Hey there, I bought an 85 Z28 a year ago blablabla. its got 230,000kms on it (had it since 203xxx) and so as far as i know its completely stock. No record of any upgrades or rebuilds.

The engine is an LG4 (305 carb)and my book says my compression should be 100-150psi with a max 20% difference. So i did my test today and i got 175psi across all cylinders (lowest i got was 170..2.9% difference) So i realize that my engine is in really nice running shape.

Do you think that somethign in my engine is modified for higher compression? or do you think its just had a rebuild? or is it just doin the chevy thing

Im just not quite sure where to start becuase my plan for the motor was to rebuild it and put 350 heads and a 350 cam in it, and have a 305 from hell.

But perhaps the heads arent stock? would that raise compression? maybe i got awesome pistons? im not really knowlegable in the engine department, im just wondering if you guys have heard of this, is this normal? or is there something odd about my car.

EDIT: forgot to include, when i pulled my plugs they all looked perfect and clean. If that has anythign to do with anythign.

 

The actual number isn't so important. If you tried it with a different compression gauge you could just as easily get 90 psi in all cylinders.

 

Well, that will definately drop the compression...
from 9:1 to 8.1:1 , dropping a considerable amount of horsepower in the process...

 

1985 was the first year the LG4 got the same higher compression ratio as the high output L69 (9.5:1). Combine that higher compression with the itsy-bitsy LG4 cam (very low duration and relatively tight LSA) and you get considerably higher cranking compression than you might expect.

There is a reason that was also the first year the LG4 got a knock sensor like the L69 HO motor.

 

Crankin PSI does not equal "compression". You do not need to drop the compression of your 305. An 8.5:1 motor with a stock TINY cam will easily make the cranking PSI you are seeing, with carbon buildup included. Remember an old crusty engine w/ tons of carbon on the piston tops will have higer CR (although be detonation prone). There is zero chance your 305 has too high compression, it's just not possible. The smallest 58cc stock type head on a 305 w/ stock dish tops is not even going to be much over 9:1 if you blue print it.

 

and as mentioned, 350 heads and a 350 cam don't make it faster. Get the right parts for your engine. 350 parts aren't "better" then 305 parts because they go on a bigger engine normally

 

there is a big difference from static and dynamic compression. reading from a compression gauge is dynamic (in PSI) and static is the ratio to which the volume of the cylinder is condensed. dynamic can read totally differently due to many factors such as the cam that is in there (duration and LSA).

also compression gauges are not always all in par with eachother, as Aperion stated. the main importance of getting an engine compression tested is that to see if the cylinders are consistant (as yours are), so you at least know that the engine is mechanically sound from that aspect.

 

I'll add that 1985-up LG4 engines used the same flattop pistons as the L69HO engine and the same heads. That's how they achieved the same 9.5:1 compression ratio.

84 and older LG4s used the dished pistons and had the lower 8.5:1 compression. The difference in compression was the pistons.

 

As stated already putting 350 heads and a cam from a 350 will be far from making the 350 from hell, it would in fact result in an engine with less power than it had before. You need to get parts that work well with what you have rather than just throwing random parts that were on a bigger engine and expecting magic to happen. If the engine needs a rebuild I'd suggest swapping a 350 in its place rather then going through the trouble of rebuilding a 305 only to get less power for your $$. Basically if the 305 short block requires machine work to get it back into shape buy a rebuildable 350 block for around $50 and rebuild that instead, it will cost the same to rebuild except for the initial $50 purchase of the block and will net you much better results power wise.

 

Alright guys thanks for that info, it cleared up alot of stuff.

Someone at my work told me that if i put a 350 heads and cam on the engine that itd be alot better, but i guess their full of ****.

This car is my daily driver and is by no means a race car (runs 15.7@85mph) i just thought the cam and heads woudl give it a bit more grunt. I wanna drive the 305 till it dies then goto 350 for gas mileage purposes mainly. I dont plan on rebuilding the 305.

thanks for all your help.

 

Buy a set of 305 vortec heads , cam , headers,and perforner rpm type intake. Then let it eat.