Compression Ratios

[C20-67_N_MO]

Okay for most street engines you need to have your engine below like what? 10:1 or lower right?

So what the balance on power vs. compression ratio and still have a good powerful engine and still be able to use pump gas?

I've got like .30 over 400 sbc forged pistons and have been told by my hot rodding friends that is going to be a dog and not have the performance that a flat top would have. While this might be true, the trade off seems to be pump gas vs. 110 octane race gas at 6-7 bucks a gallon, or face denation with higher compression.

But with 64cc aluminmum heads and these dished pistons I should be just fine at 9.9:1 compression ratio, right?

So I know its a fine line for a street/cruiser engine vs. what you guys deal with on your ultra high performance motors right?

So give me some kind of insight of what I should be looking for as far as compression ratio goes for street use.

Thanks!

[Super73]

Static compression can be a bit higher than 10-1 and still drive on the street. I run 11.25-1 on 91 octane in california. One thing to play close attention to is dynamic compression. Cyl head chamber design is going to play a big roll in what you can do.

[prostreetC-10]

Cool thing is with todays tech and much more efficient heads, you can build some serious HP under 10:1 comp.

[68longhorn]

We have a 350 with 10.5:1 and a 292 I6 with 10.25:1 and both run just fine on 91 or 93 octane. A friend has a mustang with 11.5:1 and runs it on 93 octane with no issues.

[Marv D]

First,, where in the heck are you getting 110 octane for only $6-$7. Just bought a barrel of C-12 for $580 and a barrel of C-14 for $680,,, can we say OUCH!!! to the racing budget!!!

Lots of things are doing to dictate how street friendly 10:1 is. A 18cc D-dish, zero decked, 400 motor, with a 64cc chamber aluminum head is going to LOVE 91 pump gas with a moderate cam. IMO 'HOW' you get to 10:1, and running the appropriate cam for the combination is much more important than the end number.

Leave the stock deck, run a 76cc head and I'm afraid your friends will be right. 8.2:1 isn't very conducive to making big power numbers. In the end that doesn't really answer your question does it.

I don't believe it is an exact curve. For example the difference between 8:1 and 9:1 on a 400" motor may well make a 30HP difference in a properly set up combination for each compression ratio. And the difference between 9:1 and 10:1 may make 30+ HP difference in one motor, and COST 30HP in another combination.

Here's a few old engine builders rules of thumb.

The fuel you run, dictates the maximum compression you should build, which in turn dictates the generally accepted 0.050" duration of the cam you use, which in turn dictates effective the RPM range.

For example,, 9:1 seems to like 240° @ 0.050" valve timing,
10:1 is very acceptable to 250° @ 0.050
11:1, likes 260° @ 0.050,
12:1 for 270°,, etc

Keep in mind that is maximum 0.050" duration for the given static compression.

Because cubic inches gobbles up duration, you have to use your imagination a little. Lifter type / valve material and total lift not withstanding.....

generally in a small block 23° head motor,
230@ 0.050 will respond well up to 5500rpm
240 @ 0.050 - 6000
250 - 6500
260 - 7000

Obviously you can't use the same cam with the same duration and rpm range for a 302 as you would a 400. But it's a starting point.

Back to fuels,, The MOTOR octane number is the only number I pay any attention to. It's the octane measured under LOAD, not steady state operation. And the closer the two numbers are (motor and research) the more stable the fuel is under load. Some fuels (name withheld to protect the guilty) will advertise 115 octane. But you look at the 'real' numbers and it's 115 research octane and only 105 motor octane. My fuel of preference is VP. VP's Motor octane and research octane are seldom more than 1 or 2 numbers apart. It's a 'consistent' fuel and whether your a bracket racer, or running the motor on the ragged edge, CONSISTANCY is a dead serious matter to go rounds, and not hurt parts.

Pump gas is all reasonably the same station to station, brand to brand (as it typically all comes from the same tank with just an additive package being the difference). Rather than R+M/2 I sure wish each would post the 'real' numbers they derive that octane number from.

