1970 LS5 vacuum inHg at idle?

[Siegfried K.]

I finally have my 70 LS5 coupe on the road after 4 1/2 years of hard work, an awesome feeling of accomplishment I must say. I am running an RV cam, so it has a very nice idle. The engine is 30 over, stock heads with a very nice port and polish job, roller tip rockers, stock iron intake, stock rebuild on Quadrajet carb, and distributor has been upgraded to electronic ignition. Upgraded to a Dewitts radiator and billet aluminum high flow water pump from Meziere, runs nice and cool now even sitting for long periods of time. I have had it dyno'd at the crank, and it put out 435 hp and 550 ft/lbs of torque.

My question is what is the factory vacuum "Hg spec for this engine at idle, 700 RPM, and at approximately 3000 RPM? I have around 15-16 "Hg at 800-900 RPM in neutral on the non ported carb outlet. I have not checked at the vacuum at the main manifold port as of yet. It also seems to run very well at 16-18 degrees of initial timing which seems rather high? I have no experience with big blocks until now, so I have nothing to compare to.

[Mark E.]

Re: 1970 LS5 vacuum inHg at idle?

I don't recall a factory spec for vacuum at idle. My stock (not counting the ignition advance curve) '70 LS-5 pulls 17-19" at 750-800 rpm. This is with centrifugal @ 12 deg. BTDC plus full vacuum to the VAC.

You have a different cam, so your results are probably different.

[Mathew S.]

Re: 1970 LS5 vacuum inHg at idle?

Mine is pulling about 14" at around 800, running 8 deg BTDC, per the stock spec. I a, not sure the cam, as it was done by previous owner, heads were ported and polished, along with exhaust manifold entry.