Hey does any one know what is to much compression for a 4K? My new engine is pinging too much when set to correct timing. If I set timing to TDC it is ok. Engine guy says it is about 13:1 compression ratio. Do I need to run AVGAS? Where can I get it from?

 

Thanks

 

B-Rad

Try some E85 if its available in your area/state.

 

13:1 is pretty bloody high. Pulling out timing is only a band-aid solution which is altimately costing you power.

Are you running a different cam profile? if so you should calculate the Dynamic compression.

 

http://www.wallaceracing.com/dynamic-cr.php

 

You'd have all sorts of trouble with 13:1 on a stock motor.

 

What sort of fuel are you running - 98 RON?

Thanks for the help

 

The engine is a long way from stock it runs kevlar bearings and all the good things (I got the block with bottom end all done (over 5k spent) for a 3K block.

The cam is a clive cam but not sure which one. and it was running on 91 RON (that is what was in the tank)

 

B-Rad

Wow, sounds like a rocket of a motor. Post pics! What's it in?

 

Educated guess would say that it's a fairly hot cam.

 

I'd try it on premium for the moment and see what that does for it.

You can't legally run avgas on the road. it's easily detectable if you do, as it has a very unique exhaust smell.

 

13:1 Is high. Try running premium 98, but if it's a genuine 13:1, it might not cope.

 

Seriously, you would wanted to have dropped the 91 out of it before you even started it. It was never going to cope.

You can't legally run avgas on the road. it's easily detectable if you do, as it has a very unique exhaust smell.

 

13:1 Is high. Try running premium 98, but if it's a genuine 13:1, it might not cope.

 

Seriously, you would wanted to have dropped the 91 out of it before you even started it. It was never going to cope.

 

The engine is for my rally car. It is in my daliy just so I can run it in. I was thinking of porting another head to drop it down to about 11:1. The bloke I got the engine off said that you can machine the head down to the indicator mark and it will be fine. I decide that was a bit to much and only machine a few thou or so off it. He said he has had them reving to 10,000rpm and about 120hp with standard exhaust and inlet manifold.

 

B-Rad

You can't run Avgas in a rally car, either. Pump fuel only.

 

Yes, you cantake the head down to the indicator mark. In total you can take up to .125 off them.

 

He's having a lend of you if he says you can get 120hp with standard manifolds.

 

what you reallt need to do is find out what compression you really have. If you've only taken a few thou off it, I can't see how you'll have 13:1.

My bullshit-o-meter has gone way off the scale.

My bullshit-o-meter has gone way off the scale.

 

+1

why go to all that effort modifying it for pump fuel if you are using it for rally anyways? the 5k head i am getting built now is going to be about 10.7:1 so i can still use 98, much more than that and things don't run well, i am also running a large 4k cam with 4age pistons

:: sigh ::

 

You should have put the valves & plugs in, flipped the head upside down and covered the combustion chamber with a piece of perspex with a hole in it, sealing it with a little grease. Then you take a burette and measure how much kero it takes to fill the combustion chamber.

 

You repeat it for each cylinder, because you need to know that they are all the same AND you need to know what volume they contain to calculate your compression ratio BEFORE you get the head skimmed!

 

Don't they teach you kids anything at EngineMods 101 ??

 

THEN you tell the engine dude how much you want skimmed off to get the compression you want, and when you get it back you volume check it again to make sure he didn't make a mistake...

 

 

(and if you had a decent cam in it to use your 13:1 the valves would probably smack a piston!)

Edited October 9, 200917 yr by altezzaclub

why go to all that effort modifying it for pump fuel if you are using it for rally anyways? the 5k head i am getting built now is going to be about 10.7:1 so i can still use 98, much more than that and things don't run well, i am also running a large 4k cam with 4age pistons

 

For one thing, you have to use pump fuel in a rally car.