Slant Six Forum :: Cams and overlap fun...

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Cams and overlap fun... https://slantsix.org/forum/viewtopic.php?t=29484	Page 1 of 1

Author:	DusterIdiot [ Thu Jun 12, 2008 7:31 pm ]
Post subject:	Cams and overlap fun...
Think about what your engine is going to do during the camshaft events... in particular there's a time in the cycle when both valves are open briefly... this promotes some scavenging in the engine as the spent 'charge' is going out the exhaust valve, some of the intake charge is drawn into the chamber... if you pick a camshaft with a 'short' cycle where both vavles are open this allows more time for the engine to build up some pressure and make the best use of what's going in and out... so if you have a 'street' engine and or making torque and or lower compression the short duration cam provides a way to not scavenge so much and keep this event short... since raising compression also raises pressure in the cylinder which also promotes some detonation and or the need for running higher octane to make it work, using a camshaft with a longer event (longer duration) will allow the cylinder to bleed off some of this higher pressure during the scavenging event by keeping the valves open a bit longer (if it goes too long or the timing isn't right the intake charge can get 'muddled' with the dirty exhaust charge and make things a bit inefficient)... if you put the shorter cam into the high powered engine it would ping, have high cylinder pressure and would be hard to tune... conversly if you put the long cam into a 'stocker' type scenario the compression would drop in the cylinders, it would lose power and become 'lazy'/inefficient.... When picking the cam and your combination it's good to think about a realistic goal and then figure out what the engine might do while looking at the wanted compression ratio and what the cam's profile is going to do... (9:1 compression short to moderate duration... 12.5:1 long duration cam/not streetable... 10:1 moderate+ cam, but may still have to run higher octane to keep it in check with other factors).... another camshaft event to look at is the LSA... or Lobe Separation Angle... this determines how far apart centerline to centerline you lobes are in 'degrees'... as a rule of thumb: smaller = torque, lumpier idle (anywhere from 105-108 LSA)... middle range= smoother idle, compromise in torque vs. power/top end power (109-111)... 112-116 is used for higher horsepower (mostly top end power/higher revs... smoother idle sometimes seen in today's FI cars... also these have a shorter overlap event so they can also make good 'Blown' application cams where the valve opens then is slapped shut about the same time the other valve is about to open....) You can go online and read several cam articles, (it's a pretty well covered topic that is still highly misunderstood by enthusiasts).... like everything it's subjective... what may work for a 350 chevy may not work as well for an inline 6... -D.Idiot

Author:	Aggressive Ted [ Thu Jun 12, 2008 7:42 pm ]
Post subject:
DI, That is a great start, kind of like wetting the appetite........ So what are your favorite cam pics for the SL6? Could you provide at least 3 examples? by compression ratios, like 8.5 to 1, 9 to 1, 10 to 1, etc....

Author:	DusterIdiot [ Thu Jun 12, 2008 8:05 pm ]
Post subject:	Nuts!
This was supposed to be a reply to help cardpar think about how the camshaft kind of relates to certain events in the engine... but the page timed out and dumped me when I hit the post button.... Everyone has their favorites for cams so it's pretty subjective... and what works for one may not be what the other is looking for... (kind of like carbs... some guys like X and others have just had good luck with Y...) The 252 has been a good 'bread and butter' stick for what I've asked of it... when the 1972+ mech cam was 'low miler' it was a good cam for a 'stock' engine (once they get about 100K on them it kind of loses some of it's 'pep')...I've been very conservative in my cam selections since I daily drive my cars... One thing that my machinist said to look at is how your mileage compares to your torque band.... (so far he's been spot on with a 'fat wide' band giving good 'average' mileage in a wide range of rpms... whereas the 'peaky' cams lose mileage when you aren't in their 'sweet spot'). Maybe someone else wants to jump in with a bit more to help some of the new guys understand the bumpstick shuffle... -D.Idiot

Author:	Aggressive Ted [ Thu Jun 12, 2008 9:20 pm ]
Post subject:
I wonder if there is much difference between the Comp 252 cam verses the 254 Erson grind. I am wondering if the lobe profiles are different...? one making more torque over the other?

