Lol... Ummm. NO!

I think I'll edit this and be just a little bit nicer

on 11:1 with a cam with that much overlap you would kill it driving it on the street as an everyday car. You would have so much carbon buildup in the intake from the overlap at idle after awhile.

11:1 your looking more toward race gas, your engine might run fine at idle, but above that the overlap gets less and your looking at a pinging nightmare.

A driveable street engine would be more like 9.5:1 and duration at about 280, or 290 adv. max. A more useable cam would be in the 260 range. But even at 9.5:1 your looking at running 91octane and up.

 

Wait a second, he didn't give the cam specs other than the .050 number, you cannot tell how much overlap there is by that alone.

If you have all the specs on the engine we can figure whether or not if it will be pump gas friendly.

One of the biggest things is what is the quench height? If the quench is tight .035 - .040 then I think it should be fine. Still need to know all the cam specs, to be sure. Is this a "TRUE" 11:1 or according to the parts book 11:1? There is a difference.

Royce

 

The entire purpose of increasing the engine's compression ratio, regardless of cam profile that goes with it is to increase cylinder pressure, dynamic cylinder pressure, which is the cylinder pressures the engine is realizing under actual running conditions. Regular pump fuels cannot live under these conditions, and resist engine-destroying pre-ignition. If you play with the cam timing or spark lead , just to lower the cranking compression, you are just putting a band-aid on a bad situation, which is not enough octane for the cylinder pressures of a racing type engine. Do not use racing type compression ratios with street type fuels, it's a losing situation.

 

Nairb, I agree and disagree. You can make a healthy running small block on pump gas. 11:1 with aluminum heads is doable. That is the practical limit in my opinion. I have ran several like that. This is the reason I asked for all the specs, I would not suggest running that high of compression and then using other means to prevent detonation (ie.. retarded timing). You can run 11:1 with full timing on pump gas IF everything is setup right.

With the specs I could then check his DCR (dynamic compression), and this is where the compression counts.

I agree high cylinder pressures are not pump gas friendly in most cases.

It will also depend on how much gear, stall, car weight etc... This is where the cam choice comes in. As you know you can't just build a high compression engine throw a big cam in it and run pump gas, and that is not what I was saying at all.

Royce

 

I am running a true, calculated 11:1 compression on my SBC 400. I have iron heads. I only have access to 91 octane gas in my area.

I have a 234o duration @.050" cam.

My timing is at 34o total, with 18o distributor timing.

Up hill, 100o weather whatever, it doesn't ping.

My car is real life proof. With aluminum heads I could go 11.5:1 no problem.

I would run that with a cam between 230o and 240o. Go with the higher duration if you want more performance than idle quality.

People have a lot of misconceptions about high compression ratios. I have done a lot of research on the subject and can give you a few pointers if you need any help.

 

lluciano77,
If you don't mind can you post your engine specs. A true 11:1 with iron heads is ummmmmm not going to be easy to pull off on 91 octane. I am not saying you are wrong, I would just like to see how you managed to do it. 10:1 is pushing it with iron heads, you MIGHT be able to squeeze 11.5:1 with aluminum, but then you have to "assist" it so much to prevent detonation you get into the situation NAIRB was talking about.

The tiny bit more power you make would not be worth the trouble and you would likely give it all back with the "under" tuning required.

What I would like to know is deck height, bore size, gasket thickness and cam specs, piston part number. If you don't mind.


True = measured

Calculated = estimate

Royce

 

No problem.

Cylinder head vol.(64cc)

Piston head vol. (-7cc)

Gasket thickness (.039")

Gasket bore (4.2")

Cylinder bore (4.145")

Deck clearance (.05" below deck)

Stroke (3.75")

rod length (5.7")

Cam specs:
234o @.050"
114o lobe
.488" lift (.520" with 1.6s)

I have Keith Black KB125s

Dart Sportsman II iron heads
There is no special tuning required. I haven't tried anything below 91 octane so I don't know the effect of lower octane fuels.

 

You can make a healthy running small block on pump gas. 11:1 with aluminum heads is doable. That is the practical limit in my opinion. I have ran several like that.

