Rebuilding a L72 for pump gas

[Steven S.]

Looking for some input on compression ratio selection for rebuilding an L72 to run reliably on pump gas. I have experience with two prior 396's that were built to 10.5:1, and with optimal tuning I was able to run them on pump hi-test, but that experience tells me any more compression than that will be a problem. I found this magazine article awhile back that I kept bookmarked... http://www.corvettefever.com/techart..._engine_build/ I thought it was an interesting build, the end results appear to prove out the changes they made to the engine, and the only real additional expense would be the additional relief machining on the pistons. Thoughts?

Thanks,
Steve

[Duke W.]

Re: Rebuilding a L72 for pump gas

Most Flint and Tonawanda engines as built don't detonate on modern premium fuel. So the first question is: Does it currently have a detonation problem?

As far as rebuilding an L-72 is concerned, I only recommend OE equivalent replacement parts with the exception of the rods. It may be less expensive to buy aftermarket high strength rods than qualify used rods.

Many parts like the OE oil pump and most valvetrain parts can be reused if they pass inspection. There is a question on valve springs since the second design don't appear to be availalbe.

The camshaft is an excellent design for a high performance road engine. Massage the heads and it will make power to 7000 without sacrificing low end torque and tractibility.

Duke

[Steven S.]

Re: Rebuilding a L72 for pump gas

Duke,
This is an engine I'm putting together for my car, so I have no prior experience with it. I will have to purchase pistons and a cam, so any deviations in this area wouldn't cost me any more if they were advisable. I like the idea of just building a blueprinted L72 as long as running pump gas wasn't a problem.

On the topic of pistons, does anyone know what the dome size in cc's was on the factory part?

Thanks,
Steve

[Larry B.]

Re: Rebuilding a L72 for pump gas

The stock rods will work fine unless you intend to turn alot of RPM But a set of Eagle Sir Rods don't cost much more than reconditoining yours. You can run 10 to 10.5 compression ratio without a problem.I like to use a piston that is lighter than the stock because it in effect makes the rod stronger.The biggest decison should be camshaft, yes the stock wasn't bad for it's day but that was 40 years ago. Todays custom grinds can give you so much more.I'm not talking a Summit special.I'm talking one built to work with your comp ratio ,automatic or stick,gear ratio,power brake,etc.

[Duke W.]

Re: Rebuilding a L72 for pump gas

Duke,
This is an engine I'm putting together for my car, so I have no prior experience with it. I will have to purchase pistons and a cam, so any deviations in this area wouldn't cost me any more if they were advisable. I like the idea of just building a blueprinted L72 as long as running pump gas wasn't a problem.

On the topic of pistons, does anyone know what the dome size in cc's was on the factory part?

Thanks,
Steve

The dome volume I have for the L2268F-30 from an old Federal Mogul catalog is 35.5 cc, but John McRae measured a different number. Search for threads started by him, and I highly advise that you contact him.

I recommend you target a not to exceed CR of 10.25:1, and this can be done with OE components. Read the article published in the Fall 2009 Corvette Restorer in order to learn how to manage your CR.

By far the most cost effective way to get "more power" is to massage the heads. The torque/power measured in the article you referenced is about equal to Chevrolet's early advertised gross ratings, so I doubt if the aftermarket camshaft did much other than hurt low end torque because it has more overlap than the OE cam.

The F-M CS-165R OE replacement cam will have to have a groove cut in the rear journal, and be sure you use the correct "three-hole" rear cam bearing.

You will also need to select a valve spring that matches the second design OE spring, and you should be able to get some help from other big block owners who have wrestled with this issue.

There appears to be no aftermarket replacement oil pump that provides correct oil pressure, so you should reuse the old one. Oil pumps rarely "wear out", and are seviceable if they pass a simple visual inspection. Most will unless they have ingested debris due to something disintegrating inside the engine.

The performance and quality of what you end up with will be proportional to the up-front research and planning you do.

Duke

[Steven S.]

Re: Rebuilding a L72 for pump gas

The dome volume I have for the L2268F-30 from an old Federal Mogul catalog is 35.5 cc, but John McRae measured a different number. Search for threads started by him, and I highly advise that you contact him.

I recommend you target a not to exceed CR of 10.25:1, and this can be done with OE components. Read the article published in the Fall 2009 Corvette Restorer in order to learn how to manage your CR.

By far the most cost effective way to get "more power" is to massage the heads. The torque/power measured in the article you referenced is about equal to Chevrolet's early advertised gross ratings, so I doubt if the aftermarket camshaft did much other than hurt low end torque because it has more overlap than the OE cam.

The F-M CS-165R OE replacement cam will have to have a groove cut in the rear journal, and be sure you use the correct "three-hole" rear cam bearing.

You will also need to select a valve spring that matches the second design OE spring, and you should be able to get some help from other big block owners who have wrestled with this issue.

There appears to be no aftermarket replacement oil pump that provides correct oil pressure, so you should reuse the old one. Oil pumps rarely "wear out", and are seviceable if they pass a simple visual inspection. Most will unless they have ingested debris due to something disintegrating inside the engine.

The performance and quality of what you end up with will be proportional to the up-front research and planning you do.

Duke

Will do, thank you for the advice! Unfortunately the oil pump was replaced once already. What can you tell me about the OEM pump and the pressure issues with the replacements? Perhaps I can find an NOS pump.

Steve

[Duke W.]

Re: Rebuilding a L72 for pump gas

Do you know what the replacement pump is and what hot oil pressure it delivers? The OE big block spec is very broad - 50-75 psi at about 2000-up fully warmed up, but aftermarket pumps often deliver more, which is not necessary.

