Squish or no squish
Discussion
How much effect does squish have?
I am building a Rover V8 that's going to get some nitrous abuse on the strip but will be used now and then NA on the road in my TVR Chimaera. The block is crossbolted and has top hat liners fitted but awaiting final machining depending on pistons. My budget is struggling to justify full on custom pistons so ive been looking at whats available off the shelf at places like Summit.
My crank is an offset ground 3.4" stroke with 2.0" journals and the block can be bored from 94mm to 96mm.
Ive found some Chevy Icon forged pistons for the 305 that are 3.736" bore that would take me to near 4.9 ltr.
All calcs with a 5.7" rod (chevy machined down) work to giving me the compression ratio I need but as the compression height is 1.433" I end up with as much as 100 thou piston to deck clearance depending on final deck finishing of the block.
So do we consider this an issue or does it not make much difference as squish is poor on a RV8 anyway due to the piston and chamber designs.?
I am building a Rover V8 that's going to get some nitrous abuse on the strip but will be used now and then NA on the road in my TVR Chimaera. The block is crossbolted and has top hat liners fitted but awaiting final machining depending on pistons. My budget is struggling to justify full on custom pistons so ive been looking at whats available off the shelf at places like Summit.
My crank is an offset ground 3.4" stroke with 2.0" journals and the block can be bored from 94mm to 96mm.
Ive found some Chevy Icon forged pistons for the 305 that are 3.736" bore that would take me to near 4.9 ltr.
All calcs with a 5.7" rod (chevy machined down) work to giving me the compression ratio I need but as the compression height is 1.433" I end up with as much as 100 thou piston to deck clearance depending on final deck finishing of the block.
So do we consider this an issue or does it not make much difference as squish is poor on a RV8 anyway due to the piston and chamber designs.?
Zero squish is a backward move Derek. Squish speeds up combustion and pushes fuel from the outside of the cylinder back towards the centre to be burnt. It reduces the chance of detonation as well. You'll have less unburnt fuel in the engine.
I've used off the shelf pinto 2.3 forged pistons but can't remember the stroke. The one's I used had a 12 cc semi circular dish which when machined fully gave about 24 cc. 96 mm bore to but will have a ford pin size.
Also, look for 327 'fuellie' rods. They're 2'' journals and 5.7" long. The chevy pin size .927" is bigger then the ford .912".
I've used off the shelf pinto 2.3 forged pistons but can't remember the stroke. The one's I used had a 12 cc semi circular dish which when machined fully gave about 24 cc. 96 mm bore to but will have a ford pin size.
Also, look for 327 'fuellie' rods. They're 2'' journals and 5.7" long. The chevy pin size .927" is bigger then the ford .912".
Edited by Boosted LS1 on Thursday 31st January 09:49
Edited by Boosted LS1 on Thursday 31st January 09:51
In terms of burn rate and detonation resistance squish is critical. However, the problem is that squish simply delays, rather than prevents detonation, meaning that should it occur, which will be later in the cycle and hence at a lower average chamber volume and therefore a significantly higher enthalpy, it'll likely be significantly more energetic and potentially damaging.....
On the WRC engines i developed in the early 2000's we spent a huge amount of time optimising squish to try to get the ultimate spark efficiency out of those air limited engines, and we had great success, running up to 13:1 static CR and 3 bar boost on 102 octane fuel. The problem was the mapping was CRITICAL, with 0.25 deg of spark timing and 0.1degC of intake charge temp being the difference between practically zero detonation and 160 bar knocking pressure!!! (which was enough to crack and "tulip" the tops of the bores, even on closed deck blocks!).
The other issue is that charge motion and flame propagation is highly dynamic and dependent upon a myriad of factors, both geometrical (port geo, valve timing, piston shape etc) but also conditional (charge density, internal EGR, ignition energy, AFR etc etc). We resorted to massive CFD programs to get to a directionally correct design, before then hitting the dyno with lots of test chambers with subtlety different geometries to try to work out what was "best". And "best" was often both counter intuitive and highly non linear!
The other problem you may find with an old engine (like the venerable old Buick) is that good spark efficiency necessarily drives high peak cylinder pressures and increased engine structure metal temps. Old engines were mechanically weak by modern standards, and simple often cannot put up with those sorts of mid and high cycle fatigue limits.
