Compression ratio help please 327 (331)

[Beemerlite]

Hi one and all from the land far down under, New Zealand to be exact.

I’m looking for some info on a suitable compression ratio for my engine.
A little back ground on my engine and what I can remember about it!!
I had the engine built 27 yrs ago when I was in my early 20’s.
Ok here go’s, please again it’s what I can remember.

Small Block chev 327 (30 over 331 )
Casting # 3782870 unable to read the deck casting numbers as they have been machined off.
Heads double hump fuelie 461’s with lots of work done to the ports.
202 inlet 160 ex 3 angle valve grind, stainless steel.
Titanium retainers screw in studs.
Treble isky valve springs
Anti lube/pump hyd lifters.
Yella Terra roller rockers.
True roller timing.
Small journal 2 bolt mains steel crack shot peened and balanced.
Rods are standard chev, shot peened and balanced.
Pistons forged TRW 12.5 high domed with chrome moly rings.
Edelbrock Victor Jr intake.
Holley 750cfm 3310-2 carb.

I have no idea what the cam spec is as I lost the spec sheet. But when I had the trans rebuilt 25 odd years ago TH350, I had the torque converter done to match the cam. The torque converter has 2400RPM stamped on it.
Had one hell of a lumpy idle! and could pull very high RPM…

Now for the fun part… I haven’t used the engine for about 20yrs.
Probably only done 15000 miles on it way back then.
About 9 years ago I had the engine completely rebuild from the crank up.
It’s been sitting on my engine stand ever since.
The valve springs have not been torque down so there isn’t all the tension on them.
I turn it over once every two months or so and spray CRC down the bores.
I used the engine in my street car, then a 1975 Chevy Monza Spider, daily driver.
Took it out when I sold the car and placed a 305 back in it.

My Problem… I think…Compression ratio.
I would like to put this motor in my 1989 GMC C1500 at some stage. But I need to lower the compression ratio? So I can run it on todays pump gas. The GMC’s my daily driver.
What would be a suitable compression ratio and a good piston type, to run in my engine these days. We have 91, 95 and 98 octane fuel here in New Zealand at most petrol stations.
Since I’ve had the engine for over half my life I’m not selling it... lol
But would like to be able to use it in something… one day.
My truck has the stock 350 in it and runs sweet as, 246000miles on clock. Was rebuilt at some stage in the States before it was shipped over here.

Sorry for the long winded 1st post but thought a might as well throw it all out there...

I'm gona try and add a few pictures.

Thanks for any replys...

[RidgeRaceCars]

To make it perform with the rest of what u have your going to need to know what cam is in it!
Pull the timing cover off and the timing chain the cam should be stamped with p/n on the end of it then go from there.... the compression is determined by cam,head flow,intake etc.. around 10.5:1 would be as far as i would go on 94,98 is good!

[RANZTRANZ]

Those heads should be 64cc chambers...unless someone worked on them to change the chamber size. Just another item to consider on compression ratio figures....

[Captainfab]

Welcome to the forum

I hope you're not planning on putting the TBI on this engine. If you are going with a carb, I would not use that intake. You will be much better off with a dual plane intake such as a Performer or Performer RPM.. Also you will not be able to run the serpentine accessory drive on this engine.

[geezer#99]

Welcome!!
So! Is the tiller on the left side of your GMC or is it converted to rhd?
Where in NZ do you hang your hat?

While it's easy to do I'd pop the timing cover off, pull the intake, get a degree wheel and a dial indicator and check out the cam specs. You might find a p/n on the end of the cam like Ridge says but you might spend a lot of time searching for what cam it is. You'd want to know how the cam is degreed in the motor anyway.
Then I'd pull a head and determine what your compression really is.

[Marv D]

Your issue is definately compression. The advertised "12.5"1" compression ratio is typically assuming the old 64cc heads and a stock 0.025" deck. Being aftermarket pistons for the 0.030 bore it is 'likely' to be the 12cc dome typical of the TRW 12.5 pistons.

Thing is,,, bore it +.030,,, Zero deck that motor (as we know it has been decked but not how much) and it's 13:1. You need a MINIMUM of 108 - 110 MOTOR octane to keep it from eating it's self alive with detonantion.

I'd say your first step is to find out how N.Z. rates fuels. Here in the US they use R+M/2
If NZ shows only research octane at the pump,, (which is always considerable higher than MOTOR octane in pump fuels) YOu need to find theh motor octane of what they are pumping there MOTOR OCTANE is the ONLY number you should consider in a motor on the street, or the strip.

All that said,,, ~84 MOTOR octane typically works well with the typical mild hydraulic performance cam (i.e. the Comp 268H - 274 EX grinds) with a 9.0 to 9.5:1 mechanical compression ratio, with iron heads.

To get there with all the assumptions in deck and such,,, your going to need a 10cc dished piston. If you can find a 'D' dish which will keep a tight quench on the closed side of the chamber,, would be ideal. I think Mhale has one,, but they are pretty proud of them at $700 a set. OUCH

[Beemerlite]

Thanks guys for the replys.
Ridgeracecars I pulled the timing cover but no luck. End of cam is as smooth as a babys butt... not a thing on it, but worth a try.

Ranztranz, not sure if they are 64cc heads, hopefully are are. The heads (bear when I got them) and block came out of a 55 Chev dragster when they changed over to running a big block.

Captainfab, no not putting TPI on it, was going to run a carb. Thanks for mention of intake because you reminded me that when I was running the motor in my Monza the intake I had on it was a Edelbrock Torker 2 because the Victor Jr wasn't right for a daily driver.
A mate wanted a "lend" of the Torker 2 so i gave it to him. Had the Victor Jr spare sitting around so put it on to cover the hole...
Yes i did have problems in the Monza with the serpentine accessory drive because of no bolt holes in the heads...

Yes Geezer99 the tiller is on the left hand side of GMC... lol
Live a little below Auckand about 40min a little out in the sticks...
We also have a 1997 Pontiac TransAm LT1 with 63000miles on the clock. Tiller is on the left hand side, had it about 7yrs. Stays in shed under blankets...

Marv D thanks for the info, yes engine use to "Ping" pretty bad at idle when in Monza.

Thanks Again guys your help is most appreciated.

[geezer#99]

Originally Posted by Beemerlite

Thanks guys for the replys.




Yes Geezer99 the tiller is on the left hand side of GMC... lol
Live a little below Auckand about 40min a little out in the sticks...
We also have a 1997 Pontiac TransAm LT1 with 63000miles on the clock. Tiller is on the left hand side, had it about 7yrs. Stays in shed under blankets...

Couldn't tell from your pic if lhd or rhd. I looked at a 90 SS454 back in '91 on our first trip of four to NZ. It was converted to rhd. Was in Albany. Priced at a little over 73,000 NZ bucks.
Sure a blast driving a lhd vehicle on rhd roads. Had the pleasure of that in Wellington with my buddy's 67 Chevelle.
Anywho, again welcome to the forum!!

[66 C10 383]

I'd also look at your dynamic compression. If that cam was even close to working with close to 12-1 compression and you drop in pistons to get you to roughly 9-1 compression, that cam will have so much overlap that your dynamic compression will be too low to have much low end torque at all. I'd look to replace the cam, too.