it all depends on the rest of the "combo". if your running a radical cam with a tight lobe seperation and lots of duration you will bleed off a lot of compression at lower rpm's. it can be driven on pump gas, just not at its full power, you'd have to back the timing down and stay out of the higher rpm's. 9.5 is usually fine for an iron headed mild motor. if you post the cams your interested in(or already have) we can give a better idea of what octane you might need. if you pick up a copy of desk top dyno, you can learn a lot about volumetric effiency, and talk to people at hotrod shows and get that stare

i have a copy of dd2000 if you would like me to run a combo for you just give a holler.(btw i found it to be pretty acurate when provided all the info)

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pics of my car
<A HREF="http://www.chevelles.com/showroom/chrisfast66hb1.jpg" TARGET=_blank>
www.chevelles.com/showroom/chrisfast66hb1.jpg
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I don't think it is possible to say a certain compression ratio is the absolute limit for an iron headed SBC. 9.5 to 1 is the rule of thumb. Sure you can get away with higher but then you have to be careful with quench, cam, piston type and things like that if you want to stay away from detonation. I would suggest staying with a flat top pistons (they are better than domes as far as the flame front and quench goes). Like Fast66hb said the cam can be used to help too. I don't think .5 or so is worth all the trouble. Sure compression helps make power, but not as much as some people think (I'm not comparing 13-1 to 8-1, but a 1/2 a point in a street application). 9.5 to 1 is enough to make good power if you combine that with a good set of heads and the right cam. Of course aluminum heads will allow more (I know you don't want to run aluminum).

Royce

 

Thanks for the help and here is what I am considering.

Keith Black Hypereutic Piston
KB125*.030 over
Head vol.-
Comp.ratio
58cc -12.3
62cc - 11.7
64cc - 11.4
76cc - 10.1

Eagle Balanced & Rotating Assembly for .030 over 400 SBC. Eagle SIR 5.7" RODS and Eagle 400 cast steel crank. Flat Top KB Pistons = 10.1:1 with 76cc heads. Ported and polished 2.02/1.60 76cc smog heads for now. Because I have them from years ago. "Will probably add 74 cc AFR 195 Aluminum Heads later on when funds allow." Comp Cams XE274. 1 5/8" Headers with complete 2.5" exhaust. Edelbrock RPM Manifold. 780 VAC Secondary Holley cause I got one of those from years ago laying around too.

If it matters BM Holeshot 2000 stall torque converter(should be slightly higher with 400 SBC torque) coupled to 4:10 Posi. This 71 is just a street car that I enjoy driving from time to time. It is not a RACE CAR or a daily driver but in a pinch it might have to do the last one. In that case the rear gears get changed out and pump premium are a necessity.

I've seen several postings from people who say they are running 11.1:1 or so on pump gas without ANY problems. There are a few Iron Head Crate Engines that advertise 10:1 or better being compatable with pump gas. Which to me indicates just fill it up with no additional additives or whatever = no problems. In my book to high a compression ratio is worse then over camming because of the damage and defeating adjustments necessary to operate on 92-93 octane.

I can step the CR down to 9.1:1 with 76cc right now because there is still time to change the order. Afterwords it is too late. I'm having bad flashbacks. Does anyone remember the water alcohol injectors? The failed attempts at double stacking head gaskets to try and save those expensive Lightweight JE pistons and rings? Forged pistons take detonation better but the rings are a different story. Running water down the carb to clean out the carbon then changing plugs immediately. Rejetting and juggling the plugs to cooler ranges in different cylinders. Carrying the distributor wrench to adjust the timing advance so you could start then retarding it. Trying to keep the engine very cool running and so on and on. Ever seen what a RATTLED motor looks like inside after a severe knock down drag out bout with detonation? I'm gun shy and thinking about changing my order for the Lower CR kit EVEN if it cost more! Funny how times have changed because at one time it used to be the other way around for a Higher CR. One thing about it I can't just change the heads to lower the CR if it is necessary with the suggested present setup. That other kit is looking a whole lot better.

