Hi,

I picked up a bird not too long ago. It is an iron headed 383 tpi setup that is 11:1. It has an EBL computer and on premium gas it has spark nock so I have been adding boostane to the tank to keep the knock counts down. My question is, if I retard timing enough, could 11:1 run on regular 87 octane gas? I do not really care about power and only bought the car because it was rust free and interior was mint. I would not have even minded if it was v6 tbh. The SA table in EBL has most of the timing between 10-24 anyway so it seems like a lot of timing is already.

 

I would run 93+ all the time pings are bad and without proper spark control and knock sensing you wont be able to get away with with 87

 

Not for long.

 

Saying it's 11:1 doesn't necessarily paint the entire picture. Cam timing plays a large part in the octane requirements.
The results of a compression test will give an indication of what's needed and whether pulling timing is a workable solution.

 

If it's detonating on premium it's not going to run on 87 no matter how much timing you pull. Remember that pulling timing also increases exhaust gas temps and a hot cylinder can suffer from pre-igntion as well. You can cause rolling detonation if you pull too much timing and stay in the throttle - detonation, once started, tends to increase and if pulling 5 degrees out doesn't stop it then pulling more won't stop it either - it will actually make it worse.

Drop the compression. 11:1 on iron heads without a nice big cam isn't going to play well. And increased compression (in the range of a few points from say 9 to 11) isn't going to dramatically increase power output. My 350 at 8.5:1 with AFR aluminium heads still puts down 300 at the tires. Compression was chosen for supercharging but as yet haven't installed the charger. Tuning it at 8.5:1 is just a REALLY pleasant experience - it takes all the timing you can throw at it (I've had it over 40) and it refuses to detonate regardless of the octane of fuel I have used.

This happens all the time - people build high compression engines and wonder why the beat the bearings out of the bottom end and have no end of trouble tuning it due to pinging in hot weather, etc. IMO it's largely more trouble than it's worth for a street engine. You can make PLENTY of power without going nuts on the static compression ratio. Displacement, good quench, and proper modern cylinder heads and roller cams go a LONG way now without stupid high compression ratios.

GD

 

Than you for the information. I think this summer I will do swap to aluminum heads and lower the compression to 8.5:1 like you. The car used to run nitrous and the system (except for wiring and switches) were removed prior to the sale. I know the previous owner said the cam was picked for nitrous, would I need to change the cam as well if lowering the compression?

 

Nitrous and Super/Turbo charging have similar cam suggestions. Will still work fine with NA. I have a Comp 276/288 Nitrous/Blower cam in my 350 along with the 8.5 compression and it works nicely for NA also.

Just make sure you get the quench less than 0.065" and lower the compression. If you aren't going to run nitrous or a charger then I would target about 9.5 compression. Will be easy to tune with aluminium heads. If you plan on forced induction later then 8.5 is a good place to be.

GD

 

install alumnium heads and drop the compression to 8.5:1? madness. this thread is full of misinformation. if you don't care about power get a stock engine, install it and be done with it. chances are whoever built your engine didn't plan it very well and ignored the things that every one tells you not to worry about. this is probably why it detonates vs the static compression ratio. And as for keep throwing timing at it and it doesn't detonate, 40* WOT timing? what is this the 60s? i recently built a 10:1 355 with TBI heads, 26* all in timing the less advance to make the most power tells you how efficient your engine/cylinder heads are.

 

You need to slow down and read. I recommended he target 9.5 if he's not planning a power adder. I'm at 8.5 in order to run a ProCharger at 15 to 18 psi. And I'm still making 300 RWHP on my DynoJet with 8.5 and no Procharger (yet). Adding compression, in and of itself, doesn't actually add as much power as you and many others think it does. Here's a chart for reference:



So the entire span from 8:1 to 10:1 is only good for about 3.7% of total power and efficiency. So when talking about a 350 HP 350, the difference in power is.... 13 HP. Nothing you'll be able to feel I assure you.

