78 Engine rebuild

[1978L48jafo]

Two questions I am using Northern's HP rebuild kit with the flat top pistons and crane street cam. I have also purchased performance heads 58cc casting number 14014416. The heads have larger valves and competition springs basically the works done.

Question 1 Did I take it too far for a Daily driver regarding compression? I obviously started at 8.5:1 and my best guess is I am at least 10.5:1????

Question 2 The sender hole is smaller on the new casting any suggestions? I checked and the heads could have been used on a 78-86 305. I also purchased an aluminum performer intake maybe if I am lucky there will be a plug I can use and move it up top??

I would appreciate suggestions
Thanks
John aka JAFO

:confused

 

[Stallion]

Compression means power, and if you want more power, than I'd say you did the best thing (but I think the original '78 L48 was an 8:1 compression)! It's good to put a little boost on that engine. I think it's rated at about 180bhp. One of the lowest in Corvette time, I'm sure.

Good luck!

TR

 

[MaineShark]

10.5:1 is pretty high for a street driven small-block.

Joe

 

[JohnZ]

10.5:1 is stretching your luck even with aluminum heads; 9.5:1 is about the practical limit for iron heads on pump gas without detonation, especially with a low-overlap stock cam.

 

[1978L48jafo]

10.5:1 is my best guess at the moment. I am going to measure deck hight tomorrow and put the numbers through again. The company who sold me the heads stated that it should raise the compression to 10:1 and I also changed out the dish pistons with flat tops. I am going to check and see if extra thick head gaskets might be available to keep me in a reasonable range.
John

 

[Jack]

10.5:1 is my best guess at the moment. I am going to check and see if extra thick head gaskets might be available to keep me in a reasonable range.John

John:
In general, using thick head gaskets is not a good way to get cr down. Most flat tops actually have a negative volume around negative 5cc. IMHO ... I agree it may be stretching it ... but 10.5:1 cr sbc with aluminum heads may be street-manageable ... but only with at least 92 octane & some ignition curve tweeking ... and maybe some cam overlap.
JACK:gap

 

[1978L48jafo]

Hi Jack thanks for the post.

I am using cast heads that have had a lot of work done including larger valves. I measured my deck height and come up with .039 plus a crushed gasket thickness of .039. I checked the volume of the valve relieves in the pistons and I think they are 5 or 6 ccs. I am going to recheck tomorrow with a medical syringe Using the calculator I end up with 10.1:1 or 10:1 depending on the cc accuracy. This engine will still need the high octane fuel.

I am not sure what you mean by cam overlap. I installed a Crane 100052 energizer.

Let me know if you still think this combination will be street able. I feel pretty good being around 10:1.:cool

John

 

[Jack]

I am not sure what you mean by cam overlap. I installed a Crane 100052 energizer.John

John:
Overlap: the time/distance when the exhaust valve is closing (but still slightly open) and when the intake valve has already begun to open slightly. As overlap (measured in degrees) increases, its effect is to lower dynamic compression ratio. It appears 100052 has about 60 degrees overlap at 0.000" timing or about 52 degrees overlap at 0.004" timing ... not a great deal ... but should be fine.

If you truly have 0.030"- 0.040" deck height, the CR should be manageable w/ 0.039" gasket. If you truly have 0.030"- 0.040" deck height, I surmise the block's deck has never been cut/decked.

Post your piston mfg & pn (& how many valve reliefs ... trough, 2 or 4) ... maybe I can look up the volume & save you some time & aggravation.
JACK:gap

 

[1978L48jafo]

Jack thanks for explaining the overlap.

My block is low mileage (57K) and the deck height has not been touched. The only reason this engine is apart was one bad cylinder and the problem was the cam had two lobes wiped out, it shook at idle for several years so I wanted the bearings replaced, and I wanted to clean away 20 years of neglect.

The pistons are flat SL H345NP (Sterling) they are part of a kit from Northern Auto Parts. 1097 HP350KH. I emailed Northern yesterday requesting additional information on the pistons. I didn't specifically ask about the relief's.
Appreciate the time and trouble you are taking to help me!!
John

 

[LT4hitsthespot]

I'd like to hear how your engine comes out. I have a 78 and was thinking about putting a super charger on it but have decided that I probably better do a rebuild instead. I don't know alot about mine as the work was done by the previous owner. Is an L48 with L82 heads, Edlebrock High RPM roller cam, carb and High RPM intake with headers and true duals.

I liked it until I got an LT4 and now feel that the power is lacking in it. I am considering just pulling the engine out and doing a complete rebuild and not sure what parts I want to put in it. I also have found a complete 454 pretty reasonable. I want to swap my tranny out for a 5 or 6 speed while I'm at it because my second gear is going and I want the over drive.

 

[1978L48jafo]

LT4hitsthespot,
I agree the stock L48 is pretty lame. I will keep the posts coming I hope to finish in early spring. If you decide to supercharge I notice Northern has a book on it. I built a car about ten years back and installed a paxton super charger. This was not a difficult install and it performed very well.

I too would like to upgrade to a 6 speed. but I need to keep things reasonable for now. The motor has the potential of being a screamer but I stopped a few weeks ago because I thought I took it too far with regards to the compression. I am ready to move on but man is it cold here.
John

 

[Jack]

The pistons are flat SL H345NP (Sterling) they are part of a kit from Northern Auto Parts. 1097 HP350KHJohn

John:
yer welcome ... no prob. I don't have a lookup for sterling ... I believe they are essentially asian reverse-engineered pieces (nothing wrong with that) .

