C1 (1960) 245 or 270 build decision - HELP

[Joe L.]

Re: C1 (1960) 245 or 270 build decision - HELP

Well I guess my 1962 Motors manual must have a typo since it clearly states 10.5:1 for both the 270hp and 290hp engines. Seems a little odd since cams and compression ratio usually go hand in hand. Why would GM have run such a low compression ratio given the duration of the Duntov cam?

Ted------


Although it's usually pretty accurate, there are some errors in Motors Manual specifications section and I think this is one of them. For 1957-60 all Corvette engines had 9.5:1 compression except fuel injection with Duntov cam. In fact, the pistons used for all 57-60 NON-FI with Duntov cam engines were the same, GM #3739886. 1957-60 fuel Injected with Duntov cam engines used pistons GM #3746289, left side, and 3746290, right side.

Also, compression ratio and camshaft were not generally "paired" by GM. For example, the GM #3733431 and 3896929 hydraulic lifter camshafts were used with a variety of compression ratios from 8.5:1 to 10.5:1. The 70-72 GM #3972178 LT-1 camshaft was used with compression ratios ranging from 9.0:1 to 11.0:1. The GM #3896962 camshaft has been used, over the years, with compression ratios ranging from 8.5:1 to 11.0:1.

[Donald H.]

Re: C1 (1960) 245 or 270 build decision - HELP

It will be a while before I cam really look at the cam or pistons. I'm about a month away from pulling the body, then I am going to start working on the chassis and drive train.

I will test the lifters again when I get home from Carlisle.

[Duke W.]

Re: C1 (1960) 245 or 270 build decision - HELP

Well I guess my 1962 Motors manual must have a typo since it clearly states 10.5:1 for both the 270hp and 290hp engines. Seems a little odd since cams and compression ratio usually go hand in hand. Why would GM have run such a low compression ratio given the duration of the Duntov cam?

What Joe said, except according to the NCRS Spec. Guide the '61 283/275 HP FI engine was rated at 11:1, same as the 283/315HP FI engine, and both used the then brand new 461X heads with 1.94/1.50" valves.

In the mid sixties when I was working in a gas station I had a customer with a 270 who used regular gasoline. I was aghast the first time SHE came in and said "fill it up with regular", but she stated the engine ran fine and didn't ping.

As a general rule, the later the inlet valve closes, the higher static compression ratio the engine will tolerate.

Considering how easy it is to design a cam - especially if you use an existing, proven lobe, I'm surprised Chevrolet used the same cams well into the emissions era. Modern LS cams have relatively short duration, and relatively low overlap, but are phased late relative to sixties designs, so they can tolerate high compression. The secret to their broad torque bandwidth is excellent head flow and well "tuned" valve timing.

In the next issue of The Corvette Restorer you will learn how this design philosophy, along with some other internal tweaks, was implemented on vintage Corvette engines to provide base engine low end torque with SHP type top end power for base engines and optional engines with the base engine cam, which would include the 245 HP engines and 250/275 HP FI engines and the ubiquitous 327/300.

Duke

[Domenic T.]

Re: C1 (1960) 245 or 270 build decision - HELP

Both solids and hydraulic lifters have the hole to run the oil up the pushrod to the rocker arms and upper end.
IT only takes a few seconds to remove the clip at the top of the lifter and look for the hydraulic unit. Turn the lifter over and tap it on the bench and you will see.

DOM

[Joe L.]

Re: C1 (1960) 245 or 270 build decision - HELP

What Joe said, except according to the NCRS Spec. Guide the '61 283/275 HP FI engine was rated at 11:1, same as the 283/315HP FI engine, and both used the then brand new 461X heads with 1.94/1.50" valves.

In the mid sixties when I was working in a gas station I had a customer with a 270 who used regular gasoline. I was aghast the first time SHE came in and said "fill it up with regular", but she stated the engine ran fine and didn't ping.

As a general rule, the later the inlet valve closes, the higher static compression ratio the engine will tolerate.

Considering how easy it is to design a cam - especially if you use an existing, proven lobe, I'm surprised Chevrolet used the same cams well into the emissions era. Modern LS cams have relatively short duration, and relatively low overlap, but are phased late relative to sixties designs, so they can tolerate high compression. The secret to their broad torque bandwidth is excellent head flow and well "tuned" valve timing.

In the next issue of The Corvette Restorer you will learn how this design philosophy, along with some other internal tweaks, was implemented on vintage Corvette engines to provide base engine low end torque with SHP type top end power for base engines and optional engines with the base engine cam, which would include the 245 HP engines and 250/275 HP FI engines and the ubiquitous 327/300.

Duke

Duke------


Yes, for 1961 both fuel injected engines used the same cylinder heads and pistons unique to to those applications and that model year. The pistons were GM #3769731, left side, and 3769732, right side. I've corrected the above post to limit the information to the 1957-60 model years.

