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Discussion Starter #1
RB25 and RB26 squish pad removal adds a LOT of extra volume to the combustion chamber, yet when I see this recommended or suggested I never see any mention of how the compression ratio is brought back up to a sensible level. Off the shelf pistons seem to be used and a 0.9 mm head gasket is as thin as any I have come upon. So what CR are people getting with a full squish pad removal, and how are they achieving a figure close to stock?
 

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isn't squish removal mainly used in drag applications so a lower CR + more boost = bigger power?

my guess would be the only way to get back to a decent CR would be custom pistons.
 
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Squish

A drag engine certainly in my experience does not need lower compression............the sqish areas certainly affect port flow at up to about 7/8mm lift and also increase the liklehood of detonation. If you are not after economy or low rpm driveability you are better off without them.

Tony
 

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c.r. ratio

simple way to bring compression ratio back up is to skim whatever required off the head. that said you will have to have a target c.r ratio, then cc the combustion chamber, piston above deck, head gasket and do the maths. this will let you know how much has to be skimmed off to reach that level .removing the squish pads, especially the inlets de-shrouds the valves allowing for better flow in as well as reducing det. a compromise would be to trim them back a bit, rounding the sharp edge back and down. and in both cases the combustion chambers will all have to be volume balanced. time consuming ! oh and whenever removing metal from either the head or block make sure and check valve to piston clearance.
 

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Don't skim the hell out of and wreck a perfectly good head and intoduce all sorts of other problems, if you want to remove squish pads and get CR back up just get some custom pistons made.

Rob
 

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Don't skim the hell out of and wreck a perfectly good head and intoduce all sorts of other problems, if you want to remove squish pads and get CR back up just get some custom pistons made.

Rob
how much does it take to wreck a head ? and what sort of other problems ?
i have seen me skim up to 80 thou off rb26 heads in extreme cases with no issues. thats roughly 11cc off the combustion chamber, it wont take that to bring it back up again. custom pistons seem an unessecery and exspensive route when very little money will skim the head. i would'nt even put a head on again, or a block without skimming it, even if its just to clean it and leave it on the new. plus a 1mm gasket will get you started. thats 1.1 cc less than a 1.2mm gasket.
and, unless you are chasing big numbers, i wouldnt go too far back, just round the edges back and down a bit, it doesnt need to be a perfect circle. its not as if you are trying to mirror the piston.
 

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isn't squish removal mainly used in drag applications so a lower CR + more boost = bigger power?

my guess would be the only way to get back to a decent CR would be custom pistons.
so is this not true then? if i skim the pistons, lower the cr, add more boost i can increase the power? yay or naay?
 

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so is this not true then? if i skim the pistons, lower the cr, add more boost i can increase the power? yay or naay?
skim pistons?

are you talking about removing squish pads now or just reducng CR?
 

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im talking about reducing the cr so i can add more boost to make more power! i was told you can skim the pistons to achive this? i sometimes confuse myself! what simonh stated above,was to lower cr by removing squish pads, im saying,cant i do the same thing by skimming the pistons?
 

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Discussion Starter #13
From the Simple Digital Systems site, Tech Page whom in my opinion, are excellent people to deal with, both for their basic EFI systems, and their advice:

Fuel Octane vs. HP

03/13/98

In turbocharged engines there is a fine balancing act when it comes to making a lot of power on low octane fuel. In most cases, ignition timing must be retarded as the boost pressure rises above a critical point and finally there reaches a further point where the engine simply loses power. If the timing was not retarded with increasing boost, destructive preignition or detonation would occur. Normal combustion is characterized by smooth, even burning of the fuel/air mixture. Detonation is characterized by rapid, uncontrolled temperature and pressure rises more closely akin to an explosion. It's effects are similar to taking a hammer to the top of your pistons.

Most engines make maximum power when peak cylinder pressures are obtained with the crankshaft around 15 degrees after TDC. Experimentation with increasing boost and decreasing timing basically alters where and how much force is produced on the crankshaft. Severely retarded timing causes high exhaust gas temperatures which can lead to preignition and exhaust valve and turbo damage.

We have a hypothetical engine. It's a 2.0L, 4 valve per cylinder, 4 cylinder type with a 9.0 to 1 compression ratio and it's turbocharged. On the dyno, the motor puts out 200hp at 4psi boost with the timing at the stock setting of 35 degrees on 92 octane pump gas with an air/fuel ratio of 14 to 1. We retard the timing to 30 degrees and can now run 7psi and make 225hp before detonation occurs. Now we richen the mixture to 12 to 1 AFR and find we can get 8psi and 235 hp before detonation occurs. The last thing we can consider is to lower the compression ratio to 7 to1. Back on the dyno, we can now run 10psi with 33 degrees of timing with an AFR of 12 to 1 and we get 270 hp on the best pull.

We decide to do a test with our 9 to 1 compression ratio using some 118 octane leaded race gas. The best pull is 490 hp with 35 degrees of timing at 21 psi. On the 7 to 1 engine, we manage 560 hp with 35 degrees of timing at 25psi. To get totally stupid, we fit some larger injectors and remap the EFI system for126 octane methanol. At 30psi we get 700hp with 35 degrees of timing!