OE uncut total block height = 9.025"
1/2 stroke = 1.74"
rod = 5.7"
OE factory down in the hole = about ~ 0.040" (many piston variants)

Federal Mogul/SpeedPro/TRW/Sealed Power has same number piston. Your deck measurement of about ~0.039" would roughly coincide w/ FM pn's:
H345P+
345NP
both are flat tops & have a comp distance of 1.540" & have 4 VR's & minus 5 cc volume. With either, I would expect an uncut deck height of about ~ 0.045" down in hole.

FM pn H345NP is essentially same as above two, but has a CD of 1.560" ... with it I would expect an uncut deck height of about ~ 0.025" down in hole.

Slight piston rocking in bore can affect down in hole measurement. Dunno what head gaskets you were sent? ... but your CR should be manageable with a composite gasket (not thin steel shim). Extra-thick gaskets bad for optimum squish. Regardless, you should check that gasket is compatible with your aluminum head ... because some h gaskets' fire ring not ok for aluminum ... some can brinnel the aluminum (damage it). Hope this helps more than not.
JACK:gap

 

[1978L48jafo]

Jack thanks again. I purchased the upgrade on the bearings rings and gaskets. The gaskets are high end felpro. The heads are not aluminum (http://www.carshopinc.com/heads.shtml) they are small chamber cast units which have been reworked. Based on your post I am going to finish it as is next weekend.

I could be off on the deck measurement I used a metal straight edge and a feeler gauge. I am not setup with a good selection of machining tools....;-) I think it's .039??

If you think just to be safe I should get a thicker gasket just let me know.
John

you helped a great deal!!!

 

[Jack]

The gaskets are high end felpro. The heads are not aluminum (http://www.carshopinc.com/heads.shtml) they are small chamber cast units which have been reworked. Based on your post I am going to finish it as is next weekend.

John:
Pardon me, please ... but for some reason I mistakenly thought you were working w/ 58cc ALUMINUM heads.

BTW, think those IRON heads in link are from an sbc 305. Yes, the valves are oversize for a 305 ... but 1.94/1.50 is standard size for L48 ... so NO valve size increase over L48.

IMHO, I think 10:1 - 10.5:1 CR w/ IRON heads is WAAYY TOO MUCH CR for IRON ... and may not be street-manageable. Also, I don't think your cam overlap is near enough to kill off enough of the CR ... and if you go with more cam then you probably won't like the power band. If there's any possiblity, I suggest you exchange flat tops for dished. Or return the 58cc heads & just have your old 75-76cc iron heads serviced. If possible, I'd service the old heads given this situation ... I would not run 58cc iron on flat tops on street. Again, I apologize for mistaking IRON vs ALUMINUM.
JACK:gap

 

[mmvette80]

Ive been reading the engine posts. Ever since I installed the 700R4 ive been thinking about rebuilding the old L48 I have sitting in the garage. What would be the best engine build to get the car to 100mph the fastest? Im gona put in 3.55 gears with a bigger hp engine.

 

[1978L48jafo]

Hi mmvette80,
I am probably not the best one to ask. I went with the hipo kit from Northern and for the most part pretty happy unfortunately I choose the wrong heads.
There are some really savvy engine guys on this forum. I am just not one of them.
John

 

[1978L48jafo]

Jack, How much compression will the cam overlap you mentioned earlier compensate for? If I am at 10:1 and the cam helps a bit.......you said on a earlier post 60 degrees. I am checking to see what I can arrange on the heads. As I said on the email the book I am using says not to bother rebuilding the 624s but at this point I wish I had.
No need to apologize this is my mistake not yours.
Thanks
John

John:
Pardon me, please ... but for some reason I mistakenly thought you were working w/ 58cc ALUMINUM heads.

BTW, think those IRON heads in link are from an sbc 305. Yes, the valves are oversize for a 305 ... but 1.94/1.50 is standard size for L48 ... so NO valve size increase over L48.

IMHO, I think 10:1 - 10.5:1 CR w/ IRON heads is WAAYY TOO MUCH CR for IRON ... and may not be street-manageable. Also, I don't think your cam overlap is near enough to kill off enough of the CR ... and if you go with more cam then you probably won't like the power band. If there's any possiblity, I suggest you exchange flat tops for dished. Or return the 58cc heads & just have your old 75-76cc iron heads serviced. If possible, I'd service the old heads given this situation ... I would not run 58cc iron on flat tops on street. Again, I apologize for mistaking IRON vs ALUMINUM.
JACK:gap

 

[LT4hitsthespot]

Ive been reading the engine posts. Ever since I installed the 700R4 .

MMVette,

How do you like the 700R4 in that car? Did you install it or have someone do it for you? If you did how hard was it to convert? I'm thinking about changing and that was one of my options at one point.

 

[mmvette80]

I had never done an engine or tranny swap before I swapped the t350 for the 700R4. Yes, I did do it myself. It was very easy. I had to shorten the drive shaft and have a piece on the shifter changed. The crossmember I got as a package with the tranny. It changed the whole car. Just the other day I showed the wife on the highway what it used to sound like, I pulled it out of OD into 3rd. She was like, Oh yeah I remember.

 

[1978L48jafo]

mmvette80
Just courious did you consider switching to a manual 5 or 6 speed?
John