With all that rambling,, my real point is you can't put any exact 'number' to what compression ratio is 'ideal' for street with pump gas, without having the whole picture.

Like Super is running 11+ on pump gas, (and successfully) the next combination at 9.5:1, with a stock converter, iron heads and not enough cam will be a detonation pig that rattles it's self to death in short order.

Does any of that make sense?

[KQQL IT]

My 9.7:1 400 ran its best pass on 89 octane... And that was 12.19 @ 4160lbs

[Bad70sbchevy]

I'm running 12:1 compression on my 355 and it can run on 93 octane all day, it can even run 87, but im running a big cam so dynamic compression is probably saving it from knocking. 10:1 should be no problem at all. Higher compression increases both fuel efficiency and power as long as its not causing the engine to knock.

[C20-67_N_MO]

Marv,

That quote on 110 octane fuel was given to me by the local speed shop that he just threw out there was he was telling me I can not run flat top pistons on my 400 motor. And needlesstosay they would not take back my dished pistons. Stating they were special orders that they do not even stock that brand. I told the gentlemen I did not specail order the pistons but bought them from one of your sales staff that returned them. I now have them on Craigs list for sale.

Below is the camshaft grind I plan on using on my 406 SBC:

Duration Advertised 296° Intake/300° Exhaust
Duration @ .050'' 234° Intake/238° Exhaust
Lift @ Valve .539'' Intake/.548'' Exhaust
Lift @ Cam .359'' Intake/.365'' Exhaust
Lobe Separation Angle 112°
Intake Centerline 107°
Intake Timing @ .050" Open 10° BTDC
Close 44° ABDC
Exhaust Timing @ .050" Open 56° BBDC
Close 2° ATDC

The thing I truely do not understand to this day is where the cam manufactures or engine designers are coming up with this Duration @ .050" ?? at one point does this become important? As all cam makers seem to base their specs off of it. As lift and duration are important COMPRESSION ratios play an important role to match to the engine as well I would assume.

Originally Posted by Marv D

First,, where in the heck are you getting 110 octane for only $6-$7. Just bought a barrel of C-12 for $580 and a barrel of C-14 for $680,,, can we say OUCH!!! to the racing budget!!!

Lots of things are doing to dictate how street friendly 10:1 is. A 18cc D-dish, zero decked, 400 motor, with a 64cc chamber aluminum head is going to LOVE 91 pump gas with a moderate cam. IMO 'HOW' you get to 10:1, and running the appropriate cam for the combination is much more important than the end number.

Leave the stock deck, run a 76cc head and I'm afraid your friends will be right. 8.2:1 isn't very conducive to making big power numbers. In the end that doesn't really answer your question does it.

I don't believe it is an exact curve. For example the difference between 8:1 and 9:1 on a 400" motor may well make a 30HP difference in a properly set up combination for each compression ratio. And the difference between 9:1 and 10:1 may make 30+ HP difference in one motor, and COST 30HP in another combination.

Here's a few old engine builders rules of thumb.

The fuel you run, dictates the maximum compression you should build, which in turn dictates the generally accepted 0.050" duration of the cam you use, which in turn dictates effective the RPM range.

For example,, 9:1 seems to like 240° @ 0.050" valve timing,
10:1 is very acceptable to 250° @ 0.050
11:1, likes 260° @ 0.050,
12:1 for 270°,, etc

Keep in mind that is maximum 0.050" duration for the given static compression.

Because cubic inches gobbles up duration, you have to use your imagination a little. Lifter type / valve material and total lift not withstanding.....

generally in a small block 23° head motor,
230@ 0.050 will respond well up to 5500rpm
240 @ 0.050 - 6000
250 - 6500
260 - 7000

Obviously you can't use the same cam with the same duration and rpm range for a 302 as you would a 400. But it's a starting point.

