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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, 07:43 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

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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)

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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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Old 11-28-2008, 02:25 PM
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Hello Fbird on my carb its a 4 corner idle control circuits. I am going to swap out cams anyways and go with the comp 288hr wich will give me some more hp and torque and somewhat a better idle and still be a choppy sound just not as bad as the thumpr cam. I will never recommend a thumpr cam to anyone.Thanks for your help. How do I advance the initial timing without changing my advance timing? thanks eric
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Old 11-28-2008, 02:40 PM
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id have to say that theres no such thing a a piece of crap cam, just a cam not matched to the engine its in. from the SOUND of your motor, id say that cam is just to big........ even your converter sounds a little to big, i wouldnt run much more than prob a 2200 in an s10. as far as your cam goes..... id run something more along the lines of that 292 you had. lunati makes the bmII series but it isnt roller. im sure you could find one tho with similar numbers. as far as your car dying, try adjusting your idle.... with a cam that big, its going to have a higher idle speed than normal.
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Old 11-28-2008, 03:09 PM
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This is a clear case of overcamming.
I wonder if CompCams recommended the BIG Muther? The biggest in their series.
I certainly wouldn't recommend a cam with 30* too much overlap for their combination.

A 350 cube SBC with real good heads will make a strong midrange and peak horsepower at 6100 rpm with a 224* @ .050 single pattern cam on 107* LSA and a mid 9s compression.

Unless you have very high compression and a desire to regularly turn above 7300 rpm, a cam with 236+ intake and a big exhaust blow out will be a pretty high rpm bugger.

Crutching a cam with more than 4* advance from straight up just says that it is the wrong cam for the combination in the first place.
Wide LSA and long exhaust durations need lots of advance to crutch the mismatch.
JMO FME, FWIW

Last edited by ScoTFrenzel; 11-28-2008 at 03:19 PM.
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