What's my compression ratio?

[bwalat]

I have a 68 Delta 88 with a 455-2bbl mated to a TH400. This engine came in two compression ratios:

A= 9.0-1/310 HP

B= 10.25-1/320 HP.

The information I have is conflicting. According to the dealer brochure, engine B is standard/available at no charge with a TH400 in Deltas. However, a service bulletin tells me that it would be engine A since it comes with a black air cleaner as opposed to red. But I understand that all 4bbls have a red air cleaner and all 2bbls have a black one.

The only sure way is to look at the pistons, but barring that, what are your views here? Thanks for your help.

PS: I plan to convet this engine to a 4bbl and duals, FYI.

[Sgt Art]

I'm going to go out on a limb and say if your car's engine came with a stock 2bbl carb, then your CR is 9:1. My reasoning is that the higher CR 10.x:1 would be for performance/greater horsepower. It would seem self defeating to use a higher CR and a 2Bbl carb. Another way to know, is how does the car run on 87 octane gas? If it's a 10.x:1 CR, it will knock. However, I could be wrong.

[Guest]

I concur with Art just on general principles alone. The higher compression engines would typically be a 4 bbl, because they are the ones that could take the greatest advantage of the additional fuel at greater rpm ranges. You might just to a compression check and figure the average between all the cylinders, accounting for some additional wear loss of maybe 10% or so after averaging. The compression gauge will typically have a compression ratio index right on it.

[Oldsmaniac]

In 66 the Delta's standard eng. was a high compression 2bbl that had a red/orange air cleaner. Optional was a low compression 2bbl version also. This had the black air cleaner...If they followed this pattern over to 68 as well you would have a low compression eng with that black air cleaner....A high compression 2bbl eng will run very well and straight line acceleration will egual or better a 4bbl car until,I'd say 50mph...then the 4bbl will take the lead!

[bwalat]

SgtArt:

Speaking of 87 octane, what was the rating on "regular" and "premium" in 1968?

I know that there was 100+ octane readily available then. I use 92 from the local convenience store. But I last used Ultra 94 before I put the car away.

If you are right, I can save beaucoups on gas.

[Sgt Art]

I'm not real sure. I believe they used two different methods back then of expressing octane. Seems like some gasolines were rated at over 100. Now a days, they average two methods to arrive at one universal figure. Then main thing was all gas, as far as I know, had tetraethyl lead added to increase octane and as a "cushioning" agent (for lack of a better term) for valve seats. If you read my battles with an overheating problem, you'll see where the older heads need hardened valve seats to compensate for the lack of lead. I did have a 69 model Corvette with 10.25:1 CR about 15 years ago. I once filled up with 87 Octane regular unleaded and it sure let me know it too. It sounded like a hammer hitting the tops of those pistons. That's what makes me think your running the lower compression ratio. I've used 87 octane in my 455 engine without a peep. The engine is a 1976 (I think) casting and they were designed to run on no lead low octane gas. A high compression engine can ignite low octane gas by means of it's compression alone like a diesel does. Once the engine has started running and gets warmed up, the low octane gas will actually ignite or burn too quickly causing the piston to want to return back from the way it came instead of continuing in it's upward motion to top dead center and then returning to it's down or power stroke. In other words, low octane gas can in a sense advance your ignition timing to much. The knock is the connecting rod slamming against the crank shaft journal and that's rough on the bearings etc. If I were you, I'd bring it down to the compromise gas, 89 octane. Wait till near empty, put in $5.00 of 89 and drive the car as you usually do. If it doesn't knock or ping, trust me if it knocks, you'll know, you're good to go. If that lower octane gives you trouble, just add the 91 or higher stuff.

[bwalat]

If that's the case, I'll have an Olds engine that was never made: a 455 with a 4bbl and dual exhausts with 9.0:1 compression, once I add the 4bbl and the duals.