What Cam Should I Use???

[84ke70]

Hey everyone i'm new to rollaclub, great site heaps of info,

 

I'm building up my 84 ke70 as a street/ motorkahana/ kahanacross, hill climb car :wink:

So far i have converted the car from auto to manual using a k50 box etc,

i'm running hurricain headers with a 2" exhaust system

i have another head i'm working now, running 30mm exhaust valves over stock 29mm

and i have changed from stock inlet 36mm to 38mm, ported and polished head with a three angle vavle grind.

i will be shaving the head to get the compression up to around 9.5:1

The car will be running Twin DHLA40's on redline manifolds.

 

i'm trying to work out what cam i'll need for my application i have spoken to dean tighe at Tighe Cams

and he suggested a 424 or 140 but i'm worried that in going to lose too much down low torque (with over 280 duration) any help or suggestions would be appreciated,

 

Thanks, 84ke70

[philbey]

I'd be looking at a bit more static comp if I were you, remember that as you increase duration of your cam, your dynamic compression ratio will decrease, so you up the static compression some more.

[84ke70]

I'd be looking at a bit more static comp if I were you, remember that as you increase duration of your cam, your dynamic compression ratio will decrease, so you up the static compression some more.

 

 

what sort of compression should i be chasing philbey??

do you agree that that the cams suggested are too large and manily high rpm use for what in planning on doing or ??? :wink:

[philbey]

I should have been a little more specific;

 

Firstly, have a read of this, it's a good start.

 

When people start modifying their engines like this, they often start talking about compression ratio. Once you start putting hot cams in, the difference between dynamic and static compression ratio really becomes clear.

 

Once you have settled on your camshaft design (what revs it comes on, how revvy the cam is, top range, midrange etc), then you need to think about the dynamic compression that you want to achieve and what fuel you will run. Without looking too close at your cam grind, I think your 9.5:1 is what you want to aim for Dynamic. If you start with a 9.5:1 static, you will get a lower dynamic ratio.

 

There are various programs out there to calculate it, just search for Dynamic Compression Ratio Calculator and trya few of them. You basically enter your static compression variables, your camshaft specs and it will calculate DCR. Aim for ~8.5 or so and you should be right.

 

EDIT - I just read up a little bit more. THIS article is also quite nice and also that linked to a dynamic compression calculator

 

They're talking petrol with octane as low as 80RON whereas we can get 91-95 pretty easily.

 

Mine is tuned for 98RON.

Edited April 14, 2010 by philbey

[84ke70]

i've had a look into things and i have found that the maxium static compression i would really what to run would be 9.8:1

here in Central Queensland Gladstone there is only two servos that i know of that offer 98 octane fuel but even then their supply is not constant and not of the best quality.

 

but i'm saying that is it possible to build my motor to have around 8.5 dynamic which is up around 10.5:1 static and retard my timing to suit the fuel aviable??

 

These are the cams suggested by tighe

 

Advertised timing duration lift

in ex in ex in ex

424 31 70 276 281 0.450" 0.450" Rally (PRC

 

Advertised timing duration lift

in ex in ex in ex

140 37 71 287 288 0.488" 0.488" Twin Carb/Fast Road :wink:

[Felix]

but i'm saying that is it possible to build my motor to have around 8.5 dynamic which is up around 10.5:1 static and retard my timing to suit the fuel aviable??

 

You can do that, but would make less power (and have a flatter running engine) than if you ran a bit lower compression with more aggressive timing. Read Graham Bells "Modern Engine Tuning" book, he goes into optimising compression ratios to suit available fuel, quite interesting.

 

I build for 95 octane. I live in the country and need to drive 20 minutes to the closest place to get 98, whereas I can get 95 5 minutes up the road. A good thing about building in conservative compression is if you get caught out at a servo and can only get 92, you don't have to worry so much about it pinging or running hot like you would if you ran high compression.

 

Is your motor a 4k or 5k? Knowing that would help with recommending a cam. Personally I think a Tighe 113 or 104 is ideal for a 4k with a very wide powerband and a decent topend. I'd go a little bigger with a 5k, like that 424.

[84ke70]

Is your motor a 4k or 5k? Knowing that would help with recommending a cam. Personally I think a Tighe 113 or 104 is ideal for a 4k with a very wide powerband and a decent topend. I'd go a little bigger with a 5k, like that 424.

 

The engine is just a 4k, looking at the spec's the 104 looks like the go, but my local engine builder thinks that a 25 65 cam is a little small and thinks i should be loking at a 30 70 like the 240

 

arghhh so confusing :wink:

[coln72]

The worst thing you can do for motorkhanas etc. is to go to big with you cam. You will be on and off the throttle and if you drop off the cam you are just leaking time. Used to see a lot of cars compete and everything would be coming on song when it was time to back it off again.

 

Personally, I found a Wade 169 grind ideal in a 5k. No noticable lag down low and a good top end. Would be more aggressive in a 4k though.

 

Approx specs from website - I lost the cams specs a while ago

 

lobe lift In .269 Ex .268

timing ad open/close In 41-70 Ex 75-36

duration @ 0.050 218

valve lift .395

lobe separation 109

Edited April 15, 2010 by coln72

[Evan G]

Is your motor a 4k or 5k? Knowing that would help with recommending a cam. Personally I think a Tighe 113 or 104 is ideal for a 4k with a very wide powerband and a decent topend. I'd go a little bigger with a 5k, like that 424.

 

+1 for Tighe 113. I'm going to using it in a mild engine built for torque!, you can use standard valve springs aswell WIN!

[84ke70]

after looking at all my options i have come to a decision of then tighe 104,

i'm going to be running heavy duty valve springs and going for static compression of 9.8:1

when going for a high lift cam (.423) will i have to machine down the rocker shaft blocks to suit?? :D

also is it worth getting or modifing the cam gear to have multi keyways to advance and retard the timing( is it really worth it??).

 

Thanks for everyone help so far :laff:

[snot35]

It's got nothing to do with lift, Tighe will have to grind down the base circle of the came to get the extra lift. Depending how it lines up and if you've had anything skimmed from the head, you may need to get the rocker blocks machined. The more lift is ground into the new cam, the more machining you'll need.