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Old 09-13-2010, 03:55 PM
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SBC, Iron heads DCR for Pump gas !

Tried Search without any result.

was wondering if anybody knew what would be the optimum DCR for iron headed SBC running 91 Octane Pump gas, That's with 0.04" Quench.

Would be well appreciated if some one could tell me what DCR 93 Oct will handle ?

Best regards.

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Old 09-13-2010, 04:34 PM
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Quote:
Originally Posted by monzter
was wondering if anybody knew what would be the optimum DCR for iron headed SBC running 91 Octane Pump gas, That's with 0.04" Quench.

Would be well appreciated if some one could tell me what DCR 93 Oct will handle ?
I am of the opinion that your questions cannot be answered as asked. There are variables such as ambient temperature and humidity, engine operating temperature, final drive gear ratio (will you lug the motor or not?), where you buy the fuel, who formulated the fuel, what is the efficiency of the carburetor/throttle body and the intake manifold (how well will your combination pack the cylinders and how much cylinder pressure will the combination make?), etc., etc. A motor using a Performer intake will have very different cylinder filling characteristics than a motor using a Performer RPM intake, even with the same cam profile.

Over the past several years, I have noted that using the Keith Black DCR calculator, a reasonably safe DCR for pump gas is 8.0:1 to 8.3:1, but that can vary widely due to the aforementioned variables. I have read of fellows who are running 8.8:1 and 8.9:1 DCR on pump gas and getting away with it. There is the possibility though, that their motors are detonating and they just aren't hearing it due to loud exhaust components.

Using 91/93 octane fuel, I personally would keep the upper limit SCR under 10.0:1 and cam the motor for 8.3:1 DCR. That should leave you a little cushion for dialing in enough spark timing to make the motor operate efficiently without rattling. This is just my opinion based on my experiences and the experiences of others.

Others on this board may have an opposing viewpoint and that's OK too.
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Old 09-13-2010, 05:51 PM
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Quote:
Originally Posted by techinspector1
I am of the opinion that your questions cannot be answered as asked. There are variables such as ambient temperature and humidity, engine operating temperature, final drive gear ratio (will you lug the motor or not?), where you buy the fuel, who formulated the fuel, what is the efficiency of the carburetor/throttle body and the intake manifold (how well will your combination pack the cylinders and how much cylinder pressure will the combination make?), etc., etc. A motor using a Performer intake will have very different cylinder filling characteristics than a motor using a Performer RPM intake, even with the same cam profile.

Over the past several years, I have noted that using the Keith Black DCR calculator, a reasonably safe DCR for pump gas is 8.0:1 to 8.3:1, but that can vary widely due to the aforementioned variables. I have read of fellows who are running 8.8:1 and 8.9:1 DCR on pump gas and getting away with it. There is the possibility though, that their motors are detonating and they just aren't hearing it due to loud exhaust components.

Using 91/93 octane fuel, I personally would keep the upper limit SCR under 10.0:1 and cam the motor for 8.3:1 DCR. That should leave you a little cushion for dialing in enough spark timing to make the motor operate efficiently without rattling. This is just my opinion based on my experiences and the experiences of others.

Others on this board may have an opposing viewpoint and that's OK too.
I'd agree with your statement.

Bogie
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Old 09-13-2010, 06:01 PM
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Depends on the spark advance too I think. You are better off with less advance and more compression if you can. People disagree with me tho too. I don't really mind tho

Last edited by Dirty Biker; 09-13-2010 at 06:09 PM.
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Old 09-13-2010, 06:49 PM
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Quote:
Originally Posted by Dirty Biker
Depends on the spark advance too I think. You are better off with less advance and more compression if you can. People disagree with me tho too. I don't really mind tho
You know, I've no idea where you come to any of the conclusions that you so often voice here. But until or unless you have some actual hands on experience to back up these kinds of blanket statements, you need to type less and get your hands dirty more, IMO.

What I am getting at, is you have no experience- by your own admission you've yet to build a single engine, unless I have misunderstood you. I would think that you would realize there's a lot more to the engine building procedure than reading a few (or many) lines off the 'net. You are in no position to be giving advice- again, IMO.
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Old 09-13-2010, 07:03 PM
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Quote:
Originally Posted by cobalt327
You know, I've no idea where you come to any of the conclusions that you so often voice here. But until or unless you have some actual hands on experience to back up these kinds of blanket statements, you need to type less and get your hands dirty more, IMO.