Back to fuels,, The MOTOR octane number is the only number I pay any attention to. It's the octane measured under LOAD, not steady state operation. And the closer the two numbers are (motor and research) the more stable the fuel is under load. Some fuels (name withheld to protect the guilty) will advertise 115 octane. But you look at the 'real' numbers and it's 115 research octane and only 105 motor octane. My fuel of preference is VP. VP's Motor octane and research octane are seldom more than 1 or 2 numbers apart. It's a 'consistent' fuel and whether your a bracket racer, or running the motor on the ragged edge, CONSISTANCY is a dead serious matter to go rounds, and not hurt parts.

Pump gas is all reasonably the same station to station, brand to brand (as it typically all comes from the same tank with just an additive package being the difference). Rather than R+M/2 I sure wish each would post the 'real' numbers they derive that octane number from.

With all that rambling,, my real point is you can't put any exact 'number' to what compression ratio is 'ideal' for street with pump gas, without having the whole picture.

Like Super is running 11+ on pump gas, (and successfully) the next combination at 9.5:1, with a stock converter, iron heads and not enough cam will be a detonation pig that rattles it's self to death in short order.

Does any of that make sense?

[Super73]

Let me see if I can help you a little:

Most Cam manufactures list 3 duration numbers .006 / .050 / .200 as well as lift for each lobe, intake lobe center line and exhaust center line. All of these specs are important!!

With the .006 number and the intake center you can figure out your dynamic compression ratio if you already know your compression ratio. This is very importartant when it comes to figuring out what fuel to run. Also, getting as high a dynamic compression as possible for a fuel is going to aid in making more power. Add in the exhaust side of things looking at overlap here. Remember, the more overlap you have here, the more chance for reversion (burnt gasses going backwards in to the intake). This kills bottom end TQ, so the less overlap here the better. Since exhaust has soo much pressure behind it and moves at supersonic speeds, looking at the true opening point of the exhaust (.006) tells you when your starting to bleed off the exhaust.

.050 numbers are really going to help you understand where the motor is going to want to make it's power in the rpm band. Looking at the valve timing, IE where the intake vavle closes and also the over lap here. The more over lap here, typically the higher the power band will be in the RPM. IE it's moving your TQ production from say 3000-4500rpm to 4000-5500 rpm. Since HP is a function of TQ * RPM / 5252, this will make for a lot more HP but will reduce power under the curve (IE less TQ down low because again, the overlap moved it up in RPM).

.200 numbers, well I use them to see how agressive the lobe is. I look at all three, .006 / .050 / .200 as well as total lift. You can get a good idea of how soft or agressive a lobe is by looking at all of these data points:
.006-.050-.200----Lift
296 - 234 - ??? @ .359 lift doesn't tell the full story
298 - 245 - 166 @ .379 My intake lobe
318 - 260 - 175 @ .368 My exhaust lobe
As you can see my exhaust lobe has more duration in every point but less lift. This would be a less agressive lobe and would be less likely to go in to valve float or bounce using the same valve spring for both (assuming they were used on both intakes ect since the intake and exhaust valves typically weigh less).


There is a lot more to it. The reason guys like David Vizzard has written a book on it.

[C20-67_N_MO]

Originally Posted by Super73

Let me see if I can help you a little:

Most Cam manufactures list 3 duration numbers .006 / .050 / .200 as well as lift for each lobe, intake lobe center line and exhaust center line. All of these specs are important!!


There is a lot more to it. The reason guys like David Vizzard has written a book on it.

Looks like I need to go check out this guy's book so I can have a better understanding how all the timing works out.

Just as long as it shows its all relative to compression ratios. As that's what it all comes back too. I know they are not related but if you can match your CR with your camshaft are you not better off?

Thanks guys!

These are some fantastic data coming out and I am really enjoying all of it!

BRAVO!

[Super73]

In my opinion, you are thinking about it backwards.. Pick your cam based on RPM band and your intended useage, then mill the heads, select head gaskets, ect to obtain obtimum dynamic compression based on the fuel you are going to run.

