I've got a '68 with a blown big block in it. I don't know much about the motor since I bought it pretty much "as is" but I will post what I know. It's a 454 with a set of cast iron 840 heads on it. The cam is a CompCam grind with a .575 inch lift on both the exhaust and intake side and is a hydraulic flat tappet design with a 110 degree lobe separation. Sitting on top of this is a Dyers supercharger which is either a 6-71 or an 8-71 with two Holley's of unknown size. The engine is currently five percent underdriven and makes between 10 and 12 pounds of boost. The car runs great but I've been thinking about upgrading the cast iron heads to an aluminum version for weight savings and improved heat dissipation as well as better breathing with the larger valves in the aluminum heads. I want to stick with a direct replacement head which eliminates some of the heads that are available because of their raised exhaust port design. The 840 head is a closed chamber design with about a 107cc combustion chamber. My questions are as follows: What is the intake and exhaust port volume on a stock 840 cylinder head? Would the compression be significantly reduced if I went with an aluminum head that had a combustion chamber volume of 110cc? Do any of the members have practical experience with either the Edelbrock performer rpm marine 454R head or the Brodix Race Rite BB2Plus head? These two are the closest match in combustion chamber size to the 840 heads on the motor now. The intake port volume on the Edelbrock head is 290cc while the Brodix head has an intake port volume of 312cc. Would more port volume on such a motor be better and how much would be too much (there are some heads that have upwards of 350cc intake port volume or more)? Thanks for any replies and opinions. Joseph Evans
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Re: Opinions wanted on cylinder head swap
Joe,
This is a restoration web site. I would suggest to pose your questions that are on modified engine performance on another site that deals in performance modifications. -
Re: Opinions wanted on cylinder head swap
This question is no out of line than another member that constantly tells members to switch to some modified cam he designed or to switch from a hydraulic lifter cam to a solid lifter cam.
I say its a fair question and there are knowledgeable engine people that could help him, but I will defer to the Moderators here.
DonOThe light at the end of the tunnel has been turned off.Comment
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Re: Opinions wanted on cylinder head swap
I understand that this is a restoration website and I have posted questions in the past about my other cars. I own three C2 cars that are all bone stock and original but I also have my "toy" that I can tinker around with and not worry about affecting its originality or value by modifying it. I simply thought that some of the members who may have the same diverse automotive interests that I do would have some input or practical experience with the parts I mentioned that might help me regarding the changes to this vehicle that I'm contemplating. Thank you for your opinions. Joseph EvansComment
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Re: Opinions wanted on cylinder head swap
Joe, if HP is what you're after, stick to the iron heads. HP is heat and iron withstands heat better than aluminum. They are just darn heavy.
DonOThe light at the end of the tunnel has been turned off.Comment
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Re: Opinions wanted on cylinder head swap
Joseph,
Have you thought about buying GM aluminum heads, then porting them? A quick search turned up these part numbers:
3919842 3946074 3904392
Here's an article on compression ratio that might help you, scroll down to page 5:
JoeComment
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Re: Opinions wanted on cylinder head swap
Your cam has too much overlap. Moderate to high boosted engines at WOT with reasonably low back pressure exhaust systems have higher inlet pressure than exhaust pressure, so high overlap short circuits part of the fresh charge right out the exhaust system. Some aftermarket cam manufacturers (check the Crower online catalog) offer "supercharge" cams that have relatively wide LSAs compared to a similar duration cams designed for headers and open exhaust. A LSA of 114-116 degrees at that duration would be much more effective as would later phasing of the valve events.
Headers are also of little value on boosted engines for the same reason. The rarifacation wave that headers can generate at the exhaust valve during overlap is small compared to inlet pressure. That's why Top Fuel/Funny Cars just have short individual pipes to get the exhaust clear of the chassis and driver and up out of the way with as little friction as possible. They aren't "tuned".
On a road car large manifolds and exhaust pipes/mufflers to keep backpressure as low as possible are best on supercharged engines.
DukeLast edited by Duke W.; February 26, 2014, 10:50 AM.Comment
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Re: Opinions wanted on cylinder head swap
Thanks for the input from all who responded to my inquiry. According to the dyno sheet that I have for this engine when it was freshened in 2008, it makes 820 horsepower at 6200 rpm and 760 ft lbs of torque at 4400rpm. Perhaps I'll follow the old saying of "if it ain't broke, don't fix it". For a street car, it runs extremely well with enough power to melt the tires if one chooses to do so. I may, however, take Duke's suggestion to heart and put in a more appropriate blower cam as opposed to the cylinder head swap I was thinking about doing. I'm planning on pulling the motor to detail both it and the engine compartment when things warm up a bit here and this would be the ideal time to do any mechanical upgrades to the motor as well. I sincerely appreciate the input of those who responded. Thanks again. Joseph EvansComment
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