Hi-comp 18RG cam selection help

[Damo82]

Hi, I have searched the forums, the old forums and google to no avail so this question goes out to all the 18RG gods/gurus.

I have an 18RGU which I am rebuilding. I am looking at getting a set of 10.5:1 compression pistons. However I have concerns about valve clearance due to the dome height of the piston among other things. What I was wondering though was what am I going to need to look for in camshafts? Longer duration, more lift?

I have a basic understanding of cams but am a bit lost as to what I should be looking for cam wise. Preferably not something that makes power between 6500 and 7500rpm however if that is my only option then I will reassess my piston choice.

Any help and advice much appreciated.

damo

[tricky]

So you're getting around to building the bugger good stuff! As far as the dome is concerned, there are plently of high comp 18RG's out there running huge lift and long duration. Where are you getting the pistons from? When u get them made, if you give them a sample of the original pistons, they'll keep the new flycuts at the original depth, and design the dome accordingly. In all honesty, I don't think there is a significant difference between 10.5:1 pistons and 9.7:1 pistons, so if u can track down a set of early RG pistons, you'll be laughing.

Also, check out toysport's website. They are in america, but they sell 10.5:1 forged pistons for a very reasonable price.

Cam wise, as soon as you run higher compression, you need to run longer duration for lower dynamic compression. 272 degree cams should be about right, and the power will start to be useable around 3000rpm. You'd wanna run higher lift than stock too, and you should also consider getting the head ported. If u do get the head ported, it'll be put on a flow bench, and they'll be able to recommend a profile to suit the flow characteristics. So in a word... Don't worry about clearance issues with 10.5:1 pistons.

[Damo82]

Yeah I'm finally getting around to it :-)

I was looking at getting the pistons from Toysport, the exchange rate ain't too bad at the moment. I will check with them about flycuts in the pistons when I order if thats the route I go down. When I take the head to get ported I will talk to them about cams then I guess.

Thanks tricky, I'll see you at the next CCG meet and we can continue this discussion.

[Mr DOHC]

surecam made my cams, for 9.7 they reccomended 282/282 at 12mm lift, if i had the choice again, i'd go for 292/292 and stock lift

[oldcorollas]

eek...
i generally recommend to go the highest lift that still increases flow, and then pick duration to match your powerband....

lift/flow = torque...
duration = rpm...

no point having a 6-9000rpm powerband with buggerall lift

work out what your maximum lift is.. then find out what lift gives maximum flow from the head (get it tested? just 1 intake/exhaust port)
then choose duration from rpm..

280deg cam, maybe.. 4000/4500rpm and up..... (to over 8000)

hmm, maybe i should rephrase that... what rpm are you likely to max out at? choose the duration based on max rpm you will daily use, and then live with the bottom end that gives you

[Celica RA45]

i would go the trd rally profiles 304 inlet and 288 exhaust this way you get the best of both worlds PS my mate has the master for the profiles

[Damo82]

Thanks guys, a heap of info there for me to go through. Looks like high lift and 280ish duration might be the go. I'll get the head tested after I take the block in to have the bores cheked.

Celica45, what is the benefit in having a lower duration for the exhaust cam?

[Celica RA45]

most of the trd rally profiles had the same lift as std but more duration ,with lift on these motors about 430 thou lift is as big as they run or valve to valve can hit and after 300 degress it becomes a racing cam and hence not much fun driving around town 10.5 to 1 comp would be fine and static ign timing about 5 before tdc and would need a set of 45webbers to get the best out of it

[tricky]

i would go the trd rally profiles 304 inlet and 288 exhaust this way you get the best of both worlds PS my mate has the master for the profiles

... I could become good friends with someone who has those profiles at their disposal! That is exactly what will make it's way into my 92mm bore 10.5:1 18RG with quads

[Damo82]

I'll try an get mine up and running with similar specs for you to have a drive of tricky.

[tricky]

Sounds good Damo

.. But not if I beat u too it! ... but I doubt that'll happen, I gotta rebuild my other car first.

Just for your interest, my proposed specs are as follows:


-blueprinted block
-bored to 92mm (2148cc)
-10.5:1 forged pistons
-balanced reciprocating masses and probably shotpeened rods (average velocity @ 8500 rpm = 4461ft/min )
-O/S valves (undecided spec)
-304in/288ex. I'd like slightly higher lift than stock... If only 18RGs were VVTL-i
-head ported to buggery
-inlet runners tuned to 7500-8000rpm
-20v silvertop throttles
-megasquirt ECU running CoP ignition (I'll make it work...)

All in all, I think that'll move nicely. I don't want to set a hp target yet though... Could be rather dissapointed.

And if u want to have a drive of an 18RG with decent sized cams and 45 DCOEs to see how piggish it is below 3000 rpm, feel free to give me a bell.

[Mr DOHC]

what size are 20V throttles, wont they be too small

get ferrea valves, they are cheap and reliable

[tricky]

43mm. But theres' four of them. That is enough flow for about 500hp, so it isn't gonna be a restriction. my 45DCOEs are more of a restriction, in fact, due to the venturis.

[Celica RA45]

well tricky go the hole hog and use 48mm webbers with 40mm chokes then

[tricky]

nah. Like I said, It's getting a dose of EFI. Silvertop quads is what I'm using. I'va also done some calculations. mean peak runner velocity is about 305ft/sec @ 8500 rpm... A bit high, so maybe blacktop throttles would be better... (Or in a few months of saving be able to afford some of your throttles) But in the end, my EFI setup will be cheaper than the outright purchase price of 48DCOE's, have less restriction and make more consistent power across the entire rev range.