EddieJ wrote the following in a different thread. I would like to branch this in another direction, but didn't want to hijack the thread.


"Matt it's hard to say exactly how much compression a certain octane will allow. It has a lot to do with the quench of the piston to head, the shape of the combustion chamber in the head, the maximum advance on the timing, and the timing/duration of the cam. All of these things work together in controling detonation.

If your not taking any of the above into consideration, I would say that the safe comperssion ratio on 93 octane would be 9.5:1 on iron heads or 10:1 on aluminum head.

That being said, if you do your research and take all of the above and build for high compression you can get away with a lot. I'm running 11.8:1 on the street with 93 octane. It runs great and gives me no problems. I do have a .031 quench, alum trick flow heads with a good combustion chamber, I limit the advance to 32*, and have a cam with a long duration. Some guys run higher compression than me. It's all in how you set it up"


I have an L-88 spec, 427 with ~12:1 compression ratio. This currently requires ~100 octane to avoid pinging. This car is used for street applications and I am looking at ways to reduce the octane dependancy and possibly increase power in the 3-5K RPM range. I know that this design was meant for revving and power generation in the 6-7K range

I have spoken with Isky about a custom grind and am wondering about other options here. Do you think retarding timing alone would reduce my need for octane? Or will I need some other changes, like a cam. I have the 074 alum heads and and LS-6 low-rise, dual plane intake.

Thanks

 

Run E-85 in it.

BigBlockk

Later.....

 

does it pull in cold air?
how much advance now?

 

u want a new cam and pistons.
but,
it would be fun to make it run good on 93(but only a 50/50 chance of success)
after the usual tricks, i'd get a knock sonsor and use it to trigger water/methonal spray.
it would self medicate.

 

It does not pull cold air now. I am trying to figure that one out also. Not a lot of room to work with here. I have the timing set at 36* all in ,with another 12 coming from Vac Adv. (it idles much better with the extra advance, I think because of the big cam - I was only getting about 5" of vac. prior to this)

What are the "usual tricks?" I am ok changing the cam for now, but would rather not change the pistons until I ready to make permanent decisions on the potential rebuild (I am still considering leaving it the way it is).

 

You have 12:1 cr then look for a cam with the correct intake closing angle to get DCR to 8.5:1 or lower and you can run 93 octane. You probably need cam in 250+ deg at .050" range ( in a solid roller cam )

....and don't run a vacuum advance, don't think L88's did in the first place anyway but not sure on that

 

If you have a L/88 cam I see that Crane makes a blueprint
of it, 264/269 at .050, 560/580 valve lift, 112 LSA power
range 4400-7200.

4400 on up power range around town most likely miserable
with street gearing.

If the combo of 264 at .050 int. dur., aluminum heads is not
calming that 12.1 comp. down, changing to less duration cam
with the same pistons will just make your problem worse.

Big chamber big block heads are going to need a dome to
maintain any good compression. The combo of big cylinders,
dome pistons makes it more prone to problems then
a stroker small block with flat surface pistons that has good comp.

I would put up with it the way it is untill you can afford to
rebuild it with smaller dome pistons and a lower duration
more street gearing friendly cam in it.

 

i have the l88 hood, the cowl cold air was worth 4 octane for me, from 91 to 87.
so hooking up the hood is #1.
.
what is the cranking compression?

 

You can rebuild what you have, build something else that is more street friendly and sel the L88 spec motor, buy a new crate or run higher octane. With 12 to 1 and dome pistons you will have to raise the cam duration to the point where it will be pretty much unstreetable to run on 93. Getting rid of the vacumn advance might help but the DCR is already built into the motor. I would run 110. and drive it hard.

 

Plugging up the vacuum advance helped my 10.5 to 1 engine. I also mix about 20% 110 leaded racing fuel with pump 91 or 93. Makes finding fuel a bit easier. If I could change to aluminum heads I could probably run just pump gas.

 

lr172, Controlling detonation is all about controling heat. The L-88's were amazing engines for the day, but technology has improved greatly since then. The shape of the pistons and the shape of the combustion chamber of the heads are what's killin' you.

That being said, there are other ways to control detonation, the tricks as some call it. Reducing as much heat as possible will help you but I don't konw if it can stop the detonation of an L-88 on 93 octane.

Try these first......

Make sure the cowl hood is sealed properly and pulling only cold air

Larger Aluminum radiator- pulls more heat than copper/brass

Cooler thermostat- 165 range with the large flow thru openings

Wrap the headers- will shorten their effective live but will take a lot of heat out from under the hood

Richen the carb a little- not enough to foul the plugs but enough to color them a little- cools the combustion process

Run a cooler spark plug- drop down a couple of heat ranges

Back the total timing down a couple of degrees- lower will stop the pinging but will affect performance- try to find a happy medium

Run octane booster

Then the more drastic solutions.....

Change the heads to moddern aluminum performance heads

Change the pistons- can get any compression ratio you want out of this one

Change the rear end gears to a taller gear- i.e..3.73, 4.11- the motor won't have to work as hard and will produce less heat.

Be aware that some or all of these changes will reduce performance. Some more that others, but may get you what you are looking for.

Good luck

 

I will take my shot at this. I dont think you will ever make a 12 to 1 engine run on pump gas without pulling a ton of timing out, and dealing with the crappy performance that comes from that. A cam wont fix it.
The cheapest way that you can correct this will be to pull the pistons and have a machine shop mill some material off of your domes. About 10cc removed will get you down around 11 to 1, which should be doable with aluminum heads. After you get the compression down, then a cam change will be in order.
I would suggest something like this. http://store.summitracing.com/partde...1&autoview=sku
Its got enough seat timing to bleed off a little cylinder pressure at low RPMs, and should be a lot stronger down low than your current cam. With this combo, you should be able to run an acceptable amount of timing without worrying about detonation. My .02 .