Author:	Doc [ Fri Jun 13, 2008 9:33 am ]
Post subject:
I would think that the minor amount of difference is one of the following: -Measurement error or the measurement was taken at a different starting lift, common ones used are ..010, .020 & .050 -The lash ramp is slightly different, designed to be used with a different diameter lifter. The main thing to look at is the lash ramp, I would pick the cam with the longer / slower lash ramp because that give you more "adjustability" and it is easier on the lobe & lifter. On cam events.... the overlap event is important but also the intake valve closing point, combined with the static compression ratio has a big influance on dynamic cylinder pressure, idle quality and power production. I sure wish I had paid more attention to (and logged) the intake valve closing point and static CR on every SL6 I have ever built... that data would really help me in selecting better cam designs. DD

Author:	Aggressive Ted [ Fri Jun 13, 2008 11:05 am ]
Post subject:
Doc, That would be a nice record to have for history. What is your analysis of the Erson 254? That is what mine is reground to. Will it provide more bottom end torque over the E 270? or E 280? Will your RDP cams provide more low end torque over the E 254? If so, which one? the RV15 or RV20? I like the low end torque. We may be getting a 84 truck soon and it may need rebuilding. I have two spare motors that I would like to rebuild for the truck or the Dart. Thanks!

Author:	Doc [ Fri Jun 13, 2008 11:41 am ]
Post subject:
The RV 10 is Erson's 254 lobe, the RV 15 is their 260 For low end, the RV 10 (254) on the intake with the MP 244 on the exhaust. Here are the event numbers. DD

Author:	ceej [ Fri Jun 13, 2008 12:35 pm ]
Post subject:
I highly recommend the RV15M for truck applications. I'm running one in the Hooptie right now. According to Fopar's software, I make 104 HP at 2000 rpm with it. This has made my launches problematic. I have to run big tires with very little air to keep from torching them. That in turn limits me at the top of the track. This winter, I may get a bit more aggressive with my cam selection on the stroker motor. I'd like to be able to spin it a bit higher. It's pretty well spent at 4000 RPM. The cam card from Erson shows 106Â°. I asked my builder about that. He set it at 104Â° to allow for chain stretch. I may have to re-degree the cam. Doc, didn't you recommend 108Â° ? Or was that another spec? It's all fuzzy now. Some projects just take too darn long. CJ

Author:	Aggressive Ted [ Fri Jun 13, 2008 1:47 pm ]
Post subject:
Ceej, What is the diameter of the tires your running? and what rear end ratio are you running? I tried several different rear end ratio's and eventually settled on a 2.76 and had Randy's Ring and pinion build me a nice unit with a Auburn Posi-traction. I thought this would stop the tire spin by going to a posi and a higher gear ratio. Well so much for that! Now I wish I was running a taller ratio yet, like a 2.45. I had no idea that the SL6 had so much torque potential. My 254 regrind cam is pretty impressive. It would be interesting to run together or at least running both our motors on the same dyno. I am running P235 60 x 14's Goodyear Grand Ams that are sipped on 14 x7 aluminum rims, but they still break loose at the drop of the pedal. I am thinking of going to a 15 x7 just to be able to strap on some taller rubber that has the same width foot print. I was running a P205 70's but they would brake loose way too easy. The car was just too squirely. If we could run taller rubber to reduce tire spin maybe 4000 rpm might be enough in the quarter mile. I have seen a few big blocks run taller tires to keep the rpm down to 4000.

Author:	ceej [ Fri Jun 13, 2008 1:52 pm ]
Post subject:
My street tires are Toyo Eclipse 205/70R14's I have been running Fopar's street tires on the strip. 225/70R15's Air on the open rear is 28/20 to keep them from torching too bad. Road gears are 2.80:1 and Strip gears are 3.50:1 CJ

Author:	Dart270 [ Fri Jun 13, 2008 2:01 pm ]
Post subject:
Ceej, You are probably too far retarded already. I wouldn't run more than 104 deg installed c-line on ANY slant cam. I have my mild MP 0.436" cam at 103 deg and it is top end heavy for that cam. My wild 0.510" cam does best with 98-100 deg for all around power. 102 deg on that one was too much high end for my street tastes. More retarded makes higher RPM power, more advanced is better down low. Lou

Author:	Aggressive Ted [ Fri Jun 13, 2008 2:14 pm ]
Post subject:
Mine is at 104 degrees, 110 centers. Tons of bottom end.........bigger diameter tires are in my future.

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