For something like a old relatively large capacity V8 where fuel economy and absolute peak power are to a large degree secondary concerns, i'd tend to go for a "bit more boost and a bit less spark" as the safer option, if necessary with spending a bit more money on a better boosting system (more efficient compressor and increased intercooling performance for a turbo, lower EBP and better cams for a Nitrous engine). This also makes your engine a lot less susceptible to mapping issues and variance, which in a road car is a good thing....
On the WRC engines i developed in the early 2000's we spent a huge amount of time optimising squish to try to get the ultimate spark efficiency out of those air limited engines, and we had great success, running up to 13:1 static CR and 3 bar boost on 102 octane fuel. The problem was the mapping was CRITICAL, with 0.25 deg of spark timing and 0.1degC of intake charge temp being the difference between practically zero detonation and 160 bar knocking pressure!!! (which was enough to crack and "tulip" the tops of the bores, even on closed deck blocks!).
The other issue is that charge motion and flame propagation is highly dynamic and dependent upon a myriad of factors, both geometrical (port geo, valve timing, piston shape etc) but also conditional (charge density, internal EGR, ignition energy, AFR etc etc). We resorted to massive CFD programs to get to a directionally correct design, before then hitting the dyno with lots of test chambers with subtlety different geometries to try to work out what was "best". And "best" was often both counter intuitive and highly non linear!
The other problem you may find with an old engine (like the venerable old Buick) is that good spark efficiency necessarily drives high peak cylinder pressures and increased engine structure metal temps. Old engines were mechanically weak by modern standards, and simple often cannot put up with those sorts of mid and high cycle fatigue limits.
For something like a old relatively large capacity V8 where fuel economy and absolute peak power are to a large degree secondary concerns, i'd tend to go for a "bit more boost and a bit less spark" as the safer option, if necessary with spending a bit more money on a better boosting system (more efficient compressor and increased intercooling performance for a turbo, lower EBP and better cams for a Nitrous engine). This also makes your engine a lot less susceptible to mapping issues and variance, which in a road car is a good thing....
^ Thanks. It's been many years since I looked. This page suggests .912" though. It's the US engine.
http://www.merkurencyclopedia.com/Motor/enginespec...
Interestingly I just found these mahle's with the chevy pin size and for a 5.7" rod
https://frsport.com/mahle-pin090810f01-2-3l-ford-4...
http://www.merkurencyclopedia.com/Motor/enginespec...
Interestingly I just found these mahle's with the chevy pin size and for a 5.7" rod
https://frsport.com/mahle-pin090810f01-2-3l-ford-4...
Edited by Boosted LS1 on Thursday 31st January 14:57
Max_Torque said:
In terms of burn rate and detonation resistance squish is critical. However, the problem is that squish simply delays, rather than prevents detonation, meaning that should it occur, which will be later in the cycle and hence at a lower average chamber volume and therefore a significantly higher enthalpy, it'll likely be significantly more energetic and potentially damaging.....
On the WRC engines i developed in the early 2000's we spent a huge amount of time optimising squish to try to get the ultimate spark efficiency out of those air limited engines, and we had great success, running up to 13:1 static CR and 3 bar boost on 102 octane fuel. The problem was the mapping was CRITICAL, with 0.25 deg of spark timing and 0.1degC of intake charge temp being the difference between practically zero detonation and 160 bar knocking pressure!!! (which was enough to crack and "tulip" the tops of the bores, even on closed deck blocks!).
The other issue is that charge motion and flame propagation is highly dynamic and dependent upon a myriad of factors, both geometrical (port geo, valve timing, piston shape etc) but also conditional (charge density, internal EGR, ignition energy, AFR etc etc). We resorted to massive CFD programs to get to a directionally correct design, before then hitting the dyno with lots of test chambers with subtlety different geometries to try to work out what was "best". And "best" was often both counter intuitive and highly non linear!
The other problem you may find with an old engine (like the venerable old Buick) is that good spark efficiency necessarily drives high peak cylinder pressures and increased engine structure metal temps. Old engines were mechanically weak by modern standards, and simple often cannot put up with those sorts of mid and high cycle fatigue limits.
