Using a camshaft to lower static compression ratio

[jmac]

It'll work Na, to some extent, because after certain rpms, there's less chance of detonation.

On a turbo engine? You're outta luck (and I lost enough engines on the 'guaranteed' advice of people in the early 90s to be adamant about this).

The money spent on a cam should get you a diy water injection setup, which is definitely more capable of preserving the engine.

[stockymcstock]

okay well i'm glad i asked cheers

water injection *may* be on the cards if i can't fit a decent sized front mount totally hidden behind the front bar.

reckon it's worth the extra 'easy power' that a simple cam change would give? i was thinking it would give me another 10-15hp at least, for free, which would be money well spent methinks. the engine produces 93kw in factory form i think, with the turbo i'm looking at around 130 at the crank.

[jmac]

if it produces 93kw NA, getting 30-35% more from a turbo would easily be doable with a std cam. the thing to look at, perhaps, is the fact that a turbo tends to expand the useful upper rpm range by as much as a thousand rpm on a small 4, so you might end up with something that works well but puts the motor in dangerous territory. If the motor can handle far more rpm safely than it sees in stock form, then for sure go for it.

It's a big maybe - the problem being to ascertain how good the stock cam is. In something like an old holden, you'd definitely want something bigger than 90% of factory cams, since they are so asthmatic to begin with.

[Masier]

Stocky have you talked to Alex on this board about turboing the hd?? He would be the one to ask. If not try him on daihard.

[RB30-POWER]

Stocky look at this link(chart at the bottom), with a wider LSA you lower dynamic compression ratio and reduce detonation, you get it hanging on more in the top end, get better idle vacuum. only thing that drops is low end torque a bit.

http://www.popularhotrodding.com/eng...ion_angle.html

[Greg Rust]

I used this calculator to select the cam and comp ratio for my last engine,

The program is free and is down the bottom of the page,

You just plug in all your specs and it tells you whether it will blow up or not.
http://http://www.empirenet.com/pkelley2/DynamicCR.html

[stockymcstock]

i still can't seem to find stock cam specs unfortunately, but i know tighe does a couple of cams for this engine - should ring the old man up and see what the go is.

Jmac i'd keep it to 6500 which is where power starts to drop off at the moment - it's got a haltech so the rev limit can be whatever i want. i have no need to rev it over 7000 cause the gearbox is a close-ratio job so when you drop into the next gear from 6000rpm, it's still usually doing above 3000 which would be full-boost territory i'd hope. still unsure of which turbo to use but that's for another thread later.

Masier who's alex?

thanks for the links guy's i'll have a look.

[stockymcstock]

I found the stock detomaso cam specs (the engine i have at the moment)

Standard HD-EG Detom cam specs Exh 52°/8° 240° 0.310" 0.012" In 8°/52° 240° 0.325" 0.01" rocker ratio 1.5

Do you guys reckon this would lend itself to turbocharging ok? it's a 16 valve head and compression is 9.5:1 i think. worth changing this cam to a tighe grind with springs? only really want to produce about 140kw at the engine on 9-10psi hopefully.

[stockymcstock]

BTW SteveS1 if you are reading this, send me a pm cause i can't send you one, something about it being disabled? i wanna see if you still have that crank/block/pistons etc in your shed!

[400HPONGAS]

CAms and compression ratios

So i'm considering putting a T25g onto my 1.6L charade. it's a 16 valve head, 9.5:1 compression standard. i can't find standard cam specs but i'd be looking at rebuilding an engine (applause engine is the same) and whacking in a cam from tighe with some isky springs, so was wondering how does one go about selecting a cam which will drop the static CR down to around 9:1 or 8.5:1.

i want to run 9-12psi boost maximum as the standard gearboxes are weak as piss and i don't wanna keep smashing the fuckers. looking to put a mild "street" cam in it. how does one determine what the final CR will be if you change overlap etc?

The short answer is "NO" the static compression ratio is not affected by the cam ,its pure ly a mathematical answer when dividing the Compressed volume into the swept volume .
What you mught be after is the "Dynamic Compression Ratio" This has a huge difference to the real compression the engine actually works with .
Remember inlet Valve actually close at xx degrees after Bottom center (ABDC)
DEpending the duration,the lobe center and hence the overlap
So with a ABDC event of 46 degrees that takes say a 10.6 static down to a 9.4 Dynamic (6 litre engine)and with good alloy heads it may be even lower again.
So it comes down to what engine , whaqt fuel,

[stockymcstock]

thanks that was very helpful

[TK]

The stock cam is on 112 lobe centres (going by your figures) - should work fine.