Just throwing together a 357W with the pistons and heads I got it should be around 11.5 CR....Can I run it off pump gas?.....I'll be throwing in a mild or custom cam.. Thanks
11.5 Compression Ratio on 92 Pump Gas Possible?
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I_AM_CANADIAN
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My 10.6:1 347 (274XEhr) pings with lighter advance springs in the dizzy at anything over 14 degrees initial on 94 octane. Good luck! 
I got into a good discussion with Nikwoac in this thread about high compression vs. pump gas: http://forums.stangnet.com/770697-any-benefits-aftermarket-distributor-2.html
A properly built, cool running engine will do it easily. Read through the thread that FastDriver linked to. I had a friend that ran a competitive 13:1 CR (supposedly, haha) engine on pump gas. 11.5 is very do-able.
This depends greatly on the cam. Specifically the intake closing event, overlap has nothing to do with it. I've got a cam for a 10:1 306 (my personal toy) that runs just fine on pump gas but would require race gas in a 347. If you go custom you can do it no problem. Keeping the cranking compression around 185 is key and that is done by delaying the intake closing event and thus dynamic CR.
Now a distributor with an adjustable advance curve is important as well.
If you are interested we do SADI customs for 349 and Billets, HR or SR, for 449. If you need a flat tappet they are cheaper at 249 but do not include lifters.
Now a distributor with an adjustable advance curve is important as well.
If you are interested we do SADI customs for 349 and Billets, HR or SR, for 449. If you need a flat tappet they are cheaper at 249 but do not include lifters.
squeak93
15 Year Member
Ran a 357 11:1 with a 236 241 duration 549 554 lift at .05 on 91/93 with a conservative tune. can be done esp with a custom grind cam as I had.
What you've written here makes a lot of sense to me, and I can definitely see how the intake valve's closing event is the most important part with its relative effect on detonation. However, I have to respectfully disagree that overlap "has nothing to do with it," though.
While you're right in that the dynamic compression ratio is affected by only the intake closing event, and is not affected by the overlap. Overlap still significantly affects volumetric efficiency, proportionately with RPM. And volumetric efficiency is a factor in determining the detonation threshold of an engine. The higher the VE, the more prone it will be to detonation.
However, detonation is most likely to occur at high load and low RPM, where low piston speed and yet a constant flame front velocity conspire to create higher peak cylinder pressure. Conversely, engine's are less likely to detonate at higher rpm. High overlap cams have lower VE at low rpm, and actually a better VE at high RPM, which helps to offset the tendency of the motor to detonate at its lower rpm without restricting the ability of the engine of the motor to make power at high RPM.
It was a confusing bit to type out. Did any of that make sense?
Chris
Right on, Chris. I thought it was a good explanation. The only thing I'd add is why VE affects detonation resistance. Chris, I'm sure you know this, but I thought I'd throw it in for the people who are following this thread who may not.
VE is a measure of the actual Air/Fuel flow through an engine versus the theoretical Air/Fuel flow through an engine (calculated from engine displacement and speed). For a given engine displacement, an increase in VE means more Air/Fuel mixture is in the cylinder after the intake valve closes, and thus, more mixture gasses are present during combustion. More Air/Fuel in a fixed space (combustion chamber volume) lead to higher pressures, and finally, more likely to detonate.
Obviously, the goal in building an engine is to have the greatest VE possible. Its why we port our intake manifolds, grind custom cams, and install superchargers. However, when running on the ragged edge of pump gas, VE actually starts taking affect on detonation, for the reasons described in the above posts.
To the OP, I'd suggest talking with a sales rep (for an off the shelf cam) or a cam grinder on what your best options are for your specific build. I'd think its a good idea whenever buying a cam, but probably even more so important in your situation.
Also keep in mind operating temperature. In any application, a motor with coolant temperatures in the 200 degree range is going to be much more prone to detonation than a motor running 180 or less. Keep this in mind when building the cooling system and/or tuning the EFI.
VE is a measure of the actual Air/Fuel flow through an engine versus the theoretical Air/Fuel flow through an engine (calculated from engine displacement and speed). For a given engine displacement, an increase in VE means more Air/Fuel mixture is in the cylinder after the intake valve closes, and thus, more mixture gasses are present during combustion. More Air/Fuel in a fixed space (combustion chamber volume) lead to higher pressures, and finally, more likely to detonate.
Obviously, the goal in building an engine is to have the greatest VE possible. Its why we port our intake manifolds, grind custom cams, and install superchargers. However, when running on the ragged edge of pump gas, VE actually starts taking affect on detonation, for the reasons described in the above posts.
