I have an all original factory 1968 350 hp corvette 327 with the pop up pistons and original heads(needs gone through) what would the compression ratio have been on one of these?? anyone know? thanx

Wow, the .100 dome pistons?? With a 461 head and a steel shim head gasket they would be about 11:1

11:1

I have these little pistons below in my 327. They provide 250ish pound cranking compression with the curent cam. So far in a chevelle that weighs 4080 pounds with a 360 pound driver it has ben 12.182 @110.04mph with 1.68 60' times and more to come after it is tweaked

http://www.chevelles.com/showroom/data/500/Photo_070707_005_1_.jpg

I have an all original factory 1968 350 hp corvette 327 with the pop up pistons and original heads(needs gone through) what would the compression ratio have been on one of these?? anyone know? thanx

The '68 327 Vette Hi-Perf is an 11:1 forged (TRW) piston unit with a "nitrided" L/J steel crank. The block casting number should be 3914678, the heads should be #3917291 (Fuelies) with 2.020"/1.600" valves. No accessory holes in heads.

We just delivered this particular build about 3 or 4 weeks ago.

(Add) The "462" heads were not used on 68 Vettes!

Thanks, Gary in N.Y.

P.S. These are extremely rare crankshaft's to locate. They ended up in the '68 Vette and "marine" units only! G.M. went to the 350" SB in '69!!

so what could i do to not change heads or pistons to get it to 10.5:1 to be a little more pump gas friendly?? i have heard that 11:1 would have pining issues with the crap gas (no lead) of today??

I think the heads are 291`s as stated above I have a 327 in the vette now but i got this 327 from another guy that just pulled it from his 68 convertible car that had been wrecked yrs ago and it sat in a garage for lke 10 yrs it spins free but deff. needs gone through. So would thicker head gaskets work to lower cr or what else can i do?? thanx

Couldn't you use a thick head gasket?

It will ping worse with a thicker head gasket dueto the quench being .065", that piston above i have ran on pump gas on the street just without hammering it and i have 240-250 pounds cranking compression

Run a steel shim head gaskets with 93 fuel with liquid horsepower octane boost from manhatton oil or run som AV gas 50/50 with it, if you use AV gas only use 100LL as this is the same as sunoco blue, i have the spec sheets on booth and the 100LL is only $4.5 per gallon, the purple AV gas will give you all kinds of problems, the 100LL is usually 104-108 motor octane

Airports are required to have each batch tested, they will give you a printout of each batch if you request it.

gosfast Gary in NY,

You have the right info!:thumbsup:
The 68 327/350 Vette engine has Chrome valve covers and a Q-jet atop an iron intake.....big valve 291 heads, domed 11:1 pistons, rare LgeJournal forged crank. Typical of all 327s, a 2-bolt block. Very peppy.

GM went to the LJ 350 in the Camaro in 67 while changing the 327 to LJ in 68 and the Vette finally got the 350 in 1969.

A thick head gasket will reduce the compression a bit. Thing is, raising the head with a thicker gasket reduces the quench effect. With a less squished fuel-air mixture the engine can be just as prone to detonation than before we started the exercise. Maybe worse off!

The cheapest thing to do is find the posts by I believe Harold Sutton re real octane booster. Spend $100/year, less if you drive less.....

Another option is to run a really big duration cam to help bleed off the cylinder pressure.....your #151 cam has 222 degrees @ 0.05, 0.447" lift (with 1.50 rockers) on both the intake and exhaust. Something with 235 or 240 deg @ .05" will help bleed cyl pressure.... Run the widest LSA you can find to further reduce cyl pressure...... Gee whiz, we're just about to the 1970-72 LT-1 solid lifter camshaft

Those pistons were/are .125" and are rated for 11:1 compression but I don`t know how close they are because my old 327 with KB .100" domes and a .006 deck height only figured out at 10.3:1 and I ran pump gas no problem but my cam was a little bigger than yours and reduced the cylinder pressure to 150psi but still ran low 12`s. Summit sells replacment 350hp pistons and rates them at 10.3:1. If you have to go through the motor anyway, have a machine shop take .025" off of the tops of the domes and set thr quench up good and you should be ok depending on the octane you have out there. might want to step up the cam a little.

camarofreak, by chance do you know the casting number on the crank my 327 has the same block number but the heads are 3917293. i looked up the casting number on the crank and it was for a 350 engine. il have to get to the shop to check the numbers again on the crank, just wanted to know.

Run the widest LSA you can find to further reduce cyl pressure......

How's that again ?????

kivaka tried to reply to your email but it says you have chosen not to recieve them??

well thanx for all the tips! it is gonna be in a road driven 68 vette so i wanted to keep it as factory as possible but im not gonna run 5 dollar a gallon gas in it so i guess i will keep the short black factory and put a set of newer heads on it with a little more cc like 70cc or something. its not an ncrs car just a toy for me and the wife to mess with thanx again for all the help Tony
the different piston idea is something i will keep in mind as well ! i will talk to my engine builder later this week and see what would be the best route!

so what could i do to not change heads or pistons to get it to 10.5:1 to be a little more pump gas friendly?? i have heard that 11:1 would have pining issues with the crap gas (no lead) of today??


The manufacturer (now SpeedPro division of FM) lists those pistons at 10.35cr with a .039 gasket. I ran them for years in my 327 Vette (with a .039 gasket) on pump gas. When octane dropped from 92 to 91 I got some surging under cruise conditions and had to cut back on the amount of vacuum advance.

thats odd there all checked to get them,

How's that again ?????

A cam with a LSA of 114 deg will have less cranking compression than one of 110 deg