Calculation of 70 LT-1 compression ratio

[Patrick B.]

I was calculating compression ratios of various combinations of 350 Chevy parts to get some insight on its sensitivity to deck clearance and head gaskets. I started with the baseline combination of stock 70 LT-1 parts. Duke Williams has said that Chevrolet overstates the compression ratio on some 327's that have been discussed here so I expected it to compute to about 10.5. However, I was quite surprised that my computation was actually 11.04. I wonder if the LT-1 is an outlier as far a GM reporting or that there other factors affecting my computation.

Here are the components:
Cylinder Volume of 4.00 in bore with 3.48 in stroke: 716.6 cc
Combustion Chamber of 186 head with 2.02/1.6 valves: 64.0 cc
Dome Volume of TRW L2304F: 2.4 cc
Volume of 0.025 deck clearance: 5.15 cc
Volume of 0.021 steel shim gasket: 4.6 cc

The total volume = 716.6 + 64.0 -2.4 +5.15 +4.6 = 787.95 cc
The compressed volume = 64.0 -2.4 +5.15 +4.6 = 71.35 cc
The compression ratio = 787.95/71.35 = 11.04

The original LT-1 pistons were made by TRW, and the TRW L2304F pistons have an identical dome design and only slight differences around the piston pin bore. I am confident that the TRW dome volume spec represents the LT-1 OE pistons.

The deck clearance of 0.025 in is the design clearance you see in the Chevrolet Power books, but I don't think the actual blocks are machined intentionally for a greater deck clearance. I measured the deck height on a 70 350/300 engine that had never been taken apart (with stock flat top pistons). The deck height at one end was 0.035 but the height at the other end was only 0.020. I was surprised at this variation across the length of the block. The compression ratio of the back cylinder in this case would be only about 10.8 but the compression ratio of the front cylinder would be about 11.15 with LT-1 pistons. (a whole compression point difference results from about a 8 cc change in a 350).

If GM overstated the compression ratio of the LT-1, the only place left to look is the cylinder head volume. I have not measured the volume of the 3927186 LT-1 head, but the figure of 64 cc is widely published. Does anyone have a real measurement of a nominal 64 cc Chevy head? Comments?

[Duke W.]

Re: Calculation of 70 LT-1 compression ratio

Deck height/clearance is usually the biggest variable. As the broach tools wore deck height/clearance increased. I've seen them up to .015" high and up to .010" different side to side, but I've never seen any with significant slope from one end to the other.... maybe two or three thou at most.

The '64 vintage ....461 heads were the first with the 2.02/1.6" valve set, and these were nominally about 64cc chamber volume, but broach tool wear could also cause them to be larger.

The '67 vintage ...462 head eliminated the small quench zone on the spark plug side of the chamber, which increased chamber volume with the big valves to about 66 cc, and this carried forward with later small chamber heads such as the ...186 used on the LT-1, and AFAIK the only difference in these heads is the accessory bracket bolt bosses added to the ...186. Chamber and port geometry were essentially the same.

Of course, beginning in '71 head chambers got bigger to drop the CR so engines would operate satisfactorily on the 91 RON unleaded gasoline per edict from the fourteenth floor.

The sixties vintage OE SB head gasket installed at Flint was .018", but the service gasket was either .022" or maybe .026". It was assumed that if heads were removed for valve reseating that they would also be surfaced. So a thicker shim gasket was provided for service, so as to not raise the CR excessively, which could cause customer detonation complaints.

Since the CR calculation involves dividing a relatively large number by a relatively small number, a small change in that small number, the total combustion chamber (or "clearance') volume, has a big effect on the larger number, which is clearance volume plus swept volume. A few minutes playing with a compression calculator will give you an idea of the sensitivity. It's been awhile since I did this but I recall that increasing deck height or gasket thickness by .010" drops CR by about two or three tenths, and a 1 cc increase in head chamber volume has nearly as much effect.

About 10 years ago I co-authored an article with John McRae that goes into detail on managing the CR during a rebuilt project, and it has a link to an online CR calculator that I was using. I recall it was published in John's regional newsletter and later the Corvette Restorer. It might also be here on the TDB, and I know for sure it's on the CF in a thread started by SWCDuke.

Duke

[Larry E.]

Re: Calculation of 70 LT-1 compression ratio

[QUOTE=Duke Williams (22045);885220] volume. A few minutes playing with a compression calculator will give you an idea of the sensitivity. It's been awhile since I did this but I recall that increasing deck height or gasket thickness by .010" drops CR by about two or three tenths, and a 1 cc increase in head chamber volume has nearly as much effect.