So have I, many a time.
It's the PACKAGE, not just the static compression ratio that matters.

 

So have I, many a time.
It's the PACKAGE, not just the static compression ratio that matters.

Hey that is what my wife said to me before we got married.:mwink:

 

lluciano77,
Your compression with the specs you gave me is 10.12:1 Your quench is .089 (huge). I would expect that to detonate like crazy. If you mis-typed the deck height and it is supposed to be .005" your compression will then be 11.24 with a quench of .044 (that sounds more like it).

If I had to bet I would say if you cc'd your heads and those pistons you would see the volumes are larger than advertised.

To calculate the DCR I would need all the cam specs, advertised duration, LSA, ICL (intake center line), intake/exhaust opening and closing points.

Sounds like you have a very similar set up to what I have run except in a 383, but I was running aluminum heads.

If it works for you great. You are probably the first guy I have come across that got away with 11.2:1 with iron heads and a smallish cam. How long has it been together?

Royce

 

Hi Guys! I have been going over some of the same compression decisions
for piston ordering for a ford FE. The kb-silvolite.com site has several
calculators for static and dynamic compression. I am still learning and the
factor that really jumped out was when the intake valve closes after BDC
on the compression stroke. If the intake closes late then the cylinder pressure is decreased....a 10.5 static goes to a 9.7 at 37 degrees ATDC.
Plug tour figures into the calculators and see what comes up. I remain
teachable. Tim

 

lluciano77,
Your compression with the specs you gave me is 10.12:1 Your quench is .089 (huge). I would expect that to detonate like crazy. If you mis-typed the deck height and it is supposed to be .005" your compression will then be 11.24 with a quench of .044 (that sounds more like it).

If I had to bet I would say if you cc'd your heads and those pistons you would see the volumes are larger than advertised.

To calculate the DCR I would need all the cam specs, advertised duration, LSA, ICL (intake center line), intake/exhaust opening and closing points.

Sounds like you have a very similar set up to what I have run except in a 383, but I was running aluminum heads.

If it works for you great. You are probably the first guy I have come across that got away with 11.2:1 with iron heads and a smallish cam. How long has it been together?

Royce

Sorry, I must have mis-typed it. You are correct in your correction. The piston is .005" down.

You are also correct about the Sportsman IIs. They are actually 67ccs according to Car Craft magazine. I also smoothed out the combustion chambers to a mirror finish. I'm sure that took off a little more.

It has been together since October I think. Somewhere around there.

I have been driving to work everyday and have gotten on it a lot since it has been broken in.

My compression as tested with a compression tester is 215-220 psi.

 

Generally speaking, raising the compression ratio will always increase power production. The law of diminishing returns applies to the application of compression, however. Jumping from a true 8.5:1 to 12.0:1 will net huge gains in torque and horsepower, but going from 12 to 13 will not realize as large of a return.
An engine's ability to fill the cylinder (volumetric efficiency) dictates to an extent of how high the compression should be for maximum horsepower. Notice that the huge mountain motors of nearly 600 cubes or more will generally run upwards to 16.0:1 compression ratio as even though they have huge cylinder heads, they still are not able to achieve a volumetric efficiency of let's say a top notch Winston Cup engine, which will only need 12.5:1 to get the job done. Of course, this is a generalization and many other factors will contribute to finalizing an engine's compression ratio. There are other terms such as dynamic compression , which is a term used describing real world cylinder pressures generated under operating comditions, and cranking compression, which is just as the name implies. Camshaft timing and compression go hand in hand and the proper camshaft should be used for a given compression ratio


This is something I posted quite some time ago, but it's very true. We all know that a 350 CI engine displaces 350 cubic inches of air if it operates at 100% volumetric efficiency, you can then burn x amount of fuel with x amount of air and produce x amount of power. If you increase the thermal efficiency of the engine, by changing the combustion chamber, increasing the compression ratio or adding thermal barrier coatings this can increase power production also.

What is the aim in running 11.0:1 compression ratio on pump fuel? If it's for maximum power, then surely you are also trying to maximize volumetric efficiency and then trying to maximize thermal efficiency, and then bam!!!!!!! you have exceeded the limits of pump stock unleaded fuels and preignition brings on detonation and you've smoked some pistons, your heads and your block!

