increase mid-peak power.
2) shorter durations but higher lifts combined with faster opening and closing ramps give better idle quality, emissions and bottom end power.
Only kidding!!
Am thinking of A8 or FR33. Diagrams posted earlier show A8 is actually similar to mine but slightly later opening and earlier closing but with higher
lift.
FR33 from earlier research is very similar to A8 but more valve lift.
A8 - 12.8/304/47-77/71-41
FR33 - 11.76/300/45-75/74-43
RL21 - 11.92/314/52-82/92-42
It's the exhaust valve opening less than half way down the stroke (92 BBDC) that bothers me. I'm thinking I'm not going to get any
decent power less the "rev the nuts off".
I used to be apathetic but now I just don't care.
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DarrenW
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| posted on 21/4/08 at 08:34 PM |
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Have you read books by the likes of Des Hammil and David Vizard? I recall Des had some clear viws on what cams work best in Pinto's and that
popular cams are oppposite to what is best. He was also critical of the valve train geometry.
Wasnt there an online guide by Puma race engines??
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pajsh
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| posted on 21/4/08 at 09:52 PM |
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Yep read all that and got the "T" shirt.
Alot of it I find woolly and sometimes contradictory.
Des Hamill says Pinto's are not sensitive to long duration cams but I think Puma racing notes say you want high lift shorter duration.
It was the following Des Hamill's statement that got me thinking:
"it is a proven fact that road going engines should not have a camshaft fitted that opens the exhaust valve before 70 BBDC while the wildest
racing camshafts should not open earlier than 85 BBDC"
A8 - 71 degrees
FR33 - 74 degrees
RL21 - 92 degrees 
I think the best advice comes from the sort of practical experience on here.
[Edited on 22/4/08 by pajsh]
I used to be apathetic but now I just don't care.
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