6-71 Burned Exhaust Valve!! New Heads and cam! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-25-2006, 10:36 PM
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6-71 Burned Exhaust Valve!! New Heads and cam!

Hello guys I havent posted in a while(I am sorry please forgive me) and wanted to get some feed back on what you think. My setup is as follows:


2 Bolt Main 355
TRW Blower Pistons
882 Heads
Billy Joe Camshaft?? <-----
Trick Flow Rods
HomeBuilt 6-71 Detroit blower.
3000RPM 9.5Inch converter
Th350 9"inch tail
Stock :-( 10 bolt rear


Here is the problem the other days my good old trusty 882 heads decided they no longer wanted to hold boost on the exhaust valves (one to be exact). I backfires as soon as there is boost. I am having a custom set of heads made for me by a machinist ive used before (custom CNC etc, small shot great guy). The heads will flow (with flow sheet when done) around 280cfm intake and around 200ish on the exhaust maybe a little more. 70cc Heads 220CC Intake runner.

Now this sound pretty dumb but I want to remove the cam since I dont remember what is in it and only put it in due to the 882 heads. I have picked out a cam here are the specs

Dur 234 Int, 244 Exh and 114 lobe separation, Lift .480 .510 Flat Tappet Hyd. I have had experience with 6-71s alot, but ive only used close to stocker cams or huuuugeee racing cams with them. What kind of characteristics do you forsee with this cam. Idle/surge/chopiness/lossing to much on a 350/dont need to go that big etc. The digger we run has a difinate surge, but again the cam has enought duration and lift to swallow most small animals. (I love surging on a blown car and would love to have it on the street)

Your thoughts please!! And thank you guys in advance.


Anything else 1/4 mile estimates etc??

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Old 04-26-2006, 12:50 AM
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My first thought was to ask if you'd lifted a valve cover to inspect the valve springs on the offending hole? Boost means more spring pressure (seat pressure) then an N/A motor with the same lobe profile.

Last edited by 56Maynard; 04-26-2006 at 12:56 AM.
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Old 04-26-2006, 10:11 AM
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that definately could have been the reason. Like I said I cheaped out so much to just get it on the road and slowly add better parts. I put in 1.25 od Z28 springs on it. Probably chinese. Thanks I forgot that I had done that, definately could have been a reason.
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Old 04-27-2006, 08:40 AM
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Nobody! :-(
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Old 04-27-2006, 11:52 AM
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My first thought was the same as Maynard (valve spring/s). Those heads are not very durable anyway so changing them is a good idea on a blown engine. I am not sure I follow your theory though. The large runner heads and medium cam. With the blower you don;t really need 220cc heads on a 350, unless you are really going to spin it. With that cam it isn't going to spin to take advantage of the heads. What I am saying is I see a little mismatch. What are you going to do for an intake manifold? With the larger runner heads, most blower manifolds aren't large enough to cover the ports in the heads. My 215cc heads were a problem, I ended up going with a "race" manifold for my blower. My engine is 388ci, with an 8-71 and my cam is 248/257 @ .050 (solid flat tappet). Mine does surge at idle. Your two bolt main also doesn't lend itself to high RPM's or big boost. How much boost do you plan to run? Is this a pump gas car? All of this will make a difference. You didn't say what type of car or how heavy it is? That makes it impossible to guess 1/4 mile performance. Will it be on slicks or street tires?

Royce
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Old 04-27-2006, 09:09 PM
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Well my friend you just brought up some exeedingly good points that I overlooked. As far as the heads beign to large (220cc heads), I did feel that that might be the case (heads beign to large). Im not concerned in it beign a little soggy off of boost. The way that I figured it was heads are an investment so might as well go with something that I can grow into if I later change my mind. I have a buddy running AFR 210s on the street and it feels really good. I guess the blower does compensate. I dont plan on spinning past 6000, to 6500ish once in a blue moon at the track, (mostly a street car). The front main has a flat bar on the front main(acts like a support). But the intake ports matching now has me really worried. I have a BDS blower manifold with the thermostat in front and pop off plate in the back. The heads are cnc to match a fel pro 1206. (any ideas).

The car is a pump gas car, weights 3350lbs without me in it. TH350 3.42gears(will change this later). I was getting 13psi of boost :-) with the old heads. You know when you see something stupid after you type it, like 13psi of boost on 1.25 stock od springs. By the way the 280, 200 cfm of flow on those heads are at .550. They flow 310int, and 215ext at .7 lift.

camaroman7d you made great points, now im a little concerned, specially with the intake port not having the size needed to cover the head ports :-( (Why didnt I think of that) Do you see the cam beign to small for this app, I built something similar before and while true it was soggy off boost it feelt great on boost.

Camaroman7d I cant thank you enough for your help on this!!!
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Old 04-28-2006, 09:01 AM
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With the blower, the heads won't hurt you very much at all. The blower is going to move the air, so you will have power. Like you said you won;t have as much torque down low but, hat might not be all bad.

The best thing for you to do is grab a 1206 intake gasket and hold it up to your blower manifold port and see how it looks. I will post a link to my page where you can see the issue I had with a regular street blower manifold and my 1206 gasket/215cc heads. You have two choices, 1) have stringers welded (add material) on your manifold and machine it. 2) Buy a large port manifold from The Blower Shop www.theblowershop.com I chose to go with option #2 it still has a pop off valve and thermostat opening.

If you go here http://community.webshots.com/user/camaroman7d and click on the folder "New Engine" you will find pictures of what I was talking about above (manifold mis-match).

As far as your cam, no it's not to small for 6000 RPM use but, it is not going to take advantage of the heads you have. If you had slightly smaller runners (200cc) the cam would be a perfect match. The car will run and still be plenty fast with the parts you have selected. I am only giving you my thoughts on the combo and what I would change.

Don't be surprised to see a little less boost with the better heads, you will make more power than your old heads even when showing a little less boost.

Royce
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