Does anyone remember the Crower max fuel mileage system for the small block chevy? It utilized a special cam coupled with over 12.0:1 compression, maybe it was higer than that. Produced decent mileage, but no big power gains, much like a stocker or even less powerful. High compression does not equate big power.

If I put 11.0:1 in an engine, then it's going to be a good 11.0:1, but I would only do it to help save the bearings on maybe an endurance engine. If I were to use anything that requires racing type fuels, I'm gonna run 13.0:1 and higher and see some cranking compression of over 220 psi and the dynamic pressures will challenge even racing fuel. That's my point, if I run 11.0:1, I want to match my combination to deliver as much force to the piston as possible and regular unleaded fuel ain't gonna stay glued together under those conditions, if it is, then I ain't making the power I wanted.

 

I would agree that most the time actual compression ratios end up being somewhat less that claimed. Lluciano's combo is a good one to pick at. Like Royce said, the heads and pistons are probably a little larger than advertised. This coupled with the polishing could put compression around 10:5. I am running a very similiar combination on the 400 in my 57. It has chevy 461 heads polished with large 18cc dish pistons. With virtually no quench due to the dish pistons it is a rattle trap if you are not careful. Next time I go through it, I will be lowering the compression and consentrating more on flow numbers and getting quench right.

Anyway, I am just rambling now.....I would suggest you keep compression around 9.5:1 with iron and 10:1 with aluminum heads. It is just not worth the extra headache of detonation. One thing to keep in mind is that detonation is not always audible. Alot of the time the rattle on acceleration is preignition(also bad). Many times it takes more than the human ear to hear detonation.......sometimes you find the results when it is too late.

Chris

 

The question was wether or not 11:1 with aluminum heads is possible. I listed my specs. Of course you have to give or take a little when estimating compression. I doubt it is a whole half a point lost to polishing.

I am not building a dragster here. I want my car to have good idle quality with more power all around. I didn't want to loose my bottom end like lower compression engines tend to do. I also didn't want to loose the idle quality. Later on if I want to add a larger cam I will be able to without having it run like a popcorn machine.

There is a lot inbetween my engine as it sits and the "bam!!!" theory that was stated before. I am not aiming for maximum volumetric or thermal efficiency. I may have pushed the limits a little with my compression. That is my perogative.

The question was if his engine will work. I responded saying mine is working fine. I even went on to give the specs. I have inspected my plugs and there are no signs of detonation after months of driving to work every day.

You can pick my combo apart and draw extremes all you want. I gave the guy an honest answer.

 

The question was wether or not 11:1 with aluminum heads is possible. I listed my specs. Of course you have to give or take a little when estimating compression. I doubt it is a whole half a point lost to polishing.

I am not building a dragster here. I want my car to have good idle quality with more power all around. I didn't want to loose my bottom end like lower compression engines tend to do. I also didn't want to loose the idle quality. Later on if I want to add a larger cam I will be able to without having it run like a popcorn machine.

There is a lot inbetween my engine as it sits and the "bam!!!" theory that was stated before. I am not aiming for maximum volumetric or thermal efficiency. I may have pushed the limits a little with my compression. That is my perogative.

The question was if his engine will work. I responded saying mine is working fine. I even went on to give the specs. I have inspected my plugs and there are no signs of detonation after months of driving to work every day.

You can pick my combo apart and draw extremes all you want. I gave the guy an honest answer.

I am not picking your engine apart at all, I'm sure it runs fine. We've touched on the compression ratio issue alot in here. I've got a wall full of blocks that are ruined due to detonation problems. I just think it's a bad deal to lead people to believe they can run these very high compression ratios on pump fuel. It's gonna cost somebody out there that takes this advice, that's why I'm opposed to promoting the idea

 

Good point. I did my homework, and I did a lot of other things that I didn't mention in this last post in order to keep out of detonations reach. I usually do say that it isn't easy to run when I post about my compression. It is however possible.

I don't believe in the conventional way of thinking where it is automatically assumed that you can't go above 9.5:1. If you run a big cam with that ratio, you will loose vacuum, idle quality, and bottom end that you could of had by raising the compression a little higher.

