Pump gas compression

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6pkScamp

6pkScamp

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Need input on what compression ratio your running on pump gas. I've gotten a range of 10:1 to 11.5:1. If you don't mind giving specifics on your combo I would appreciate it. I'm in the process of a small block stroker and would like to get the most out of I can. The only factor in the build that is a done deal is I have a set of Indybrocks with the larger valves and their port job. Thanks for any info.
 
I'd go no higher than 10:1, you could probably run 10.5 or 11, but for the 4-8% you loose, you gain a safety margin that would make it less touchy to station to station, season to season, and year to year differences. Not only that you can adjust the timing around to make up the difference and use an electric fan if your worried about tiny margins of power like that.
 
It will have to do with your camshaft duration, more specifically when you close the intake valve. I know people who have run 13:1 on 93 octane pump gas. I have run 11.9:1 with iron heads and .160" quench on 91 octane.
 
but do they run good like that or is it just another compromise.
 
An encyclopedia could be written on this subject. This is my latest project. 10:1 360 in a 3350# A body with 4.56 gears, 5000 stall 8" verter, 252* @ .050 cam, iron heads, good cooling system, properly tuned. Working great. Now put that same motor in a 4000# sled with a stock converter, hiway gears, shorter duration cam, timing and carb all outa wack. That thing will kill itself. Highest acceptable compression is a relative thing, but some guys just make blanket statements without taking all the variables into account.
 
Well said Skews.

6pkScamp, tell us about what type of combo you are wanting to run.

I myself have run a set of KB-107's @ 0 deck with 72cc J heads and Edelbrock heads @ 63cc. Cams have never been smaller than 244*@.050 with a 4spd manual & 4.10's. no pump gas problems at all. So depending on the head flow, the J's being the worst, I could still get into the 12's easy enough. It makes for a great street car.
 
Last fall Ford ran a new tv commercial for the Mustangs engine that did mention 10 to 1 compression. Seems the engineers finally figured it out. I have to think part of their success is in the onboard electronics making changes in tuning automaticly.
Then again they are building every day, every climate, every street, cars.
The hotrodder builds and tunes to suit himself. His price per gallon isn't even considered.
 
IQ52 said:
It will have to do with your camshaft duration, more specifically when you close the intake valve. I know people who have run 13:1 on 93 octane pump gas. I have run 11.9:1 with iron heads and .160" quench on 91 octane.
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And this so far is the best response. Static compression ratio is a useless number and really means nothing. Dynamic compression ratio is the only one relevant. This number takes into account the closing of the intake valve.

Ex. A 10.5:1 motor might have a 9:1 dynamic compression ratio and will detonate on pump gas. Change the camshaft and say now the intake valve closes later and its still a 10.5:1 motor but the dynamic compression ratio is now 8.5:1 and now it runs fine on 93 octane gas.

See the problem?

Also, keep in mind, aluminum heads keep a cooler combution chamber vs. Cast iron heads and for that reason they can run a higher dynamic compression ratio without detonating.

DISCLAIMER: The example demonstrating the difference between DCR and SCR is just that, an example. Those are numbers I pulled out my ***.
 
Unless it's a majority track car, there's really no reason to push the comp. ratio any higher then need be. A stroker will have enough torque that a 10/10.5 ratio, max, with a proper cam will give you all you need to get the job done. Do you have a cam in mind for your build?
 
Pick cam first then figure out what CR you need that will give you DCR that will work with your combo.
 
I always polish the chambers of my iron heads. Hot spots dont help detonation.
 
rumblefish360 said:
Well said Skews.

6pkScamp, tell us about what type of combo you are wanting to run.

I myself have run a set of KB-107's @ 0 deck with 72cc J heads and Edelbrock heads @ 63cc. Cams have never been smaller than 244*@.050 with a 4spd manual & 4.10's. no pump gas problems at all. So depending on the head flow, the J's being the worst, I could still get into the 12's easy enough. It makes for a great street car.
Click to expand...

It will be street/strip but not a cruiser. Not concerned about street manners, it may go to town for a car show otherwise I just take it out thash her and put it away. The indys have a 62cc chamber so I need to decide on a cam. The block is at the shop now getting decked so I dont know where that will wind up. Just took the four speed out put in a 727 with Dynamic 9.5 convertor and either 410s for street 456 for a trip to the track. The weight with the old motor was 3400 with me and 2/3 tank of fuel.
 
bad440 said:
but do they run good like that or is it just another compromise.
Click to expand...

Well, as I've stated before, we built an 11.7:1 compression 451 stroker that ran on 91 octane pump gas, the best it could manage was 787 HP and 600 LB-FT of torque. It had 500 LB-FT of torque from 3500 rpm-7500+rpm.

Does that qualify as good, or as a compromise?
 
IQ52 said:
Well, as I've stated before, we built an 11.7:1 compression 451 stroker that ran on 91 octane pump gas, the best it could manage was 787 HP and 600 LB-FT of torque. It had 500 LB-FT of torque from 3500 rpm-7500+rpm.

