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Old 11-24-2008, 06:07 PM
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Reccomendation between two cams.

Hello guys I am going to swap out my hydraullic roller cam next year in my 350s sbc cause I currently have a comp cams big mutha thumpr cam with 243/257 @50 537/519 lift and it has over 95 degrees of overlap and I am tired of trying to tune my carb to this piece of crap cam. Here is the two cams I am looking at replacing it with but cant decide wich one will give me the best results and if there would be really any difference between the two cams for the sound of the rough idle. Ok the first one is the comp cams xtreme energy 288HR wich specs at 236/242@ 50 with around 520/540 lift with 1.5 ratio. The other cam is the next step up its the XE 294HR with 242/248 @50 with 540/562 lift. Here is my engine specs 350 with 9.6 to 1 CR, 650 mech quickfuel Q series drag racing carb with Edelbrock air gap performer rpm intake. Head are Dart Iron Eagle 2.02/1.60 200cc intake runner heads and have been fully ported and gasket matched, 68cc combustion chambers. Bottom end is a Scat Steel crank and forged rods with forged flattop pistons. Hope that is enough info. I have a Chevy S-10 with 3.73 gears with a 3500 stall converter with a 350 turbo transmission. Also if fbird reads you know a lot about cams (or anyone else) will the XE 288hr give me about the exact same rough idle sound as the comp 292h flat tappet cam? I used to have it and it was nice but did not give me all the problems like the Big Mutha Thumpr Cam. Also any guesses on how much horse power I would make and torque? I estimate around 450hp but don't know on torque. Thanks guys. Have a nice Thanksgiving
Eric

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Old 11-24-2008, 06:32 PM
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To be honest, if you have to keep retuning it then it would lead me to believe the problem is elsewhere. The cam doesn`t change, so neither should the tune once it`s tuned in correctly. I used a rather small RV cam once and had to retune it everytime I drove it, I later figured out the problem was the fuel pressure was too high. Amazing how something so small and simple as fuel pressure can cause as many headaches as it did me.
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Old 11-24-2008, 06:37 PM
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Well its not that I have too keep tunning it I don't. I just don't have a very good carb siginal at idle cause it is burning too rich but I can not lean it out anymore cause when I put it into gear it always stalls and quits unless I just barely put my foot on my gas pedal. Never had so many problems like this before. My previous cam I used did not give me these problems and everything is the same. That is why I know its the cam. Fuel is a steady 6 and not more. Thanks for input though all helps.
Eric
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Old 11-24-2008, 07:23 PM
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The carb probabily needs further fine tuning. I've never used the other hyd roller you want to switch to so can't comment.

A vacuum leak will make it near impossible to tune. So will junk in the carbs idle circuits or having the wrong idle air bleeds.

I recently did a custom built holley/proform 750 on my bird and it required a bit of fine tuning to get it just right.
I found using a N/B O2 sensor+ Auto meter AFR meter a big help in setting up the idle circuit.
Ended up making my own custom size idle airbleeds.

Very smooth now.
If you don;t already have the spark timing locked out give it a try.
34-36deg timing at idle.


If you do have a carb issue then swapping the cam won't change that.
But if you want to go to a new cam something like 236-242 on 112 in on 107 will make very good power. It will likely "drive" and idle much better seeing as it has a lot less overlap. Comp can make you one.
Just don't rule out that the carb may still need some dialing in with the new cam too.

Can you try another carb?
The more overlap the cam has the more egr effect at idle/ needs a ton of idle timing.

Last edited by F-BIRD'88; 11-24-2008 at 07:50 PM.
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Old 11-24-2008, 07:43 PM
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you seem to be a little light on compression ratio and gear for those cams. 10 to 10.5:1 cr and a 4.11 gear would be much better

what kind of ignition timing are you using?? Initial and total.
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Old 11-24-2008, 08:04 PM
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Well as far as ignition goes I have a Hei with whatever springs and weights that comes with the proform hei kit with a 50000 volt coil. As far as timing goes my dad was the one who set it last summer before I knew about setting timing and such and have yet to try it but anyways he set it at 11 degrees at idle and 34-36 advance at 3000rpm I believe. He said it gets total timing just a little after you rev it up. I also have the vaccume advance hooked up but cant tell you what its making timing wise. My vaccume is 7psi at idle. My carb works good on highway and such but the idle is whats killing me.

This is a way more racing type carb then my previous edelbrock 750 performer and it worked great with my old comp 292h cam and all I had to do was change out the metering rods and springs and thats it. It would run great at idle and not dump tons of fuel in the motor but had a nasty off idle stumble but still ran good and did not foul out my plugs. I figured with my quickfuel carb with all of its better tunning features and bigger price tag would help with tunning wise and make me better response and it does too that effect. The only nice thing about my new carb it does not have as much as an off idle stumble like my edelbrock had and has way better throttle response. Thanks guys appreciate it. I really just want to take that cam out and have one that will sound and work just as good as my old 292h and just wanted to know out of the two XE roller cams would the smaller one sound as good as the old 292.

