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Discussion Starter · #1 · (Edited)
Well i purchased some 58cc cast iron cylinder heads for my 350 chevy thinking i would find smog era truck dished pistons, however what i found were forged flat top pistons with 4 valve reliefs 6.1 cc.

Any who, now my compression ratio should be at 10.2:1 give or take .1 of a point, i'm not sure what cam it has but it doesn't sound stock but not too radical either

my questions is, can i retard my timing so this engine will run good on 91 octane pump gas ?

will there be any problems if i do this ? :confused:
 

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More machine than man
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I will let the pro's answer if it is OK to retard the timing. But when I run your numbers, you are closer to 10.9:1. That assumes a .040" squish band (piston deck clearance and compressed head gasket thickness).
 

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Discussion Starter · #3 · (Edited)
Silver Surfer said:
I will let the pro's answer if it is OK to retard the timing. But when I run your numbers, you are closer to 10.9:1. That assumes a .040" squish band (piston deck clearance and compressed head gasket thickness).
you're probably right, there's a good chance the blocked has been decked to 0

i know retarding the timing isn't optimal for performance, but can i retard it just enough to get away with 91 octane
 

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If after you do the previously recommended mods, and still have problems, another option that you might consider is retarding the cam a bit. If you retard it the intake valve will close latter in the compression stroke, and lower the cylinder pressure, at least at lower rpm when its more likely to detonate.
FWIW
ssmonty
 

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Discussion Starter · #7 ·
F-BIRD'88 said:
Your cr will be 10.2:1. (4.166x.041" gasket) The other poster should try math sometime.

58cc 305 heads.::: Use a slightly cooler plug :eg champion rv8c or equal.
set the timing for about 34deg bTDC at max advance.
get a fully adjustable vacuum advance and dial it in.
a stock vac advance will be way to aggessive.

It will run fine. on premium.

If you were to finish the job and port your 305 heads with larger valves the chambers end up bigger than 58cc.

online compression calc
http://www.wheelspin.net/calc/calc2.html
apparently they used 601 heads which were originally 54cc or something like that, and after the porting and bigger valves were installed it only got up to 58cc

the guy sells another 601 casting with 62cc chambers after the work is done because those were originally 58cc heads

anyway started up the engine with 87 octane in the tank and it runs great, gotta advance the timing just a bit after i get some premium gas in the tank, right now it feels to retarded throttle response is sluggish
 

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I'm getting around 10.6:1.

727 cc/cylinder (355 cid)
58 cc heads
8.5 cc head gasket (0.039" compressed)
3.1 cc piston below deck 0.015"
6 cc piston (as stated)

(727 + 58 + 8.5 + 3.1 + 6) / (58 + 8.5 + 3.1 + 6) = 10.6

if zero decked then 11:1 cr.

I feel that is a lot of compression for a small cam

I have run 11:1 with 93 octane and a cam with 244 degrees at 0.050" and 292 advertised without any issues . But that was a pretty big cam for a street car (compcam 292H).
 

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as for the compression ratio; if you are trying to have a proper quench at around .040-.046" then yes the compression ratio will be around 10.8:1. but if you aren't trying to achive a proper quench and you go ahead and use a .041" head gasket without taking into acount what the pistons are down in the whole at TDC, you will likely end up with a quench that will be around .060-.065" (unless decked), this will put your compression around 10.2:1. since you are trying to get this to run on pump gas then i would think that achiving a proper quench would be a concern.

Just my 2 cents
 

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Discussion Starter · #10 ·
my87Z said:
as for the compression ratio; if you are trying to have a proper quench at around .040-.046" then yes the compression ratio will be around 10.8:1. but if you aren't trying to achive a proper quench and you go ahead and use a .041" head gasket without taking into acount what the pistons are down in the whole at TDC, you will likely end up with a quench that will be around .060-.065" (unless decked), this will put your compression around 10.2:1. since you are trying to get this to run on pump gas then i would think that achiving a proper quench would be a concern.

Just my 2 cents
i have a strong suspicion the block had been decked to zero, when the pistons were a top dead center they were practically equal to the top of the block

the gasket i used is 0.039 compressed thickness

don't mean to sound like a jerk, you guys have given me good info but no one has answered my question yet, can i retard my timing just enough so it can run good on premium pump gas with no adverse effects ?
 

