327_TPI_77_Maro

iTrader: (8)

Hi guys. I have a 327 MAF w/ HSR. This is a small journal 327 with a big cam (2500-6800 power band rated by Isky). Needless to say it revs high. I shift around 6300. I used to have a set of 1.94 intake, 1.5 exhaust 76 cc heads w/ big ports on this car. I would max out the MAF just barely around 6k rpms. I only had 8:1 compression with the big 76cc chambers though. I now have 1.84/1.5 305 heads to give me 10:1 compression. The car is definitely no slower with the 305 heads, it is actually more responsive because the big overlap cam now has enough compression to work with, but I do not quite max the MAF out even at really high RPMS. The 305 heads flow 200 cfms intake, around 135 exhaust, at .500 lift stock. This should be adequate flow to run into the low 13s, especially with my very short stroke 327.

Is it unusual that I am not maxing the MAF out? This car should be running high 13s right now. It ran low 14s un tuned with terrible launching and bogging, which I have under control now.

Craig Moates

I managed to max out the 165 MAF (even gutted with the fins out giving 15% low bias) when I was running a 383. That motor did pretty well, spun to 6500. Ran low 12's /w/ AT. It just barely maxed out, so I'd think that unless you're in the 12's you probably won't. Going with rule of thumb here based on experience, with air rate being proportional to power and subsequent times.

JPrevost

Did you raise the "Maximum air flow vs RPM" or are they stock? Stock these tables should be raised. I just take the stock table and multiply by a factor of 1.5 which gives you plenty of room for air flow at low RPM.
Other than that, I could see a high 13 sec car having difficulty maxing out the 255 limit. Just because it's 255gps doesn't mean it's ACTUALLY 255gps. There is a calibration table for a reason and even with the same sensor it's not the same calibration = not as accurate as people seem to believe. The MAF might be reporting 240gps but in reality your engine is digesting 255gps. That's an AFR calc range of like 11.9 to 12.6 . Meaning who knows what the actual air is, all you can assume is that if the sensor value goes up, then the air flow is going up. For this reason, I wouldn't worry about what the reported MAF is telling you so long as it's got a good dynamic range and consistent. Get the wideband on there and with a properly working ignition system you should be focusing on the AFR and the ET/MPH.

TraviZ

iTrader: (2)

hey craig can i ask, did you not start maxing out till 6500rpm?

because i took this into concern while im currently building my 383 has listed to the left here.

heads flow about 250cfm if that matters.

Craig Moates

I was running a 236/242 .560/.570 lift S/R cam with Sportsman2 heads that flowed like 270 intake 180 exhaust if I remember correctly. It started sucking wind around 5800 or so. Maxed out around 6k. But hey, it was still flowing above that point, the meter just didn't see it. So I had to compensate with the PE. No big deal really to compensate. Even if it isn't puritanically correct.

91L98Z28

my opinion would be that you've lost VE and thus you really are flowing less air (those 305 heads with the small valves are flowing worse than your old heads).

but you've probably made up the power loss difference and feel because of the compression boost.

get some nice aftermarket heads and I bet you'll see your MAF max out again and probably a lot sooner in the RPM band.....

327_TPI_77_Maro

iTrader: (8)

Aftermarket 58cc heads, to keep the compression ratio of my flat topped 327 near 10:1, are out of my price range at this point. The 305 heads I know have been used to run low 13s, which is all I'm asking for at this point. I'm going to concentrate on the tune and see what this combo will do. The first thing to go will be the cam -- the 30 year old single pattern is just not going to cut it. It is lazy until very high rpms, plus the idle smells terrible with the 108 LSA, bad enough that the car is annoying to drive. The heads I was using seeing the MAF actually just max out were 487X's. These flowed very very well for factory heads. They actually had 2.02/1.6 valves from the factory, but the previous owner had seats for 1.94/1.5s installed. The 1.94/1.5 valves were therefore fully unshrouded because the heads were cast for the bigger valves. All that flow was not doing me any good though, because I had barely 8:1 compression. The car still ran low 14s at 99 mph in this trim, even with the Isky 280 being way too much cam.

