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  #16 (permalink)  
Old 04-02-2010, 12:02 PM
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Okay I put an edelbrock 750 on it and I think they are the stock heads

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  #17 (permalink)  
Old 04-02-2010, 02:55 PM
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The iron LT1 heads outflow from the factory the alumminum lt1 heads, its limitations and reason for the 15hp loss over the f&y body cars does not matter if you are carbing this thing and using it in a swap not a factory b body car.

The stock iron heads and lt4 hot cam kit(with the 1.6 rockers) with correct tunning, not too much carb, and good supporting exhaust will get you pretty close to your goal depending on the condition of the engine itself.

Again if you are looking for rear wheel hp, you are not going to be anywhere near 400.

Hope this helps.
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Old 04-02-2010, 03:19 PM
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Quote:
Originally Posted by konnie336
What about if I port the heads and get a lt4 hot cam kit
will this help with reaching my 400 goal
If this is indeed an LT1 not an iron headed L99, for which I already gave you the casting numbers to check to be sure. The reason is most Impala's came with the 4.3 liter version and lots of vendors sell these as 5.7 liter LT1s. So knowing for sure which you've got is the first step to 400 horsepower.

As for getting 400 hp out of an LT1 or any GEN I or II small block, this is a no brainer. Something like Vortec heads, a big carb, and cam around 270 degrees duration measured from the SAE standard of .006 inch lift. In fact the LT4 cam's timing falls into that neighborhood, especially with 1.6 rockers. If you want something else the Comp XE268 or any body elses' equavalently timind and lifted cam will provide the same level of performance. But your compression with the cast iron heads will about a ratio to a ratio and a half too low to optimize the cam. Therefore, a bit of milling or a piston replacement is in order as well as using a shim style gasket. If you have a choice between milling the head or using a shim style gasket to gain compression, lean toward leaving more meat in the head deck and go with a thinner gasket. This retains more strength and stiffness in the engine's structure.

Use a standard distributor not that opti-spark thing. Kits are available that remove the opti spark and plug its drive hole and install a conventional distributor on the standard Chevy position, your carb intake will support this change if you haven't made it already.

Your big Holley is plenty adequate if not a bit excessive but should be fine when tuned. A decent set of 3 foot long headers and a good set of dual exhaust is needed.

There is no reason why an LT1 or any GEN I or II SBC with this combo won't make 400 hp or damn close to it without the need to port the heads or enlarge the valve size.

Bogie

Last edited by oldbogie; 04-02-2010 at 03:21 PM. Reason: change "cam" to "came"
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Old 04-02-2010, 09:24 PM
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How about if I get an lt4 hot cam kit and port my heads
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Old 04-02-2010, 09:44 PM
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lt1

What size transmission to put behind the. Lt1. And what gears to go with in the rearend its in a 1985 cutlass 442
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Old 04-02-2010, 11:29 PM
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Porting results depend on several factors. But to be short- assuming you do good work/not too much, or get done by someone who does good work, you should well exceed the 400 fly wheel mark, closer to 410-425 depending on quality of porting and tune setup on carb.
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Old 04-03-2010, 03:02 PM
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Quote:
Originally Posted by konnie336
How about if I get an lt4 hot cam kit and port my heads

I'm trying to assume my way through your questions as to what you're trying to do. In this case, I'm assuming your thinking about porting to gain power instead of milling to regain compression. These are both things that improve an engine's power output but are mutually exclusive in process, result and intent. Yes you can port the heads but that doesn't solve the problem of getting the compression ratio up that the milling is intended to do.

Compression ratio deals with the density to which the mixture is raised inside the cylinder, that is the quantity of molecules in a given space. In general increasing compression increases an engine's thermal efficiency, therefore, it's power output also goes up and its fuel consumption goes down in the process. That's basic physics but engineering brings in other issues of design and how you go about optimizing the design to get all the physics you can from the design. For an engine there are two predominate factors regarding compression ratio. Static compression which is simply all the volumes contained within a closed cylinder divided by the volumes above the piston when it's a Top Dead Center (TDC). This is often shortened to SCR for Static Compression Ratio and is the number most people mean when they talk about compression ratio. But there is a compression ratio called the Dynamic Compression Ratio or DCR, this is the one that really is getting the work done. It is always less than the SCR for an unsupercharged engine because it is the SCR times the reverse pumping effect of the intake valve closing point. This point is always after Bottom Dead Center (BDC) therefore the piston is rising up on the compression stroke. At this point if the valve is still open, some amount of the intake charge is blown back out of the cylinder, which is one way reversion is created. The later the valve is held open the greater the DCR is reduced. The effect is greatest at low RPMs especially when combined with large ports and valves.

