big block cylinder heads (long)

[Michael]

This one is long and loquacious….

For some time now I’ve been tossing around the idea of getting good aluminum cylinder heads for my 454 big block. Currently I have 346236 GM heads – standard equipment on 1970’s trucks. I have been following the various discussions about heads for small blocks, power vs. torque, the significance of cfm ratings, piston speed, etc. – and while the basics of course apply here as well, the particular choices are very different. Granted, it’s the full combo that matters, not the individual parts. But decent aluminum BBC heads are $2000 ($3000 with serious porting), so I first want to get a feeling for the cylinder head selection. I AM in principle willing to spend $3K on heads – or more – but only if the expense is justified. The price vs. performance curve is not a straight line – there is a knee in the curve, which for big blocks evidently occurs at around 500 hp.

 

A cursory summary of my situation: 2700 lbs car (with cast iron heads), to be driven regularly (in good weather) and suitable for occasional drag racing and maybe autocross-type stuff. The transmission is a Doug Nash 5-speed with 3.27 first gear, and the rear is a stock 3.54 R200. I want ferocious off-idle torque with a usable torque band up to maybe 5500 rpm. Anything beyond that is nice, but not important. It must be compatible with 92-octane gas and pull decent vacuum at idle. The goal is, tentatively, the aforementioned 500 hp. The very highest-flowing heads may be overkill, since the rest of the intake-combustion-exhaust flowpath may not be able to make full use of the high flow volumes at this relatively tame hp level. However, when I say 500 hp, I mean a LAZY 500 hp – the engine is not straining, stock cast crank and 2-bolt mains are doing just fine, pistons speeds are comfy low and I don’t have to constantly keep staring at the oil pressure gauge, and maybe even the carb and ignition tuning isn’t 100% there. The point is, there’s plenty of extra power available with the eventually combo – but 500hp is the notional benchmark for the first iteration.

 

There is no particular goal in the quarter mile. This car is so weird that it would not be competitive in any class for which it would qualify (refuses to join any club that would take it as a member ), and I’m not interested in bracket racing.

 

Information on big blocks is MUCH harder to find than on small blocks. Chevy High performance ran a series of cylinder head flowbench articles (available on their web site), also covering big block heads. This is my starting point. I plotted the various flow numbers vs. lift. Of the heads that they considered, I included:

 

GM 049, 156, 236, 290 and 702 oval port heads;

Brodix OEFI, Edelbrock Performer RPM, Dart and GM Performance Parts aluminum oval port heads;

Merlin iron oval ports heads (they don’t make an aluminum oval head!);

and Canfield and Merlin VR rectangular port heads.

 

These were apparently all unported. I excluded the larger rectangular port heads based on the sentiment that their port volume is probably too large for my application (though port cross sectional area, or rather hydraulic diameter is a more significant parameter, I assume comparable port geometries and runner lengths). Of course, this is just a flow rate comparison. Combustion chamber design, spark plug location, etc., are not considered. Bore diameter was 4.250”. Some heads, like the new AFR heads (see below) flow much better with the larger 4.500” bore, for reasons attributed to valve unshrouding. The main results were:

 

* all the stock GM heads, except for the 049, all almost dead equal on both intake and exhaust (especially up to 0.500 lift) and dead last. The 049’s are considerably better, especially on the exhaust side, with numbers half way between the stock GM heads and the aftermarket heads.

 

* Among the oval port heads, the Brodix (and Merlin) heads have good low-lift intake numbers, but become mediocre by 0.400 lift (intake) and are in the back-of-the-aftermarket-pack on the exhaust side.

 

* Among oval heads, Dart heads are middle-of-the-pack for intake at lift below 0.3, but after 0.4 they are clearly in the lead, and have the best E/I ratio because they’re really good on the exhaust side. The Dart heads reach 300 cfm intake flow at 28” of water at 0.550 lift – that is, at about the max lift that plan to run.

 

* Among the oval heads, the GMPP heads are probably all-around second to the Darts (surprising!).

 

* The Canfield and Merlin VR heads have similar intake flow at 0.4 and lower; then the Canfields pull ahead. Both are superior on intake to all the oval port heads. Canfield is also the hands-down winner on exhaust, where the Merlin VRs are mediocre.

 

Not included in the original CHP series were the new AFR BBC heads (available only with rectangular ports). But a recent article in Hot Rod and AFR’s web site give some data for the 305cc (or rather 315cc) head. Unfortunately it’s apples-to-oranges, because these heads were CNC ported. With that in mind, the AFR heads were amazing, reaching 300 cfm intake flow at 0.370” lift, with the best E/I ratio (for 4.5” bore).

 

At this point, if I choose aftermarket oval, the choice is Dart. If I go rectangular, it’s Canfield or 305cc AFR. The sentimental favorite is AFR.