Does that qualify as good, or as a compromise?
Click to expand...
Good if it's you. Bad if it's the guy in the other lane! :burnout:
 
Can somebody explain why dynamic compression ratio matters enything to detonation, because when engine is running in maximum cylinder filling (highest torgue when intake charge doesnt flow back to intake manifold)its "dynamic compression" ratio is same as static compression ratio in that moment. Looks like only static compression ratio affect detonation. I hope someone understand what i mean.
 
Dynamic is effected by the cams timing when it opens and closes the valve during the pistons travel. The can build more or release more compression driving the PSI in different directions which can be very different that just a dead straight up mathematic number of static ratio.

It is possible to run a 12.5-1 static ratio engine on pump gas if the cam timing allows it. On the flip side, a 9.0-1 can ping to high heaven if the cam is wrong.

A static figure is a starting point to red the possibles.
Dynamic is much better a d is often seen in cylinder see PSI.
Couple it all with the cams known events (timing) a d the gas o game can be figured out.
 
I answered in the post before in the Race section - expanding on that it's the combination of parts that makes the engine prone or less prone to detonate. Looking at this build, I would not want it under 10:1.
IMO it's giving up a lot and the bigger engines need the compression. 8% is the difference between a 500hp and a 540hp engine and it will affect torque to a greater extent. That 40hp will affect ET and IMO there's no down side so long as you can get somebody's 91 octane with 10% ethanol out of the ground.
If you were having me build it I'd go with that 4.125 stroke you want, the shortest rod that clears the counterweights, and a 20cc dished piston coming proud of the deck .005". I don't know if the 5.7 Chevy rod will clear with the smaller Chevy piston pin size but I'll always trade piston height for rod length in a race engine.
All that being said - conservative is always the safer course - your builder knows that.
 
Dusterguy said:
Can somebody explain why dynamic compression ratio matters enything to detonation, because when engine is running in maximum cylinder filling (highest torgue when intake charge doesnt flow back to intake manifold)its "dynamic compression" ratio is same as static compression ratio in that moment. Looks like only static compression ratio affect detonation. I hope someone understand what i mean.
Click to expand...

It's all in the name...lol. as a point of review (and it sounds like you already get this part) - "Static" is the ratio of the volume of area swept by the piston when it's at the bottom of it's bore plus the volume of the chamber and gasket and area on top of the top ring vs. the volume of the piston at the top of it's travel plus the volume of the chamber and gasket and area on top of the top ring. In both positions the cylinder is sealed - both valves closed.
the issue is - the cylinder isn't sealed when the piston is at the bottom of it's travel. Both the exh and intake valves are open. So pressurization doesn't start happening until after the intake valve closes as the piston's moving up the bore on the compression stroke. The "Dynamic" ratio takes that closing event and the rod ratio into account and gives the builder a much better picture of what might be happening in the cylinder. The reason the others are saying the rest of the combo is so crutial is there is a whole bunch of physics that happens in a running engine that is based on what the cam, intake tract, and exh tracts are comprised of. In a running engine - the more efficient the ports and airflow is, the more intake charge is drawn and pushed into the chamber each time. On a racing engine that effectiveness comes at a higher rpm range where the physics works better. So while you can say there's "x" amount of pressure based on the formulas, that amount will rise in a properly designed intake system and the cylinder pressure will go higher than calculated.

Now to answer you - in my opinon - you're saying at peak torque the compression ratios are the same. I don't believe that's the case. The ratios never change. The volume of intake charge that is compressed does. Static and dynamic ratios are a calculated means for a builder to base decisions on. Cylinder pressure and volume of intake charge in the cylinder are (or can be) empirical measurements. That is exactly what volumetric efficiency is calculated from: the measured amount of torque made from a measured amount of air and fuel taken in.
 
"The can build more or release more compression driving the PSI in different directions which can be very different that just a dead straight up mathematic number of static ratio."

as far i understand engine doesnt release eny compression when its operating in area of higest torgue in rpm range. Therefore it has same cylinder filling% = compression psi as smaller cammed engine. Thats why i dont get it why bigger cammed engine wont detonate like smaller cammed engine.

Ok. maybe I just think too much.
 
No - I think you're just fine. I think you're stopping motion in your head. At peak torque it sure as hell does bleed pressure - but it does it only out the exh port rather than out both ports like it does at idle or low rpm. Plus the "bleeding" action along with the exhaust system evacuation and intake ramming over-fills the available space. So while it bleeds some it's overfilled anyway. So the net change is an inctrease in pressure. But again - the ratio of that volume to be compressed never changes.

cylinder filling % does not = compression ratio which is exactly what I mean. It equals the difference of the cylinder filling as measured by the dyno sensors, and the calculated empty volume - and - that % will rise as the engine approaches peak torque and can remain for rpms after peak torque. It should be well beyond 100% VE at peak torque.

edit - this is all assuming that the engine in question is built properly and has no inherant detonation or preignition inducing features and is being run on the proper fuel.
 
Compression ratios/numbers never change i agree. What i meant higer cylinder filling leads to higer pressure in compression stroke. And thats what lead to detonation.
 
6pk,

I am running a pretty close combo to yours, with 10.6 compression. I run 92/93 octane with about 24 initial, 34 total. Cam is Hughes solid flat #5256. Have about 1500 miles on the engine, so far so good. You can see all my specs in the member photo section. By the way, love the stance and paint of your ride.
 
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