might actually make less power, less then optimal heat in the combustion chamber

 

The main argument I hear is more BTUs/lb in lower octane gas, but I've seen that argued both ways as well. Either way, I bet the effect on performance would be imperceptible.

The effect on the wallet on the other hand... I just spent $80 filling up my truck. Had to swipe the charge card twice!

 

OK, gotcha

 

I didn't say either one. What I did was TRY to 'say', it's obvious, you don't know chit from shinola, about WHAT the ECM does or WHEN it does WHATEVER it does, without hurting your little feelings.

THat's what I SHOULD have 'said', instead of trying to be politically correct and give you a little much needed insight into the technical aspects of what's going on under your hood (or dash) depending on where your ECM or PCM is located.

Please refrain from any additional technical discussions until you have developed the necessary neuron links to competently participate.

Really???? And what was your ORIGINAL question?
You actually asked about removing the heads and replacing them with lower compression ones...... over your concern about the 'quality' of fuel you may encounter...... and you BURN YOUR OWN CHIPS?????????????????????
Shame, shame, shame.

Ahhh... I feel better now. As they say, no good deed goes unpunished.

Notice: I'm not running fior ANY political office.

VetNutJim..over and out of here.

BTW... Scientist 'say' there are more BTU's contained in a specified amount of 87 octane fuel than in the same amount of 93 or 100 octane fuel.
Could THAT be the reason the performance of an engine that doesn't NEED the higher octane fuel may experience some measurable loss of performance while operated with the higher octane fuel?

As CentralCoaster said:
I you don't NEED those bigger pants it ain't going to do you a bit of good to wear them AND it may slow you down a little if you do.

 

Your bluntness is amusing. It's like an unmanned firehose.

 

Must all the floating around in your head!

Its clouding your judgment.....

 

Sometimes I wonder what is better between the two of these methods:

Use a lower compression ratio with a lot of spark advance (to create cylinder pressure)

Or

Use a higher compression ratio with a slight amount of spark advance...

Sounds like either of the two extremes would still have drawbacks...

 

Right on.
That's freakin' clever
---Thanks for the laughs---

 

Here is the first part of the question:

I have a 1990 L98 shortblock (which means there are no heads on it now) sitting in my garage and a choice to select 76cc heads or 58cc heads that are immediately available.

I was wondering if anyone has experience going to the bigger cc heads so that they could run regular gasoline? I never thought of running regular gas in my stock Corvette and was surprised to read some of the answers here.

I figured that with lower compression I would not risk damage to the engine running low octane fuel and the Corvette should still have enough power to keep up with traffic.

The stock 1990 block with 113 series 58cc heads was 9.5:1.

And here is the rest of the question:

I have five C4 Corvettes and I thought it would be interesting to have one that is built to run on regular gasoline. It could be driven routinely in Mexico as part of the baby boomer migration to Mexico - see my new/low traffic website http://peso.com that I setup so I could be the first to get leads on property for sale in Mexico.

The 1985 is my first choice to install it since there is no engine in it now, but it is certain that one of my other cars will eventually need a replacement engine and I could have one ready to go.

On my 1989 I have programmed customized 256K eproms that are double the size of stock and added a switch so I can select the upper or lower half of the eprom. - and this works well. I could do something similar on the 1985 but maybe it doesn't need any computer teak at all with lower compression.

 

A 256 with a switch! Now thats down right clever!

I use a PROMinator and change the bin as required. Alvin has done the tunes. Just can't seem to get the best fuel economy in the 1600-1900 RPM range. As soon as the engine is over 2200 the MPG holds at 23-4.

 

I started off with 256K eproms because they were cheaper than 128K and also available. After burning a few with identical images in the upper and lower halves I decided to try my luck at burning two different images.

I am using a 5.1K resistor to pull the upper address bit high to address one prom image. Putting a jumper on the connector above the little window brings the high address bit low and selects the other image. It works great.

