Compression ratio for 91 octane

[donraye]

I have a burning ( no pun intended) question for someone who is knowledgeable out there.
I am building a 418 and need to make a piston choice. The choice is all about copmpression ratio and what I can go to and still use 91 octane pump gas without any detonation problems.
Someone on another web forum just wrote that with aluminum heads you can go 1 whole step higher than is possible with iron heads and still not get detonation. My questions are:
1: Is that true?
2: Considering I have some monster 210cc aluminum fully ported and chambered CNCed heads that flow pretty big numbers, so what is the maximum SAFE compression I can go to for a 91 octane motor without running into any detonation or other problems.
I do not really want to "push" the edge, but I do want to get as much power as I can for a stereet/pump gas motor.
I had always heard 10 to 10.5:1 was it, but now I hear I may be able to go 11:1 or even more. I'm an old fart so the 10:1 may have been the rule before aluminum heads were even out there.
What's the story with this? What are some of you builders going to these days with no problems.

[eschaider]

DonRaye,

I think you are about there with 10.5:1. 11:1 might be pushing the envelope. There is a early detection / prevention tool that not only detects the detonation but corrects for it in real time. If you don't use electronic engine management then it pulls spark from all cylinders simultaneously

The engineering side is just one step short of black magic but the performance is stunning. It uses all its wizardliness (is there such a word?) with electronic engine management. Without electronic engine management I understand the function is similar but it affects all cylinders the same. Basically what happens is when it first hears the sonic signature associated with detonation it pulls several degrees of timing (you can set the number). If the knock is detected on the next turn of the crank it pulls more timing. As soon as it hears no further knocking it begins to add timing until it detects detonation again. When it finds this point it backs off the timing just enough to prevent detonation and holds the mark until bad things happen again.

The website is click here => http://www.jandssafeguard.com/MarineRacing.html

Great insurance for the time you run low on fuel and the only thing available for the next 40 miles is a Farm Tractor fueling depot with 89 octane.

Ed (another old fart)

p.s. I may have miss-spoken at the beginning of this post. You ought to give John a call. As I was reading his page for the marine/racing unit it may not require an electronic engine management system to adjust timing on cylinder by cylinder basis. He is easy to talk to and will give you the straight story right out of the box.

This is the unit for individual coil direct ignition engines. It has more explanation, a testimonial link at the top of the page and some explanation of the theory of operation. Click here => http://www.jandssafeguard.com/Vampir...S_Vampire.html

[CoupedUp]

10.5. Or, you can ask someone who really knows - Keith Craft.
Doug

[Hotfingrs]

I'm running 11-1 on 94 octane, aluminum heads. I would suspect that with 91 octane somewhere in the 10-10 1/2 range would be the max.

[bobcowan]

What you're really interested in is cylinder pressures. You want to calculater Cold Cranking Pressure (AKA- Dynamic Compression). For pump gas, you want to keep the pressure below about 190psi or so.

Static compression ratio is part of the equation - but only part of it. You're also interested in intake valve closing point, altitude, and a couple of other things.

RB Racing has a nifty calculater for this. It's a harley site, but the calculater works for nearly any engine. http://www.rbracing-rsr.com/comprAdvHD.htm

A good racing cam will also increase cylinder pressures as the rpm's rise. Even though the engine will run pretty good on pump gas, you'll need true racing gas for the track.

Aluminum heads can withstand higher pressures because they're better at transferring heat through to the coolant. Excess heat will increase detonation.

[Tommy]

I learned a little about detonation after ruining the pistons in my engine. As has already been said, static compression ratio and octane are only parts of the equation. You also have to look at timing advance, air and engine temperature, and how hard you're running the engine (e.g., will you be taking it on the track?). After my rebuild, I went one step below the automated knock detection system described above. Because sidepipes made it impossible for me to hear detonation, I installed a MSD Knock Alert system that gives a visual and audible warning in case of detonation. I then adjusted distributor timing to where I thought it should be and went for a ride. It didn't allow me to optimize the timing constantly as would the automated system, but it did give me confidence that I wasn't experiencing detonation under ususal conditions.

[WildBill1965]

93 octane 10.5 427 Roush, I asked about the compression and they said ther were at the edge!

