From what I have heard, you can assume around 20 hp per point of compression. This is just an estimate, and will vary. As far as you combonation running on pump gas, the only way that you will have any chance is if you are going to be running aluminum heads, and your quench is right (distace from top of piston to deck of head, should be around .045).

Adam

 

it depends on the rest of the combination....lower RPM engines will have less impact than higher RPM engines as far as total increase in power.

 

how much hp?

How much gain for each increase in ratio depends on the engine, how its built, head design, etc. There is not hard fast rule on it.
Higher compression shows its greatest effects at higher rpm, where the breathing time is much shorter, and cylinder filling isnt quite so good.

With your engine setup, cam wise, you will probably see some good gains from going to the higher compression. The biggest concern is whether you can run that high on 93 octane, due to other factors like combustion chamber size, etc.
Around these parts, with a steel head engine, about the maximum anyone is running with any big blocks (427 and up) is about 10-10.5:1. Thats with a heavy cam and steel heads.

 

compression increases also help with large cams in the lower RPM ranges. It helps when the overlap is hurting performance... More compression from a diminished charge will feel like the cylinder is filled more completely than at a lower compression level. It will make large cams seem less radical or enable engines to operate more smoothly at lower RPM. It will also help torque and vacuum slightly as well.

 

I can't remember the exact specs. I have it written down somewhere. The way it works is you get 4% more power each point from 7:1 to 10:1. From there each additional point nets around 2%. I know that is close enough for what you are asking. If you want I can go digging through the archives and get the article, but I am in the ballpark.

You can't say how much hp. Every engine will gain more or less. That is why it is more accurate with a % of hp.

 

Thanks for all the replies thanks to everybody for the great help...

 

Just wanted to know if it is possible to run this set-up on pump gas 93 octane?

 

Thats gonna be very border line for sure you may have to knock back the timing a bit on pump gas which will take away any gains you made from the increased compression.

 

Robinson Robin, I think you hit the nail on the head. With iron heads and 11:1 it is not going to run on pump gas (and if it does it won't for long). You honestly need to be in the area of 9.5-10:1 to run iron heads on pump gas (I think that's about the practical max). The slight gain you "might" get from 10:1 to 11:1 will be negated with reduced timing and broken parts.

Royce

 

Bah humbug!!! lol. ANyway I run about 10.5 or 11:1 on the street. I admit I have had someproblems with spark knock, but I think I've got them taken care of, and we noticed the plugs to look a bit lean, so it needs a bit more fuel anyway. That coupled with a new set of 3.73 gears in leu of the 2.73, and I think I'll be fine. It depends a lot on the car and the conditions. I wouldn't want an 11:1 truck motor that will be towing a boat, but in a car, you're much better off. I run 93 and some marvel mystery oil, but I also admit that I've had to come down to that from mixtures of 110. I also haven't CCed my heads, so... who knows. But I think it can be done...

K

 

I read somwhere that 1 point increase in comp usually nets an average of 15 hp. I have no proof. But I did read it somewhere in a pile of my old mags.

 

Robinson Robin, I think you hit the nail on the head. With iron heads and 11:1 it is not going to run on pump gas (and if it does it won't for long). You honestly need to be in the area of 9.5-10:1 to run iron heads on pump gas (I think that's about the practical max). The slight gain you "might" get from 10:1 to 11:1 will be negated with reduced timing and broken parts.

Royce

I have said it a million times now. My 400 is 11:1 with iron heads and 91 octane gas. My timing is set at 18o initial and 16o mechanical.

What you gain from 10: to 11:1 is more vacuum, better throttle response, better gas mileage, and more power.

You have to do your homework before attempting to go over 10.5:1.

 

lluciano77,
How did you calculate your compression ratio? Were the heads cc'd, pistons cc'd, how far is the piston in the hole, how thick is the head gasket? What is your quench? Without actually measuring, which most people don't do or have the tools to do (home builders). There is no way for you to know your TRUE compression. If you buy a set of heads that are supposed to be lets say 64cc, when you actually measure them they can/will be from 63 - 66cc in some instances. The same goes for pistons.

If you are estimating based on what the advertised compression ratio of the pistons are, then you can easily be 1.5 points off. there are a lot of "11:1" engines running around and in reality they are closer to 9:1.

I am not trying to start an argument you, may have very well measured everything. I just have a hard time believing you are running 11:1 with iron heads on 91 octane.

While the engine is new you might get away with it for a while but, once you start building up carbon and the compression goes up slightly this will end the party. Nevermind a bad load of gas.

If your engine is actually 11:1 with iron heads more power to you. I can tell you I have seen first hand what happens to "true" iron headed 11:1 engines on the street (broken ring lands, hammered bearings, poor performance).

Just cause you "have said it a million times" doesn't make it a fact.

Royce

 

lluciano77


Sent you a PM

If not pump gas how about aviation fuel? I have a gas station that sells it for air boats. Aviation fuel is what octane? 110?

