Pat 69BigBlock

I have decided to rebuild my matching numbers 427 engine rather than go the crate engine route. One of the mechanics has suggested lowering the compression to 9 to 1 because he said that if it is rebuilt to 10.5 to 1 (original specs) I will have to run octane boost. Is this true? How much horsepower will
be lost by doing this? How can I maximize horsepower and torque and keep the engine looking stock. I want to keep stock heads, intake, and carb.
Cam suggestions? My car has AC,PS,PB,4 speed, so I cannot go too radical
with the cam.
Thanks

MikeC

I would say at 10.5 to 1 with a carb, you are pushing it. I will let the engine guys tell you for sure.
Someone once told me a rule of thumb off x% HP loss for each point of compression( either 1% or 5% but I can't remember) but its not that much in the overall scheme of things.
As for horsepower, port your heads and intake, then your main stumbling block is the exhaust manifolds, headers will greatly improve power but will not look stock( but they are easy to remove if you ever wanted it to look stock again)

Pat 69BigBlock

Sorry guys,
The original compression from the factory was 10.25 to 1.
Not 10.5 :D

Tom454

I have the original LS5 454 in my 70 (published 10.25/1) and I have no problems with knock/ping running 93 using factory timing specs. This is with a Holley 6211 spreadbore. I tried using ported vacuum advance instead of non-ported (manifold) vacuum with the 6211, and did have ping. Using ported vac adv with TriPower did not have ping, so I think the 2300 carb has a less effectve port. Regardless, 10.25 is possible as long as you don't get aggressive with advance. Other forum members have posted that they have problems with ping in this engine. Carbon buildup in the combustion chamber can also cause ping. So, if the engine has a lot of miles on it, chances are there is some buildup & ping can be a problem.

Pat 69BigBlock

I didn't have a problem with pinging before, but my mechanic said that was due to a loss of compression as the engine gets older. Anyone have a newly
rebuilt engine with 10 to 1 compression or higher?

A C

Yep, my 11:1 small block was rebuilt 3 years ago. I get pinging only when it's hot outside (like over 85 degrees). Even at that temp, it's only under fairly hard acceleration that is pings. I can put in about a quarter tank of the really good stuff (110 octane here), and it goes away. It's kind of a pain in the butt, but I guess it depends on if your Vette is a daily driver or not.

AC

Jvette73

Stay with the stock compression. Camshaft selection will be important since duration will effect overall cylinder pressure. 10.25 to 1 will work fine on high test pump gas so long as you choose the right cam. Im running 10.5 to 1 on pump gas and never heard a ping or detonation. Polishing the combustion chambers will also help prevent the pre ignition condition. If you select a cam that has too short of duration, its possible to have problems with excessive pressure. I would guess that a cam that was a step or two above the stock cam for that moter will be close to what you need.

Pat 69BigBlock

Anyone want to make some cam recomendations for stock compression. Remember power steering, power brakes, AC, manual trans.
Thanks,

Jvette73

Is you 427 all stock now? If so, what are the cam specs of the L36 427. Need to know duration@.050lift and lift. Knowing what you have to begin with is the best way to proceed with a selection.