[donraye]

Thanks to all for your valuable input. I also ran this question on the FordMuscle.com tech forum and got some good advice there as well. Taking in the consensus view of those that had experience in this are I have made my choice which was somewhat limited as i had to get the piston that worked with the crank, cam, rods I alreadt had. etc.
I decided on Mahle froged flat tops with 6.6cc valve reliefs and all edges are factory radiused for detonation control.
This choice would give me 11.1:1 with the usual .040 gasket thickness, but I have decded to run with a copper gasket and to increase the compressed thickness to .064. This will bring the compression down and put me in the 10.6:1 range at which I feel much more comfortable.
Thanks to all for this discussion and your valued input.
I'll let you all know how it came out after the build and dyno tuning, etc.
Don

[Paul F]

Don,

Was the 418 a typo or did you change the build from the original 408? If you changed your plans, what influenced the change?

[vettestr]

Hey Don,
In the end you have to do what you feel good about and I appreciate that fact. Just a couple of items to throw into the mix for you. I ran copper gaskets for years on my supercharged engines but have been using Cometic layered gaskets with great luck and highly suggest you check them out. They will supply about any compressed thickness you need. I run them in an alum BBC with 11.72 compression on pump gas and 39 degrees total timing. I have a knock sensor to be sure but no pre-ignition problems at all.

[donraye]

Originally Posted by Paul F

Don,

Was the 418 a typo or did you change the build from the original 408? If you changed your plans, what influenced the change?

Hi Paul, Yes I changed the plan as the shop was having trouble getting the crank for the 408 and it would have delayed things for a while with no certain date for the arrival of their backorder. They had a 418 crank and offered to use that instead so I accepted the offer to avoid any further delays.
Don

[Paul F]

(Homer drooling sound) Mmmm. More cubic inches. Good stuff.

[donraye]

Originally Posted by vettestr

Hey Don,
In the end you have to do what you feel good about and I appreciate that fact. Just a couple of items to throw into the mix for you. I ran copper gaskets for years on my supercharged engines but have been using Cometic layered gaskets with great luck and highly suggest you check them out. They will supply about any compressed thickness you need. I run them in an alum BBC with 11.72 compression on pump gas and 39 degrees total timing. I have a knock sensor to be sure but no pre-ignition problems at all.

Interesting you mention this as I was actually going to use a thicker .064 head gasket to lower the compression down to around 10.6:1 but now I've been told that a thicker gasket might actually increase the chances of detonation as the engine tolerates detonation best in the .035 to .045 deck range. so the .040 gasket would be the best choice with my 0 decked block.
This is new information to me but the source sounded very knowledgeable and experienced so now I am probably sticking with the .040 gasket as originally planed or perhaps go to the cometic as you mentioned. but I think their gasket are .050 unless ordered custom. I'll check into that.
I was hioping to run one of the self ealing type gaskets that do not require any RTV or other goop, but now I have to re-think the whole deal.
does the Cometic require silicone around the water ports? What did you not like about the copper gaskets?

[map]

My old 84 RS 2.3 Turbo Capri street car, daily driver 22lb on pump gas (92) Boosted Compression Ratio is 19.68 : 1 at the track 25 to 28Lb (22.95 : 1) with a Qt of Torco accelerator... 12.30 bracket car..

[wtm442]

I've got a mild 351W (357 in3) with 10.5:1 CR and it runs like a dream with 89 octane. Yes, 89 octane.

[olddog]

Originally Posted by donraye

Interesting you mention this as I was actually going to use a thicker .064 head gasket to lower the compression down to around 10.6:1 but now I've been told that a thicker gasket might actually increase the chances of detonation as the engine tolerates detonation best in the .035 to .045 deck range. so the .040 gasket would be the best choice with my 0 decked block.
This is new information to me but the source sounded very knowledgeable and experienced so now I am probably sticking with the .040 gasket as originally planed or perhaps go to the cometic as you mentioned. but I think their gasket are .050 unless ordered custom. I'll check into that.
I was hioping to run one of the self ealing type gaskets that do not require any RTV or other goop, but now I have to re-think the whole deal.
does the Cometic require silicone around the water ports? What did you not like about the copper gaskets?

This is the squish/quench theory. There are quite a few believers out there that if you get the edges of the piston extreamily close to the head, that this squishes the air/fuel charge into the center at high velocities. All this turbulance mixes things up better. It is supposed to eliminate hot spots, as well. Therefore reduces tendency to detonate. They claim the piston to head clearance must be very tight for this to work. I think there is something to all this, but I do not understand it enough to know exactly what is needed or not.

[vettestr]

I used the copper head gaskets for years on blown engines because they were O-ringed and frequently taken apart. I would reuse the copper gaskets many times and they worked well from that application but also demanded higher maintenance or steps. Most of these engines were dry blocks but I have used copper with coolant in other applications too. You touched on one of my biggest gripes with copper and sealing of any coolant passage.