91L98Z28

You might consider the aluminum corvette L98 (aka ZZ4) heads. They go pretty affordable from time to time on ebay or wherever, and i believe they have the 58cc chambers. They may not be AFR's or even vortecs, but they could be the cheapest way into a decent set of heads that won't hold you back a lot.

Just a thought is all. I can understand the direction you're going in, but just keep in mind the MAF isn't lying to you either - you lost flow going to those 305 heads, and the flow loss is actually worse than "reported" - you gained some efficiency back with the compression.

With regards to compression, you might try running your combo through a dynamic compression ratio calculator. with that short stroke and the big cam, you might be suprised to find that you're still out of your ideal compression range. just another thought.

TraviZ

iTrader: (2)

get some nice edelbrock performers 58cc like 700 bucks a pair. and u get 2.02 valves.

327_TPI_77_Maro

iTrader: (8)

I know the cam is way too big, I'm sure it is slowing me down a lot. I know guys running 11s and 12s with carbed setups running a very similar cam. For a 13 second car it is just killing my low end power. I am going with a split pattern cam with a much wider LSA and less lift/duration on the intake side in the next week or so -- something like the old crane fireball .465/.488 224/234 cam on a 112 or 114 LSA. I can't spend $700 on heads right now, this is a car that rarely gets driven at the track and the cost just wouldn't be justified. I got a set of carbide bits and am going to pocket port the 081 305 heads and gasket match them to the stealth ram -- I'm sure I'm losing quite a bit of power just by the fact that the Stealth Ram is matched to 1205s, which are quite a bit bigger than the 305 ports. I knew a guy running 1.74 valved heads on a 327 with a .500 lift cam into the 12s.

Grim Reaper

What you experienced with the head swap was an increase in tq in the bottom end due probably to the higher compression but at the expense of some at the top end due to the smaller valves.

It is quite possible to have equal or better track times if the increase in the bottom-end tq exceeds the little power you loss on the top end.

I would concur that if you went to small cc heads that flowed well like AFRs, you would probably experience a TQ increase throughout the rpm range and your top-end would improve as well.

JP86SS

iTrader: (1)

You should look into that.
In looking at my 224/230 cam, I'm not running a high static comp ratio to make up for the long duration overlap. Advancing my cam 4 degrees will boost my DCR by almost 1/4 point. There are trade offs but this one (for me anyway) is worth doing.

kdrolt

At 100% VE, 327*6000/3456 = 568 cfm

At 75% VE, it's 426 cfm.

So assume, for now, that you have 75% VE at 6000 rpm and WOT.

426 cfm = 7.10 cubic ft per sec

7.10 cu feet/sec = 0.20 cu meter/sec

0.20 cu m/s * 1.2 kg/(m³) = 0.241 kg/s = 241 g/sec

241 is close enough to 255 g/sec, especially with my assumed 75% VE, that you have maxed out the sensor. A slight change in rpm, or increase in VE, or an increase in air density (I assumed 1.2 kg/(m³), or any combination would be enough to put you over.

Note that this is just math so it doesn't necessarily apply to your engine with your modifications. It is what it is, under the assumptions given above. The math tells you that at 6000 rpm with 75% VE, that you would pull nearly enough air to max out the sensor.

If the sensor isn't maxed out, as it was before your engine changes, then the VE probably did change enough and the smaller valve heads (with the poor exh flow) would certainly be a reason. HTH.

JPrevost

Excellent explanation and applied math skills. No salt needed

327_TPI_77_Maro

iTrader: (8)

Thanks guys. I am doing a full port and polish on the 081 heads, I got a set of carbide cutters and a Standard Abrasives deluxe port and polish kit. The 305 heads needed a ton of material removed to match a Felpro 1205, which is the size of the Stealth ram ports. Air was crashing into almost 1/4" of flat surface at the top of the 305 ports. There is also a lot of potential to remove really big ridges where the valve bowl cutter stopped in the casting. This low buck, high time-investment approach should get these heads flowing enough to do what I need.

junkcltr

iTrader: (1)

It might be worth it to get a copy of David Vizard's cylinder head porting book. It has some good pics and talks about flow, port biasing, where the most cfm is gained vs. time spent grinding, etc.
Be careful if you start unshrouding the valves in the chamber. Mark where the gasket covers the head.
It is worth the $20 bucks.