I am still taking out my thumpr cam anyways too much overlap and its killing my motor. And on compcams camquest software the 288hr actually gives me over 40 more hp and 30 pounds of torque then the big mutha thumpr cam.
Thank you for help fbird I am going to change out the cam and go with the 236/242@50 520/540 lift XE compcam it has a 110 lsa and a 106 or 107 centerline. It will work better and I did use a speed demon carb and had all the same issues but when I used it with my previous cam it worked just fine. It was the cam change and all the overlap that killed my good running motor.
Eric

Last edited by eric32; 11-24-2008 at 08:11 PM.
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Old 11-24-2008, 08:36 PM
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You have to realize that by changing the cam (especially one with a lot of overlap) you have to retune the carb.
Expensive carb or other wise. There is nothing wrong with your carb. Its just not tuned right to idle with your high overlap cam.
11deg is not enough at idle. for this camshaft. Remove the weights an springs and lock the advance mechanism so it cannot move, using two plastic cable ties.
reisntall the rotor and retime the motor with 36deg at idle.
The advance does not move with rpm when its locked out.
This cam requires a full 36deg of timing at idle. Not when you rev it up.
(therefor the locked out mechanical advance curve)

then:
Remove the carb, flip it over and look at the throttles blade position and idle transfter slot exposure. Reset/preset the throttle curb idle setting adjustment on both then primary and sec throttle so they are even and both only show about .030" of slot under the throttle blade at idle. Install a 3.5" power valve and reinstall the carb.

What is the diameter of the idle and high speed air bleeds on the carb?

All the carb and timing setting up (fixed 36deg ) is done with the vacuum advance disconnected. when you are all done, reconnect the vacuum advance to the carb.

If you were to install a comp 292H magnum cam you'd have to do the same thing. (dial in your QFT carb and spark advance curve).
Get your Dad to help you. Show him this thread and ask him to try my recomendations and bear with me on the tuneup.
Need the idle fuel feed restriction size and the idle and high speed air bleed size.
Yes these high overlap cams are a bit more of a challenge to get set up right.
Once set up, it won't stall out on ya or stink at idle.
probabily will need fresh spark plugs too.

I realize Comp offers this cam in their catalog ground on a 110LSA.
Call them a get them to make you one just for you ground on a 112LSA.
Custom cam grind requests don't upset them. Its their job.
Give 'em a call.

Last edited by F-BIRD'88; 11-25-2008 at 01:25 AM.
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Old 11-25-2008, 12:22 AM
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eric, I wouldn't expect much power at all out of the motor at 9.6 with that much cam. Waaaaay overcammed in my opinion and too much head for a 350. But maybe you're not concerned with power and are just wanting to make max rump-rump.

Here's a 383 I ran on the DynoSim with the same c.r. and those heads. Check out the cam. Of course, I will admit I was going for torque rather than horsepower.
383
Dart Iron Eagle 200
Performer RPM
750 carb
1 3/4" headers
9.70:1 SCR
CompCams XR270HR 270/276, 218/224, 0.528"/0.535" (1.6 rockers intake and exhaust), 110 LSA, installed retarded 2*
RPM HP TQ
2000 153 403
2500 194 407
3000 248 435
3500 311 467
4000 370 485
4500 419 489
5000 458 482
5500 472 451
6000 461 403
Over 450 ft/lbs 3200-5500
Calculated using the following flow figures for the heads......
.200 129/114
.300 186/145
.400 229/164
.500 261/172
.600 252/174
Tried 7 different cam grinds. More cam will make a little more horsepower, but the bottom end torque goes away. This cam is the best all around choice. Use the 2000-2200 stall converter you mentioned previously. Plan to buy tires often. (This combo makes 360 ft/lbs @ 1500)

Last edited by techinspector1; 11-25-2008 at 01:17 AM.
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Old 11-25-2008, 01:19 AM
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Quote:
Originally Posted by techinspector1
eric, I don't understand how you can expect any power at all out of the motor at 9.6 with that much cam. Waaaaay overcammed in my opinion and too much head for a 350.