To the OP, I'd suggest talking with a sales rep (for an off the shelf cam) or a cam grinder on what your best options are for your specific build. I'd think its a good idea whenever buying a cam, but probably even more so important in your situation.
Also keep in mind operating temperature. In any application, a motor with coolant temperatures in the 200 degree range is going to be much more prone to detonation than a motor running 180 or less. Keep this in mind when building the cooling system and/or tuning the EFI.
Michael Yount
Mustang Master
FWIW - there are a number of additional factors that enter into the whole detonation picture that haven't been touched on in the thread. Detonation is a complex phenomenon and often misunderstood. The areas that haven't been touched on yet include - 1) intake air temperature, 2) quench dimension, 3) fuel/air dispersion and 4) combustion chamber design.
Google detonation or ping and there are lots of good technical articles about what actually happens. While we think we have a homogeneous air/fuel mixture in the chamber just before the plug sets it off - we actually don't. There are an almost infinite number of different a/f ratios in the chamber during the compression stroke - and they're moving around. Detonation occurs when one of these small pockets of LEANER a/f mixture sees the right conditions to self ignite, usually before the spark plug sets off the entire mixture. So at the heart of reducing the chances of detonation under any operating condition are trying to assure as homogeneous an air/fuel mixture throughout the chamber as possible.
Newer OEM's are running 10.5-11-even 12:1 static CR's these days. Direct injection has made this possible. Fuel is injected directly into the chamber (not the port) at pressures that are 80-100 times higher than what our cars use (3000-4000 psig). This much higher pressure does a MUCH better job of evenly atomizing the fuel throughout the chamber. Timing the injection closer to the ignition timing also allows less time for detonation to occur. With this approach, the mixture is much more homogenous -- which reduces the chances of detonation. These newer engines also have knock sensors which allow timing to be run/real-time-adjusted to get right on the ragged edge of detonation. In fact, some systems run a small current through the spark plug after it's fired to measure the effectiveness of combustion in each chamber - and with distributorless ignitions can alter ignition timing BY CYLINDER to optimize things. Yes - our 25 year old efi and ignition system technology is a dinosaur compared to what's available today.
We can't go to direct injection on our engines -- however, within reason we should do whatever we can to atomize fuel more effectively. Fuel pressure on the high end of the acceptable range for a particular set up helps; and running the smallest injector possible that will still meet the flow needs of the engine helps. At high load/small throttle openings not much fuel is moving through the system. And a smaller injector does a more precise job of metering small amounts of fuel than a larger one does.
In our cars, in addition to the things already mentioned, several things can be done to help. Be sure the air intake is delivering the coldest air charge possible. The cooler the chamber - the less the chances for detonation. Keeping coolant at 180F helps. Keeping the air charge cool helps too. In fact, cooling the fuel would help as well. I don't think anyone has mentioned material choice in the heads yet. Aluminum heads pull heat out of the chamber more effectively than cast iron heads do. This feature also helps control detonation - all things equal, you can run a higher CR with an aluminum head than with a cast iron head. Not to mention the weight savings.....
Keep the quench dimension as tight as possible. This is the distance from piston to head OUTSIDE the combustion chamber. Ideally - .035" or so. Keeping that dimension tight forces every bit of the air fuel mixture into the chamber as the piston comes over the top. As it's all forced into the chamber at TDC (note - the ignition process has already begun at this point) it becomes very turbulent in the chamber -- which further homogenizes the air/fuel mixture. This is a detail that's often overlooked by people. In fact, sometimes folks will put a THICKER head gasket in thinking that reducing overall static CR will help with detonation - only to find that the big quench dimension they've created significantly reduces turbulence in the chamber resulting in detonation potential being even worse.
Combustion chamber --- and even port/valve configuration plays a huge role in how turbulent the mixture is before combustion. Later model chambers/intake designs are MUCH more effective at introducing turbulence/air-fuel mixing than older stuff was. The reason it came to be was to reduce emissions and increase efficiency (fuel mileage). But that also helps hp/torque production. One of the benefits of the GT40P head is it's efficient combustion chamber design. So the shape of things is important.
Google detonation or ping and there are lots of good technical articles about what actually happens. While we think we have a homogeneous air/fuel mixture in the chamber just before the plug sets it off - we actually don't. There are an almost infinite number of different a/f ratios in the chamber during the compression stroke - and they're moving around. Detonation occurs when one of these small pockets of LEANER a/f mixture sees the right conditions to self ignite, usually before the spark plug sets off the entire mixture. So at the heart of reducing the chances of detonation under any operating condition are trying to assure as homogeneous an air/fuel mixture throughout the chamber as possible.
