Robbie2734
Lambcharger
What will be the effect of fly cutting the exhaust valve chamber side on a Magnum head? I need to lose some compression and this was suggested.
unshrouding exhaust valves
- Thread starter Robbie2734
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71Scamper
Mopar runs thru my veins!
Do you mean unshrouding the valve in the chamber? Its best to unshroud the intake more than the exhaust side. See the intake charge has the hardest time clearing the valve to enter the cylinder than the exhaust charge does leaving the cylinder. The reason being is because the intake charge has to be drawn into the cylinder under vacuum whereas the exhaust is just being forced out of the cylinder by the piston with positive pressure. Watch this video on freshening up heads from Power Block TV.
http://www.powerblocktv.com/episodes/HP2012-09/budget-cylinder-head-refresh
Robbie2734
Lambcharger
Thanks, I'll talk to the machinist this week. I have new Magnum heads going on my 90 roller motor. With the KB 107 pistons I have a little too much squeeze. This was suggested so I can run pump gas. The motor is for my convertible so I'm not looking for a monster, just a big fat torque curve and some decent HP.
71Scamper
Mopar runs thru my veins!
Your very welcome. What Comp Ratio is it right now before you unshroud the valves?
Robbie2734
Lambcharger
The static C/R with fel pro 8553PT gaskets (.053) is 9.68 and the dynamic is 8.531. That's with the Magnum heads opened to 70cc's from 64cc's. With the heads left as is and a .039" gasket it's 10.687 static and 9.404 dynamic. A little too much for pump gas. That's the numbers I get from KBs web site.
What size,you cam? Just curious.
OldmanRick
Well-Known Member
With those numbers, must be mild?......
X2.
Robbie2734
Lambcharger
Here are the specs for the cam. The specs are for a 1.5 ratio, I'm using 1.6 rockers so they will be a tad larger...It is a mild cam, but it's a convertible and a cruiser.
OldmanRick
Well-Known Member
Well, the cam is very mild on the duration and intake close. Probably designed for a stock comp. engine.
With that said, your already figuring a fairly thick gasket and if the 8.5 dynamic number works out correct, with a really mild/good advance curve, you may be alright for a cruiser type application on 91/93 pump swill? If any performance work is involved, larger chambers or more cam would be required in my opinion. Sorry i didn't check, but your altitude may have some bearing also?
I checked and looks like your vicinity is around 400ft above sea level, so not much help with the density altitude, especially this summer.
With that said, your already figuring a fairly thick gasket and if the 8.5 dynamic number works out correct, with a really mild/good advance curve, you may be alright for a cruiser type application on 91/93 pump swill? If any performance work is involved, larger chambers or more cam would be required in my opinion. Sorry i didn't check, but your altitude may have some bearing also?
I checked and looks like your vicinity is around 400ft above sea level, so not much help with the density altitude, especially this summer.
Correct ,Rick. Why,a 110 l/c? I run the same basic setup,carb'd 224@ .050 ,114 l/c .Tuned correctly.21" vac,at 800 in "d" mode.
OldmanRick
Well-Known Member
The lobe seperation @ 114* and the centerline at 110* shows 4* of advance built into the cam. While that helps build cylinder pressure, that's not really what your looking for. It might be possible to install it a few degrees retarded to cure your problem. Hopefully a cam guru will chime in with a opinion. I'd hate to see you cut the heads if it wasn't necessary?
P.S. I may have misinterpreted in post #10 your early numbers? What i'm getting at is, run your calculator with the .053 gaskets, uncut heads and a intake close of 36.5ABDC and 38.5ABDC.....2 and 4 degrees retarded, and see how that effects the dynamic ratio?
P.S. I may have misinterpreted in post #10 your early numbers? What i'm getting at is, run your calculator with the .053 gaskets, uncut heads and a intake close of 36.5ABDC and 38.5ABDC.....2 and 4 degrees retarded, and see how that effects the dynamic ratio?
Robbie2734
Lambcharger
I should explain the cam, it's a reground stock roller I'll use for now. I want to get the motor up and running and I've put a lot of money into it so far. I was going to use the 308 heads but got a great buy on new magnum bare heads. The 308's are 73 cc's and would have been no problem. They just needed new valves, springs etc. I put the work into the new castings. I still have to get a clutch and clutch linkage for the 4 speed.
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