[Donald H.]

Re: C1 (1960) 245 or 270 build decision - HELP

Ok, so here is a picture of the lifter out and apart. Given the spring and floating section, I assume this is a hydraulic lifter which means the cam was also changed at some point.

My decision that I need to make right now is whether to have the carbs rebuilt as 270hp versions or leave as 245hp versions. I have read elsewhere that some folks change from 270hp to 245hp carbs because the cars ran better.

If I choose to leave the carbs as they are (245hp) and then switch the cam when I get into the engine later, what is the impact:

1. Driveability - What do I give up, or gain? I'm not going to race the car. But I would like for it to run and drive as good as possible. I also like the idea of having the idle of the solid lifter cam.

2. Judging - I looked at the score sheet and I assume I would take a 40 points hit for originality, but if they are restored and look like new, will I loose the 20 points for condition?

Please give me some feedback. I need to let Bob Kunz know if I have changed my mind about the carbs. Right now he is looking into modifying from 245hp to 270hp if he has the parts.

Thanks,
Don Harris

[Duke W.]

Re: C1 (1960) 245 or 270 build decision - HELP

Yes, those look like hydraulic lifters.

Ultimately whether to go with a 245 or 270 HP configuration is your choice based on the tradeoffs you are aware of.

The 245 HP configuration will have better driveability around town.

One thing you could consider is to use the L-79 cam rather than the original Duntov. Both have about the same effective duration, but the L-79 has less overlap so the idle is somewhat tamer, but is a lot closer to the Duntov than the low overlap OE cam.

Zero lashing the L-79 cam will probably cause enough valve train clatter for most to assume that it has the original Duntov cam.

You could also use the 3896929 cam, which is the replacement for the original 245 HP cam. A low overlap cam can be zero lashed for a little valve train clatter, then the idle speed can be set to SHP values and the mixture leaned to the point of some idle roughness.

It might be enough to fool a PV judge.

How does the engine before you took it apart? The OE cam idle was smooth, 600 at about 18".

Duke

[Donald H.]

Re: C1 (1960) 245 or 270 build decision - HELP

Yes, those look like hydraulic lifters.

Ultimately whether to go with a 245 or 270 HP configuration is your choice based on the tradeoffs you are aware of.

The 245 HP configuration will have better driveability around town.

One thing you could consider is to use the L-79 cam rather than the original Duntov. Both have about the same effective duration, but the L-79 has less overlap so the idle is somewhat tamer, but is a lot closer to the Duntov than the low overlap OE cam.

Zero lashing the L-79 cam will probably cause enough valve train clatter for most to assume that it has the original Duntov cam.

You could also use the 3896929 cam, which is the replacement for the original 245 HP cam. A low overlap cam can be zero lashed for a little valve train clatter, then the idle speed can be set to SHP values and the mixture leaned to the point of some idle roughness.

It might be enough to fool a PV judge.

How does the engine before you took it apart? The OE cam idle was smooth, 600 at about 18".

Duke

I don't have the engine apart yet. It is still in the car. But I took the carbs to Carlisle to give to Bob Kunz for restoration. The car had been sitting for a number of years when I bought it. It ran, but bearly. Spitting and idling at something over 1k rpm by my guesstimation. I ran enough to pull on a trailer then off and into my garage when I got home. Hasn't been started again in 9 months.

If I went with something like the 350HP L-79 cam, does it matter if I leave the carbs as the 245hp versions?

Thanks,

Don Harris

[Duke W.]

Re: C1 (1960) 245 or 270 build decision - HELP

Either should work okay - might need a little tuning beyond just the idle speed/mixture adjustment. I dont' know the detailed differences between the two versions, but since the L-79 cam is closer to the Duntov than the OE 245 HP cam, having them converted to "judging correct" 270 HP specification will meet the goal of the carbs matching the engine code.

I recall reading here that some owners have used the L-79 cam in lieu of the Duntov and were happy with the results.

Duke

[Donald H.]

Re: C1 (1960) 245 or 270 build decision - HELP

After more research on this forum, I have decided that I want to go with a solid cam. That's what the car had in it originally.

What I have found in a number of recommendations is the Sealed Power CS113R cam and the AT992 lifters.

[Duke W.]

Re: C1 (1960) 245 or 270 build decision - HELP

The CS113R comforms to the 3736098 drawing, which is the Duntov cam for '57-up Gen I blocks.

The AT992 lifters are the piddle valve type, which is about the only game in town, now, for mechanical lifters.

Duke

[Donald H.]

Re: C1 (1960) 245 or 270 build decision - HELP

Duke, thanks for all your good advice.

Hopefully in the end when I get this restoration finished, I will be happy with this choice. I really want to have the original corvette high performance sound, i.e. cam, lifters, and off road exhaust.

Again Thanks,

Don