[C20-67_N_MO]

Originally Posted by Super73

In my opinion, you are thinking about it backwards.. Pick your cam based on RPM band and your intended useage, then mill the heads, select head gaskets, ect to obtain obtimum dynamic compression based on the fuel you are going to run.

Well I do have a hydralic (sp) roller camshaft that I will be using by Edelbrock, its there #2201 grind. And hoping it will work well with the CR and 91/93 pump gas I intend to use.

[KQQL IT]

Ive a comp cams xr288hr in my 406 9.7:1 dart 200s and a rpm air gap, its a riot to drive and not a pinger with 89 or better fuel and alot of timing 14 initial and 36 total all in@ 3k rpm

[Marv D]

Originally Posted by C20-67_N_MO

The thing I truly do not understand to this day is where the cam manufactures or engine designers are coming up with this Duration @ .050" ?? at one point does this become important?

0.050" duration is the duration (in crankshaft degrees) that the lifter is raised off the cam base circle 0.050" or more. It's become a 'standard' so to speak, and for good reason. Every cam grinder has their own recipe on how to treat the opening and closing ramps. And add the 'XE' , Voodoo, and Extreme grinds and the 'seat to seat' numbers are meaningless.

Here's an example. I bought a MOON smallblock cam kit way way WAY back in the 70's. It was advertised on the box as 301° duration and said "HOT street / strip cam". WOW,, 301°!!! Saying 301° and 'street' on the same box caught the eye of every wanna-be racer on the block (me included!) The actual duration @ 0.050" tappet as about the same as the a Comp 284XE. But MOON had this thing ground with such mild opening and closing ramps the 'seat to seat' numbers were HUGE, and meaningless. The old GM performance cams were the same. Loooooong mild ramps that made the seat to seat timing look HUGE, but was a .420 lift, 230° @ 0.050" cam. That's less than the Edelbrock RPM cams of today's fame.

Like Super is telling you,, ALL of the numbers are important, and will show you how aggressive the lobe is. BUT the 0.050" duration is a 'standard' measure for cams more unaffected by the ramps to lift the tappet and valve off the seat gently, and set it back down gently. Ever heard a Comp XE grind that didn't have some valve train noise??? That noise is the aggressive ramp beating the hell out of the lifter, pushrod, rocker and valve stem tip.

In the end, long quiet ramps makes for happy customers for a long long time. Short aggressive ramps tossing the valve all over the place makes for more power, at the cost of reliability / longevity of valve train parts. You HAVE to pay attention to the ramps, but the 0.050" duration will give you a better 'feel' for how that particular cam will respond in 'you' combination.

Does that help?

[C20-67_N_MO]

Originally Posted by Marv D

0.050" duration is the duration (in crankshaft degrees) that the lifter is raised off the cam base circle 0.050" or more. It's become a 'standard' so to speak, and for good reason. Every cam grinder has their own recipe on how to treat the opening and closing ramps. And add the 'XE' , Voodoo, and Extreme grinds and the 'seat to seat' numbers are meaningless.

Does that help?

Aye!

Indeed it does, makes perfect sense to me.

So going back to my orginal quesion of compression ratios, basically having a 10:1 CR should work just find with my Edelbrock #2201 grind then right?

Again here are the specs on my cam as I am hoping to get 400-450hp when its all said and done.

Duration Advertised 296° Intake/300° Exhaust
Duration @ .050'' 234° Intake/238° Exhaust
Lift @ Valve .539'' Intake/.548'' Exhaust
Lift @ Cam .359'' Intake/.365'' Exhaust
Lobe Separation Angle 112°
Intake Centerline 107°
Intake Timing @ .050" Open 10° BTDC
Close 44° ABDC
Exhaust Timing @ .050" Open 56° BBDC
Close 2° ATDC

Any thoughts on this cam?

[Super73]

With:
4.03" bore
4" stroke
5.7" rod
70cc head
.045" gasket
-13cc piston
0 deck

I get 10.05 static compression and 7.46 dynamic compression with the cam you mentioned. Now, I'm not going to say this will exactly work for you but your leaving a lot of power on the table..