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Sounds like you run a good chance of overheating the engine by just retarding the timing:
http://www.hotrodders.com/forum/timing-overheating-160577.html

It sounds like you have a performance engine and now want to try to get away with something for nothing. Might I suggest getting a set of cheap used Vortec heads with 64cc combustion chambers? That would bring your CR down to a nice 10.1:1 (although the factory cast chambers will actually measure out to 66-68cc which would put you around 9.7-9.9:1). And you have a good chance of running 91 octane at these numbers.

Also ssmonty had a good thought about retarding the cam to help bleed of cylinder pressure. But that might not be enough for a 10.9:1 CR. Retarding the cam will shift the powerband to higher RPM's, so hopefully you don't have a heavy over geared truck.
 

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retarded timing will not fix a engine component miss match

Surfer, yes you touched on a important point. The vehicle, what is this going into and what is the trans and rear gear etc?
 

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Silver Surfer said:
IDK dude, looks legit to me. Even with the 4.166" bore gasket its not going to go from 10.9 to 10.2.
Your numbers are close as anyone's to being correct (who used the calculator right), given the info provided. Maybe someone SHOULD "try math". :mwink:



Many SBC 4" nominal bore head gaskets are either 4.09" or 4.1" bore diameter. I usually use 4.1. I used 4.030" bore. If the OP's bore is standard, the CR will be about 11.05:1.

68NovaSS said:
You're using a .0015 gasket, F-Bird is using a .041 gasket.
Yes, he's using a 0.015" gasket- with a 0.025" piston deck clearance. Added together this is 0.040".

I have no idea how fb88 came up w/his numbers, unless he was using a 64 cc chamber size and not the OP's spec'ed 58 cc chamber size.

xzero117:

You can retard the timing and get some added detonation protection but that is not a cure for anything. There will still be conditions under which the engine is going to detonate w/that much CR- unless where you live all the roads are down hill. The engine will (as has been said) tend to run hot, and that exacerbates the tendency to detonate. Add to that the power will be way down, and any hopes of good or even acceptable mileage is out the window.

Bottom line is maybe you can get away w/it. But each engine and vehicle and situation is different so no one can say w/certainty exactly what will be the result. You can either fix the problem or you can try it and see if there's a sweet spot in timing that will let you run premium and not detonate.

I would suggest you use stronger advance springs and a vacuum can w/a lot of timing in it so under light throttle cruise conditions you have advance but as soon as the engine goes under a load, the vacuum advance should drop out to prevent detonation. That's about all you can hope for under these circumstances.
 

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Other possibilities:

If you have E85 in your area, you can set up a carb to run on it, and you'll be fine.

Anything you can do to reduce the engine temperatures will help:

• Cold air intake
• Effective cooling system: fan shroud, as big of a fan and radiator as you can run, use straight water w/water pump lube added, 180 degree thermostat
• Block off the heat cross over under the intake
• Insulate the carb from the intake, an "air gap" intake might help a tad
• Richen the carburetor's power circuit by using a stronger power piston spring (Q-jet), or stronger step up springs (Edelbrock/Carter) or a higher number power valve (Holley-type)
• Retarding the cam timing a few degrees might help- but this is getting to the point where it might be as easy to address the root cause instead of applying band aids.

Recently, a member here said adding Marvel Mystery oil to the fuel will reduce detonation. This might be a good experiment to see what your results would be. A search will probably get you more on this.

I'm sure there are other things, this is what I came up w/for now.
 

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Discussion Starter · #15 ·
thanks guys, i got a 180 degree thermostat, my cooling system is pretty good, i don't want to change the intake so is it okay to just add a 1 inch spacer ?

i said 91 octane but i can get 93 octane like a mile away from my house

maybe i'll get lucky and find a good sweet spot with 93 octane and the timing

and for the record this is going to my buggy style jeep, 3.73 rear gears with a 5 speed manual and it weights 3100 lbs tops
 

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my KARMA ran over my DOGMA
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A light jeep and a 5 speed will help as long as you are running stock tire size? 3:73's quickly loose there effect with 30+ inch mudders or taller.

Use a 4 hole spacer if you go this route, better mixture dispersion, also the effect is to shield the carb some from the heat, what is the carb?

Personally if it were my 11:1 motor I would want to know exactly what cam was in there. At the very least you could dial gage the cam at the intake rocker/push rod to get an idea as to the lift, this can give a hint to the type of cam. Lift x rocker ratio = lift

Be interesting to do a compression test on this mill and post the cranking pressure numbers.
 
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