 

First question: what is the potential of my 346236 heads? If I spend some time porting them, and have 2.25/1.88 valves fitted with a good valve job, what hope do they have of supporting 500 hp with the above-mentioned conditions and, say, a .550” lift flat-tappet hydraulic cam? By gut feeling is that money spent on getting these slugs to “perform” is money wasted.

 

Second question: should I limit my choice to oval heads, as the local engine builders recommend? This is a relatively low-rpm engine, and I do want the low-end torque. But in the big block world, mine is a very light car, and it is deeply geared.

 

Third question: why are Canfield heads so rare? Do they have some hidden flaw that racers know about, but don’t divulge?

 

Fourth question: the Dart heads really wake up at large valve curtain areas, despite having among the smallest port volumes (and hence port cross-sectional areas). My guess is that there’s a big loss due to separation at the “pocket” just upstream of the intake valve seat, so that even at moderate valve lift the flow past the seat is not attached (low discharge coefficient, so to speak). Does this imply that pocket porting would really improve these particular heads – more so than their near competitors?

 

Fifth question: I think that there are reasons (consequences of secondary flow) why oval-shaped ports should flow more “cleanly” than rectangular-shaped ports, when normalized to the same flow rate – especially for “wet” flow (carburetor). So then why is it that maximum-effort race big blocks are based on rectangular-port heads, instead of very large oval-port heads?

 

And by the way, how can I post the Excel plots with the port flow data?

[grumpyvette]

I build more bbc engines than sbc engines so I know something about them, your goal of 500hp is fairly easy to reach with most good flowing heads, look here,

(1)you could reach your goal with bigger valves and porting on your presant heads but it would be far easier with the 305cc afr heads,

http://users.erols.com/srweiss/tablehdc.htm

http://www.chevelles.com/racing/BBCombo.html

 

If I was trying for your goal of 500hp just the 305cc AFR heads (un-ported) and a good cam, headers and a good flowing intake should get you there,If you call CRANE and AFR and get their input I think youll be pleasently surprised.

(3)as to the CANFIELDS they work fine they are just not well known, and don,t advertize much.BRODIX and DART both advertize much more and give package deals (combos of heads, valve covers and intakes) so they get more sales but from what Ive seen a good EDELBROCK intake and the AFR heads is the way to go now.

(2)because your car is so light and has a resonablely low rear gear I think you would be better off getting the small rectangle heads, while its true the ovals will get you to your goal too, the light car will have no trouble at all with the slightly lower, low end torque and higher mid range the small rectangle heads will provide, you will have a hard time keeping ANY STREET TYPE TIRE FROM SPINNING (after all excessive tire smoke will not do you any good) and if your going to the track with slicks, just launching a few hundred rpm higher will get you the torque differance.

(5)the pro-stock heads tend to be huge oval ports , about the same size as the standard rectangle ports but remember they are designed for 8000rpm and .800 lift cams and are shaped totally different than stock heads.and they are designed for well over 1000hp

(4) Ive used several rectangle port dart heads but not the oval ports so I can,t answer that.

 

CALL AFR AND CRANE, THEY WILL BE GLAD TO HELP YOU!

http://www.airflowresearch.com/

http://www.cranecams.com/

[Kevin Shasteen]

Cant really argue w/Grumpy...appears he really knows his stuff. I just wanted to throw out a few curves to watch out for. In your quest for the 500hp range dont get to caught up in high HP numbers for a street engine...especially from a Big-Block.

 

If, as you say, this isnt really going to be a car/engine for the strip...then torque is what you want; and w/a BBC it shouldnt be to hard to obtain at lower RPM's. If/when anyone go's looking for big HP #'s your gonna be dancing w/the "Limited Properties" of Pump Gas...even w/a BBC. W/all the torque of a BBC (like Grumpy said) you're gonna have more than enough of a problem just getting traction: this is where gearing/tire size will really come in to play.

 

For such a light car your 3.545 rear gearing is gonna be more gearing than you'll be able to handle w/a massively torquey BBC... Eh, bust the factory rear & then go the way Scottie did w/an altered Vette rear w/the gearing of your choice.

 

If you wish to do more research on your own; I'ld suggest buying the book;

 

"How to Build Max Perf.Chevy Rat Motors" by Ed Staffel & published by SADesigns; 1996 Copywrite.

 

Ed's book is very informative & gives the readers much to think about...in ref to modern rat motors as well as the earlier rat's.

