Compression ratio

[morpheousssv]

What's the general consensus for best comp ratio in a L98 when running 98 octane in ultimate performance and/or general street performance with reliability ???

[morpheousssv]

What's the general consensus for best comp ratio in a L98 when running 98 octane in ultimate performance and/or general street performance with reliability ???


No responses ???

Question to hard to answer ???

mmm :confused:

[peter b]

Engine built?? cammed ???

[swingtan]

It's like asking "How long is a piece of string?"

The target Compression Ratio for any motor depends on many, many factors. More than just the mechanical side of things.

Simon.

[morpheousssv]

Engine built?? cammed ???



Sorry I should had emphasised static comp ratio not after dynamic ratio, built or cam is irelevant.

[ADAM 26]

if your still running the thumpr cam, i recon 12:1 would make it wake up a bit.

[morpheousssv]

if your still running the thumpr cam, i recon 12:1 would make it wake up a bit.

Yer thanks Adam, is that the reliability side or performance side for that comp ratio on a stock bottom end ??

Actually surveying for answers of those that have run off spec comp ratios with these engines successfully

[macca33]

Why not give Nathan Higgins, or a local WA engine builder a call and ask what they reckon a strong, safe SCR would be? I'd imagine that 11:1 - 11.5:1 would be in the ballpark.

cheers

[morpheousssv]

Why not give Nathan Higgins, or a local WA engine builder a call and ask what they reckon a strong, safe SCR would be? I'd imagine that 11:1 - 11.5:1 would be in the ballpark.

cheers

Yer thanks Macca that is also happening, Higgins heads is in the make up for the upgrade. Trying get get a wide audiance on this as we all know some workshops (specially in WA) and their ideas should not be in business.

[boggers007]

Street Quick Performance here in perth should be able to help you out if not Eddy at active or mitch from WA perf

[VYSHSV8]

Would be good to get hold of 1 of the new mazda 6 in Jap land to see what they have done coz there comp ratio from factory is gonna be 14:1 but we wont see it because people over here dont want to pay premium fuel prices and we will probably end up with 12:1......


I no that its a new designed piston in them to help with everything and no doubt the fuel system has alot to do with it

[Jag530G]

Would be good to get hold of 1 of the new mazda 6 in Jap land to see what they have done coz there comp ratio from factory is gonna be 14:1 but we wont see it because people over here dont want to pay premium fuel prices and we will probably end up with 12:1......


I no that its a new designed piston in them to help with everything and no doubt the fuel system has alot to do with it

The Mazda might have some fancy head design that won't be radily copied for LS use. A bit of history, Jaguar, in the early 80's released the HE (high efficiency) version of the 5.3L V12 which had "May" (Bloke called May designed them) heads with a static compression ratio of circa 12:1, astronomical back in the day.

A very old school trick for running high compression is to use sodium filled exhaust valves. This idea dates back to the 1920's aero engines. Pre-ignition, or pinging, is generally caused by hot spots in the combustion chamber causing the mixture to ignite where it shouldn't and therefore generating two flame fronts (the other caused by the spark plug) within the combustion chamber. The exhaust valve tends to be a hotspot in a combustion chamber, so making it cooler by allowing it to quickly disappate heat (ie, filled with sodium) helps.

The issue of hot spots is one reason alloy heads can allow higher compression ratios than iron heads. Alloy conducts heat better and therefore allows any potential hot spots to disappate heat to the surrounding area better than what an iron head can do. Since an LS head is alloy already, nothing to be gained there.

Cheers, Matthew

[morpheousssv]

Since an LS head is alloy already, nothing to be gained there.

Not neccasary, you state 12.1 ex factory and back then it would have been super as the best available fuel which from memory is 93 octane so I would say then that 12.1 :1 is a safe comp with alloy heads. Mazda on the other hand have a wierd fuel system were it runs from sequential to full group at vaious stages of it's cycle - maybe they have found a way to make this work with raised comp ratio. Be good to have the tech on this.

[ADAM 26]

my old ls1 had a 232/234 @ 112 cam and had cnc ported heads with very close to 12:1 comp. it made 309kw. so no reason why a l98 wouldnt be a problem with 12:1.

