cam duration

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ir3333

ir3333

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for an iron headed small block at about 9.5:1 compression.
..what is the least amount of duration you can get away with
before you have detonation issues.
 
I think the real question would be how much before you end up with a dog on the bottom end. With 9.5 compression the 2 barrel cam would work depending of how efficient your cooling system is, fuel quality and advance curve. Where do you want to make the power and what RPM will it see regularly?
 
You are trying to eat the wrong end of the horse.

It's about the system, the package.

If you just pick a CR or a duration number. You'll be eating the poo end.
 
looking for shortest possible duration without having major detonation problems
 
Depending on how much lift you want there will be limits on how short you can go. A roller will allow a steeper ramp than a flat tappet. Sounds like you may need to call one of the custom cam grinders and let him help you make a choice.
 
mguner said:
Depending on how much lift you want there will be limits on how short you can go. A roller will allow a steeper ramp than a flat tappet. Sounds like you may need to call one of the custom cam grinders and let him help you make a choice.
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i have sourced both Lunati and Comp with my complete build and they are suggesting 230 / 236 int and exhaust.
i was hoping something around 216 / 220 (intake) might bleed off enough...I also would like a fairly clean,smooth idle
 
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What size engine. Cid will make a huge difference.
What are you gonna use this engine for; as in DailyDriver,Street/strip/,etc
How much flywheel?
Car weight?
Gears?
TC?
Fuel?
Altitude?
Your tuning skills?
In a 360 lumpy starts around 220* on a 110LDA. But a 230/110 can be tuned to be smooth, but lumpy. By 240/110 things are not smooth, and by 250/108 there is nothing but lumpy
 
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you guys are over thinking this...
who is running an iron headed 9.5:1 small block with no detonation problems on 91 octane?
 
I think 216 could work in a teener. I'm pretty sure I could make it work. I'd like to see closed chamber heads, a tight quench, and one of the following cams;a 256/114, or 262/112, or 268/110 cam. This makes a Dcr of about 7.8 and cylinder pressure around 153. Total chamber volume around 77 cc. Allowing 14 for the gasket and eyebrows, that would need 64.5 to 65 cc in the heads.
All these cams can be installed at a 58 to 60 degree ICA, and would not affect the Dcr to any degree,varying cylinder pressures about 3 psi
Choose the cam according to useage.
If you're stuck with open chamber heads, getting 9.5 is tough.
If you have an engine other than a teener, nothing above applys
 
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Call a cam company who knows what they're doing! Duration isn't the end all be all for determining if the engine will detonate or not. I'm sure others will correct me if I'm wrong, but what's more important is when the intake valve closes because that's when in the compression stroke the piston/cylinder actually starts building compression. In other words, an earlier intake closing point will build more cylinder pressure than a later one and be more prone to detonate. A legit cam company will be able to build you the right cam to fit your needs and put your dynamic compression right where you want it.

Do yourself a favor and play around with a dynamic compression calculator online and see how intake closing event changes dynamic compression.
 
Comp is right. You would need around 230/236 duration to keep around 7.5 dynamic (the "magic" number) that iron heads can run on 91 pump gas with. You might be able to go one size down. But that's probably max. Anything smaller and you most likely will have issues.

Why don't you wanna run a 230/236 sized cam? Why do you wanna run smaller?
 
ir3333 said:
you guys are over thinking this...
who is running an iron headed 9.5:1 small block with no detonation problems on 91 octane?
Click to expand...
Everything smaller than the stock 340 cam I've run has had detonation issues with 9.5. The little 340 cam was in trucks and tow package cars with like 7.8:1 360s, they didn't think it was too much for those purposes. The 2bbl 318 cam won't run 87 on 9.2:1 easily.
 
Lustle said:
Comp is right. You would need around 230/236 duration to keep around 7.5 dynamic (the "magic" number) that iron heads can run on 91 pump gas with. You might be able to go one size down. But that's probably max. Anything smaller and you most likely will have issues.

Why don't you wanna run a 230/236 sized cam? Why do you wanna run smaller?
Click to expand...

as mentioned earlier...something that has a smooth idle, doesn't load up and gets fair mileage (read short duration hi lift)
.. emphasis on lo rpm torque,don't want a big hp number,no racing, street car
.. stock 340, 210 / 220 would be fine but i don't think it's doable?
 
ir3333 said:
as mentioned earlier...something that has a smooth idle, doesn't load up and gets fair mileage (read short duration hi lift)
.. emphasis on lo rpm torque,don't want a big hp number,no racing, street car
Click to expand...

Well I read that. But I don't see why you think a 230/236 cam won't do any of that? I run comp XE274H (230/236, 488/491) and it idles great. I idle at 1000 for best vacuum with power brakes/steering. Idle is not rough by any means (to me at least, it has a noticeable lope to it). No loading up. Pulls HARD right off idle. If I mash that pedal she just burns tires and GOES. The thing runs like a raped ape. I'm running 10:1 on iron heads with that cam. I could run 91, but I stick to 94 so I can run some more initial timing and really put a tune on it. It's a great cam.

Only thing I can't speak to is mileage. I bought a scamp with a 400 horse 340. Not a prius.
 
In a 340, The 340 cam should easily be tuneable. IIRC that was a 268 cam on 114LDa
That puts the ICA at 64* and that sets the DCR at 7.57 and the pressure to 148.
that should be easy to work with.
The first thing I would do is to check your cylinder pressure.Then I would check the timing and rate of advance.Your dizzy may be in need of an overhaul.
 
How do you know the engine has 9.5:1 compression? Did you measure it to verify or is it just an advertised number for that engine??
 
If you run premium you can go as short as you want.

I would pick something in the 220ish range. No more than 228 @ .050. You want SOME performance I am sure.
 
tx guys...new build,looking to get some real world input.
..i have used the XE268..great cam,lopey idle but despite what Comp says,with 3.23 and a decent tire it needs a bit of a converter..i don't want to go that route this time..i think it's 224 / 230... 218 / 220 may work
 
Duration at .050 isn't what is used to determine DCR. How many degrees after BDC the intake valve closes at is. A cam that has an aggressive lobe will close the valve sooner and have a higher DCR then a lazy lobe does. Compare a stock 340 cam duration("advertised") to a XE268 cam for example. After that compare the duration at .050.
 
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ir3333 said:
you guys are over thinking this...
who is running an iron headed 9.5:1 small block with no detonation problems on 91 octane?
Click to expand...
I am running a 9.5:1 iron J headed 340 small block with an aggressive roller cam (286 duration/.544 lift on a 110 lobe sep) and she put down 421 at the flywheel. Listen to your audience and keep in mind that this is all about the total package on the build, not just the cam. I run 91 fuel with no detonation issues at all.
 
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