Bdeliman

I'm building a 350, LT1 with stock bore & stroke and stock LT1 dome pistons. If I use:

.027 Cometic MLS Head Gasket, giving quench area .051
Aluminum 64cc heads

Gives:

Static Compression 10.9:1
Dynamic Compression 8.9:1

Is this a little too strong to run on premium pump gas? I've heard numbers like 8.5 and 9.0 for a max on the dynamic compression ratio for running on premium . I could lower the compression ratios a bit by going to a thicker MLS head gasket, or more by going to 67cc heads. What experience have you had? Thanks in advance.

Bill D.

GCD1962

I went with Dart .72 cc heads. No problems with regular high test 93 octane unleaded, 10% ethanol

Jims66

Mine is 11.1:1 and I've been running 93 pump premium with no problems since we built the motor back in 1990. Only thing I did was increase jets (2 sizes) a few years back when they added ethanol to the mix as it started running a bit lean. You'll be fine.

devildog

My DART 400 with 64 cc aluminum heads is right at 10.9 and run OK on 93. I have mixed 50/50 91 & 93 and run OK

I run about 14 A/F ratio.

Joe

Bdeliman

There is a big difference between running a cam like the Duntov 30-30, which has the intake valve close very early after BDC and the LT1 cam, which closes much later after BDC. Thus the Dynamic Compression ratio. A cam with a late close of the intake valve may run fine, but a cam with an early close may ping like mad. I'm trying to find out how late can you close the intake valve, the resultant dynamic compression value, and still run premium gas without ping.

Bill D.

427Hotrod

There is so much to dynamic compression ratio calculations that the little tools just can't predict. For example...the biggest is if you have a real good flowing set of heads with a decent intake/exhaust setups...eventually those cylinders are going to get real full if things are working well. If the heads are poor flowing, they will never fill as much and you'll get by with more dynamic compression. Think of the old 9.0 compression Busch motors. Incredible heads and cams that were fairly healthy. You better have the best fuel in the tank or it's going to detonate like crazy due to the incredible efficiency. Not the same for our stuff..but the concept is.

You can do a lot to control detonation by running a tight quench, lowering coolant temps, no exhaust crossovers, cool intake air etc etc.

I can tell you that you'll be on the edge IMHO with the LT-1 cam but it can surely be done with proper tuning and 93 octane. Which heads are they? The chamber design makes a big difference, but overall I wouldn't be scared of it.

JIM

Bdeliman

I'm not actually running the LT1 cam. I'm running a comp cam that has the intake valve close around 60 degrees ABDC. It is a little milder than the LT1 cam, made for a lower RMP range, with .500 lift with 1.5 rockers. If I use 64cc heads, it gives me a DCR of 8.9, but if I use 67cc heads, it gives me 8.5. I'd like to use the 64cc heads, but I'm afraid it would have more of a chance to ping than with the 67cc. I was just trying to see what some folks are running out there around the 8.5 - 8.9 range and their experience with ping on premium gas.

Scott Marzahl

I think you are going to have trouble. Is that intake valve closing spec at .050" or .020" for the comp cam. Doesn't the LT1 intake closes closer to 73+ degrees @.020" because at .050" the intake closes at 51 ABDC and it has much slower ramps than the typical comp cam which allows for more bleed off. A few 327s here running LT1 cams are around 10.5:1.

JohnZ

Actually, the "30-30" intake valve closes LATER than the LT-1 (and later than any other factory-installed small-block cam); that's why a "30-30" will run with no problem on pump premium and 11:1 static compression - it bleeds off cylinder pressure at low-to-medium rpm, reducing the dynamic compression ratio. If you took a '64-'65 L-76 or '67-'69 Z/28 (all had 11:1 and the "30-30") and yanked the cam and replaced it with a base-engine hydraulic cam, it'd ping like a coffee can full of marbles.

