???'s about compression ratios

[lilrebel68]

Hey guys, does anyone know what is the highest "safe" compression ratio for use with 89 octane gas that can be done on a 3.1 engine?? would it be better to try to do a "stroked" rebuild, or just get the bump in compression ratio from new pistons???

which camshaft would get the best performance from the motor??

I know I cant mention fuel efficiency when talking about performance, but if I just wanted the highest compression ratio to get the best fuel efficiency, what ratio would I be shooting at??

I am not going to supercharge or turbo this motor, but it would be nice to have a solidly efficient naturally aspirated engine under the hood.

I already am planning the switch to the 3400 ported intake and 3400 heads,
I just want to make sure I have a damn good bottom end to go with that..

thanks in advance for all the help..

[3400-95-Modified]

no more than 9.8:1 the 3500's are that and I think they can get away with 89 on 9.8 because of the design of the combustion chamber...

[Joseph Upson]

Hey guys, does anyone know what is the highest "safe" compression ratio for use with 89 octane gas that can be done on a 3.1 engine?? would it be better to try to do a "stroked" rebuild, or just get the bump in compression ratio from new pistons???

which camshaft would get the best performance from the motor??

I know I cant mention fuel efficiency when talking about performance, but if I just wanted the highest compression ratio to get the best fuel efficiency, what ratio would I be shooting at??

I am not going to supercharge or turbo this motor, but it would be nice to have a solidly efficient naturally aspirated engine under the hood.

I already am planning the switch to the 3400 ported intake and 3400 heads,
I just want to make sure I have a damn good bottom end to go with that..

thanks in advance for all the help..

You should really consider talking with a camshaft manufacturer since you are considering a compression increase and camshaft change together. The reason is due to the fact that compression ratio and cylinder pressure work together to produce the actual compression the engine sees.

For example, you can build your engine for 11:1 compression and then add a long duration camshaft, your cylinder compression will not be the equivalent of 11:1 at low speed rpm because the long duration will allow intake air to leak out into the intake before the intake valve closes, that's why long duration cams typically have low vacuum at idle, which means the power curve is shifted in the higher rpm range where the valve timing vs airflow starts to even out with rpm and air trapping becomes more optimized, the very reason that VVT makes a big difference.

If you plan this properly with the help of some calculations and guidance from anyone with access to the proper info on the best combination characteristics such as camshaft manufacturers I see no reason at all for you not to be able to use 10:1 compression on regular gas. The non direct injected 3.6L does it with 10.2:1 compression and although there is no comparison between the two engines the theory is the same.

If you keep your engine temps in a reasonable range you should be able to go above 10:1 compression without having to use premium. Some of the iron head fieros are in that range. Todays cylinder heads especially the aluminum ones are much more efficient and combustion stabile.

Another issue that hasn't hit us heavily yet is how the advent of ethanol use is going to hurt older non ethanol optimized engines. The reason specific cars are labeled E85 is because their fueling system is capable of detecting E85 so that timing can be optimized to take advantage of it by compensating for the reduced energy of combustion with more aggressive timing, in addition to having corrosion resistant upgrades also.

In other words, the 10% ethanol fuels have an anti-detonation characteristic and are a benefit to high compression and forced induction engines.

I am no expert but through my own experiences and what I've read, there is plenty of cushion to get into and above 10:1 compression and not have to pull up to the premium pump.

The 3500 is rated by GM for 87 octane.

[Superdave]

I ran about 10.4:1 last year with a huge cam and 89 octaine/E-10 with no problems.

this year i'm at 9.8:1 with the same cam, still no problems.

[lilrebel68]

ok.. so about 10:1 ratio.. got it! how will the 3400 prted intake and port matched head fit into the equation??

Would it be better to change the stroke or just change compression using new pistons??

what cam should I use to get the best low end torque, but be better than stock??

[95Grand-Am]

You shoud consider using the whole 3400 engine. When you choose a cam you need to know where you want your engines power band.

[Superdave]

Hey guys, does anyone know what is the highest "safe" compression ratio for use with 89 octane gas that can be done on a 3.1 engine?? would it be better to try to do a "stroked" rebuild, or just get the bump in compression ratio from new pistons???

a 3.1 is allready a stroked 2.8.. adding additional stroke would require offset grinding the crank and custom pistons, also clearancing the block. Not worth the money invested IMO

which camshaft would get the best performance from the motor??

What will you be doing with the motor? drag racing, hill climbing, autox, dirt track racing???

I know I cant mention fuel efficiency when talking about performance, but if I just wanted the highest compression ratio to get the best fuel efficiency, what ratio would I be shooting at??

I get over 30 MPG highway making over 300 crank HP with a huge cam.. it's all in the tuning and parts you decide to use.

I am not going to supercharge or turbo this motor, but it would be nice to have a solidly efficient naturally aspirated engine under the hood.

Not going to happen with a Gen2 engine, building a hybrid with a top end (heads, LIM and UIM) from a 3400 SFI would help but your best bet is a full engine swap. The Gen2's like to wipe out cam lobes, the gen3's are roller engines with a ton of upgrades that make them a better platform to start out with.

I already am planning the switch to the 3400 ported intake and 3400 heads,
I just want to make sure I have a damn good bottom end to go with that..

thanks in advance for all the help..

I've been there.. don't blow your money on a hybrid. If you want a proven engine then just use a complete 3400.. or if you want some serious power the 3500's are laying down great numbers.


designing an engine combination takes lots of time and work. I spent weeks on my cam alone.. lol

[ForcedFirebird]

Joseph hit the nail on the head. It all depends on your cam. The dynamic compression is going to make the difference in performance and fuel requirements.

My engine and cam combination, with 11.5:1 or so of static compression is going to be running at 9.48:1 dynamic compression at lower RPM's, and that number will increase as the engine rotates faster until it reaches the static compression.(intake valve closes at about 70 degrees after bottom dead center on my cam)

[lilrebel68]

this is my daily driver.. and I am really not interested in an engine swap, because i dont have the money for it ( engine, ecm, wiring harness, then all the extra $$ to build that engine up ).. but I can do stuff in stages (uim/lim, heads, cam, etc)...

[Superdave]

i've got the original 3.1 computer and harness running my 3500.. 3 out of 4 original engine mounts too