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Discussion Starter · #1 · (Edited)
I need y’alls honest opinion. At this point I just can’t leave my truck alone!

I have a “built” standard bore 350 with flat top pistons, a 540ish lift 260ish duration cam, screw in rocker arm studs and some rollers on solid lifters/stud girdles. Running an Edelbrock intake and a q-jet carb. Standard .041 head gasket.

“Problems” is that these are some 305 H.O. 601 heads. The gentleman who built the engine says they have the 1.94 intake 1.6 exhaust valves and have been machine to a 62-63cc chamber. He claims they are flowing as good or better than a set of vortec heads. Says he’s be super surprised if slapping on the vortec 64cc heads would yield any better power. Obviously the valvetrain has been upgraded for the high lift cam.

After getting of a disagreement with the builder, some of you may remember previous posts of mine, he has given me that addditional info. He says SCR should be right around 10:1.

I know running pump 93 with 36 degrees total timing there is no detonation, so I’d assume the heads were machined. Stock these heads are 53cc chambers and with flat tops I would think it would hardly run on pump let alone keep its self from detonating to death. I just don’t feel like I’m getting the power I should. It’ll roast the tires and chirp third gear but I feel like it’s got a 0-60 time of more than 6 seconds. Running this in a single cab 5 speed nv3500 c1500 with 3.08 gears so maybe it’s in the gearing. I manage a repair shop so I get the chance to drive quite a few cars. I’d assume with the flat tops and the cam I’d be putting 400 to the wheels with the right heads? Or am I over estimating that? My butt Dyno says I’m pushing no more than 300-325 or so.

Opinions? Waste of time swapping the heads? Minimal gains? Try gearing first (which will be don either way)?
 

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Race it, Don't rice it!
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The fact that the previous owner thinks the 305 head is a good head has me walking away from the conversation.
A stock Vortec head is better performing everywhere regardless of the how well the 305 heads are worked over.
 

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The fact that the previous owner thinks the head is a good head has me waling away from the conversation.
A stock Vortec head is better performing everywhere regardless of the how well they are worked over.
I agree, even a full time head porting pro will have a tough time getting the 305 head to flow as good as a stock Vortec head.....a small time machine shop/engine rebuilder has about a snowballs chance at the equator in July of achieving that.

400 HP at the wheels means you need to make 500 Hp flywheel....you are not making that right now....but Vortecs won't get you that either.
 

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I have a 1968 Chevrolet 350 CI engine in my 1962 Chevrolet with 1962 double hump heads that have 64 cc combustion chambers and .041” deck clearance. . The compression ratio is calculated at 10.3:1 with the following fuel mixture.

I got detonation under load with pure Mobil/Exxon 92 octane premium unleaded, winter blend and summer blend. I pour in five gallons of VP C12, 112 (R+M/2) leaded pump octane race gas with fifteen gallons of 92 octane premium pump gas and there is no detonation even on a hot day with 210 degree engine temperature. That mixture creates about 97 octane in the tank.
(% A) x (octane A) + (% B) x (octane B) = octane of mix
(.25 x 112) + (.75 x 92) = 97 octane.
A = race gas
B = pump gas

The five gallon sealed container of VP (green) C12 race gas costs $70. The local race shop keeps that fuel in stock. It is the most popular VP leaded fuel to mix with pump gas that will work for street performance engines with up to 12:1 compression ratio. If you have more than 11:1 compression ratio, you should use 100 % VP C12 rather that mixing it with any pump puke.
 

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Discussion Starter · #5 ·
Well, I should trust my gut in these scenarios but I always trust y’all a opinions. I’ve read, heard and seen how well the Vortec/aftermarket heads flow and I knew these 305’s were chocking down the engine. Vortecs/aftermarket it is. I’m super tempted to Dyno it before and after.
 

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look at Profiler heads before you randomly buy some "aftermarket" Vortecs. 0-60 in 6 seconds is pretty good. Whats your 0-100?
I doubt your 305 head engine is making 400 hp?
 

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You'll see coolant long before you can make a 305 head flow enough for 400 horsepower. About 350 is all they have in them and that takes a lot of tricks in other places. This is a head where high lifts are mostly a waste of time. Here you need to deal with the inability to make these smallish ports flow to the amounts that 400 horses need to feed on. The answer here is more duration with not so much lift. The excess lift cannot be used by the port as it cant flow enough to feed the bigger curtain area at the valve; the same holds to some extent of valve size. So all the big lift numbers do is incease wear and tear on the valve train be requiring high spring pressures for no power benefit.