 

1st i want to see cranking psi.
i doubt it has 12:1
it is a "clone"
it could be anything

 

Thanks for all of the ideas and thoughtfull guidance here.

The cranking comp was 160-170. I thought this was low for the 12:1 CR. However, in speaking with the cam folks, they indicated that the agressive cam would show that type of cranking compression readings, as it does not build up dynamic compression significantly until higher RPM's. I have seen pictures of the actual L-88 heads and I am pretty confident that the dual springs and pushrods are stock GM also. I have not seen the pistons, but all other components have "checked out" to be original.

I have found a source for lead additive to bring the octane up to 100 for only about $1.50 a gallon. This is pallatable over the $6.50 I was paying for racing fuel. It seems that any hope of getting to run on pump gas will be too much of a sacrifice. I guess I will continue the 100 routine and look for ways to increase the low end power.

Maybe I can sell it for enough to build a more streetable Big Block.

Any thoughts on a new cam that would sacrifice some top end for low to mid-RPM power. In speaking with Isky, they indicated that a custom grind could be made to increase the low end power, even with the 12:1 CR. They did indicate that I would lose quite a bit of top end though.

Tough choices here, but it is hard to leverage power between 4-7K around town.
main

 

A few points

1)The cranking compression is that low because your cam is HUGE, especially the seat timing. By the way, your DCR calculates to be pretty good for pump gas, but it doesnt work, does it?

2) The only way to increase power down low is going to be shortening the cam duration which will make the engine even more likely to detonate without race fuel.

3) As long as you run that low rise intake you will never make good upper rpm power anyway, so you are much better off with a smaller cam & less compression on the street.

4) Selling a used engine for enough money to build a new one is very unusual unless you had a real L88.

 

A couple of questions:

Can I add a second head gasket or a bigger head gasket to reduce the compression ratio?

Can I replace the cam with a shorter duration, and thereby move the power curve down, without reducing compression if I still run racing gas.

Must I reduce the compression if I go with a shorter duration cam?

I think it is a real L-88 based upon the history I got. Can I get the casting numbers off the Pistons and crank by just pulling the pan with the engine in the car? (so far I have confirmed the block, the heads, valve springs and flywheel. The intake is an LS-6).

Thanks for everyone's assistance here. I am anxious to finally get a plan in place for this engine as I get nearer to completing the paint job.

 

I've been thinking a bit more here. The 454 LS-6 used the same head and manifold that I have and it produced 465 HP (albeit with a few more cubes) and I thought that was without racing fuel. Can I change the pistons and cam to get a respectable power level down around 4-5K?

Does this make more sense than trying to make this work with the configuration that I have now?

I really appreciate you guys helping out a new enthusiast.

 

gkull has 11.8:1 and gets 17.4 mpg. and 600hp and a smaller cam.
so, 12:1 doesn't scare me one bit.
and 160-170psi is so low it could idle on 87.
so if i had this car, i'd be tuning it and ignore the naysayers.
i like the L88 cam, but to make it more streetable i'd advance it and re-check the cranking psi. If that brings it up to 180-200 we are in business. And i'd want to drive it right away, so i'd store the hood for now, so it gets cold air, and will be easy to work on. Next i'd weld up the cut-down divider in the dual plane. that will bring back gobs of TQ and better MPG. It would really be fun to run an L-88!

 

I assumed you wanted to get rid of the problem of buying high dollar
high octain fuel or mixing fuel, would like to get where you could just
pull up to the pump buy gas. I ran 12.0-12.5 comp. with iron heads
in both small and big block with cams ranging in the 254 to 260s
.050 int. dur. but that was a long time ago when you just pulled
up to the pump bought 100-102 gas. 1970 454/450 HP chevelles
with 11.0 domed pistons, closed chamber iron heads, fairly small
solid lifter cam, just bought fuel at the pump. chevy did build a
454, 425 hp, 475 ft lb torque LS6 1971 vette with aluminum open
chamber heads, mild soild lifter cam, but it only had 9.0 comp.
If your still willing to run high octain fuel, buy a wider LSA 114
cam with 245/250 .050 dur. modern cam with high lift. that will
drop your power take off range down to 3200/3500 rpm, but still
give you better mid range power and keep some of the high end HP.
4400 on up power range with street gearing not a good plan, unless
you just want your car for a freeway flyer or road racing.

 

My intention here was to find a way to tame this thing in a way to run pump gas. However, it is beginning to seem that may not be an acievable goal and still keep the power I would like to have I can stomach the octane cost if that is the only way to maintain performance.

so it seems like my options are:

1) keep running 100 (or whatever blend of octane keeps me from pinging) and reduce the size of the cam to bring my power range down to a more realistic level for the street. It sounds like I would have quite a few options here to adjust things. It also sounds like the LS-6 manifold would work well here. Is this a correct assumption?

2) Find a way to reduce compression through new pistons or shaving the existing pistons and then replace the cam. This would allow me to get power down where I want it AND run 93. This may be better in the long run. QUESTION: Will this approach net less power than option 1?

3) Get an L-88 hood and a hi-rise single plane and run like hell. I have 3.73 gears and a 4 speed Super T-10. I just don't have enough experience to know how this will be on the street. Seems I would be leaving a lot of rubber at each green light to get into the power range.

Thanks for all of your support here. I don't know what I would do without you guys.