For something like a old relatively large capacity V8 where fuel economy and absolute peak power are to a large degree secondary concerns, i'd tend to go for a "bit more boost and a bit less spark" as the safer option, if necessary with spending a bit more money on a better boosting system (more efficient compressor and increased intercooling performance for a turbo, lower EBP and better cams for a Nitrous engine). This also makes your engine a lot less susceptible to mapping issues and variance, which in a road car is a good thing....
Ive had this comment on a facebook group, 'Squish gets very dodgy at around 0.100, well away from that either way is ok, not necessarily ideal but ok. At around 0.1 it will detonate and cause all kinds of mayhem. 0.3 or 0.4 down the bore is fine for power with enough boost but not terribly efficient'On the WRC engines i developed in the early 2000's we spent a huge amount of time optimising squish to try to get the ultimate spark efficiency out of those air limited engines, and we had great success, running up to 13:1 static CR and 3 bar boost on 102 octane fuel. The problem was the mapping was CRITICAL, with 0.25 deg of spark timing and 0.1degC of intake charge temp being the difference between practically zero detonation and 160 bar knocking pressure!!! (which was enough to crack and "tulip" the tops of the bores, even on closed deck blocks!).
The other issue is that charge motion and flame propagation is highly dynamic and dependent upon a myriad of factors, both geometrical (port geo, valve timing, piston shape etc) but also conditional (charge density, internal EGR, ignition energy, AFR etc etc). We resorted to massive CFD programs to get to a directionally correct design, before then hitting the dyno with lots of test chambers with subtlety different geometries to try to work out what was "best". And "best" was often both counter intuitive and highly non linear!
The other problem you may find with an old engine (like the venerable old Buick) is that good spark efficiency necessarily drives high peak cylinder pressures and increased engine structure metal temps. Old engines were mechanically weak by modern standards, and simple often cannot put up with those sorts of mid and high cycle fatigue limits.
For something like a old relatively large capacity V8 where fuel economy and absolute peak power are to a large degree secondary concerns, i'd tend to go for a "bit more boost and a bit less spark" as the safer option, if necessary with spending a bit more money on a better boosting system (more efficient compressor and increased intercooling performance for a turbo, lower EBP and better cams for a Nitrous engine). This also makes your engine a lot less susceptible to mapping issues and variance, which in a road car is a good thing....
The pistons I want to order will be 100 thou down the bore and + 60 thou MLS gasket so looking at 160 thou squish. Shall I be concerned or just go for it?
Max_Torque said:
On the WRC engines i developed in the early 2000's we spent a huge amount of time optimising squish to try to get the ultimate spark efficiency out of those air limited engines, and we had great success, running up to 13:1 static CR and 3 bar boost on 102 octane fuel. The problem was the mapping was CRITICAL, with 0.25 deg of spark timing and 0.1degC of intake charge temp being the difference between practically zero detonation and 160 bar knocking pressure!!! (which was enough to crack and "tulip" the tops of the bores, even on closed deck blocks!).
That sounds rather fun to try and get the best of, what kind of resolution would you get from the sensors?At 160 thou there won't be any squish. Your engine may run ok on boost but off boost it'll be inefficient but it may be fine for racing. I guess it depends on your priorities. GM (years ago) did experiments on squish and after .060" the engines started detonating and running poorly. These were old SBC engines, a bit like an rv8 design wise.
From my estimations and certainly from the 3.9 RV8 they run about a 1mm - deck height and inc the head gasket thats a squish clearance (or lack of) inc head gasket thickness of 1.25/1.28 of at least 2mm plus 
just for example I would estimate the 4.0/4.6 (excluding the 5.0) being about the same , these are all stock unfettled engines obviously
just for example I would estimate the 4.0/4.6 (excluding the 5.0) being about the same , these are all stock unfettled engines obviously I really wouldn't fret about it. When you look at how zero squish engines like the Ford Crossflow run (flat head, bowl in piston) it might not be quite as efficient as a squish engine but they manage ok. If you're trying to extract the last few bhp from a given configuration then sure it all helps but no engine will detonate if you wind the ignition advance down low enough. Ideally you want about 1mm squish clearance or even a bit less if you know what you're doing. Then there seems to be a danger area between about 2mm and 3mm clearance where detonation actually gets worse and after that it doesn't seem to matter anymore.
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