[BLACKVE]

Dynamic comp is more important than static, my L98 with 239/247 cam runs good with a 11.7 comp and heads milled 60 thou, big flycut of the pistons required to fit that cam thou

[morpheousssv]

Dynamic comp is more important than static, my L98 with 239/247 cam runs good with a 11.7 comp and heads milled 60 thou, big flycut of the pistons required to fit that cam thou

Agreed however for a discussion point it is easier for the general populous to understand static comp. 60thou brought you up to 11.7 - what head gasket do you use ??

[BLACKVE]

Agreed however for a discussion point it is easier for the general populous to understand static comp. 60thou brought you up to 11.7 - what head gasket do you use ??

Standard head gasket, i should say i guess 60 thou milled as i wanted 40 thou and ended up with 60cc chambers whick is more like 60/70thou but the chamber work etc all change the volumes and calculations.

Using some of the internet compression calculators etc give different results:confused: i also allow for the flycuts on pistons which would have to be 1 to 2 cc not as easy to work out as i though.

Even with 80 thou flycut i had to relly diall my cam in to fit, thats a big intake valve to kiss the piston so be careful, seen mine doing 7000rpm on dyno and still together so must have got it right:)

match the cam to the compression, i might have a big cam but she drives really nice

[abi]

To the original poster, ignoring Dynamic compression will be your foly my friend.

Two engines with 12:1 compression, one with the factory cam with 196 deg@50" while the other with a 242deg@50", which one will like more timing?

Take the various current Porsche (spelling) models, the road and track model (GT3 I think) with the factory fit half cage and fire extinguisher run about a half point to three quarter point more compression than the cooking models all the city bankers buy (did buy prior to the GFC). This is due to the dynamic compression among other things as the recommended fuel for both is 98.

On another point, I recall the early 80's super being 97 octain.

Adrian

[ls2 cruiser]

Once you use a cam with lots of duration, the static compression is just a figure and at no time does the engine have this figure. Dynamic compression ratio is the one you have to get correct. Usually between 7.5 - 8.5:1 is ok for various octane pump fuel.. It is complicated to work out yourself as you have to know exactly where the intake valve closes after bottom dead centre on the compression stroke. Most cam cards state degrees after bottom dead centre at .050" tappet lift. At the valve it is 1.7 times this figure, so the valve is not quite closed. The table that i have been using says that you add about 15 degrees to the figure at .050". The calculations involve sine cos and tan and the easier way to calculate dynamic compression ratio is to use a calculator like kb silvolite which is available on the internet. You will need a lot of info about your engine which is readily available. Then when you use large cams as one I am building at the moment (266 / 270 @.050" .720" lift ) you should allow for the effect of lots of fuel and air which builds up the DCR and I usually calculate so that we have a DCR of 7.5 to 7.8. This engine is a 404 cu ford windsor with Dick Johnson heads on it. So if you want to have a look at that calculator, just have a look and put your figures in.

[ls2 cruiser]

If you put up all your engine details I will try to calculate how much to shave off your heads using the kbsilvolite calculator. It will be a lot closer than just guessing.

[morpheousssv]

Thanks all keep it coming great info, this is exactly why the thread was started as this is a grey area in building HP. Search button shows very little on this subject.

[abi]

One other test which is related directly to Dynamic compression is the old and faithful compression check. Though only helpful after a build its still a tool.

I recall pressures in the 185psi to 200psi being the target for 98 octane fuel.

I had a 302ci chev in an early Monaro which ran 12 to 0 with 220psi cranking pressure and hated 98, had to run BP100 or Avgas to sm the pinging. These were the old iron fuellies and the cam had a tight lobe separation of 106deg both of which didnt help

Adrian

[caj39]

Hi, I have a 232/242 560 lift 112 lsa cam, standard heads never been removed and did a static compression test and all cylinders came up with 240psi. Engine has 112,000 kms on it with mostly highway use before mods. Run 98 octane fuel on a mafless tune and makes approx 420 rwhp. Hope this helps.

[caj39]

Hi again, just found out my compression gauge is almost 40psi out! Compression should be 200psi.