Thread: Engine rebuild advice.....Need Help!!!

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Ok I scored a 010 block that had been sitting for awhile, it is a 350 made around 1980 I believe the date code is b108. The head numbers are 462624 with 1.72/1.5 valves. I think the combustion chambers are 76cc. I do need to know what kind of compression I would have with flat tops and with dish pistons.

I need advice on how to build this engine for lots of torque, it is going in a 4x4 blazer with a 700r4 tranny.

Here is what I was thinking:

Q-Jet carb (I already have a good one)
Performer intake (I already have one)
rebuild kit from northern auto parts (block will probable have to bored 30 over)
268 adv Xtreme Comp Cam
stock exhaust manifolds with dual exhaust into a single muffler with dual outs, no cats
stock ignition

Any thought would be greatly appreciated!!!

Bill

I appreciate the advice Tech, and others of course! I agree the valves are small, can I get my heads cut to larger valves like 1.94/1.5 or 1.94/1.6?

Will any cam below the comp cam 268 work well or is there a specific grind in any manufacturer that will give me lots or torque?

Bill

I'm sorry- but I can't belive what I just read. Increased velocity by choaking airflow thru a mismatched intake & heads combination....is that really what I just saw? You are saying that he can achieve more mass mixture into the combustion chamber that way? By cutting the flow will increase volume to the chambers??

Well I finally finished the tear down! The pistons were very hard to get out, I will be suprised if .030 will be all I have to bore the block.

Tech, I agree with your flow assumption on the edlebrock intake and 624 heads. I took out the caliper and did some measuring and you advised, the ports on the performer intake are only slighty larger then the head ports, I should be able to clean the ports on the head up with a die grinder and get the flow profile I need.

I believe, with all of yall's help and my limited budget that I have come up with what I am going to build:

010 block bored .030 over (the block is on its way to the machine shop now)

Stock 624 heads with 1.72/1.5 valves, some porting to match intake

stock exhaust manifolds with 2.5 pipes into one muffler with dual outlets behind the rear wheel, no cats

northern master rebuild kit with comp cam 12-205-2

performer intake with Quadra jet

stock ignition

Please if you feel I am messing up with any of this combo please advise.

Thanks Again
Bill

Should I use flat top pistons or dish? What about compression ratio for both with 76cc heads.

Is more compression better or worse for torque?

Bill

Thought I'd just toss in some amplification. Tech wasn't decreasing flow, he was increasing velocity. You increase velocity by cutting down the cross-sectional area of the manifold runners.

Fluid flow, excluding compressibility:

V = Q/a, where,

V = fluid velocity
Q = fluid volume
a = cross-sectional area of the passageway.

Q will try to remain constant at a given RPM, since the engine is simply a big heat-producing air pump. Therefore, if a (the area) decreases, the V (velocity) increases.

This is a very simplistic approach, but it illustrates Tech's premise.

Thank-you Henry.

Richard- I don't think that I deserved such a response from you. Not everything I ask is an attack on you or your I.Q. As for "getting my shit togather" I think I will by not asking any questions here anymore.

Originally posted by techinspector1
I have a lot of respect for Streets, but in my opinion, for this combination, these heads will work fine. They'll flow all the air you need up to 4,500 rpm's and that's where the cam is gonna sign off anyway. THIS IS A LOW-RPM, EXTREME TORQUE COMBINATION. It will provide maximum grunt, smooth idle and excellent mileage on crap gas. If you want different characteristics, DO NOT USE THIS COMBINATION.