Newer OEM's are running 10.5-11-even 12:1 static CR's these days. Direct injection has made this possible. Fuel is injected directly into the chamber (not the port) at pressures that are 80-100 times higher than what our cars use (3000-4000 psig). This much higher pressure does a MUCH better job of evenly atomizing the fuel throughout the chamber. Timing the injection closer to the ignition timing also allows less time for detonation to occur. With this approach, the mixture is much more homogenous -- which reduces the chances of detonation. These newer engines also have knock sensors which allow timing to be run/real-time-adjusted to get right on the ragged edge of detonation. In fact, some systems run a small current through the spark plug after it's fired to measure the effectiveness of combustion in each chamber - and with distributorless ignitions can alter ignition timing BY CYLINDER to optimize things. Yes - our 25 year old efi and ignition system technology is a dinosaur compared to what's available today.
We can't go to direct injection on our engines -- however, within reason we should do whatever we can to atomize fuel more effectively. Fuel pressure on the high end of the acceptable range for a particular set up helps; and running the smallest injector possible that will still meet the flow needs of the engine helps. At high load/small throttle openings not much fuel is moving through the system. And a smaller injector does a more precise job of metering small amounts of fuel than a larger one does.
In our cars, in addition to the things already mentioned, several things can be done to help. Be sure the air intake is delivering the coldest air charge possible. The cooler the chamber - the less the chances for detonation. Keeping coolant at 180F helps. Keeping the air charge cool helps too. In fact, cooling the fuel would help as well. I don't think anyone has mentioned material choice in the heads yet. Aluminum heads pull heat out of the chamber more effectively than cast iron heads do. This feature also helps control detonation - all things equal, you can run a higher CR with an aluminum head than with a cast iron head. Not to mention the weight savings.....
Keep the quench dimension as tight as possible. This is the distance from piston to head OUTSIDE the combustion chamber. Ideally - .035" or so. Keeping that dimension tight forces every bit of the air fuel mixture into the chamber as the piston comes over the top. As it's all forced into the chamber at TDC (note - the ignition process has already begun at this point) it becomes very turbulent in the chamber -- which further homogenizes the air/fuel mixture. This is a detail that's often overlooked by people. In fact, sometimes folks will put a THICKER head gasket in thinking that reducing overall static CR will help with detonation - only to find that the big quench dimension they've created significantly reduces turbulence in the chamber resulting in detonation potential being even worse.
Combustion chamber --- and even port/valve configuration plays a huge role in how turbulent the mixture is before combustion. Later model chambers/intake designs are MUCH more effective at introducing turbulence/air-fuel mixing than older stuff was. The reason it came to be was to reduce emissions and increase efficiency (fuel mileage). But that also helps hp/torque production. One of the benefits of the GT40P head is it's efficient combustion chamber design. So the shape of things is important.
Very nice write up, Michael. Glad to see you're still around.
You didn't read the link, as most of the things you mentioned in your write-up I also mentioned there. However, you did add some depth to the topic.
One thing that I would like to clarify is that there is a difference in terminology between detonation and preignition. Preignition happens before the spark. Detonation happens after it. Many confuse the two, but they are distinctly different phenomenon with differing causes. Preignition is much more dangerous, and harder to detect before it causes engine damage.
Chris
You didn't read the link, as most of the things you mentioned in your write-up I also mentioned there. However, you did add some depth to the topic.One thing that I would like to clarify is that there is a difference in terminology between detonation and preignition. Preignition happens before the spark. Detonation happens after it. Many confuse the two, but they are distinctly different phenomenon with differing causes. Preignition is much more dangerous, and harder to detect before it causes engine damage.
Chris
Rather than try and explain it, anyone with interest in this subject should read an article from someone that's more credible. There are a lot of them out there. I like this article: Engine Basics: Detonation and Pre-Ignition: Streetrod Stuff
Michael,
Very good points.
I particularly like the direct injection technology although it's not germane to the OP question but it's really cool stuff. Ford's new EcoBoost V6 uses this. They inject the fuel at something like 3000 PSI and even have designed the piston to help shape the charge. Very cool stuff which uses high compression and turbo charging.
