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| Effect of Oversized Valves on Camshaft Selection https://slantsix.org/forum/viewtopic.php?t=10183 |
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| Author: | FastDart [ Sat Aug 21, 2004 4:54 pm ] |
| Post subject: | Effect of Oversized Valves on Camshaft Selection |
im currently rebuiling a slant six for my dart. the block has been overbored .30. the head has been given a good porting, polishing, has been decked .100 and is being fitted with oversized valves. induction will be a super six with bbd. the spent gases will be handled by the stock exhaust manifold fitted with 2.25 in piping and a flowmaster 40 series muffler. the tranny is a 904 and up to this point still retains the stock converter. the rear is an 8.25 with 3.23 gears. my question deals with the camshaft. i want good low end torque as this car will be street driven daily and rarely see track time. i want it to be as quick as possible with my given combo though. i know the comp cams 252s has good low end torque from about 800 rpm, but how will the cam work with oversized valves? can i keep the stall stock with the 252s? will the larger valves kill some of the torque? might a 264s be a better choice? with that i will have to use a higher stall converter, correct? the 264s will put the torque curve at a higher rpm and im not sure that is what i will desire in this street car. im sorry to ask this camshaft question again but i am unsure of how my 2 choices might behave with the oversized valves. thank you all for any advice. ive learned many things from the great people at this site. |
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| Author: | FastDart [ Sun Aug 22, 2004 1:37 pm ] |
| Post subject: | |
Does anyone have any answers to my questions? |
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| Author: | DusterIdiot [ Sun Aug 22, 2004 2:10 pm ] |
| Post subject: | Some answers... |
Well I don't have all the answers, but... O/S valves would be wasted for the 'close to stock' 252 with a super six. You'll want more cam, more carb, and more compression to make use of those. Sadly, when you go more duration you'll need a better stall convertor, and the cam does bump the torque curve 'up'. The fix to put the torque curve 'back' to where you want it, is to run a better rear gear ratio (i.e. if you have 2.76's, then you'll want to run some 3.23's ), this puts your cruise rpms back into the 'sweet spot' for the engine combo. Regardless of which you may lose gas mileage with some increase in cam, rear gearing, etc.... Like any anthing in the real world, you have to make of list of 'what's important' and make a compromise in what you want. I built my engine combo for 'street use' and made the compromise that I wouldn't see the 26mpg the 1 barrel setup would get, but didn't want to get 18 mpg either... If looking for a daily driven, 'fun' slant with just the stock manifold, and 3.23's, the super six, 9:1 compression, stock valves, bowl blending/port matching and the 252S would be OK, and you might still have decent mileage. It's up to you where you want to go with it, but there are so many variables no one answer will be the key in your decesion. -D.Idiot |
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| Author: | Dartvader [ Sun Aug 22, 2004 7:30 pm ] |
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I think the oversized valves will still help you with the stock cam. It will still flow more than stock valves, and yet you will not lose compression like you do with a bigger cam as compression loss is caused by over lap of the intake and exhaust ramps. So you should not see a loss at low RPMs. I did exactly as you are doing as I was building my turbo slant. After I had installed the new head, with stock compression, ported head, bigger valves, and mild cam, but before I had put the turbo on, I got an additional 500 "easy" RPM's out of the slant. That is, before I put the bigger valve, and portred head on, she was cranked out tight at 4500RPM. The bigger valves and porting allowed the engine to rev to 5000 with the same ease. |
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| Author: | FastDart [ Mon Aug 23, 2004 12:44 pm ] |
| Post subject: | |
Will the 252s continue to make good power up to 5000 rpm? so theres one yay and one nay for O/S valves and the 252, anyone else? |
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| Author: | 73dart_swinger [ Mon Aug 23, 2004 2:52 pm ] |
| Post subject: | |
the comp cams web site has the 252's opperating range from 800 to 4500 rpm. im not sure the price of oversized valves and the machine work that goes with it are worth the money for what power you might gain. i dont know though, what does someone with more knowledge think? |
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| Author: | Thropar [ Mon Aug 23, 2004 7:11 pm ] |
| Post subject: | bigger cam |
i would put an even bigger cam in than the 264, i ran the BBD with stock converter for a year with a pretty big cam, no need to put the trans in neutral at stoplights or anything (cast stock exhaust man, 2 1/2 pipe, 9:1, stock valves, idled great) if you're scared, run the 264 and you could advance it a few degrees later if needed to bring the range down |
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| Author: | panic [ Mon Aug 23, 2004 8:03 pm ] |
| Post subject: | |
"compression loss is caused by over lap of the intake and exhaust ramps" Has no effect on compression. |
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