There are two big block pump gear lengths, and it's the length of the gears that determine pump volume vs. RPM. The longer the gear, the higher the volume. Also, current replacement pumps don't seem to have the exact same relief spring as OE. I believe the pump casting is the same, but they were machined to different lengths for the two different length gears. There may still be some controversy as to which length gear was in certain configurations, including L-72.

I know Mark Nagy has done a lot of recent research on this issue. We've talked on the phone about the subject. I can't recall if there is a thread on his effort, but do TDB search and shoot Mark an email via the TDB.

Duke

[Steven S.]

Re: Rebuilding a L72 for pump gas

Duke,
It's a Sealed Power Z2244153 "High-volume, standard pressure". I'll look into contacting Mark on the subject.

By the way, I dug out that Fall 2009 Restorer, great article! It answered many of my questions regarding compression ratio and head gasket selection.

Thanks,
Steve

[Larry B.]

Re: Rebuilding a L72 for pump gas

The pump to use is a 10771 Melling,Sealed Power doesn't make pumps they get them from Melling.This pump has some anti cavation groove in the cover. The does not always control pressure bearing clearance and weight of oil have a large effect.

[Steven S.]

Re: Rebuilding a L72 for pump gas

The pump to use is a 10771 Melling,Sealed Power doesn't make pumps they get them from Melling.This pump has some anti cavation groove in the cover. The does not always control pressure bearing clearance and weight of oil have a large effect.

Is it a 10770? Couldn't find 10771, I see the 10770 is a high volume standard pressure. If I understand you right the Sealed Power pump has the anti cavitation groove, correct?

Thanks for the input,
Steve

[Larry B.]

Re: Rebuilding a L72 for pump gas

you are right it is a 10770 Melling and it will have the grooves. Think about the cam to as a custom can be done with .002 lobe taper instead of .oo1 .Makes breakin easier and big blocks can have a problem with breakin because of around 110 lbs seat pressure and 1.7 rocker ratio even with good breakin oil. I wouldn't cut pistons I'd releive the heads around the valves which will increase air flow by unshouding them. Use a modern custom cam instead of 40 years ago and you can run your 10.5 ratio.Compression ratio won't hurt you the static compression is where it is at.I've built motors with 9to 1 and had 225lbs compression and I've done 14 to 1 and had 225 lbs compression all done with the cam. Compression starts to build when intake valve closes. If you want more compression close the intake earlier.It doesn't take much.

[Steven S.]

Re: Rebuilding a L72 for pump gas

Larry, thanks for the clarification, I will take all this info into consderation.

Duke, I spoke with Mark and he also confirmed the Melling 10770 pump the closest out there to the factory pump. He also stated if you were to use the factory blue spring out of an original pump the pressure would be right on. I believe the Melling has a pink spring out of the box.

Steve

[Larry B.]

Re: Rebuilding a L72 for pump gas

The spring does not tell you what your pressure will be it only tells what it will pop off at.Bearing clearance has more influence on pressure.You can take the cap of the oil pump and put air pressure aginst the pop off and know where it will open and shim if needed.

[Duke W.]

Re: Rebuilding a L72 for pump gas

It may be time for a brief review of how oil pumps work. They are a member of a family called "constant volume pumps". If you dump the output into a bucket, delivery volume will be approximately linearly proportional to pump speed until the very top of the range, near the point where cavitation occurs.

If a restrictor is placed in the output, delivery volume will remain the same, but pressure builds up, and the pump will caviate sooner or a material failure will occur due to excess stress in pump components. In an engine this restriction is the sum of various internal restrictions in the engine such as bearings.

The oil pressure relief spring serves to limit pressure to an amount that is necessary to deliver sufficient volume to the various internally lubricated parts. Once relief pressure is attained. delivery volume to the main gallery is based on this head (pressure), not pump speed, so delivery volume to the main gallery remains nearly constant with further increases in engine speed, and excess pump delivery volume is routed back to the pan or pump inlet.

For a given internal engine restriction, a higher pressure relief spring will not achieve relief pressure until a higher pump/engine speed.

Likewise, for a given relief spring, increasing internal engine clearances will require a higher pump speed to achieve relief pressure.

Chevrolet pumps are sized to provide proper oil pressure with production range engine clearances in normal road use. Traditionally, engines prepared for competition are assembled with higher clearances and greater design oil pressure that may require a higher volume pump to maintain design range oil pressure, however, such a pump on a road engine will just consume extra power and heat up the oil because power is required to pump the excess, only to be bypassed back into the sump or pump inlet by the pressure relief valve.

Most small block pumps are designed to achieve 40-45 psi at 2000 engine RPM, but mechanical lifter engines beginning in late 1963 were equipped with a 55-60 psi relief spring to the same basic constant volume pump.Big block pumps are not as straight forward with two length gears for different volume and a very wide specified pressure range that was achieved with two different relief springs depending on the engine configuration.

The new LT1 engine has a two stage oil pump/pressure system. Above about 5000 revs the second stage increases oil volume delivery in conjunction with higher delivery pressure. This means the the small volume primary pump absorbs minimal power in normal driving with the second stage available when needed to deliver a sufficient volume to maintain higher pressure at high revs.

Duke

[Steven S.]

Re: Rebuilding a L72 for pump gas

Wow, great explaination! You have to excuse me, I'm taking in a lot of new information here and I may get my wires crossed a few times along the way. Thanks to all who have contributed to this discussion so far!

Steve