Chevy Nova Forum banner

1 - 14 of 14 Posts

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #1
I am preparing to assemble my '91 L98 TPI motor. The stock compression ratio was supposedly 9.3 to 1, and 91 octane was Chevrolet's recommended fuel. The block has been overbored .040 which I'm told will raise the compression ratio using the stock heads. The stock, cast iron, heads are supposed to have 64cc combustion chambers and the stock head gaskets were supposedly .028 thickness.

My goal is to have a high torque, more fuel efficient, motor. The predicament is that I want to use 87 octane fuel and for the engine to tolerate the 10% ethanol used in the midwest. I suspect, then, that I have to lower the compression ratio. I have been told that the stock L98 heads won't work, to lower the compression, because of the small combustion chambers. I've been told to look for heads with 74 cc heads to use 87 octane. Is this advice correct? If so, what Chevrolet heads should I look for? Keep in mind that Vortec heads won't work with my TPIS Big-Mouth TPI intake. I don't know that I can afford aftermarket aluminum heads but perhaps a replacement Chevrolet cast iron head.

Thanks for the assistance,
Ed
 

·
Registered
Joined
·
249 Posts
I am preparing to assemble my '91 L98 TPI motor. The stock compression ratio was supposedly 9.3 to 1, and 91 octane was Chevrolet's recommended fuel. The block has been overbored .040 which I'm told will raise the compression ratio using the stock heads. The stock, cast iron, heads are supposed to have 64cc combustion chambers and the stock head gaskets were supposedly .028 thickness.

My goal is to have a high torque, more fuel efficient, motor. The predicament is that I want to use 87 octane fuel and for the engine to tolerate the 10% ethanol used in the midwest. I suspect, then, that I have to lower the compression ratio. I have been told that the stock L98 heads won't work, to lower the compression, because of the small combustion chambers. I've been told to look for heads with 74 cc heads to use 87 octane. Is this advice correct? If so, what Chevrolet heads should I look for? Keep in mind that Vortec heads won't work with my TPIS Big-Mouth TPI intake. I don't know that I can afford aftermarket aluminum heads but perhaps a replacement Chevrolet cast iron head.

Thanks for the assistance,
Ed
74cc heads will give you a lower compression ratio than like a 64cc head, 74cc is usually used on block with low compressions, what pistons are u running, 4 valve relief, 2 valve, d cup dished? Whats the quench?, all that with the cylinder head choice will help determine what you are going to come up with
 

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #3
74cc heads will give you a lower compression ratio than like a 64cc head, 74cc is usually used on block with low compressions, what pistons are u running, 4 valve relief, 2 valve, d cup dished? Whats the quench?, all that with the cylinder head choice will help determine what you are going to come up with
Stock type, .040. over, flat top pistons with, what appears to be 4 valve relief, stock crankshaft, stock rods...and I have no idea what the quench is.
 

·
Super Moderator
Joined
·
12,005 Posts
The cost to modify the engine may not offset the extra cost of the 91 octane fuel. Is this a daily driver?
Since it has not been assembled the easiest way to lower the compression is to use a dished piston, the heads, stock gasket and existing quench will all remain the same.
With the information you have provided my calculations put the static compression 10.72:1 with the flat top and 64 cc head. A 12.5cc dished piston will drop the compression to 9.34:1 with the existing heads and stock gasket. This assumes the deck height is stock and the flat top pistons has a stock pin height.
IMO using the dished pistons offers more advantage than a larger chambered head.

My stock TPI motor had dished pistons from the factory and would run on 87 octane but lost power when the ECM retarded the timing due to detonation. Running 91 octane cost me about $3 more per fill up.

 

·
Super Moderator
Joined
·
12,005 Posts
That piston ate up a flat washer that was sucked in through the intake. If the MAF screens had not been cut out by a previous owner it would not have happened :mad:

After removing the pieces that were left the engine ran great, I drove it another 25k miles before selling the car.
For those that have not pulled the top of TPI motor apart you are in for treat if you ever do.
 

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #8
The cost to modify the engine may not offset the extra cost of the 91 octane fuel. Is this a daily driver?
Since it has not been assembled the easiest way to lower the compression is to use a dished piston, the heads, stock gasket and existing quench will all remain the same.
With the information you have provided my calculations put the static compression 10.72:1 with the flat top and 64 cc head. A 12.5cc dished piston will drop the compression to 9.34:1 with the existing heads and stock gasket. This assumes the deck height is stock and the flat top pistons has a stock pin height.
IMO using the dished pistons offers more advantage than a larger chambered head.

My stock TPI motor had dished pistons from the factory and would run on 87 octane but lost power when the ECM retarded the timing due to detonation. Running 91 octane cost me about $3 more per fill up.

