LS7 BBC compression question

C

CorvZ061

Member
i have a 1969 chevelle with an old LS7 454 swapped in. my dad bought the engine in a crate back in 1991. The car is an original SS396 car however the 396 is long gone but the muncie 4 speed remains. The car hasn't been driven much since the engine swap, mainly due to the 12.5:1 compression and needing 110+ octane fuel. We are in the process of trying to get the car on the street again but want to consider dropping compression enough to have the engine live on pump gas. Is there anyway to to do that short of changing pistons? It wont be a daily driver by any means, but would like to drive the car around with my dad, it's just not feasible with fuel costing $10+ gallon and only getting 6 mpgs.. We have my corvette to ride in for now and do enjoy that, but it is a car from my youth and the chevelle my dad bought back in the 70's when he was 18..
 
Grumpy hasn't been around today still.
I don't think he's feeling tip top.
Must be sick.

I don't think there is any practicle means of lowering the compression ratio without tearing down to to replace the LS7 Raised dome pistons with Flat tops or Dished pistons 15-30 cc volume.
Grumpy may recommend a pump gas friendly cam & more modern performance cylinder heads.

One option short term is to use 100 LL Aviation gasoline.
Recurve distributor advance & rejet the carburator as needed.
$5.86 to $5.60 per gallon for 100 Low Lead Aviation gas near me current.
I used to pay $2.06 per gallon 100LL AV gas from 1994-2008.
Race gas 110 motor octane was $2.99 a gallon. Sunoco & Cam2.
Then that A hole luberal POTUS Saved us all.
87 octane pee water was $1.60- 1.99 per gallon prior.

Another option is to buy an E85 Carburator from QuickFuel.
High octane Corn Moonshine Fuel.
 
I have looked into the E85 but that was quickly laid to rest, there is only one station around with the stuff. Thought about pulling the heads, would a thick head gasket help much? If I have to change pistons then I may find a truck 454 cheap and build it up, leave my XCH coded engine intact.
 
No I don't think so.
A thicker Headgasket would have to be around .100-.150" inches thick to drop static compression to 9.5 -8.5 :1.
Loose all the Quench area & engine will run like a dog.
Need to gain 20-30 cc chamber volume or loose the piston dome LS7 Forged TRW's.
I would use 100 LL AV Gas myself.
If you want to do the Hotrod Power Tour From West Coast to East Coast in June & July,
A dedicated pump gas street BBC Build be best to meet your wants & needs.

Grumpy never misses a day.
Hopefully back in Tommarrow.
He is the BBC Go to guy.

Brian R.
 
dropping the compression may help a bit but its generally not going to be cheap to do it correctly and for every percentage point the compressions reduced you generally see a 3% reduction in torque, if you drop a 12.5 compression engine down to lets sat 9.5:1 too in theory allow it too run well on the crap swill the pumps currently dispense, as "high test" thats about a 11%-12% reduction in torque and an a LS7 big blocks rectangle port heads and solid flat tappet cams is designed to operate in the 5000rpm-7000rpm power band.
personally id think your best bet is adding an octane booster , to the fuel, and enjoying the car the way it was meant to run , or pulling it down and rebuilding it with a 4.25" stroker crank as a 496 with about 10:1 compression.
think about it a bit, adding a couple gallons of toluene, or race octane gas, to the tank might add $20-$40 to the cost of a fill-up but its certainly less expensive than rebuilding the engine if your not driving it frequently and it will tend to still run well.

now you can also retard the cam 4 degrees and change your ignition advance curve , which will also help, or/and you can change to MPFI as the more modern injection units have heat, oxygen and knock sensors that if properly tuned will allow you to get by running crappy fuel.
there are other options like adding a water/ethanol injection unit, that dumps a rich fuel mix into the engine under load significantly reducing or eliminating an engines tendency to get into detonation,
you can also try installing a much larger radiator ,efficient fans and a large oil cooler,with a powered fan, as reducing engine operational temps significantly reduces the tendency to get into detonation


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If mixing the race gas with 93 would keep the engine alive that is what I will do. I just wasn't sure if that would be enough to do much for it. I do have a local station that sells ethanol free 93 octane I could mix with 112 I can get local also. If I had a place to get AV gas I would but nowhere around here would have it, there is a small air field but they won't sell unless you rent a hangar.
 
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