Detroit 2008: Ford launches EcoBoost gas turbo direct injection engines - Autoblog
About the overlap discussion. Yes overlap (at .050) is used to increase VE at high RPM's and I suppose the lowered VE at low RPM's may help control detonation but this is better accomplished with spark timing. A properly sized intake tract will eliminate the need for large amounts of overlap while still maintaining good VE at high RPM's. Large amounts of overlap (for street/strip applications) is a throwback to the days when we had very few choices in cylinder heads and intakes. Today that is not the case so you can put a 200 cc head on a 306 with a large single plane intake, time the valves for negative overlap at .050 and still run run it over 7K. This set up does loose some low end like your typical thumping "race" cam of days gone by but it has excellent idle vacuum and does not load up. By changing the intake closing event Dynamic CR can be adjusted to accommodate high static CR with no need to increase overlap.
Now the OP was asking about a 357 with Vic Jr heads (210cc intake). Assuming this a street strip application and is coupled with an open intake and good exhaust a high level of overlap will not be required. He is looking at more than the 306 example above but not a cam that will only pull 9" of vacuum at idle. Of course a thumping idle can be had if desired, by manipulating seat timing, without drastically changing the performance of the motor off idle.
This is where customs pay dividends. By designing in high lift and fast ramp rates, idle characteristics can be controlled, resistance to detonation can be maximized and free breathing high RPM can be accomplished.
Very good points.
I particularly like the direct injection technology although it's not germane to the OP question but it's really cool stuff. Ford's new EcoBoost V6 uses this. They inject the fuel at something like 3000 PSI and even have designed the piston to help shape the charge. Very cool stuff which uses high compression and turbo charging.
Detroit 2008: Ford launches EcoBoost gas turbo direct injection engines - Autoblog
About the overlap discussion. Yes overlap (at .050) is used to increase VE at high RPM's and I suppose the lowered VE at low RPM's may help control detonation but this is better accomplished with spark timing. A properly sized intake tract will eliminate the need for large amounts of overlap while still maintaining good VE at high RPM's. Large amounts of overlap (for street/strip applications) is a throwback to the days when we had very few choices in cylinder heads and intakes. Today that is not the case so you can put a 200 cc head on a 306 with a large single plane intake, time the valves for negative overlap at .050 and still run run it over 7K. This set up does loose some low end like your typical thumping "race" cam of days gone by but it has excellent idle vacuum and does not load up. By changing the intake closing event Dynamic CR can be adjusted to accommodate high static CR with no need to increase overlap.
Now the OP was asking about a 357 with Vic Jr heads (210cc intake). Assuming this a street strip application and is coupled with an open intake and good exhaust a high level of overlap will not be required. He is looking at more than the 306 example above but not a cam that will only pull 9" of vacuum at idle. Of course a thumping idle can be had if desired, by manipulating seat timing, without drastically changing the performance of the motor off idle.
This is where customs pay dividends. By designing in high lift and fast ramp rates, idle characteristics can be controlled, resistance to detonation can be maximized and free breathing high RPM can be accomplished.
Mike and Brian- Very good info. I see you guys have a good grasp on this stuff. Where were you guys before when Chris and I were firing things off in this thread:
http://forums.stangnet.com/770961-so-you-think-long-rod-motors-way-go-huh.html
And this thread?
http://forums.stangnet.com/770697-any-benefits-aftermarket-distributor-2.html
I take it you guys don't wander over to talk too often, huh?
http://forums.stangnet.com/770961-so-you-think-long-rod-motors-way-go-huh.html
And this thread?
http://forums.stangnet.com/770697-any-benefits-aftermarket-distributor-2.html
I take it you guys don't wander over to talk too often, huh?
Concur with everything you just said, Brian. As for me, I would not purposefully choose a cam with high overlap specifically to mitigate detonation - there are better ways to do that. It was just a conceptual point. I really just enjoy discussing the concepts and theory involved in engines.
I like to understand the physics behind the mechanics. In particular, Brian, I enjoyed reading your point about the intake closing event. I would like to learn more about cam timing events, and other specs, but the majority of the things that I read are expressed like "this is the way I like to do things" instead of a more mathematical or scientific approach. Can you recommend some good articles or books that approach the subject more from an engineering or scientific perspective?
Chris
I like to understand the physics behind the mechanics. In particular, Brian, I enjoyed reading your point about the intake closing event. I would like to learn more about cam timing events, and other specs, but the majority of the things that I read are expressed like "this is the way I like to do things" instead of a more mathematical or scientific approach. Can you recommend some good articles or books that approach the subject more from an engineering or scientific perspective?
Chris
Wow, there's a blast from the past. How ya been Mr. Yount? We've missed your well thought out, tech rich posts. Between you, HISSIN, and jrichker, you're going to make the rest of us look bad.
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