Stock pin and deck height. I just reconfirmed that they are flat top pistons with 4 valve relief. As of today, 87 octane is 3.79 per gallon, but 91 octane is 4.69 per gallon. This car is destined to become a touring car. I've always wanted to travel Rte. 66, and other roads, with a classic.

Thanks for the info on the rise in compression. The stock TPI L98 heads have to reworked because a "certified mechanic" friend of mine got a little crazy with grinding on the valve seats, and now some of them have to be replaced. The cost to rebuild, with my new valves, is around $250.00. I'm trying to determine the best buy for the buck and still keep the compression around 8.7:1 to 9:1. Or at least that's what has been suggested.
 

·
Banned
Joined
·
26,231 Posts
that should tell you that you don't want him machining or working on any more of your components... friends are friends but machine work is machine work.
 

·
Super Moderator
Joined
·
12,005 Posts
The 8.7 is going to be hard to get with a flat top piston. If using a thicker gasket you risk losing power and mileage to prevent detonation.

I drive mine a lot. Just returned from an 1887 mile trip through California. With 10.5:1 static compression 91 octane or better is used. I averaged 18.7 mpg for the trip.
This was traveling at a little above posted speed limits with the AC on with luggage and heavy tool box. It also includes some WOT 120 + mph fun at the El Toro air strip and short low speed cruising with other cars.

Point I am trying to make is to consider all possible ways to increase your mpg and not just by detuning your engine. Compression, proper quench, cam choice, gears and transmission all should play a part of the equation.
I love the power my car has and it is still a pleasure to tour with. Given your gas prices my trip would have cost me an extra $100 in fuel between the price of 87 and 91 octane to drive nearly 1900 miles. For hundred bucks I would not give up 1 horsepower or shed 1 pound of the creature comforts.
 

·
Premium Member
Joined
·
2,161 Posts
Discussion Starter #13
The 8.7 is going to be hard to get with a flat top piston. If using a thicker gasket you risk losing power and mileage to prevent detonation.

I drive mine a lot. Just returned from an 1887 mile trip through California. With 10.5:1 static compression 91 octane or better is used. I averaged 18.7 mpg for the trip.
This was traveling at a little above posted speed limits with the AC on with luggage and heavy tool box. It also includes some WOT 120 + mph fun at the El Toro air strip and short low speed cruising with other cars.

Point I am trying to make is to consider all possible ways to increase your mpg and not just by detuning your engine. Compression, proper quench, cam choice, gears and transmission all should play a part of the equation.
I love the power my car has and it is still a pleasure to tour with. Given your gas prices my trip would have cost me an extra $100 in fuel between the price of 87 and 91 octane to drive nearly 1900 miles. For hundred bucks I would not give up 1 horsepower or shed 1 pound of the creature comforts.

Is there a simple way to measure, or calculate, quench?

I'm absolutely open to suggestions to create a fuel mileage motor. The advantage to a TPI motor is that is creates such great low end torque. I'd like to be able to obtain maximum fuel mileage at approximately 1800 to 2000 rpm, or around 65 mph. I am trying to avoid detonation by lowering the compression ratio, increased because of the overbore and stock 64cc combustion chambers. I thought that maybe the easiest way is a cylinder head with a larger combustion chamber, perhaps a 74cc or 76cc chamber.

This is intended to be a cross country cruiser with a good ride/handling characteristics, but otherwise not a high performance vehicle. I'm not disrespecting folks :no: who want higher compression motors for street, drag, or autocrossing. I have cars for that purpose but, for this one, I want a classic cruiser that obtains great fuel mileage (25-30) without a lot of concern about coefficient of drag...another factor of course because our cars are shoeboxes. :D
 

·
Super Moderator
Joined
·
12,005 Posts
Quench is the distance between the flat part of the piston and the flat part of the combustion chamber portion of the head including the gasket thickness and the distance the piston is from the top of the block at TDC.
Simply put if it is not within .040" - 050" power and economy can be lost. A low compression engine with to big of a quench area will be more prone to detonation than a high compression engine with proper quench.
Obviously the compression ratio needs to be kept within known parameters that will work with pump gasoline's. An aluminum cylinder head will allow a little more compression to be run over an iron head.
Your timing curve is controlled by the ECM so unless you are way off it should not detonate, downside is when the ECM retards timing due to detonation power and mileage both decrease.
If your computer controlled engine needs premium for normal driving when on long trips of freeway driving a lower grade fuel can be used. My son drove his LT1 powered Firebird to California and back. I suggested while on the freeway he use 87 octane and go back to 91 when in town. It worked well and increased his economy by 2 mpg.

Here is a link to an article written a few years ago. Has some good info on engine building including quench written by Steve Dulcich, the quench part is down towards the end of the article.

pump gas secrets
 
1 - 14 of 14 Posts
Top