 

One last point; in your search for that 500hp BBC...DONT FORGET ABOUT "DYNAMIC COMP.RATIO's!!!" If you're gonna be running on Pump Gas then you'll need to know your limitations & how those limitations are determined. Dont get stuck on "Flat Tappet Cams. In this day & age-especially for your "Low Intesity "But I want 500hp-Now" goals you'ld be much better off going w/a Mechanical or Hydraulic Roller Cam. W/all the technical break thru's of the 90's..you should use that tech to your advantage. If you're gonna go w/AFR's (excellant choice I might add) then why would you not want to also go w/Roller Cam/Lifters to compliment your cyl.head's & intake...something to think about anyway. If nothing else the roller cam/lifters would free up some hp & get you closer to your mark that much easier.

 

Ok, one last point..after my previous last point. Buy yourself a few of those "Auto Math" books & learn how to compute your "g's" and learn how to work the math backwards: I'ld enjoyed & still read two auto math books specifically:

 

1) "Auto Math Handbook" by John Lawler; published by HPBOoks w/a 1992 Copywrite

 

2) "Automotive Math Handbook" by Forbes Aird; published by MBI w/a 2000 copwrite.

 

Most people will have an engine they know has this much torque/hp because its been dyno'd; then they do the math from the flywheel, thru the trans, coupled w/the driveshaft torque, multiplied by the diff.gearing to the drive wheels & then to the rolling radius of the wheel/tire. My point is this-if your learn how to work the math backwards then you can determine what drive wheel torque is needed by the [g's] & the weight of your car,

 

(not withstanding friction-another debate too long for this thread)...,

 

and work it backwards thru the driveline back to the flywheel; Voila! You now have the power requirements needed for your engine to do whatever you're wanting it to do for whatever purpose it is intended. BTW-a factory stock Vette pulls anywhere from .72% to a .75% of a [g] from a standing stop & clicks off a high 13 to a low/mid 14 second ET; very respectable for the street..., learn the math & many of your gearing/tire size problems will aid in your engine power decisions.

 

Good Luck & have fun.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

Kevin Shasteen

good post,BTW you might want to look at this stroker 454/496 build,youll have over 600hp/ft lbs with thr AFR heads which flow about the same as the ported merlin heads in the article and a similar engine build and all at under 6000rpm too on pump gas!!! and you don,t need to go for (all) the expensive parts to make it work either.

 

http://www.skunk.net/boatengines-496.htm

 

heres the DYNAMIC COMPRESSION RATIO SOFTWARE

http://cochise.uia.net/pkelley2/DynamicCR.html

(bottom of the page)

heres the other math calc formulas

formulas to play with

http://www.performancesimulations.com/transmission.htm

http://www.prestage.com/carmath/dynochart.asp

 

http://www.prestage.com/carmath/

 

need some other question answered, heres how to get it.

http://cochise.uia.net/pkelley2/crc.htm

 

http://www.convertalot.com/meelinks.html

 

http://cochise.uia.net/pkelley2/DynamicCR.html

 

http://epics.aps.anl.gov/asd/me/FilmPressureDrop.html

 

http://users.erols.com/jeepfan/tirerpmratio.htm

 

http://www.prestage.com/carmath/calc_ETMPH.asp

 

http://www.prestage.com/carmath/calc_HP_fromETandWeight.asp

 

http://www.turbofast.com.au/Drag.html

 

http://www.turbofast.com.au/turbomap.html

 

http://members.aol.com/fastnuf1/calc.html

 

http://www.holley.com/HiOctn/TechServ/TechInfo/TchArtcl/Artcl07.html

 

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

 

http://www.engr.colostate.edu/~allan/fluids/page4/effarea/effarea.html

[Michael]

Guys,

 

Many thanks for the replies. When I got through writing my follow-up post, I realized that it ended up as more of a rant than a technical point, so I put it in a more appropriate place in the forum (the “I’m tellin’ ya” section). Please look at it if you get a chance, as it hopefully applies to a broader context than just big blocks.

 

One quicky tech question though – has anyone tried "porting" an Edelbrock Performer RPM oval-port manifold to fit rectangle-port heads? This might not be as foolish as it sounds, because the Edelbrock oval manifold’s runner shape tends to be "square shouldered" (I’ve heard it referred to as “roval”) and there ought to be plenty of meat in the port walls. The reason for doing this, besides using what I already have if I go with the AFR heads, is the flow-quality issues for oval-shaped ports that I mentioned earlier, especially with a carburetor.

 

Grumpy – would it be possible to continue this discussion off-line? My e-mail address is ol_70@hotmail.com.

[Guest JAMIE T]

Hey Grumpy, I've got the original 496 build-up article that Lingenfelter did from back in 1984. Considering, I'm only 26, that means I bought this magazine off the news stand when I was 8 That motor made that much power with ported Oval Ports(Holley strip dominator and Roller cam). I tell ya, with a poker face and a Performer intake and mild cam, you could easily clean some clocks with that one.

 

BTW Micheal,

A buddy of mine has Edelbrock Performer RPM heads on his 454, and makes over 500hp and great torque in an afforable package.