Ideal Compression Ratio??

[Deeken]

Hey guys, finally found a rod/piston package for my 4age but they sell it in 10.2:1 and 11.5:1 compression.. Really wish I could go the higher comp ratio but I have no idea what kinda compression you can run on 98 in a 16v.. if it helps my engine will run a 7rib with oil squirters, forged rods, forged pistons, with a 260º 8.4mm(PRETTY sure) cam set.. oh in a bigport head.

Really thinking I'll only be able to run the 10.2 comp pistons with like a 0.8mm head gasket, shave the head, deck the block.. etc, to get it to around 10.5~~

Any ideas? Wanting this to make as much power with this set up as possibly :/

[FoldKing86]

For the stats you have stated 11.5:1 will be fine, yo wont need anymore and you probably wouldn't benefit by having anymore comp.

Bigger cams then what you've stated then yes maybe more comp will help, to be honest those cams aren't that much bigger then stock so throw the forged rods idea out the window and save some cash, pretty sure stock will handle it fine.

If your cams were up around 288 and higher then maybe more compression.

As with most things, its the way the cams are dialed in and tuned that effect the final outcome.

Just my thoughts.

[Skylar]

I don't think there's a need to go to the higher comp with those cams. The 11.5:1 comp pistons probably have a high protrusion into the combustion chamber which is not good for flame front propagation and increases combustion chamber surface area, meaning more heat is likely to go into the piston and less of it to making power. Piston is better off being flat or slight concave dome.
Going higher comp only allows you to run longer duration cams, you don't really make any more power from increased comp.

Aside from that, for max power:
Polish the combustion chamber for maximum heat rejection, ceramic coat the piston tops and ceramic coat the combustion chamber as well if you can.
Get the piston skirts coated in anti-friction stuff, same for bearings, cams, buckets/whatever else that moves in the engine.
Align hone the mains.
Knife-edge the crank.

A lot of that stuff will only get you a few hp each but it all adds up and it can save an engine if something does go wrong.

[ke70dave]

its all in the tune.

[Deeken]

Cheers for all the info guys. Will have a think about it all

[lolwat]

going to do cams atleast go 270 degree range

[ke70dave]

lolwat - can you please supply some scientific reasoning to your suggestion of 270+ cams?

[lolwat]

it what everyone does........ nah jokes, worth doing (noticable) with out having complete poo low end, and still be streetable without it being a dog
personally if i was going to bother doing a mad rebiuld i would be going with 288 8.5mm but if you want it to be daily it will be a dog until its up there

[Deeken]

the head i got with my engine happened to have some 858c tighe cams in it (260 8.4mm i think lift) so I decided I would just use them.. the specs say around that much lift.. and i though, longer duration would really change the power curve more than increase the power?? ..

anyway, they are near new cams, and i was thinking around 272 is what i wanted to go before i found out they were aftermarket.. could even possibly sell them? They dont even have ANY wear marks on them yet..

[Sam-Q]

it wouldn't be worth selling and rebuying for just a 10 degree change. Who sells this piston/rod combo?