ok guys i just bought a 4k off ke ping it real, now its had mild headwork to it, I'm going to pull off head and calculate the current compression ratio, what i want to know is what other peoples compression on their 4k is and what they would recconmend i have.

 

the 4k has elec dizzy, extractors, 2 1/4 inch exhaust, reground cam (unknown), and yeah,

 

i was thinking 10.5:1, what would i have to change so that it wouldnt f@$k up at that high or higher? medicine man said i could go upto 12:1 but i would have to get it tuned so that it wouldnt blow up, could i do the tuning myself or would i need a dyno?

 

cheers travis. i want power!!

10.5:1 should be ok with your timing set right and running decent premium.

i always run 98 in it no matter what, when you say timing set right what do you mean?

yeah I'm curious to know aswell, i got a 4k with a 3k head... stuff knows what the ratio is, but I'm interested in knowing what i have to set the timing too

In the Knowledge Base/FAQ there’s and article called "how to build a tough k motor".

In the distributor section SuperJamie talks about ignition timing:

 

“I think a good rule is to set the timing to suit the octane of the fuel. Stock is 8 degrees on 96 RON leaded petrol, so 10 degrees on 98 RON BP Ultimate works pretty well. In the same fashion, you should reduce your timing if you're running 92 RON regular unleaded, maybe to 4 degrees, maybe 6, try it and see.”

 

He also talks about compression:

 

“I wouldn't suggest running much over 10:1 in a K motor. The head design really isn't the best for promoting good combustion, and detonation sucks once it sets in. I run 10.2:1 in my motor, and it get piston shakes quite badly below 2500rpm, but I think that's more to do with my amateur head porting. One of the guys on toyotastarletracing had a 4K he didn't care about anymore, so for a laugh he shaved the head down to 11:1 and slapped it back on. He said it absolutely FLEW but didn't last very long. If you want to push the limits, work out what dynamic compression you're running and keep it below 8.4.

A good rule is to take the octane number of the fuel you'll be using, and divide it by 10. BP Ultimate has a RON of 98, so 9.8:1 is pretty ideal compression for an engine running on that. Standard unleaded is 91 RON, which is alot less compression. You can usually get away with deviating 0.2 to 0.3 compression points from this rule, if your head porting and fuel supply is up to scratch.”

already read that entire faq mate, i just need to know about the timing when i have 10.5:1 compression. my timing is already set to 10 degrees

Play it by ear. Set the timing to 10 or so degrees and go for a good drive. If it doesn't "ping" great either leave as is or change it. If you hear it "ping" (usually under load) stop and back the timing off a bit. Just know which way to turn the dizzy. (Used to have to do this all the time in my HZ when I was towing).

 

By the way, I used to run around 12 deg in my modded 5K (higher comp, carbs, extractors and exhaust) on regular unleaded.

Edited September 9, 200818 yr by coln72

ok then man, sounds easyiest way, cheers