305 heads then need a lot of duration so the small ports have additinal flow time to compensate for their lack of peak flow capability. This results in a long duration cam that must close the intake valve late in the cycle, to compensate for this the compression ratio needs to go up high to gain back the compression pressure lost to reversion created by the rising piston against the still open intake valve late in the compression cycle. To overcome the reversion takes enough RPM to increase the port flow velocity to where it develops enough inertia to overcome the reverse pumping of the earlier inducted mixture taken in on the pistons downstroke. While small ports will cause this inertia to build early, the port still has its physical limits and you can't keep the intake off its seat forever to compensate for the ratty flow numbers and other operating issues that flow bench values have no way of considering as it relates to pressure building behind the closing valve as relates to pocket volume and reflected pressure waves in the port, and of course the maximun Mach number the port can sustain before it generates standing sonic shock waves that block flow. This actually happens at fairly low average port velocities. An average port is done well before .5 Mach; for a 305 probably less than .4M. So there are a lot of sticky issues hiding in here.

The other is the 3.08 probably with a large diameter tire just doesn't turn the motor fast enough to let it run in it's better power range. Then there are all the issues of getting crankshaft power to the pavement. This being the mostly an unsung zone of chassis, driveline, and suspension rocket science that has to be done along with the engine power. This is difficult, expensive, and not appreciated but if you want to go fast, you gotta put a lot of homework here. It's not as flashy as big power numbers on a dyno but if you know how to do it, you can put a lot of people with way bigger power numbers on the trailer.

Last thing is you enter into a zone where it takes increasingly large power increases to net small gains, it just gets harder to get big effects as the speeds go up. Power grows in a somewhat linear fashion as resistances and consumptions increase in exponential fashion, the physics fight you every step of the way.

Bogie
 

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Discussion Starter · #8 ·
I have a 1968 Chevrolet 350 CI engine in my 1962 Chevrolet with 1962 double hump heads that have 64 cc combustion chambers and .041” deck clearance. . The compression ratio is calculated at 10.3:1 with the following fuel mixture.

I got detonation under load with pure Mobil/Exxon 92 octane premium unleaded, winter blend and summer blend. I pour in five gallons of VP C12, 112 (R+M/2) leaded pump octane race gas with fifteen gallons of 92 octane premium pump gas and there is no detonation even on a hot day with 210 degree engine temperature. That mixture creates about 97 octane in the tank.
(% A) x (octane A) + (% B) x (octane B) = octane of mix
(.25 x 112) + (.75 x 92) = 97 octane.
A = race gas
B = pump gas

The five gallon sealed container of VP (green) C12 race gas costs $70. The local race shop keeps that fuel in stock. It is the most popular VP leaded fuel to mix with pump gas that will work for street performance engines with up to 12:1 compression ratio. If you have more than 11:1 compression ratio, you should use 100 % VP C12 rather that mixing it with any pump puke.

Crazy thing is, I can get 36 degrees out of this motor on 93 with no detonation at all. Which actually leaves me with more questions than answers!

I guess this pretty much sums up what I already figured. Now the big question would be how much gains could be made swapping over and if those gains are worth the effort? Only one real way to know I guess. It would surely suck to take the time, effort and money to see minimal performance increases.
 

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I have a “built” standard bore 350 with flat top pistons, a 540ish lift 260ish duration cam, screw in rocker arm studs and some rollers on solid lifters/stud girdles. Running an Edelbrock intake and a q-jet carb. Standard .041 head gasket.
Would you please explain the cam and rockers in detail. You have me confused. Roller, flat, rocker brand and ratio? Is this a solid roller with 260 at 0.050"? If so, it's not wonder you can't rattle the motor.
 

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Discussion Starter · #10 ·
Would you please explain the cam and rockers in detail. You have me confused. Roller, flat, rocker brand and ratio?
Sorry, flat tappet cam. No cam card but measured at about 540 lift and 260 duration. Solid lifters. Not sure of brand on rockers but they are 1.5 roller rockers.
 

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put your car on a chassis dyno.
Back off the timing and make a pull
advance 2 or 3 degrees before each pull.
When power dips you can bet you are at or near detonation.
back timing to peak power, maybe minus 1 degree ?
now proceed with carb tune
recheck timing
do a valve lash tune if you want for another pony or 2
Lash loop also tells you if the cam is too big or you can use more
 

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260 from .050

I understand. Unfortunately these don’t have knock sensors but now I’m curious how to detect detonation that isn’t audible?
Determining the minimum fuel octane requirement mostly comes from experience and help from others who have used Racing fuels. Knowing your compression ratio is a good place to start along with the engine timing advance.