 

Before trying this yourself note that I pulled the high address pin up so it would not go into the computer socket. It may cause bad stuff to happen if you short that pin out on the computer.

 

Hands down, the higher compression ratio wins.

It takes time for the A/F mixture to burn. Maximum cylinder pressure has to be reached at some (specifically unknown to me) narrow window of piston position, as it travels down the cylinder on the power stroke. The slower the A/F burns, the sooner you have to ignite it. The sooner you ignite it, the greater the time the burning A/F is pushing down on the piston while it is still rising on the compression stroke. People tend to discount the value of quench, when I try to tout it's importance. That's OK, it just makes my engines stack up better against those that don't get it. Engines with good quench have better combustion efficiency. Because the A/F burns faster, you need less spark advance, which reduces the pressure on the piston before TDC and it reduces the chances of knock. And as always in hot rodding, if some's good, more's better, and too much is just enough.

RACE ON!!!

 

I don't know the octane of the Mexican gas that will be available to you, but you can easily run low octane gas with a 9.5:1 compression ratio. Rather than putting together a combination that is an effort to reverse, later, just crank back the timing. I race my 1984 with a stock 9.00:1 compression ratio on 85 octane gasoline. I have my base timing cranked UP to 14° BTDC AND I have the knock sensor disconnected. I get no audible knock racing or daily driving. Between the stock ESC retarding the timing IF it hears knocking and the ability to lower your base timing, I don't see a need to build a super low compression engine.

You can easily do the math but cast iron 624 heads with 76 cc chambers, on the 1990 engine that only has a 9.5:1 C/R with 58 cc chambers, will result in about 7.5 - 8:1 compression ratio.

RACE ON!!!

 

So a low compression, high spark advance would create negative work and eventually reduce power... Hmmmm

 

The earlier you light the fire (more advance), the sooner (more time) pressure is exerted on the rising piston during the compression stroke. That is the wrong stroke for power production. This is a normal occurrence in all engines. When advancing the timing makes more power, the additional pressure of combustion produced after TDC is enough to overcome the additional pressure created before TDC. When more advance reduces power, the pressure before TDC is gaining faster than the pressure ATDC. Whatever you can do to reduce the timing required for maximum power, the better. A good measure of combustion efficiency is, the amount of advance needed for maximum power production. The less advance needed for max power, the more efficient the combustion.

RACE ON!!!

 

Hey Jim, Good to have you posting again...

So with your knock sensor unhooked. Does it throw you any codes? Also, when it come to quench, if I can remember correctly. Your talking thinner head gaskets, and perhaps milled heads to attain?

Best Regards, Tom...

 

 

No codes on the '84. I don't think you'll get a code on the '85 either.

The quench AREA is the flat portion of the head above the flat part of the piston away from any dish, dome, or valve notches. It is where the flat of the piston meets up with the flat of the head. The greater the area, the better. A circular dished piston rather than a "D cup" shaped dish, reduces that area. A heart combustion chamber in the head helps to make that area larger. The greater the area, the better. Regardless of size (area), what makes quench work is the tightest possible piston to head clearance. Piston deck clearance and gasket thickness combine to make up the all important "piston to head" clearance. Milling the head has no effect on that clearance. The tighter the piston to head clearance, without them actually hitting, the better. The effects of the quench area slowly diminishes as the quench distance increases, until at a clearance of approx .060", where the effects of quench all but disappear.

Your engine has a spec piston deck clearance of .025" with a block that measures 9.025" from the center of the main bearings to the deck surface. The .025" added to the .021" head gasket thickness your '85 was built with, gives you .046" squish (quench) as a "blueprint" dimension. I'll guarantee you that the deck height of your block is greater than the 9.025" spec. Combined with other tolerance variances, as built, the quench effect of your engine is weak, at best. Considering mass production, this isn't awful, but we CAN do better, on our own engines, for ourselves.

RACE ON!!!