I intend to keep the quench area very small in any case!

Just Nervous Ranting and Raving because shake, rattle and roll used to take on a whole new meaning.

Don

 

I know I'm jumping in a little late on this one, but I am fascinated by this particular subject.

In my personal opinion, 9.5:1 is a safe number if you pay no attention to quench area and/or cam overlap. As you and the others mentioned a tight quench area will get you a bit higher and a cam with more duration/overlap will get you higher yet. But picking a cam so you can run a higher compression is a misnomer IMHO - I'd pick a cam because its right for the application, not to bleed off compression as a fix for (or to prevent) detonation.

With flat top pistons, a zero decked block, and using a .039 head gasket I bet 10:1 is a sinch with iron heads, maybe even 10.5:1 with the right combustion chamber shape.

It's a chance you can take, but like you said a half point or maybe a full point of compression is worth only a few extra horsepower - but then why give up those horses if you don't have to?

Just my .02 cents - well maybe a nickle.

Eric

 

<BLOCKQUOTE>quote:</font><HR>Originally posted by Eric68:
With flat top pistons, a zero decked block, and using a .039 head gasket I bet 10:1 is a sinch with iron heads, maybe even 10.5:1 with the right combustion chamber shape.
Eric<HR></BLOCKQUOTE>

A better rod/stroke ratio really helps too.


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www.grapeaperacing.com

 

I think 10:1 will work with iron heads but beware that IMO you are at about the limit here. Same combo with aluminum heads and you could run 11:1. I have @ 9.75 to 1 with my aluminum heads on my BB but I run Nitrous which has the same effect as running a higher compression ratio in regards to possible detonation. I DO NOT have to retard the timing from the 36 degree normal timing when I run the juice either so for me the CR is perfect. Initially I was retarding the timing 4 degrees when the juice came on and that was costing me @ 35HP so you're right in not wanting to get the compression too high which results of course in uisng octane boosters, aviation gas, or retarded timing.

 

following what GrapeApe said about rod/stroke ratios effecting the ability to run high compression... check this out...
http://www.airflowresearch.com/Articles/A3-P1.htm

11:1 compression, a mild, low bleed-off, cam, and all on 87 octane.

D71,
You said you are planning on running one of Comp Cams XE cams. Comp recommends a maximum of 9.5:1 with these cams because they are designed to build cylinder pressure. You may be able to get away with some more, but this cam doesn't bleed off cylinder pressure like some others do as mentioned before.

 

Good article boodlefoof!!!

Seems like they those bigger piston, longer rod combos are worth trying. Hmmmmmm, sounds like a BB to me.

 

That is a good article Boodlefoof - that pretty much sums it all up.

The only question I have is how does the longer rod really help. I understand that the piston dwells at / near TDC longer with a better rod / stroke ratio promoting more efficeint combustion, but HOW does it create more efficeint combustion?

Doesn't the cooling effect of a high velocity charge through a tight quench area help prevent detonation? It would seem to me that if you slow the velocity of the piston through TDC you also would slow the velocity of the air / fuel charge escaping the quench area and consequently reduce the cooling effect. So wouldn't a longer rod reduce the beneficial effect of a tight quench area?

Obviously the AFR theory is valid because they tested an engine and more or less proved it, I'm just trying to understand HOW a piston moving slower through TDC promotes more efficient combustion.

On a side note I'd bet that Chevrolet opted to increase the stroke of the 327 to 3.48" because it was cheaper to produce different crank shafts and rods than different block bores. A few Dollars savings per engine would make Millions in extra profits each year. Since cars are built by businesses, they are built to make a profit first and foremost.

 

Just read the AFR article. Rhere - Morrison says rod length has no effect on HP. Who do you believe. A 302 has a 1.9 rod ratio stock.

 

I wonder where they found an .030 over 400 block for $150?

That is just one of the things I don't like about those magazine articles.