I learned the racing fuel/pump gas mixture starting with pure 100% VP C12 race gas and slowly reducing the percentage until I detect audible detonation, then increasing the amount of race gas until detonation goes away. That is risky if you have hypereutectic cast pistons. That is called a “butt dyno”.

I have used forged pistons on every engine I have put together. Forged pistons gives your engine a extra margin of safety in case the ratio of the fuel mixture is not right.
 

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260 degrees at .050 lift is a big cam with late intake closing that bleeds compression pressure so the engine will react as if the actual compression ratio is lower than the volumetric calculation.

But basically you have had to back into all the numbers since the engine builder is unable to supply them. Without actually taking the engine apart there are things you just don't know volumetric compression ratio being among those unknowns. Since the SCR which is the volumetric compression ratio is affected by the volumes be they pluses or minuses of the piston crown shape, how far the piston is in the bore at TDC, how thick is the head gasket and how large its bore, the actual measured volume of the combustion chamber; you are left not really knowing what the Static Compressin Ratio is. Without that you can't even start on calculating the Dynamic Compression Ratio which also requires you to know the length of the connecting rod.

Bogie
 

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To visualize what Bogie is talking about, choose a very dark moonless night or inside a building that is large enough to contain your car in total darkness. Remove the air filter assembly and start the motor. Let the motor idle for a little and shine a very strong light across the top of the carburetor. You will see a ball of fog, which is the air fuel mixture that has been blown back up the intake tract by the ascending piston, caused by the "long cam" intake valve closing point being strung out so far that the piston coming back up the bore with the intake valve still open, is blowing the air/fuel mixture that has just been pushed into the motor by atmospheric pressure, back out the "still open" intake valve.

This is one of the trickiest parts of engine building, to use a cam that closes the intake valve just before the piston, which is rising in its bore, relinquishes its suction and gathers enough pressure to push the mixture out the still open valve and back up the intake tract to disrupt the venturis in the carburetor bore. The venturis don't care which way the air is moving, in or out of the motor. All they know is that if they see a negative pressure at the venturi, they add fuel. So, the motor gets fuel into the airstream on the way into the motor and again as the airstream leaves the motor, pushed out by the piston. What happens when that same double-rich airstream re-enters the motor? Yep, the venturis see another low pressure and add even more fuel. This is why you see fellows blipping the throttle when they have to idle the motor for any length of time, to clean out the over-rich condition in the motor and keep the motor running.

Sorry to waste the time of you old timers, but the youngsters need to know this.

Additional understanding of fueling a motor may be had by studying two-stroke motors and the reed valve system that some of them employ. Pretty slick stuff. Visualize a four-stroke motor with too much cam and a reed-valve system to prevent reversion. Cool.
 

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Discussion Starter · #18 ·
All of that makes sense for sure. I do have to blimp the throttle every once in a while to keep the truck running. If not it will die. Good and bad with this motor. Lots of small issues/tweaks but I’ve learned a lot. I’m also running a quadrajet which I’ve read are hard to tune with low vacuum motors.

Vacuum is terrible 5-7 inches at idle. Looks like on top of the sub par heads the motor is probably over cammed also. Here are the best cam specs I couldn’t gather if y’all are curious.
 

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The Qjet should work decently enough with a big cam, the small primary venturies generate a high speed flow which increases signal, the multi-stage boosters also augment the signal so it should be easy to dial in.

Bogie
 

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I bought some new GM Vortecs several years ago for a moderate performance 355. This was a brand new build with no "before and after" comparison possible, so I can't speak to performance gains. However, I can say I would NOT go the Vortec head route again, certainly not stock GM Vortecs.

Why?

  • Max lift (with .050" safety margin) varied from head to head and valve to valve, but averaged around .450" to .460". I ended up using LS6 beehive springs installed at 1.750" with CompCams retainers and locks. You can also buy tools from CompCams to cut down the guide bosses and spring pockets, which allows the use of conventional springs.
  • Had to buy Vortec-specific intake manifold, valve covers, and gaskets which cost a lot more than conventional stuff, which I already had. That said, I really like the center bolt valve covers.
  • Exhaust ports sit around .150" higher relative to header bolt holes, so just about any header with primaries tubes = 1-5/8" or less will have to be opened up at the top. Same is true for exhaust manifolds and gaskets. The last thing you want is to block the tops of the ports where flow velocity is the highest.

Seems like there were one or two more issues I had, but don't remember them.

I'd suggest you get a good pair of conventional aftermarket heads with port volumes and valve springs matched to your engine's goals.
 
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