What I am getting at, is you have no experience- by your own admission you've yet to build a single engine, unless I have misunderstood you. I would think that you would realize there's a lot more to the engine building procedure than reading a few (or many) lines off the 'net. You are in no position to be giving advice- again, IMO.
Ditto what you said Cobalt . I have built several engines myself but they have all been stock. That's why I never give advice when it comes to anything other than basic mechanical questions . And even then I try to be careful to make sure I know what I'm talking about . Keep up the good work and thanks .
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Old 09-13-2010, 09:17 PM
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"You know, I've no idea where you come to any of the conclusions that you so often voice here. But until or unless you have some actual hands on experience to back up these kinds of blanket statements, you need to type less and get your hands dirty more, IMO.

What I am getting at, is you have no experience- by your own admission you've yet to build a single engine, unless I have misunderstood you. I would think that you would realize there's a lot more to the engine building procedure than reading a few (or many) lines off the 'net. You are in no position to be giving advice- again, IMO."
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AMEN, brother, AMEN.
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Old 09-13-2010, 09:35 PM
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Which iron heads? Whats is the purpose and power level.
What is the worst case operating conditions the engine will see. (real world)?
Think that one over.

vehicle? Which SBC? Is this planning for a real build or a (BS) theoretical exercise?
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Old 09-14-2010, 07:02 AM
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F-Bird The heads are 041's GM Castings. 64cc Chambers. Think their called Camel hump's ???
Main purpose is Off-Roading. 300+ HP and some usable low end Torque/power, along with acceptable MPG.
This is a real build, unlike some of my unrealistic builds before.
I have reached my final decision about this Jeep, It's not a Race car as my thought's have been lately.

This engine is a old Jeep CJ, That's on 36" tires, Might go 38" some day but for now it stays as is, it's equipped with TH-350 And Stock Converter. So the cam must work with that converter.
The Gears are 4.56 With 2.0 In Low range. (Transfer Case)

The worst condition this engine will see is Very deep and heavy snow for long period of time at some moderate RPM. As you can see a Heavy Load on the engine, The Jeep is fairly light Compared to others. 3500 Lbs. with Driver and Gas.

Engine is 327 SBC.
3970010 4 Bolt main Block.
Performer Intake. - Would you recommend any thing else ???
500/600 CFM 4 Barrel Carb.
1/5.8" Headers. 2.1/2" dual exhaust.
Hei Ignition - Would it be acceptable for this application ???
Comp Cams Magnum 1.52:1 ratio Roller Rocker Arms.
Cam - would like to use Hydraulic flat tappet unless you recommend any thing else.
Must use 91 Oct Pump fuel.
I haven't completed tearing the engine down, so I can't say if I will reuse my pistons or not. Will update This week !
But it was having a big problem with blow by since there was a big sky coming out of the breather's on both sides.


Not mine, but at-least you can see the snow here

If you'll need any more information about my engine or Jeep. Please Ask.

Best regards.
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Old 09-14-2010, 07:16 AM
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head design has a huge factor to play in this, especially wet flow characteristics and chamber design. A pair of "double humps" will only tolerate about a full point to point and a half less than the AFR 195's. The other tuning factors already mentioned play a significant role too.

8:1 DCR is usually a safe place, but under a high load with poor heads even this can be too much.

For your application you can probably run 9.5:1 SCR if your tune is spot on, but since durability is very important with an offroad rig I would just go with 9:1 SCR and a good RV cam. You'll be able to get by on 87 if you have too and when you plan on doing some serious "pleasure cruising" through God knows waht you can run 91 for added insurance under extreme load.

The power different between 9:1 and 9.5:1 (SCR) is about 5hp, on that rig I would trade 5hp for the increased durability.

There's a reason a lot of off road rigs stick to basically stock engines- durability. The rest of your build ideas looks pretty good, I would get a carb taht can handle off road use though, and since you like the snow I would keep the exhaust crossover.

Last edited by turbolover; 09-14-2010 at 07:24 AM.
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Old 09-14-2010, 01:00 PM
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I'we been playing a bit in the Keith Black Compression Calculator you mentioned Mr. Techinspector1 and All the small cams that would be good for my application seem to give me quite high DCR ! Especially if my heads are that poor they can't hardly tolerate 8:1 DCR.

The lowest grate gas that I can get where I live is 91 Oct. So that's why I would like to be able to use that...

Rv cam you say, I have seen this term quite often but don't know exactly what that means, Is this one particular camshaft ?
I would like to use a cam that gives me the power that I'm looking for along with good mileage and vacuum, Because the gallon of the 91 Oct gas cost's 7.6$ where I live, So best MPG possible is a must !!! 15 MPG would be The ultimate, But probably unrealistic.

I'm pretty sure i can make better engine for my application than the factory ever produced... Am I wrong ?