 

I have read that a shorter R/S ratio will actually create a little more torque by means of being a more efficient air pumping mechanism. A shorter rod and longer crank, something like a 1.52 R/S ratio (like found with 5.7'' rod and 3.75'' crank) will increase the speed at which the piston moves up and down in the bore, thus pumping more air. I don't know the extent of this, but there could be something to it.

Eric 68,
Your question is a good one and I don't really know the answer. What you said about cooling makes sense, but perhaps it is the opposite. I don't know.
The longer R/S ratio probably fights some detonation by means of the slower squishing of the charge. Perhaps a quicker compression of the incoming charge causes more heat and more detonation proneness. however, I would think that the quicker moving piston could cancel this by creating more turbulence? I don't know.

The longer rod/stroke ratio would also allow the engine to wind higher with less stress on the block and rotating assembly. This may not increase torque, but may increase HP numbers by allowing for a higher peak rpm? Also, longer rods mean shorter skirted pistons which would probably reduce the weight of the rotating assembly.

$150 for a 400 core is pretty cheap. I need me one of those.

 

I have read that the shorter r/s ratios make an engine more detonation sensative because of the rapid cylinder pressure rise (quicker accelerating piston).
One very overlooked thing about using a long duration cam to bleed of cylinder pressure is that it really only works for part throttle, light load driving. When the engine gets up "on the cam", effective cylinder pressure goes way up do to the better breathing at the cams intended rpm range. Check out some of the info from Bruce Fulper or Rock and Roll engineering...he is a pontiac engine builder but has devoted a lot of time researching detonation.

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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

 

<BLOCKQUOTE>quote:</font><HR>Originally posted by Eric68:
The only question I have is how does the longer rod really help. I understand that the piston dwells at / near TDC longer with a better rod / stroke ratio promoting more efficeint combustion, but HOW does it create more efficeint combustion?
<HR></BLOCKQUOTE>

There are a few things that benefit from a lnger rod. Not only does it reduce peak pistons speeds, it only reduces speed in the first 1/2 of the sroke, the second 1/2 of the stroke is faster. It will delay peak piston speed as well. Even only by 1 degree can allow the valve to open 0.010-0.020" more before peak airflow demand and the valve is open much more in the second 1/2 of the sroke where the longer rod speeds the piston up some.

Another reason is the dwell at TDC. The speed of the burn isnot going to change with rod length. The point where peak cylinder presure makes the bst power is still the same (or very close). With the piston move slower past TDC, you will need less ignition advance to get the same point of peak cylinder pressure. if you go from 34-32* BTDC, that is 2* less resistance on the way up.

There are other benefits as well, but IMO these two are the biggest power makers. Reduced friction will make some power also, as well as better average leverage on the crank.

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www.grapeaperacing.com

[This message has been edited by Grape Ape (edited 07-09-2001).]

 

<BLOCKQUOTE>quote:</font><HR>Originally posted by boodlefoof:
I have read that a shorter R/S ratio will actually create a little more torque by means of being a more efficient air pumping mechanism. A shorter rod and longer crank, something like a 1.52 R/S ratio (like found with 5.7'' rod and 3.75'' crank) will increase the speed at which the piston moves up and down in the bore, thus pumping more air. I don't know the extent of this, but there could be something to it.
<HR></BLOCKQUOTE>

The biggest benefit if a short rod is that the faster speeds at TDC will help scavenge the exahsut at lower engine speeds. Peak piston speeds is not as much of an issue as average piston speed. A short rod has a higher average speed in the top 1/2 of the stroke, where the valve is closed more. In the second 1/2 is it slower where the valve is opened more.

A short rod will will usually make better power in a low-rpm motor. A long rod IMO has more benefits than a shorter one.

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www.grapeaperacing.com

 

Ah-ha, so the efficiencies created by the longer rod are independent of the quench area theory.

The valve timing thing makes sense to me - max piston velocity is slightly delayed until the valve is open a little more and able to flow more. So we have a more efficient air pump, but not necessarily more efficient combustion.

Less required ignition timing with a longer rod seems to be the only benefit to actual cumbustion efficiency that I can see.

Interesting stuff.