Scott Marzahl

I show that the 30-30 intake closes at 59 ABDC @ .050", 8 degrees later than the LT1 intake. The exhaust lobes of both cams are the same.

Bdeliman

I have to check, but I think my cam closes at 60 degrees at .015. I don't know what it is at .050. I'll have to degree again.

wmf62

just chiming in... my 70 LT1 crate engine would run on 93; but, depending on timing, it could rattle like John's 'can of marbles'.....
Bill

65tripleblack

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These are relative values for a 327 engine. A 350, by virtue of its .23" longer stroke, will develop slightly higher DCR's for the same SCR's when compared to a 327.

65tripleblack

When calculating DCR you must use advertised (J604D) inlet valve closing, not 0.050 closing.

I recommend no more than about 8.5:1 DCR. Reason for this is that you have some margin to run a VERY aggressive timing map. Set your initial spark advance to 20 degrees BTDC, with 18 centrifugal all-in by 2500. Run your vacuum advance limited to 14 degrees rather than 17.5.

Get your quench down to between .038 (conservative) and .032 (aggressive, and do this only if your connecting rods are strong).

Block the crossovers in the heads.

427Hotrod

First thanks for those calculations TripleBlack......

To me...they show exactly why I don't put a whole lot of faith in the average dynamic compression calculation. Without some serious timing and fuel control, the average honest 12.3 static compression ratio 327 just isn't going to run around at WOT with a 30/30 cam on pump gas. 11.9 is going to be pretty tough for an LT-1. Now for sure there are some LS type deals doing it and even some LT-1's with fast burn heads and good timng control....but that's not what most folks have. Throw on a nice set of aluminum heads with killer ports and watch the RPM it starts to detonate drastically get lowered.

I've run around on pump gas with honest 13.5 compression motors...but only part throttle and with much larger cams than we're talking about here.

I've got an honest 11.56 static in the current version of my 555" with a 266*@.050" lobe and a 112 LSA. It does fine on 93 octane...but there isn't a whole lot of cushion left. That's with a .032" quench, cool temps, tuned timing etc etc.

In addition to the earlier closing of your cam than the LT-1...you also need to consider that it likely has faster ramps and is going to do just what it is supposed to do...fill the cylinders better at all speeds....what do you think that's going to do to the dynamic compression ratio calculations??

JIM

lawdawg

Ok,
I need help with a cam. Here is what I have:
327 bored 30
12:1. Trw pistons
Angle plug 461 heads
Will be running a fuel unit

So, this is just a stock 65 fuel motor.

To run on pump gas, do I use a 30-30 or an 097??

lawdawg

Or others?

JohnZ

At 12:1 SCR, you're going to have a problem with any of them, but the "30-30" will be the least sensitive to detonation of the group; you're going to need to pull some timing out of it and/or run a race fuel blend.

65tripleblack

Post the TRW number of your pistons and/or the piston dome volume. Also, post whether your block has been decked and/or the deck clearance. Also post the measured chamber volume if your heads have been milled. Finally, post the head gasket bore size and compressed thickness and/or the brand and number.

From this I will calculate your engine's actual SCR. Once we have that, you have to decide whether you want a GM cam or an aftermarket, a flat tappet or a roller, and a hydraulic or a solid.

With any cam you use, you can retard its timing rather than the spark timing in order to decrease its DCR. This generally has less impact on drivability than retarding the spark timing.

lawdawg

7000p on the pistons. i think these are replacements for 375hp pistons
no decking on block
no milling on heads - THESE ARE 492 ANGLE PLUG HEADS

I DO HAVE SOME 461'S I COULD USE IF IT MAKES A DIFFERENCE.

i will use whatever gaskets I need to get the compression down some. Please offer some recommendations!

Cam and lifters are unknown in the engine, so i am automatically replacing those. I am open to suggestions there too, just remember that i am running a fuel injection unit with this and need a pretty smooth vaccum signal for the unit.

Is going to 76cc heads another option?