On an engine with coolant or a wet block with copper head gaskets chasing coolant leaks was just part of the joy of copper. Iron blocks with aluminum heads tend to scrub or work the gasket because of different expansion rates. Cometic gaskets don't need special sealers around coolant passages and they recommend no additional sealers at all be used. After a few heat cycles no leaks on my engines have shown up and they do not need to be re-torqued like copper. You can and I have re-used the Cometic gaskets with great results.

In terms of ideal piston to head clearance I like a minimum of .045" with steel rods anyway. I was trying to drop C/R without replacing the pistons so increased the piston to head as far as I could without trashing my valve train geometry. I was not deciding what parts to purchase but how far I could go or had too go so I could run pump gas on an existing engine.

[donraye]

Just so all who have been following this thread know waht happened, I have just oredered the parts and made my piston choice.
I' decided to go with a set of JE pistons that will give me 10.27:1 with a .039 head gasket.
These are unfortunately very expensive pistons at around $760+ a set as they are made to order but they are the ones that will give me the ratio I wanted so I felt that it was worth the price of admission since the other options were not good ones.
My other choices were to either put me in the 11.1:1 or the 9.3:1 ratios, and neither of those were what were wanted. One was to high for my comfort (and had a very high dymanic compression with the 6.20" rod length and my cam) and the other one at 9.3:1 was just plain "wimpy" in my opinion. So... it seemed a no brainer to go with the JE #194951 17cc inverted dome and have the correct 10.3ish compression ratio for what I wanted from this motor.
Thanks to all for your valuable input!

[fkemmerer]

Originally Posted by donraye

I have a burning ( no pun intended) question for someone who is knowledgeable out there.
I am building a 418 and need to make a piston choice. The choice is all about copmpression ratio and what I can go to and still use 91 octane pump gas without any detonation problems.
Someone on another web forum just wrote that with aluminum heads you can go 1 whole step higher than is possible with iron heads and still not get detonation. My questions are:
1: Is that true?
2: Considering I have some monster 210cc aluminum fully ported and chambered CNCed heads that flow pretty big numbers, so what is the maximum SAFE compression I can go to for a 91 octane motor without running into any detonation or other problems.
I do not really want to "push" the edge, but I do want to get as much power as I can for a stereet/pump gas motor.
I had always heard 10 to 10.5:1 was it, but now I hear I may be able to go 11:1 or even more. I'm an old fart so the 10:1 may have been the rule before aluminum heads were even out there.
What's the story with this? What are some of you builders going to these days with no problems.

Maximum static compression ratio is a complex choice that depends on many factors beyond just the octane rating of the fuel that the motor will use. In general, aluminum heads will allow some increase in static compression ratio with a given CAM and induction setup (this is because of the better cooling capabilities that most aluminum heads have as compared to OEM steel heads). The rule of thumb for this is 0.5 - 1 pt increase in static CR. What really matters in terms of detonation in a well built motor is the combination of ignition advance and dynamic cylinder pressure. Consider two otherwise identical motors with 10.5:1 static compression ratios. One has a CAM deisgned to produce lots of low-end torque with a modest amount of overlap and lift. The second motor has a CAM designed for peak HP at high RPM with an agressive amount of overalap and lift. Which motor is more likely to detonate? The first one because the limited overlap will produce much higher dynamic compression ratios at lower RPMs than the more radically CAM'ed motor. The best way to go about all of this is to first get a clear picture of what you want your motor to do (is it a streeter where lots of low end torque is more important or is it a race motor where peak HP is the primary goal?). Once you've done this, select the best cyclinder heads for your applications that you can afford (aftermarket OEM heads are a good choice here for lots of reasons), then call a good CAM grinder like Comp CAMs or Crane and tell them what you are doing, what heads you are using, what the other mods are planned for the motor, etc. and let them help you pick a good CAM for you application. When this is all done, ask them what the correct static CR for you application is and pick a good set of pistons to their spec. BTW, if you are building a performance motor and you can get 93 octane pump gas, its probably a good idea to build your plane about the higher octane rating. This will help the performance of your project a bit.

- Fred

[tmareina]

I have aluminum heads and 11.4-1 and run 91 Chevron. I do put a bottle of octaine boost or about 5 gals. of av. gas when I can. I also have a manual timing retard control but have never had a problem....yet. There have been a few times I have had to run just 91 with no octaine boost without a problem.

Tom