The reason for holding the intake open, often well into the compression stroke, is that at high RPMS there becomes so much velocity in the flow of the incoming mixture that its inertia overcomes the reverse pumping of the piston and the cylinder continues to fill. This is a principle reason why as the cam timing increases the torque and horsepower peaks move to higher RPMs. Now to restore lower RPM performance against a hot cams tendency to push the power peaks higher in the RPM band we increase the compression ratio to force more work out of the less dense lower RPM mixtures that will be present. Porting is usually antithetical to boosting the bottom end power, therefore, ported heads combined with long duration cams require even more compression. There are DCR calculators out there, a decent one is at http://www.kb-silvolite.com/calc.php?action=comp You need to know in crank degrees when the intake valve is closed to use it. Estimations from similarly timed cams to the LT4 are an adaquate representation. But they also need to include the difference in LSA. So if you use a cam's closing point that has an LSA of 108 degrees where the LT4 is 115 degrees you need to take the first cam's intake cloasing point and add the difference of 7 degrees to it. You want a DCR in the zone of 8 to 8.5. Less than 8 the engine isn't making all the power it could, greater than 8.5 it will enter the detonation zone and you will have to do a better job of parts selection, oil control, richer mixtures, less advance, colder operating temps, etc. This means a less flexible engine which is fine for a race only engine but will be a PIA for a street motor.

Porting the LT1 head is like porting the Vortec head, unless you have a flow bench and are willing to throw a few heads away to learn what works, there really isn't a lot to gain simply because the factory already fixed the traditional big gainers, so pocket porting doesn't offer much on these heads and porting up the length of the ports really takes knowing what works and what doesn't. The LT4 heads offer much better ports, but you can't duplicate them in the LT1 head as the castings moved the walls around and trying for an LT4 port in an LT1 head will only allow you to discover the wet side of the port walls.

The most effective thing you can do on this engine above what you already have proposed is to change the pistons. The best answer is the D dish piston. This puts all the dish under the valve pocket and puts a flat portion of the crown in opposition to the flat area of the head inside the chamber. This maximizes efficiency and detonation resistance. This is the squish/quench zone, something I seem to write a lot about in this forum.

Squish and quench are functions of the step in the head separated by time. Squish happens first; as the piston closes on TDC the mixture on the far side of the cylinder is forced toward the spark plug. This gives the mixture one last stirring before the plug ignites it so there is greater chance of a fuel molecule being in contact with an oxygen molecule to combine with when the fire gets there. It also increases the mixture density in front of the sparkplg, this improves the chances of getting the mixture to burn, hence fewer miss or late fires, and it burns faster giving a better pressure rise without so much spark advance. This makes more power, gets better fuel mileage, and reduces the tendency to detonate because the cylinder pressures don't become excessive too early in the cycle. The next step in the sequence is quench.

Quench is a heat sink function resulting from the closeness of the piston and chamber step which shapes an area of high surface area to the volume. This draws out the heat from the increasing cylinder pressure, preventing the unburnt mixture on the far side of the combustion chamber from spontaneously igniting before the fame front get there. When the far side of the chamber explodes before the flame front gets there, the two pressure waves collide which a lot of force, enough to blow holes into the piston.

For an engine with steel rods the optimum squish/quench distance between the piston and chamber roof is .040 to .060 inch for an aluminum rod double that. Factory pistons usually have circular dishes that vary from .070 to .100 inch deep that leave a very narrow and insufficient band around the crown. When you add the crown to deck distance of .025 and a gasket of .020 to .053 plus the piston dish can equal an OEM squish/quench of .115 to .178 inch which results in a pretty weak squish/quench function that you make up for by increasing the grade of fuel you're paying for, and/or backing off the timing, and/or richening the mixture. All of these crutches cost you power and money so it's much better to engineer the combustion chambers properly to start with as all the work arounds cost what you're looking for from the motor.

Bogie
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  #23 (permalink)  
Old 04-03-2010, 07:39 PM
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To the OP- OldBogie just gave you a lot of good info. A big part of wh at I do to earn a living is build LTx/TPI/even TBI performance engines and head porting is something that takes a very good understanding of relationships to maximize your efforts. It is very do-able for a patient at home DIY to learn how to efffectivly port heads, but you must understand HOW what you are doing makes makes changes to everything else in the cylinder head, BEFORE you ever add or take away material from anything.

It has been said in regards to modern heads, The gains can still be very profitable, but all changes are much more critical than older heads. If you do no research and just stick a gasket on the ports and open them up- you are missing the point. I'm not saying you will not see improvement because I have seen dyno results that show an improvement. I am saying learn what and why to do to each type of head to maximize your efforts.
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Old 04-06-2010, 08:49 PM
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Thanks a lot
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Old 04-09-2010, 03:52 PM
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Lt1 help !

Hello guys

In the process of Id ing 350 LT1 engine . Seller says no t what vehicle it got pulled from 70 or 71 vehicle years ago. Have #s from block deck thcode and partial vin . Been on several sites can,t seem to find enough info . Help thanks larry Sorry did not intend to disrupt post !!!!Needs to be moved Larry

Last edited by old trucke; 04-09-2010 at 04:06 PM. Reason: wrong place
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