Anybody that puts the money into building an engine should read as much as they can, and be as sure as possible that what they are building will work. Throwing money at an engine is no substitute for research and common sense.

 

Aluminum heads, 11:1, 215*@.050 cam, 87 octane, no knock:

www.airflowresearch.com/articles/article03/A3-P1.htm

 

Aluminum heads, 11:1, 215*@.050 cam, 87 octane, no knock:

www.airflowresearch.com/articles/article03/A3-P1.htm

good read

 

Thanks for the repys guys. The guy that is selling the car said that he drove it on pump gas on the street and never had any problems, i just wanted to make sure that that jived with what you had experienced. The cam is a crower mechanical roller with 296 adv dur and 260, 266 dur @.050, 688 lift, 112 lobe separation. I know that that is big, but he is running comp package AFR 210cc heads on it, so you would have to run a healthy cam to match those heads on a 355. The car also has a 4000 stall and a 4.10 gear with a t350 tranny, and a trans brake.

Thanks again,
Adam

Oops, I meant 588 lift.

 

Thanks for the repys guys. The guy that is selling the car said that he drove it on pump gas on the street and never had any problems, i just wanted to make sure that that jived with what you had experienced. The cam is a crower mechanical roller with 296 adv dur and 260, 266 dur @.050, 688 lift, 112 lobe separation. I know that that is big, but he is running comp package AFR 210cc heads on it, so you would have to run a healthy cam to match those heads on a 355. The car also has a 4000 stall and a 4.10 gear with a t350 tranny, and a trans brake.

Thanks again,
Adam

that'll be one hulluva street car imp:

 

Aluminum heads, 11:1, 215*@.050 cam, 87 octane, no knock:

www.airflowresearch.com/articles/article03/A3-P1.htm

That's certainly a better PERFORMANCE motor than a 350 chevy. I actually have always thought that the 350 wasn't worth really all the credit people gave it... I mean, it doesn't have a reputation for great longevity like mopar motors, doesn't make any more power than a similarly sized motor, and quite honestly is not that well designed. It is, however, a better "all-around" motor than the 350 built in that article. The longer stroke and smaller bore isn't as good for performance apps with big valve and good heads, but for most of the guys in this world who just want their 350 to move their truck around and start up every day, its a better motor than the article 350. First, the longer stroke promotes torque with the stock, smaller valves given that it pulls in the fuel air charge more evenly and slowly. Second, the better cooling of the stock 350 block has proven more effective than that of the 400 (comon, they didn't stop making that motor for nothing!). I guess I just think they're overstating the advantages they found with this new motor-

K

 

NAIRB why are you so quick to tell people they can,t run 11.1 compression i seam to rember a guy named SMOKEY YUNICK that built an engine with 22:1 compression an had a turbo charger on it and got 60 mpg an ran on pump gas i think it was called a hot air engine.Did it this engine almost go into production by GM i guess SMOKEY don,t know what he is talking about.My point is if every one thought inside the box we would be know were.Just because one person can,t do it don,t mean nobody else can.



mike

 

You guys have me worried a little now, I am having custom pistons made by venolia, and it will be a true 11:1 compression, all the math was given to them with deck height, head gasket cc ect,ect, so it should be right on the money. I do have aluminum heads also. Is this going to be a problem for the street? Im running a roller 234,244 @ .050 .568 .595 lift 112*... Im just curious if I should back down on the compression a little... its just a weekend car..

 

Ben,
If you have a nice tight quench, you will probably be fine. Here in California we only have 91 octane. If I can get by with 11:1 and aluminum heads here, you should have no problem with 93 octane there in Florida.

Backing down on the compression a tad would not be a bad idea if you want a safety margin. Going from 11:1 down to 10.5 -10.8:1 really won't make a performance difference. You didn't mention what size engine you are working with, if I remember right it is a 351 Ford (correct?) I ran a little more duration at .050, but other than that in my 350, 385, and 383 I had no problems with pump gas. I don't have a lot of Ford engine experience so there may be some other trick with them.

Royce

Edit: Run your DCR and see where you sit. I bet you will be fine. This is what really counts.