Here's a 383 I ran on the DynoSim with the same c.r. and those heads. Check out the cam.
383
Dart Iron Eagle 200
Performer RPM
750 carb
1 3/4" headers
9.70:1 SCR
CompCams XR270HR 270/276, 218/224, 0.528"/0.535" (1.6 rockers intake and exhaust), 110 LSA, installed retarded 2*
RPM HP TQ
2000 153 403
2500 194 407
3000 248 435
3500 311 467
4000 370 485
4500 419 489
5000 458 482
5500 472 451
6000 461 403
Over 450 ft/lbs 3200-5500
Calculated using the following flow figures for the heads......
.200 129/114
.300 186/145
.400 229/164
.500 261/172
.600 252/174
Tried 7 different cam grinds. More cam will make a little more horsepower, but the bottom end torque goes away. This cam is the best all around choice. Use the 2000-2200 stall converter you mentioned previously. Plan to buy tires often. (This combo makes 360 ft/lbs @ 1500)
You're right it is a lot of cam. And probabily not the best choice. Anyways...The trick is to install it advanced in the motor on a 99 to 102 installed intake C/L. Thats 5 to 8 deg advanced from "straight up"
Doesn't make it idle any better but gets the cylinder pressure and torque output started as soon as possible in the usable rpm band. (by getting the intake valve closed early)
These cams are intended for those that just got to have to badest "BAD BOY" Pro Street idle sound as opposed to being the "right cam" for the job.

Last edited by F-BIRD'88; 11-25-2008 at 01:28 AM.
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Old 11-25-2008, 01:48 AM
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Thanks F-BIRD'88.
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Old 11-25-2008, 07:26 AM
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sounds like not nearly enough timing is your biggest problem.

Next problem is too much cam for the compression and gear.

but more timing with help a lot.

36 degrees locked out is nice as long as it will start when warm. A lot of timing like that makes it hard for the starter to turn the engine over. MSD makes a start retard system for cases like this. You need a MSD hei conversion kit anyways, so look into that.

Your engine is running rich at idle and has extra low vacuum because you don't have enough timing. I bet you can get 9 to 10 inhg of vacuum with more timing and a better fuel mixture. I bet the carb is not in the idle circuit due to the low timing, thus making it run very rich. FYI, vacuum is not typically measured in psi, but inches of mercury (inhg).
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Old 11-25-2008, 07:46 AM
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Quote:
Originally Posted by 454C10
sounds like not nearly enough timing is your biggest problem.

Next problem is too much cam for the compression and gear.

but more timing with help a lot.

36 degrees locked out is nice as long as it will start when warm. A lot of timing like that makes it hard for the starter to turn the engine over. MSD makes a start retard system for cases like this. You need a MSD hei conversion kit anyways, so look into that.

Your engine is running rich at idle and has extra low vacuum because you don't have enough timing. I bet you can get 9 to 10 inhg of vacuum with more timing and a better fuel mixture. I bet the carb is not in the idle circuit due to the low timing, thus making it run very rich. FYI, vacuum is not typically measured in psi, but inches of mercury (inhg).
I agree with 454c10 you need more initial timing than what you have.
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Old 11-25-2008, 08:17 AM
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yep, 11 degrees initial with taht cam and only 9.5:1 isn't close to cutting it. I'd start around 25 degrees initial and use the lightest springs and lock out totale mechanical to 36 degrees. Your timing will be all in before your stall flashes but you won't have as many starting problems as you would if you locked it at 36.

Do that and tuning wil become much easier and you may decide to not swap cams.

If you still want to I would go with the 288HR and have it installed straight up, rather than with the 4 degrees advance that is ground into it.

For a "rumpity" idle sound overlap and EVO play the most critical part. You want to keep your overlap fairly tame since its a street viehicle, but retarding the cam to straight up will still give you a big idle sound.
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Old 11-25-2008, 05:04 PM
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Wow Fbird and other guys thanks so much for all the info man that is bad *** on the hp and torque numbers with that other cam you used in the simulation. I am still going to take the big mutha cam out next year its too much and is taking away the potential my engine can make and have. I am probably go with the 288HR is as its not too big and works with my setup that I currently have. I will have to get me a timing light and try your method Fbird and see how it goes but I will have to wait till spring time. My Garage is reall small and dont have room. I share it with a neighbor lady. Also Fbird on my carb here is what is on my carb. Idle Air Bleeds are.070 10-32 screw in type. High Speed air bleeds are .031 10-32 screw in type. Main Jet size is 72 and secondary is 80 with extensions. Power valve is a 45. Power valve secondary is plugged. Primarry nozzle is 33 and secondary nozzle is 33. Acc pump is 30 and pump cam is pink primary and secondary and in position number 1. That is what's on my carb spec sheet. Nothing has been changed and yes I have retuned my carb since changing cams. Thanks for all your help man you know your stuff. Thanks to everyone else too.
Eric
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Old 11-25-2008, 05:10 PM
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Until you get your timing fixed don't bother trying to tune your carb- it'll never work right.

You need to get in a lot more itming at idle, either through the way I suggested or through a locked out dizzy. My way is easier on the starter, locked out is easier to do.

And even a 288 is overkill for your enigne, but to each their own.
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