perplexing situation

[Tommy]

I have my engine being built at a reputable shop and things are a bit slow but still allright. The man building the thing has mentioned a couple of times to me about ditching the big domes and lowering the comp ratio for so I can run pump gas... the engine is a L78 with L89 (842's) heads. It has been decked but was still standard bore with the original pistons. It ran fine and yes I had to run a real nice mix of high test and race fuel to make it talk to me but I didn't really mind much .... I don't know if I am going to be happy with the flat tops vs the high comp pistons.. I plan on driving it quite a bit again like I used to.. I got to tell him something in the next few days http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif

Tommy http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[SuperNovaSS]

Keep the originals. The aluminum heads should help to dissapate combustion chamber heat to reduce detination.


Jason

[427king]

You have ONLY two choices 1] Tell him you want the 11 to 1s 2] tell him you DONT want the flattops. Either choice is a winner http://www.yenko.net/ubbthreads/images/graemlins/grin.gif

[TXSS]

"Keep the originals" ditto

[92646]

I built a 350 LT1 WITH 10 to 1 flat top pistons in place of the 11 to 1 pistons but the rest of the motor was left stock with the solid lifter cam. I was never happy with the motor because it lacked the snap that you would get from a higher compression motor when you rolled into the throttle. You will be happier with the higher compression pistons and the aluminum heads will help with the detonation. Now if you want to make some real horsepower go to open chamber heads and pistons while you still can.
Mark Sheppard

[SuperNovaSS]

Looks to me that this is not a perplexing situation at all. Pretty cut and dry so far.


Jason

[RamAirDave]

Although it can get pricey if you drive your car several times a week as I do, there really shouldnt be any issue since you stated that you dont mind running a mix of hi-test and race fuel.

[TDW]

When I had the LT1 in my old 70 Z28 rebuilt, I looked for a long time to find original, NOS, Trw GM pistons for it. The engine builder wanted to cut the domes to lower the comp. I said no. Very glad I didn't let him. I had a 55 gallon drum of Torco dropped off at my shop. When it needed fuel, it was rite here. I also say don't lower the comp.

[Tommy]

I think I have my answer, thanks guys. The local speed shop has 116 in the pump and it's not far from the house at all. If it's only the gas price, I think I can justify keeping the good stuff in the engine.

Tommy http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[ORIGLS6]

Ditto on all the comments above. I just picked up ten gallon of 110 fuel at $4.99/gal at the pump. Not bad considering I filled the truck at that same time for $3.54/gal!

I just got home from VetteFest and unloaded the car yesterday afternoon. I let the car sit in the driveway and idle for a while to dissipate any moisture in the exhaust system. (Just makes you grin to hear the rumble, pop and crackle of an 11:1 solid lifter engine.)

The toys are worth the extra money, and the sounds and smell from the REAL compression and GOOD fuel..........Ahhhhhhhh yes! http://www.yenko.net/ubbthreads/images/graemlins/smirk.gif

[firstgenaddict]

[ QUOTE ]
Ditto on all the comments above. I just picked up ten gallon of 110 fuel at $4.99/gal at the pump. Not bad considering I filled the truck at that same time for $3.54/gal!

The toys are worth the extra money, and the sounds and smell from the REAL compression and GOOD fuel.......... http://www.yenko.net/ubbthreads/images/graemlins/smirk.gif!

[/ QUOTE ]

Agreed... 110 Leaded is available around here for 5.89 a gallon... been the same price since regular was $2.00. Now it is starting to look like a bargain!

[Hylton]

[ QUOTE ]
Agreed... 110 Leaded is available around here for 5.89 a gallon... been the same price since regular was $2.00. Now it is starting to look like a bargain!

[/ QUOTE ]

That fact right there tells me this whole gas crisis is a farce!

[Mr. Chevy]

I say keep the originals also. My Nova engine was rebuilt to it's stock L78 specs with the 11:01 pistons and I like it. I run straight 110 octane, Cam 2 racing fuel in it. It really runs and sounds good!! The fuel can become expensive but thats ok becuase I have fun!! Plus this fuel burns very clean and does not carbon up the chambers, exhaust etc..

Rich

[427TJ]

[ QUOTE ]
The toys are worth the extra money

[/ QUOTE ]

That hits the nail right on the head. http://www.yenko.net/ubbthreads/images/graemlins/biggthumpup.gif

[Andy]

A good website that will help you is, http://www.empirenet.com/pkelley2/DynamicCR.html You need to figure out what your dynamic compression ratio is. Download his calculator, and enter in your engine's specs. According to Kelley, long as it is around 7.5-8.5:1, you should be fine on pump gas. If your dynamic compression is over 8.5:1, then you'll need to run a higher octane fuel than 93. The more overlap your cam has, the more compression it will bleed off. I've been researching this stuff for a few years now, since I'm putting together a pretty serious 406 for my 67. The combo I plan to run will have a static compression ratio of 9.8:1 and a dynamic compression ratio of 8:1, which will be perfect for 93 octane pump gas. My engine builder said that anything over 8.25:1 dynamic would create problems with detonation in the summer, and I would have to pull timing out of it. My motor should put out around 520-540 hp and around 550 ft lbs.