If I understand this correctly then if ONE WANTED TO INCREASE THE CR WITH ALL THINGS BEING THE SAME>BY PUTTING ON A SET
OF OLDER "POWER PACK HEADS" WHICH HAVE THE CHAMBER VOLUME AROUND 59 TO 60 CC'S It WOULD INCREASE THE CR BY AT
LEAST 1 POINT?? I know the valve size in the older heads are smaller so total power my be down in comparison to the 461/186 heads.
Comments and thanks in advance>>Larry

[Patrick B.]

[Duke W.]

Re: Calculation of 70 LT-1 compression ratio

That's why I used and recommend the cgnet CR calculator. The link, if it's still alive is in the CR paper. All meaningful parameters are adjustable by the user including head gasket thickness and diameter and deck clearance.

Duke

[Duke W.]

Re: Calculation of 70 LT-1 compression ratio

If I understand this correctly then if ONE WANTED TO INCREASE THE CR WITH ALL THINGS BEING THE SAME>BY PUTTING ON A SET
OF OLDER "POWER PACK HEADS" WHICH HAVE THE CHAMBER VOLUME AROUND 59 TO 60 CC'S It WOULD INCREASE THE CR BY AT
LEAST 1 POINT?? I know the valve size in the older heads are smaller so total power my be down in comparison to the 461/186 heads.
Comments and thanks in advance>>Larry

Yes you could increase CR by installing early small port/small valse heads, but why? There are practical limits to CR, and the upper end of the power bandwidth is a question of head flow, not CR.

The practical limit for the "327 LT-1" configuration is about 10.5 (with 93 PON fuel), which can be achieved with at least two piston head gasket choices, and the nearly 300 SAE corrected RWHP it will produce at 6500 with a useable bandwidth to 7200 (incipient valve float) is a good 20 percent more than a Flint-built 327/365 with all OE replacement parts except for upgraded conn. rods. It's the head massaging that makes the top end power and the LT-1 cam makes better low end torque than the 30-30.

Installing early small port/small valve heads in place of massaged big port heads would be similar to disabling the carburetor's secondary throttle valves.

Duke

[Larry E.]

Re: Calculation of 70 LT-1 compression ratio

Yes you could increase CR by installing early small port/small valse heads, but why? There are practical limits to CR, and the upper end of the power bandwidth is a question of head flow, not CR.

The practical limit for the "327 LT-1" configuration is about 10.5 (with 93 PON fuel), which can be achieved with at least two piston head gasket choices, and the nearly 300 SAE corrected RWHP it will produce at 6500 with a useable bandwidth to 7200 (incipient valve float) is a good 20 percent more than a Flint-built 327/365 with all OE replacement parts except for upgraded conn. rods. It's the head massaging that makes the top end power and the LT-1 cam makes better low end torque than the 30-30.

Installing early small port/small valve heads in place of massaged big port heads would be similar to disabling the carburetor's secondary throttle valves.

Duke

Duke: Thanks for the response and completely understand what you are saying But>> Here is my post from a time ago>

Have a friend that has a older 327(300 HP Originally) cruiser Corvette. Just putts around town and low speeds. He has a set of #3884520 (283 Power Pack) heads on it. What I know about these heads is that they have small valves and small chambers.(CC's) With flat top pistons wonder what the Compression Ratio would be? Would this be a good combination for a car that never approaches top speed or raced??? Would low end Torque be better?? Thanks in Advance>Larry

Larry

LT1 in a 1LE -- One of 134

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April 17th, 2019, 10:30 AM #2
Duke Williams (22045)
Beyond Control Poster


Join DateJanuary 1st, 1993Posts14,081


Duke:

The small port, small valve heads effectively make it a 250 HP 327, which has excellent low and mid range torque, but only modest power and a useable power bandwidth to 4500 or so, but for the type of driving you describe, it should be quite satisfactory.

Before I started my resto on my base engine 66 Corvette we made a few runs against my friend. We both have 3.36 Gears and run
on straight Av Gas. Ran against him for aprox. 1/8 of a mile. Conclusion> He cleaned my clock bad!! From a standing start he was
gone. Larry

[Patrick B.]

[Joe R.]

Re: Calculation of 70 LT-1 compression ratio

Hi Patrick,

Here is the article Duke mentioned:

https://stlouisncrs.org/news_files/St_LouisNCRS_ChapterNewsletter2ndQuarter2012.pdf

Good luck!
Joe

[Patrick B.]

[Duke W.]

Re: Calculation of 70 LT-1 compression ratio

Thank-you for the kind words! Sometimes over on the Corvette Forum I feel like Rodney Dangerfield... I don't get no respect!

Duke