While all of these figures are hypothetical, they are very representative of the gains to be had using high octane fuel. Simply by changing fuel we took the 7 to 1 engine from 270 to 700 hp.

From all of the changes made, we can deduce the effect certain changes on hp;

Retarding the ignition timing allows slightly more boost to be run and gain of 12.5%.

Richening the mixture allows slightly more boost to be run for a small hp gain however, past about 11.5 to 1 AFR most engines will start to lose power and even encounter rich misfire.

Lowering the compression ratio allows more boost to be run with less retard for a substantial hp gain.

Increasing the octane rating of the fuel has a massive effect on maximum obtainable hp.

We have seen that there are limits on what can be done running pump gas on an engine with a relatively high compression ratio. High compression engines are therefore poor candidates for high boost pressures on pump fuel. On high octane fuels, the compression ratio becomes relatively unimportant. Ultimate hp levels on high octane fuel are mainly determined by the physical strength of the engine. This was clearly demonstrated in the turbo Formula 1 era of a decade ago where 1.5L engines were producing up to 1100 hp at 60psi on a witches brew of aromatics. Most fully prepared street engines of this displacement would have trouble producing half of this power for a short time, even with many racing parts fitted.

Most factory turbocharged engines rely on a mix of relatively low compression ratios, mild boost and a dose of ignition retard under boost to avoid detonation. Power outputs on these engines are not stellar but these motors can usually be seriously thrashed without damage. Trying to exceed the factory outputs by any appreciable margins without higher octane fuel usually results in some type of engine failure. Remember, the factory spent many millions engineering a reasonable compromise in power, emissions, fuel economy and reliability for the readily available pump fuel. Despite what many people think, they probably don't know as much about this topic as the engineers do.

One last method of increasing power on turbo engines running on low octane fuel is water injection. This method was evaluated scientifically by H. Ricardo in the 1930s on a dyno and showed considerable promise. He was able to double power output on the same fuel with the aid of water injection.

First widespread use of water injection was in WW2 on supercharged and turbocharged aircraft engines for takeoff and emergency power increases. The water was usually mixed with 50% methanol and enough was on hand for 10-20 minutes use. Water/methanol injection was widely used on the mighty turbocompound engines of the '50s and '60s before the advent of the jet engine. In the automotive world, it was used in the '70s and '80s when turbos suddenly became cool again and where EFI and computer controlled ignitions were still a bit crude. Some Formula 1 teams experimented with water injection for qualifying with success until banned.

My personal experience with water injection is considerable. I had several turbo cars fitted with it. One 2.2 liter Celica with a Rajay turbo, Weber carb and no intercooler or internal engine mods ran 13.3 at 103 on street rubber on pump gas back in 1987. This was accomplished at 15psi. With the water injection switched off, I could only run about 5 psi before the engine started to ping. I think you might see water injection controlled by microchips, catch on again in the coming years on aftermarket street turbo installations. It works.
 

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A drag engine certainly in my experience does not need lower compression............the sqish areas certainly affect port flow at up to about 7/8mm lift and also increase the liklehood of detonation. If you are not after economy or low rpm driveability you are better off without them.

Tony
removing them does not gain any flow at any lift.
 
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opinion

As seen on a flow bench but thats my opinion but hey tony life would be boring if we all agreed.
Yes but what you see on the bench does not neccesarily work on the dyno.........we have all got our opinions and I find work anything but boring at the moment.............I learn something new every day and as the saying goes there is more than one way to skin a cat. Have a merry christmas Andy
and lets hope the economy improves in 2010...............

Tony
 

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im talking about reducing the cr so i can add more boost to make more power! i was told you can skim the pistons to achive this? i sometimes confuse myself! what simonh stated above,was to lower cr by removing squish pads, im saying,cant i do the same thing by skimming the pistons?
i hope this isnt considered a thread hijack as it is on the same topic. sort of.

to be honest i have never heard of or seen anyone skimming pistons, but i dont see any reason why not. except that pistons have a shaped crown designed specifically and are treated to be hard and resistant to serious temps and pressures.

read the second link gtr-glenn posted, it is very informative.

and yes reducing CR does allow you to increase boost without the risk of detonation at the same boost level. this does not mean you should pop out to the shed and get the bench grinder going.

there is a lot to think about.

why are you asking said questions?

do you have detonation? do you want to run more boost and are worried about det? if so then you need to give more information on your current setup, i suspect that skimmimng pistons should be a much later resort, if at all. it would probably be more advisable to get more custom, spec'd pistons.

kev
 

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mate, with all due respect im not stupid enough to go and take a bench grinder to my pistons, i have availability to precision machinery, i going on what a engine builder (for the last 25 years) has infromed me on what i can and cant do to increase power while i am doing my engine build at the moment while the pistons are out, and if you read my question, it says if ican lower the cr a diffrent way could i achieve the same outcome, as [email protected] put it , we cant all agree, there are many different ways of achieving the same thing, and quote me where someone asked this question about skimming pistons to achieve lower cr? i just wanted to know if any1 has tried it, and what results they got? feedback is always good, so how many engines have you built then? or do you use a bench grinder to skim you heads?
 
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