[1railman]

If it were a daily driver, maybe so. But if not driven much, I would keep the dome top. Plus you get to smell that wonderful 116.

[musclecarjohn]

110 leaded premium runs me $6.28/gallon here with all taxes.
I blend 1/2 & 1/2 w/91 Premium pump gas which is as good as you can get here...last time I was on the dyno,operator said the A/F mixture was spot on.
My cars love it and I wouldn't have it any other way.Would not change a thing or take timing out...where's the fun in that? http://www.yenko.net/ubbthreads/images/graemlins/dunno.gifIf my cars didn't pop & bang and smell like they do,they sure wouldn't be as much fun...and I'd probably end up driving a Honda Civic.

Don't do it I say... http://www.yenko.net/ubbthreads/images/graemlins/no.gif

[Keith Tedford]

We've put over 30K miles on our L78 Chevelle since we bought it in 1997. No ill effects so far on 91-93. Being stock factory, the compression ratio might be closer to 10-1. Seems that many have been found to be down a point and more.

[indyjps]

calculate your true comp ratio before its assembled

[BBIGG BLOCK 396]

I hate butting in on you guys but here locally a lot of guys are beginning to have a lot of problems with the HIGH PERFORMANCE FUEL! Seems they are reformulating them and adding ETHNOL or some other OXYGEN BASED ADDITIVES which is causing major problems when left sitting for a couple of weeks.A lot of guys have had to have ther carbs redone because of this problem.I would surly be checking on the racing fuels you are using for additives to your Hot Rods to make sure it is not one of the reformulated brands.I will try to find out the names of some of the fuels they are having problems with and post them.

Bobby

[Chevy454]

The first L72 I put together came in at 10.2:1, after a slight clean up on the deck, with the stock TRW pistons, and the .038" Fel-Pro...unless it was the dead of summer, I ran it on pump gas...took our Y-Camaro to 12.7s @ 108+ (woulda been better had I known then what I know now!).

[jasonL78]

Tommy great topic! I am asumming this is the motor for your 68 BB nova? I was pondering the same thing with the new motor I am building for my 68 L78 nova. I have a stock l78 short block assembly with the 11:1's and my machine shop told me to change them as well. This topic just changed my mind to keep the high compression slugs. Do any of you use the high test from the pump (93) and then use an over the counter additive, like octane boost 104. Or should you just use race fuel like cam 2?

Jason

[Andy]

What are the cam specs that are going into these L78s? Have the blocks been decked? What CC are the heads, and how many CCs do the pistons displance(valve reliefs[flat top], domes[dome top pistons], dishes[dished pistons])? I bet that you could get an aftermarket grind from comp, crane, crower, lunati, or isky that would make more power, and would allow you to run pump gas. With the problems mentioned with some of the reformulated race fuels, it probably wouldn't be a bad idea to build a motor that would not have to have race gas. Most of our cars sit up at one time or another and who wants to deal with the hassle of rebuilding carbs or draining fuel tanks.

[Chevy454]

Fellas, I'm telling you, with the stock pistons, on a 34cc dome, you'd be *lucky* to see 28cc across the board (more likely gonna be 26-27cc each)...don't get wild on decking the block (a little is ok)...run the good Fel Pro .038" head gasket...and I bet you'll be right at 10:1, especially being we're talking 396s. Even better, go buy a cheap cc'ing kit (well under $100), and check it out for yourself...

I would recommend the Crane/GM blueprint '143' cam, as I believe it's a better cam and I'll just about bet it'll be cheaper...

We raced our stock spec L72 on pump gas all the time, except in the hottest part of the summer we'd add just a splash of race fuel...I'm not a fan of the octane booster stuff.

[Tommy]

Jason, the engine is for the 68. I talked with the builder today and we will be using the factory stuff. he was fixing to check the pistons out for wear and tear, the cylinders were good. I spoke with a guy at Crane today about the 143 cam and will be using it as well. Rob, thanks for the information you sent me http://www.yenko.net/ubbthreads/images/graemlins/scholar.gif.

Tommy http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[ORIGLS6]

Tommy,
That Clary kid might still be wet behind the ears,...... but he knows his stuff on these old engines! http://www.yenko.net/ubbthreads/images/graemlins/worship.gif

Gonna be interesting to follow the progress of a certain Red Chevelle now. http://www.yenko.net/ubbthreads/images/graemlins/naughty.gif