With:
4.03" bore
4" stroke
5.7" rod
64cc head
.035" gasket
-8cc piston
0 deck

Puts you at 11.54 static compression but get's your dynamic up to 8.52. If you have a decent head design, you might be able to get away with that and pump gas. 8.7 dynamic is on the edge of running 91 octane with a good set of heads. So I'm not sure what you have for gas around you, but it gives you a frame of referance.

If you are using old style heads, the above is definately NOT recomended.


Another idea is to go with a different cam.. It's not going to get you all the way there but, if you could find a cam that hat 274* total intake duration and a 106 intake center, your DCR would go up to 8.2 with your 10.05 static compression motor. But I think it is going to be hard to get a cam that has that little overall duration and still decent .050 duration with out using an aggressive solid roller. But keep in mind, most solid rolers change their advertised duration numbers to .020 due to lash.

[KQQL IT]

that 2201 is similar to my cam. 2500-6,000


Intake Duration at 050 inch Lift 236
Exhaust Duration at 050 inch Lift 242
Duration at 050 inch Lift 236 int./242 exh.
Advertised Intake Duration 288
Advertised Exhaust Duration 294
Advertised Duration 288 int./294 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.520 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.540 in.
Valve Lift with Factory Rocker Arm Ratio 0.520 int./0.540 exh.
Lobe Separation (degrees) 110
Computer-Controlled Compatible No
Grind Number CS XR288HR-10

[Super73]

Yet it has 8* less total duration on the intake side and that's gonna bump DCR..

[djracer]

What is the dish on the pistons you already have? What heads do you have? I think if you already have the heads and pistons these guys can point you in the right direction on a cam.

[pockets]

For what it's worth, my dad ran 87 octane for 17 years in his 10.25:1 404 big block chevelle. Never once put 89-91 in it, even during the host humid summers in OK and drove it all over the midwest.

[C20-67_N_MO]

Originally Posted by Super73

With:
4.03" bore *** Super- the bore will be more 4.155" which is .030
4" stroke **** 3.875" Stroke
5.7" rod **** Yes using 5.7" rods
70cc head **** But will opt for 64cc heads. I just sold my ProComp heads.
.045" gasket
-13cc piston **** Not sure how much cc the dish pistons are.
0 deck

**** Super- the bore will be more 4.155" which is .030 400 SBC block
**** 3.875" Stroke
**** Yes using 5.7" rods
**** But will opt for 64cc heads. I just sold my ProComp heads.
-13cc piston **** Not sure how much cc the dish pistons are.
0 deck

[1Bad62Pro/Street]

COMPRESSION BOOSTING MOVES:

1. Feed cold air to the induction
2. Keep water as cool as possible (170 F or less)
3. Keep the air cool in the intake ports
4. Put a heat-reflective shine on the outside of the intake manifold
5. Minimize heat transfer through the common exhaust/intake port wall
6. Keep fuel temperatures down (cool can)
7. Run with plugs a little colder than the minimum required
8. Use an ignition system that is gross overkill
9. Utilize as large a spark plug gap as possible
10. Use no more ignition advance than is necessary
11. Maximize quench action
12. Minimize head chamber volume
13. Use flat-top pistons if possible
14. Minimize under-hood exhaust heat--use coated headers
15. Do not ram in but vent out hot air through hood vents

I have 10:1 compression ratio on my truck for the street using High Test gas.

YouTube - Gotta fire the Ole' '62 up and...

[pockets]

perhaps you should clarify what compression you are talking about here, cause only a few of those items have any affect on compression.

Nice sounding truck. Do you need high test gas for that? seems overkill for a 10:1 motor

[Bad70sbchevy]

I agree, 10:1 should be no problem at all on regular 87. The factory 4.0L V6 they put in a 2003 Sport Trac came with 10:1 compression straight from the factory (granted it has aluminum heads).

[Super73]

you can have a 10-1 motor that requires race gas.. Again, it's not all about static compression ratio..