From our experience small Holley 4160 Vacuum secondary Carburetors Do very well in our environment, Q-jet's are also good if they are in good shape.


Hope you get my point of view.
Monzter.

Last edited by monzter; 09-14-2010 at 01:14 PM.
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Old 09-14-2010, 01:08 PM
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Quote:
Originally Posted by monzter
I'm pretty sure i can make better engine for my application than the factory ever produced... Am I wrong ?
Monzter.
You can improve the mileage by about 15% in a lot of cases but with a 327 in a lifted Jeep with offroad tires you're probably looking at about 12 mpg.

An RV cam is a cam with about 200-215 degrees intake duration and about the same to 10 more exhaust duration.
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Old 09-14-2010, 04:57 PM
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Rebuild the motor using flat top pistons.
Use a common summit/melling 204-214 .420-.442 112 cam.
use 1.6 rocker on the intake side only.

home port the 041 heads. Use a felpro 1205 gasket as a guide.
Do the whole port but keep the stock valve sizes.
The power is in the heads.

The distributor can benefit form moderate recurving.
Spark plug heat range selection is critical. You want something just a bit cooler
than "stock"
A Qjet is your best bet for perf and mileage and off roading.
I prefer the older simpler 4MV type pre 1975
Lean how to tune it. use edelbrock Qjet tuning parts.
See the Lars Grimsrud authored Qjet, distributor tuning guides
here...http://www.corvette-restoration.com/...o_articles.htm

Use long tube headers and 2.5" dual exhaust.

If the "stock" converter won;t stall to 2000 rpm, it is the 13" low stall version.
get the 12" th350 converter from a th350trans.

As long as the real measured compression ratio is under 10:1 it is fine.
+.030" flat tops+ 64cc heads + a stock deck height + .015" gasket
gets you a 9.75:1 cr. The heads are often more like 67-68cc.
Nothing stopping you from sweetening up the chambers when porting them.
cc yours.

This 327 combo with home ported camel heads builds a 330-340hp engine with good torque and will run fine on premimum unleaded.
any vac sec carb will do with correct jetting. A old 4mv Qjet is your best bet.

Put the DCR calc down and get to work on the heads and rebuild.

CC your cylinder heads and measure the TDC piston deck clearance NOW before block dissassembly. You need to know this spec NOW before buying parts. KB makes a nice flat top and a nice -13cc dish piston.

If the block deck height is small or 0 or the heads have been shaved down use the -13cc dished piston. You can correct a shaved head with some chamber deshrouding work.
The DCR junk is BS. You need to know the real measured engine mechanical compression ratio.

CC your cylinder heads and measure the TDC piston deck clearance NOW before block dissassembly.

Last edited by F-BIRD'88; 09-14-2010 at 05:10 PM.
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Old 09-14-2010, 05:14 PM
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Your heads are not Poor. Don't listen to the BS.
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Old 09-14-2010, 05:35 PM
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SBC Build.

Hello Mr.
Thanks for your advice, appreciate it a lot.
I'm going to finish tearing the engine apart later this week, IF the engine is a total crap, including the pistons and crank, would it be better to base the build on a 350 Cid ? Would I do it the same way ?

My heads do have 2.02"/1.6" valves in them is that worse ?
I will measure them up, and dig in them with the carbide.
Do you recommend that I would do any thing more than gasket match and clean up casting flaws ?
Is there a way to see if the valve guides are worn out ?

Do you recommend that I do any work to the intake manifold ?
Should i run spacer between the manifold and carb ?

I see you recommend The Summit/Melling camshaft, I have heard that the Summit lifters are bad ! Is that true or is it just from bad camshaft brake in ?
Are there any special valve springs that i need to run with this particular camshaft ?
What kind of Vacuum readings may expect with this camshaft and What would be accept-able Idle RPM ?

Is there any start point with timing ? Base, Max and W. vacuum advance for max power and durability ?

My converter stalls around 2000 Rpm, I thought that was to high for what I'm about to use the Jeep for, But with this big of a size converter and a good cooler It may be just perfect.

Should I run Hypereutectic or Forged pistons in this engine ?
Wouldn't it be recommend smoothing all sharp edges on the pistons to minimize detonation with the 91 Oct pump fuel ?
Any tips on ring gap's and such ?
Are stock main and rod bolts durable enough ?

About these measurement's !
What do I use to measure up the deck height from TDC ? In other words, How far the pistons are in the hole ?
I know how to measure up the Heads, and will do that soon.

That's enough for now.
Thanks again F-Bird, I appreciate your help very much

Take care...
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