 

I currently run 11.68:1 compression with IRON heads, no detonation with 14* initial, 36* total timing. I had my 305 heads cc'd at 59 instead of the stock 58 after I ported them, I have a 4.1" bore gasket with a thickness of 0.038 inches, and flat top pistons set in the bore 0.005". Do the CR math and 11.68 is what you get. I attribute my combo working from the high quench area of the 305 heads, polished chambers, special waterless coolant, and a canister of octane booster. Throttle response is awesome and it idles pretty smoothly for the cam that is in it.

 

Okay here goes....11.6:1 verified down to cc'ing the eye brow's even...BUT I have aluminum heads, a 160F thermostat huge radiator (you HAVE to keep it cool) gentle advance curve, a ton of cam which will bleed off some cylinder pressure (who cares about compression ratio, its your cylinder pressure that counts) and I live at 5000 feet of elevation. That last part makes all the dif.
Humidity, ambient temp and elevation will play an important part in where you can go with this exorcise.
And yes I run pump gas most of the time....but there are hot summer nights when I will blend some race fuel and advance the ignition just to get the low end punch back.

 

Elevinpointsixtoone,
You hit the nail on the head with the elevation. You actually need to run slightly higher compression to make power up there. You are also correct that cylinder pressure is what counts, the problem is (even with a big cam) at higher RPMs the cylinder pressure still builds, if it didn't you wouldn't make any power. You also have aluminum heads, that makes a 1 point difference right there. Not exactly the same as running 11:1 with iron heads at a lower elevation on 91 octane gas.


Mad Maggot,
It must be that octane booster, LOL
How do you know you have no detonation? Just curious

Royce

 

lluciano77,
How did you calculate your compression ratio? Were the heads cc'd, pistons cc'd, how far is the piston in the hole, how thick is the head gasket? What is your quench? Without actually measuring, which most people don't do or have the tools to do (home builders). There is no way for you to know your TRUE compression. If you buy a set of heads that are supposed to be lets say 64cc, when you actually measure them they can/will be from 63 - 66cc in some instances. The same goes for pistons.

If you are estimating based on what the advertised compression ratio of the pistons are, then you can easily be 1.5 points off. there are a lot of "11:1" engines running around and in reality they are closer to 9:1.

I am not trying to start an argument you, may have very well measured everything. I just have a hard time believing you are running 11:1 with iron heads on 91 octane.

While the engine is new you might get away with it for a while but, once you start building up carbon and the compression goes up slightly this will end the party. Nevermind a bad load of gas.

If your engine is actually 11:1 with iron heads more power to you. I can tell you I have seen first hand what happens to "true" iron headed 11:1 engines on the street (broken ring lands, hammered bearings, poor performance).

Just cause you "have said it a million times" doesn't make it a fact.

Royce

Yes. It is all calculated and accounted for. I can dig up all the specs if you want.

I have been driving this car everyday in traffic to and from work for over a year. No pinging.

My compression read with a compression tester is at 220 psi. on all cylinders.

Elevinpointsixtoone,
You hit the nail on the head with the elevation. You actually need to run slightly higher compression to make power up there. You are also correct that cylinder pressure is what counts, the problem is (even with a big cam) at higher RPMs the cylinder pressure still builds, if it didn't you wouldn't make any power. You also have aluminum heads, that makes a 1 point difference right there. Not exactly the same as running 11:1 with iron heads at a lower elevation on 91 octane gas.


Mad Maggot,
It must be that octane booster, LOL
How do you know you have no detonation? Just curious

Royce

Aluminum heads only allow .5:1 more compression. Not one point. Trust me Camaro man, I have done my homework.

11.68 wouldn't be slightly higher compression. The altitude isn't going to make up for that much difference.

 

I notice that you guys have brought up cranking compression and cams.... what are you alls lobe seps on the 11:1+ engines? and total duration?

 

lluciano77,
We seem to bump heads a lot. The general rule is 1 whole point higher for aluminum heads. I am not going to argue with you over this. I would have to see proof that you are really at 11:1 with iron heads and running on 91 octane with no adverse effects (retarded timing, detonation, heat, etc..). There are ways to bend the rules and I have been known to push the limits. "IF" you are pulling it off you must be running extremely rich.

If you believe you are doing it, that's all that matters. You can not always hear detonation (the silent killer). Cruising around on the street, to and from work is not really putting much of a load on the engine. Pull some hills or really romp on it that is when the problems occur. In most cases you only run higher compression for performance reasons, have you taken it to the track? How did it run?

Royce

 

I notice that you guys have brought up cranking compression and cams.... what are you alls lobe seps on the 11:1+ engines? and total duration?

114o lobe sep.

230o @.050"

 

That's WIDE for 11:1 on pump gas dude.

 

lluciano77,
We seem to bump heads a lot. The general rule is 1 whole point higher for aluminum heads. I am not going to argue with you over this. I would have to see proof that you are really at 11:1 with iron heads and running on 91 octane with no adverse effects (retarded timing, detonation, heat, etc..). There are ways to bend the rules and I have been known to push the limits. "IF" you are pulling it off you must be running extremely rich.

If you believe you are doing it, that's all that matters. You can not always hear detonation (the silent killer). Cruising around on the street, to and from work is not really putting much of a load on the engine. Pull some hills or really romp on it that is when the problems occur. In most cases you only run higher compression for performance reasons, have you taken it to the track? How did it run?