I said I *could* run 40 degrees. Tried it on the dyno to see where the engine made best power. It didn't complain but best power was around 34-36 degrees - and I'm not running old, inefficient heads - these are the latest CNC AFR 195's. Higher than that didn't make any additional power. I would guess that you could likely make more power if your compression wasn't so high and you could tolerate more timing. 26 is low for a SBC with stock iron heads. Put some C16 in it and stick it on the dyno and find out where it actually makes best power if you have the octane to pull through. You are right that timing is a crutch for inefficiency, but in this case I highly doubt that your 10:1 is beneficial. The amount of power you would likely gain from dropping the CR half a point and increasing the timing is a lot more than the 0.5% theoretical gain from the higher CR.

GD

 

Funny how people are willing to take their engine apart but not do something as simple as a compression test. Maybe you don't have 11:1 and the detonation issue lies elsewhere.
As for the current "nitrous" cam being suitable, there's no way of saying without knowing what it is. The cam and compression ratio are joined at the hip. Th real advantage of increased compression is how it works with a "bigger" cam. That's where the power increase comes.

 

It's all about dynamic compression ratio.

As regards power adders though - cams don't even matter that much (within reason) once you start shoving air into the engine. Same with porting, and all that business. Every little bit counts but it counts a lot less with a big blower.

And absolutely should do a compression test to see what the static is if you aren't sure (I was under the impression that OP knows it's 11:1).

GD

 

I can appreciate the forced induction perspective on cam selection. Take a totally stock engine and stuff 15 PSI into it and double your power more or less right? The idea of a centrifugal blower has always appealed to me. I'm not a turbo guy.

As for the 11:1, I kind of interpreted the engine being new to the OP and very little is known other than what's been related by the previous owner.

For the record, we know a compression test isn't going to tell what the static compression is obviously. But as mentioned, the test will tell you something about the dynamic/running compression. If that engine cranks over at 140 PSI, then there's something a little off. On the other hand, if it has 200 PSI cranking compression, then that's part of your premium fuel requirement and high knock count right there.

 

Knock counts don’t necessarily mean its knocking or detonating... they could be false, they could be inaccurate being a 383 vs 305-350 orginially designed for

i wouldnt rely solely on that but find out what the motor cranks for compression. Go from there

timing of 10-24 is not alot so without knowing whats in the tune and if there are any other timing adders i would say that doesnt seem right lol.

but i wouldnt like the idea of 87 oct on any performance motor.

 

It's actually the opposite of that. Air flow gains that help n/a are even multiplied more with boost. Everything I do to my engine would yield even more gains if I was boosted.

 

While somewhat true if you are working towards absolute max HP, that's not the way it ends up working. What happens in most cases is someone sticks a blower or turbo on it, you turn up the boost till you either can't get any traction, or are in fear for you life or fear of driving over the crankshaft and all those minor details like porting and polishing fade away from your mind since they won't make a ton of difference either way and you already have more power than you can put down or handle on several levels. And when it comes time to need more power - you just get bigger components instead. Forced induction makes EASY power. Tens or hundreds of hours spent porting out old heads is just a waste.

We find that gains with cams are usually limited as well. We have done before/after comparison's with cam upgrades on turbocharged engines and seen silly numbers - like 8 HP on a 450 wheel HP car. At some point you are limited by displacement and boost pressure. And those are where you go for more power. E85 and more boost, etc. Much easier than trying to completely optimize internals while staying with the same displacement.

GD

 

motor was broken in on the dyno. after much convincing the operator, he dialed it back 2* at a time from 32* and made the most power @ 26*. 191 heads pocket ported (no gasket matching, or port enlarging) small 214/217 @ 0.050" 112 LSA cam. quench is 0.034". 91 octane on the dyno. you should build the same engine on 8:1 and let me know how much of a turd it is, how much fuel it uses and how much it detonates.

 

My engine is a 350 (not a 355) and makes that much power at the tires at 8.5:1 with 34 degrees through a 700R. I drive it daily. My dyno graphs (from MY dyno at MY shop) are published on this forum. You are leaving a lot on the table with that high of compression and the mild cam.

