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[JamesV6CDX]

Will I gain, by fitting the 2.5 heads onto the 3.0, using the 3.0 cams?

How many BHP will I increase it by?

Cheers

[JamesV6CDX]

Also, the increased CR, is this likely to cause any damage or stress? (apart from to my wallet )

[theolodian]

Also, the increased CR, is this likely to cause any damage or stress? (apart from to my wallet )

You will need to be more careful about fuel quality.  I would think that the ECU can adapt though, just try to be consistent.  The only problem is if you year pinging for several seconds or more.

[Kevin Wood]

The 3.0 has 10.8:1 compression to start with, IIRC. I'd certainly be inclined to measure the combustion chamber volumes of the 2 heads and calculate what you'll end up with. Anything over 12:1 will probably be a headache and over 11:1 and you're talking super unleaded IME.

As to performance it probably depends on what the current limiting factors are. If, for example, the exhaust manifolds are limiting power you may not have much if anything to gain. It could also be that changes to the ignition timing would be required to get a significant improvement, for example.

It might be a good move if you're thinking of LPG though. Would make it more efficient and give it a bit more power on gas.

The best thing to do is to speak to someone who has tuned these engines before though, as this is certainly a trick they will have considered.

Kevin

[Admin]

I can't add anything to Kevins comments other than I am sure Mark will have the appropiate information!

[JamesV6CDX]

I can't add anything to Kevins comments other than I am sure Mark will have the appropiate information!

Super unleaded is fine in my book

Good comments so far,

I'll await Mark's advice too

[Kevin Wood]

For anyone who wants to get down to the nitty gritty of engine tuning I recommend a read of "Four Stroke Performance Tuning" by A. Graham Bell (not the telephone guy, apparently). It has a slight bias towards US models of engine and is a bit of a heavy read but it gives a great discussion of all parts and parameters of the engine and their effects on performance.

He does another book on Turbocharging if you really want to get serious. (you'll need 3.0 heads on a 2.5 for that!).

I'm off the read what he says about upping compression.

Kevin

[theolodian]

For anyone who wants to get down to the nitty gritty of engine tuning I recommend a read of "Four Stroke Performance Tuning" by A. Graham Bell (not the telephone guy, apparently). It has a slight bias towards US models of engine and is a bit of a heavy read but it gives a great discussion of all parts and parameters of the engine and their effects on performance.

He does another book on Turbocharging if you really want to get serious. (you'll need 3.0 heads on a 2.5 for that!).

I'm off the read what he says about upping compression.

Kevin

Haven't read that book.  Premium fuel in the US is like regular fuel here.  Also, EFI and/or knock sensors can handle more compression than carbs.  It won't necessarily be a performance benefit, but an ECU with knock sensors and learning values should be able to handle at least 12 to 1 compression on pump fuel.

[Kevin Wood]

Haven't read that book.  Premium fuel in the US is like regular fuel here.  Also, EFI and/or knock sensors can handle more compression than carbs.  It won't necessarily be a performance benefit, but an ECU with knock sensors and learning values should be able to handle at least 12 to 1 compression on pump fuel.

Yep. You have to take his octane numbers in context because when he's talking about 95 octane it's 95 MON, as used in the US, rather than 95 RON, as used here. There's a difference of 4-5 points IIRC. I got a bit worried about having built an engine with 11:1 compression when I first read this book because if you take it literally he was saying I should be running it on AVGAS.

Interestingly, what he says about compression is that you're much better off having slightly lower than ideal compression and being able to tune the ignition timing for peak power than to go slightly over the top on compression and have to retard the timing to avoid knock. He also said that a lot of production cars already have higher than ideal compression to improve fuel consumption during the cruise and are already effectively running with retarded ignition on full chat to prevent knock, which can result in a 4% reduction in power over an engine with the ideal compression ratio.

You have to understand that he's talking about US car models on US fuel but it's interesting none the less.

At the end of the day, though, it's experience of "breathing on" X30XEs that will tell you the most and I don't have any of that!

Kevin

[theolodian]

Haven't read that book.  Premium fuel in the US is like regular fuel here.  Also, EFI and/or knock sensors can handle more compression than carbs.  It won't necessarily be a performance benefit, but an ECU with knock sensors and learning values should be able to handle at least 12 to 1 compression on pump fuel.

Yep. You have to take his octane numbers in context because when he's talking about 95 octane it's 95 MON, as used in the US, rather than 95 RON, as used here. There's a difference of 4-5 points IIRC. I got a bit worried about having built an engine with 11:1 compression when I first read this book because if you take it literally he was saying I should be running it on AVGAS.

Interestingly, what he says about compression is that you're much better off having slightly lower than ideal compression and being able to tune the ignition timing for peak power than to go slightly over the top on compression and have to retard the timing to avoid knock. He also said that a lot of production cars already have higher than ideal compression to improve fuel consumption during the cruise and are already effectively running with retarded ignition on full chat to prevent knock, which can result in a 4% reduction in power over an engine with the ideal compression ratio.

You have to understand that he's talking about US car models on US fuel but it's interesting none the less.

At the end of the day, though, it's experience of "breathing on" X30XEs that will tell you the most and I don't have any of that!

Kevin

Yeah.  Btw, US is (RON+MON)/2.  95 here = 91 there, 98 here = 93 there.  Typical petrol there is 87, 89, 91 and in half the states 93.  However, oxygenation in some states knocks it back a couple points in real terms.

Ideal CR is based on type of fuel and what RPM you want your power at.  If you aren't running the fuel for it, you can lose top end with a higher CR but still have better midrange.  Also, running that much knock retard (after TDC isn't that uncommon on turbocharged) will increase EGT's - but those probably aren't a problem on a regular NA motor.

[Martin_1962]

What CR can you run with 98 at worst and 112 at best?