Royce

I am not running rich either.

If you want to do the math yourself here you go.

4.145" bore
7cc piston
3.75 stroke
pistons are .05" in the hole
5.7" rods
.039" Felpro 1014 head gasket
64cc Sportsman II heads that I removed all the sharp edges form. It removed maybe 2ccs at the way most.

If you calculate that out it comes to around 11.25:1. I say 11:1 because of the couple ccs that came out of the chamber work.

Call Edelbrock and Trick Flow's techs if you don't believe me about the .5:1 allowance with aluminum. They'll set you straight.

You can't always hear detonation, true. I look at the plugs to make sure. They have no signs of aluminum balls, (that is what detonation causes).

I haven't taken it to the track. I have taken it on WOT runs locally. It runs great.

Compression helps retain cylinder pressure at lower RPMs. With a big cam you loose some of this. When you loose cylinder pressure, you also loose metering etc. That is why I went with higher compression. Not for top end necessarily.

I have a good sized cam. It idles just a little choppier than stock. You would never know my car was fast. It makes for a good streetcar that I can rip on every once in a while.

 

lluciano77,
I am sure Trickflow and Edelbrock will tell you that. They need to be conservative if they tell you 1 whole point and you start detonating it will be on them. I have actually done it myself so I don't have to ask.

64cc Sportsman II heads that I removed all the sharp edges form. It removed maybe 2ccs at the way most.

The above statement tells me you didn't actually measure anything. So you are really just guessing as to what your compression is. Most people do, and I used to. You would be amazed at how far off the "advertised" numbers are. I am not bagging on you I just don't want you to mis-lead others.

I had to open my chambers up (by 3cc's) to lower my compression for the blower and I am here to tell you 1cc worth of material is a lot. There is NO WAY you removed 2cc worth by polishing the chambers. The piston dish and /or valve reliefs are also typically larger than advertised. This is done to keep people out of trouble. If you actually measured your heads you would probably find the chambers are larger than advertised. By the way if you removed 2cc's your compression would drop more than 1/4 of a point.

You can deny what I say is true I don't have a problem with that and if you want to tell people you have 13:1 it's not my business. Instead of calling me a fool, do a little research and you will see I am not making this stuff up. The ONLY true way to know what your compression is, is to measure everything.

When I get a minute I will run a few numbers for you so you can see how big a difference a few cc's here and there can make. Also with your piston .05 in the hole, you don't have any quench and if you were really at 11:1 you would be detonating all over the place (IMO).


Royce

 

I know I have no detonation because when I first got the engine running, I still had 89 octane gas in the tank from when I had the original motor in it. It made the odd Knocking sound when I opened the throttle blades all the way to rev it up after the cam break-in period. It only happened at around 2500 to 4000 RPM where I suspect my cylinder filling is the greatest. After changing to 92 octane with Pennzoil octane booster, I noticed no knocking or pinging whatsoever. I took it on a 1000 mile excursion to visit family in the summer, through the mountains climbing steep hills, etc. and the car ran exceptionally smooth and had great gas mileage. I reside at about 860m elevation (that's about 3000 feet), so I believe that helps a bit, but when I went to the interior of B.C. which is only a couple hundred feet, I could punch it at WOT to 5500 RPM and not hear any knocks, pings, or even the low frequency thud you might hear with partial detonation.

Like I said before, I have special waterless coolant with a high heat capacity and zero vaporization, which sucks the heat out of the iron heads far better than water or antifreeze. I figure it is worth about a full point of compression in benefits. Check it out at www.evanscooling.com, but be advised it is very expensive. I also have a huge custom rad, a flex fan instead of a clutch fan, a custom air dam to help with airflow, and right on the mark ignition timing. I also thoroughly polished my combustion chambers, deburring all sharp edges and removing places where carbon could build up, and ensured my quench is at .043", which is near the ideal thickness. The gas I use is ethanol blended 10%, and with the addition of the octane booster creates near race fuel. The octane booster I use has the highest amount of MMT allowed for use on the street, and gives me about 104 octane. Amazingly it only costs $4 CDN for each canister, so I'm not complaining. I do run my carb just a hair rich for the main circuit to ensure that any WOT punches are not running lean. I use spark plugs one heat range cooler as well and gap them at .045.

Maybe I'm lucky or my combination includes a bit of magic, I don't know. I certainly did not plan for my compression to be this high, hence all the precautions I have taken, but I was short of funds to complete my engine when I built it so I took the 305 heads I had from my old motor and ported them heavily to work on the 350. The machine shop guy did the rest of the build, telling me that flat tops and near 0 deck clearance would help avoid detonation. I guess his 30 years of experience showed me it could work.

Now that I have all the fancy heat and detonation prevention goodies already in place, I am currently porting some 58cc corvette heads to retain compression and boost top end power over the 305 heads. May as well get my money's worth out of the motor now that it is sitting for the winter.

P.S. my cam's LSA is 110* it's the Comp Cams XE268 flat tappet hydraulic.