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11:1 on pump gas or add more gasket??

13K views 43 replies 14 participants last post by  cobalt327 
#1 ·
Ok guys. I recently picked up a SBC 383. It was to good of a deal to pass up. The motor was completly assemble when I bought it. The guy I bought it from wasn't 100% sure what kind of Compression it was making so I decided it would be a good idea to take it apart and take a few measurements. Know I'm faced with a decision. To get it around 10.5:1 ill have to run a .053 MLS gasket. With the pistons .005 in the hole that is going to put the quench at .058. I guess my questions is would you guys chance 11:1 on 91(I know there is higher octane pump gas out there but I want to be able to buy gas anywhere). Or throw a little more gaksket at it and suffer on the quech a bit??



SBC 350 40 over
Scat 3.75 crank
Eagle 5.7 rods
Speed pro pistons 2 valve reliefs 5cc (.005 thou in the hole)
Edelbrock performer RPM heads 64cc
Comp hydraulic roller 236@50
Performer rpm air gap
750 doble pumper.
 
#34 · (Edited)
Cobalt & F-Bird,
Being a novice to engine building, Barry mentioned something that I've never heard of: polishing the head chamber in an effort to get a better/cleaner burn to eliminate detonation. I'm asking how effective this is and if there is a benefit for lower compression, say 9.5 - 10.5 compression when making tuning adjustments for a novice like myself. I ask because I'm going to be changing my head gaskets to get a more desirable quench distance and I could polish the chamber at the same time.

Barry,
IMHO I have to agree with others that .010 quench seems unrealistic for anything other than engine that is going to be torn down and rebuilt on a regular basis due to alot of factors such as, rod stretch - which is a number that is going to continue to increase as the rods get older and more heat stressed as the number of heat cylcles increase; as the rod bearings and wrist pins wear with normal use; as the valves and guides wear with normal use; carbon build on valve face and piston surface with normal use (especially with a carb setup.) I may be all wet with this, but I would never dare to try a such a low quench for fear of a catastrophic failure.
With that being said, from looking at your pictures how did you manage to get an electric fan on the engine side of the radiator. My son and I have put a sbc into a 77 Astre (Vega lookalike). We are running a swp with an aluminum radiator and we have 3/8" of clearacne between the water pump pulley bolts and the radiator. We mounted an electric fan on the front side of the radiator. The only time it has come close going over 220 is on the 4th run on a rolling dyno (not enough air flow.)

Cobalt, my 1st car was a 73 Vega that I paid $125 for in 1986. It needed a downhill grade to 70 mph and it melted down at 68K. Your right, it was piece of turd! :)

Trying to go fast without wrecking our engine - Jim
 
#44 · (Edited)
Cobalt & F-Bird,
Being a novice to engine building, Barry mentioned something that I've never heard of: polishing the head chamber in an effort to get a better/cleaner burn to eliminate detonation. I'm asking how effective this is and if there is a benefit for lower compression, say 9.5 - 10.5 compression when making tuning adjustments for a novice like myself. I ask because I'm going to be changing my head gaskets to get a more desirable quench distance and I could polish the chamber at the same time.
If you have covered all the other bases, and your time is available, sure- go ahead and polish away. Percentage-wise I believe it's a waste of time/effort on a street engine, but there are others who would have you believe it's a "must-do". Beyond knocking down any irregularities that could cause preignition (glowing hot spots), I just do not agree.

If you want to make a difference, do the things that will give a significant, measurable improvement- like keeping the quench tight, maintaining good ring seal by torque plate honing the cylinders, maintaining a good valve stem oil clearance which will also keep the seat concentric, a good valve job and cleaning the bowl up, etc.

Edit- Another avenue you might want to explore is the various coatings applied to the piston crowns and combustion chambers that are said to aid heat retention, etc. Not a cheap process, but there might be something to them that goes beyond polishing the chambers.
 
#35 ·
As far as timing goes, you simply don't get it. The close quench causes a higher turbulence in the combustion chamber. Instead of the flame starting at the plug and traveling across the chamber to the fuel, the fuel actually travels to the flame front. The reason that you set the timing advanced of top dead center is to allow for the lag in time of the fuel burning. You want the maximum cylinder pressure to occur just as the piston is starting down from TDC. Higher octane gas burns slower than lower octane, so the timing can be set slightly more advanced, HOWEVER, unless the engine is in detonation there is no advantage in advancing the timing and running higher octane fuel. It just costs more for gas that will waste itself out the exhaust valve (because of the slower burning). I've already made all these tests that you are recommending. My setup works the best. If you haven't tried it, don't knock it (pun intended).
 
#36 ·
Higher octane gas burns slower than lower octane, so the timing can be set slightly more advanced, HOWEVER, unless the engine is in detonation there is no advantage in advancing the timing and running higher octane fuel.
That is an internet myth. Octane rating does not impact flame burn rate, it is based on the hydrocarbon content of the fuel mix. A perfect example is Sunoco Maximal, which is their fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion.

The octane number of a gasoline has little to do with how fast it burns or how much power the engine will make. Octane number is the resistance to detonation (pre-ignition). If the octane number is high enough to prevent detonation, there is no need to use a higher octane gasoline since the engine will not make any additional power. Octane number is not related to flame (burn) speed either. Variations in octane quality are independent of flame speed. There are some high octane gasolines in the marketplace with fast flame speeds and some with slow flame speeds. It depends on how they are put together.
 
#41 ·
If you look on the valve cover,the white tag you see is where you would find my name on some of the Cosworth's.



At a 185hp with only 140 c.i. and the first EFI with 12.5 c/r,would outrun any 4 cylinder engine of it's day.Now I'm not saying it wasn't without it's problems,but the factual truth was we fixed that in the later yrs.Had it not been for the car being priced only $600 less than a vett,EPA killing the performance,and a crappy body,the car would have been continued to be produced.These days I'm told the Cosworth's Vega's are a collector's wants.Aside from the dyno tests I ran on them,having driven one,they where far from a turd.At only on the high side only 2,500 lbs,that 185 hp mattered.


Now Barry.Although I am very pleased to see your involvement in Vega's,I got to say .010 is too tight.I'm basing that off all the yrs of building engines,and also building the Cosworth.All I can think of is maybe the measurement is off. That isn't to say you don't know what your doing either.Mistakes are made sometimes by the most experienced engine builders too.:thumbup:
 
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