GD

 

Do you have the SCR and cranking compression for this engine?

 

SCR is 10.3:1, haven't checked cranking pressure. motor was built for a truck, with towing in mind. i also need to keep it relatively stock appearing for insurance purposes (cheaper), so after market heads and induction are out. i used the "door stop" heads that everyone on the internet writes off as garbage. just to be different and prove that they're not total junk with a little work and the right application.it's not always about peak numbers.

 

A 10.3:1 compression ratio IS NOT a 11:1 compression ratio.. A test at 10.3:1 cr is NOT evidence that a 11:1 cr is usable without detonation..
Ya the SWIRL PORT GM Sbc heads do run best with modest spark timing.. The very high swirl helps this but the very high swirl also really clips off high rpm airflow and power... Makes good low mid range torque but at the expence of top end power above 4000 rpm.

The 305 HO TPI hesds (081 and 4416) can be ported to flow a lot more than stock as cast port flow with good swirl and combustion efficientcy with the right work.. These (with full home hand porting) will ultimatly out power the TBI swirl port heads althou the TBI heads do make very good torque...
You only need a hair too much SCR to have a pinging mess that won't last long.. Keep it UNDER 11:1 CR especially with cast iron heads and block.
Again 10.3:1 cr is Not 11:1 cr.. Its not evidence that 11:1 is workable..on your pump gas.
A quick dyno test with cool air feeding the motor is not true evidence that yiu can do this in car at much higher air inlet temps in a typical car under typical (less than ideal) operating conditions... You can get away with a lot on a quick dyno test with good cool air for a brief running time.
Not to mention running 26° spark advance.

 

i havent said anything like that. my point was that people often go the wrong way in compression when building an engine (forced induction aside). if you use efficient heads, tight quench, etc you will be rewarded with an engine that makes more power, uses less fuel and has more protection agaisnt detonation. alot of internet lore IMO and even from the guy that has a dyno. did a trip with 2000# payload up the Rocky mountains in the middle of the summer, no detonation. granted it's not towing a trailer. tear down of the engine at the end of summer, found no signs of any internal damage. if i read everything on the internet a d believed it the thing should be scrap.

 

325 at the crank on a 383 is not very spectacular. Any decent 383 build ought to be in the 400 HP neighborhood at the crankshaft. 350 at the tires is pretty much what we expect from a 383 with decent fuel and proper timing. I'm sure you are limited by heads and cam, but also - have you actually tried a slightly lower compression and more timing? It's excellent that you are happy with the engine but it's just a statistical sampling of one unit. I've had quite a few 350 and 383 engines on my dyno and 323 HP at the crankshaft would translate to about 260 at the tires with drivetrain losses. My 350 at 8.5:1 without the supercharger it was built for is running about 325 at the tires through an auto. That's about 390 at the crank with less displacement.

Again - static compression DOES NOT make power (in this range). Airflow makes power. I stand by the graph above and the dyno results I've experienced.

GD

 

it's not a 383

 

Compression does affect power, though. In a direct A:B test you could find that increasing compression will increase power output across the entire curve 2-4 percent, and that's without optimizing valve timing for the new compression ratio. Anytime you can cram more air and fuel into the cylinder you'll create more power (I mean, that's the whole premise of boost). On the flip side, if you have to retard WOT timing because your timing events aren't optimized for the compression you're running, you're going backwards (NA, anyhow). Can you make power at low compression? Sure. But with everything else being equal, you'd make more power with more compression. And that's okay for some people - they like running cheap gas with a small sacrifice to power. Some others aren't real thrilled with the idea.

OP - I would agree with Orr. Make sure it's actually knocking before you spend a bunch of money you don't need to spend. Do a little research. Check your plugs, check your cranking compression, check your air:fuel. All of these things play a part.

 

Going back to the OP's original question, I don't think there's a rat's chance in hell of running 87 octane with 11:1 compression unless the intake valve closing event was really late, which probably is not the case. If the engine is telling you it wants high octane fuel then you better give it high octane fuel.