The small runners in these heads will keep mixture velocity up at lower rpm's and help to pack the cylinders for maximum power. You could increase the intake size to 1.94" if you want to spend the money, but I think your money would be better spent on doing some bowl work and a 5-angle valve job. Use new, STOCK Chevy springs. I think I'd use a stock cast iron Chevy 4-bbl intake manifold that was designed to accept a quadrajet instead of the Edelbrock unit. The runners on it will be slightly smaller and there again, the lower volume will contribute to higher velocity at lower rpm's. The Performer will work, but I think a stock intake would work a little better. It's all in the combination and getting all the parts to work together toward a common goal. I suspect that the exit port on the Performer is larger than the entry port on the heads and that will disrupt flow slightly. Perhaps the exit port on the cast iron unit more closely matches the head ports. It would be interesting to investigate this for yourself, don't you think? I think that with either manifold, I'd also use a four-hole phenolic spacer under the Quadrajet so the mixture could make the 90 degree turn out of the throttle bores into the intake runners more easily. DO NOT USE AN OPEN SPACER, USE A 4-HOLE. An open spacer effectively turns your 180 degree low-rpm intake manifold into a 360 degree, high rpm manifold. If I used any other cam for this combination, it would be a Comp Cams X4250H. I'd order the cam and lifter kit CL12-231-2 for $228.50.

I think your right, its kinda like running water thru a hose, the smaller the hose the faster the flow I remember Edelbrock having a manifold called SP2P, had a spread bore bolt pattern. Wouldn't work as a good torque manifold? If I remember it had small runners???

Alan

I think it depends on the compression ratio. Take a big block for instance, it takes a lot of compression (11-12.1) for rectangle heads to flow. Drop the compression and see what happens..... if you drop to say around 9.1 you'll be better off with Oval port heads.

ya sayin' that a .500 hose that is squeezed down to a .125 hose.... flows faster

Yep. Absolutely. Velocity is inversely proportional to cross-sectional area.

Water squirts further when you put your finger over part of the end of the hose, and the pressure at the house hasn't changed.

At least that's what I learned in Walter Wan Wang Hu's Fruid Mechanics class when I was a punk young engineer. . .

The following logic is for a venturi tube, but a venturi tube is only a pipe with a constriction in it.

The continuity equation is a statement of the conservation of mass in a system. Consider a pipe that is uniform in diameter at both ends but has a constriction between the ends, called a Venturi tube. Furthermore, assume that fluid is flowing through the pipe from one end through the narrow throat of the tube with cross-sectional areas A1 and A2, respectively. Let V1 and V2 be the average flow speeds at these cross sections. Assume also that there are no leaks in the pipe nor is fluid being pumped in through the sides. The continuity equation states that the fluid “mass flow rate”—the amount of fluid per unit time—must be the same at any cross section of the pipe or else there is an accumulation of mass—"mass creation"—and the steady flow assumption is violated. Simply stated,

(Mass rate)1 = (Mass rate)2

where

Mass rate = Density x Area x Velocity

This equation reduces to

p1A1V1 = p2A2V2

Since the fluid is assumed to be incompressible, p is a constant and equation (3) reduces to

A1V1 = A2V2

This is the simple continuity equation for inviscid, incompressible, steady, one-dimensional flow with no leaks. If the flow were viscous, the statement would still be valid as long as average values of V1 and V2 across the cross section were used.

By rearranging, one obtains

V2 = (A1/A2)V1

Since cross-section A1 is greater than cross-section A2, it can be concluded that V2 is greater than V1. This is a most important result. It states that the flow speed increases where the area decreases and the flow speed decreases where the area increases. In fact, by the continuity equation, the highest speed is reached where the area is the smallest. This is at the narrowest part of the constriction, commonly called the throat of the Venturi tube.

This is for an incompressible fluid like water, but the same general theory applies to fuel-air mixtures. You just have to compensate for the compressibility factor.

Ask an engineer the time . . . and he'll build you a watch.

Henry didn't have any trouble explaining this theory to me, and without the nasty, aggressive language that you choose to use. You seem to be aggressive towards anyone that questions anything that you say.
I bet living with you is a real joy, hell I bet you beat your wife. I know you brow beat her because you try to do it around here. Well "miffed" or not - you Richard Cranium- can Kiss My Ass.

Man- O -Man Look at all this fun I've been missing while I was away.

~ Vegas ~

Restrictin' the hose diameter does NOT increase the flow!

Streets, That's true, but it DOES increase the velocity, which what I thought you meant when you said "faster."

Given a constant flow rate, decreasing the area will increase the velocity.

If we're talking about volume (cubic feet per minute), then we are in total agreement.