kgkern01

E85 is 104-105 octane, very resistant to knock and loves compression. Well with 54-55cc heads, Impy head gaskets, and .010 deck I figure around 12.7 depending on the above factors. So that's probably what I'll shoot for. Worst case if I absolutely had to convert back to gas I could get the head chambers enlarged slightly and run thicker gaskets.

WS6T3RROR

OH thank you, its my first day building engines. I did not know lt1's liked high compression what a revelation. I was speaking of the quality of gasoline around here, there is one station with "good" gas that I never get any knock retard from. The rest of them will have it on the knock retard any time there is high load at part throttle. Or if I toss a splash of 100 octane unleaded into a tank from the drum in the garage all knock goes away. 93 octane at the pump aint always 93 octane.

You can dcr and scr all you want, the power gain from compression is very minimal and brings great deal of problems to the table. You also have got to factor in volumetric efficiency into your little dcr equation because that is where you will get into trouble. Most people cannot achieve a high ve with the lt1 so most get away with so much compression. What compression you can handle depends very much on how much air/fuel you can pack into the cylinder at a given time. The big deal here is the bmep you create and where in the rpm band that you make it, at lower rpm there is much more time to vaporize the fuel and heat it up which will get you into knock. The effect of increased compression is very counter intuitive when it comes to what it does to cylinder temps and pressure. I have seen data at my last job as well as designed spreadsheets for engine design classes to model what goes on inside an engine when changing several different parameters. I can tell you its not worth it for the problems you gain. The only thing dcr can tell you is if your camshaft and your compression ratio match. It can't tell you knock sensitivity or resulting ve or bmep.

People make out like a point of compression is worth 100hp. Just like they make out going from a 190cc head to a 210 cc head is going to get them another 100hp. In both cases they are wrong.

Kirby, I have never found anything in going tighter than .040" quench you will be ok with that tighter quench until you see some wear and you will probably have a shadow on the quench pads from the piston getting really close to the head. But you should decide what to do when you see how much the piston rocks in the hole with a dial indicator. Cutting your chambers and running thicker gaskets if you need to support gasoline again is a poor choice.

Also you will probably have to buy new injectors at that point, the resolution of the trims for injectors in the lt1 pcm is 0.x and when you get too big with injectors the adjustment to idle fueling will be too rough when you swtich to gasoline with such small injectors. You will be fine with e85 with the large injectors though because it has quite a bit more fuel demand and will then have the resolution to make finer adjustments to idle fuel.

kgkern01

E85 should take the 12.5-12.7 SCR pretty easily, especially since most E85 specific motors run well over 13:1. Given I am in Indiana I don't see the availability going away anytime soon. Why would bigger cc heads be a bad idea to lower CR BTW? Just wondering.

slomarao

I dont think bigger heads would be a bad idea. But im not too technical.

Again, i just think that comp. is too high. Also think your going to be kicking yourself later if you go higher that 12.5.1 because of the issues that WS6t3RR said. If your building a street car there is no reason to go over 12.1, even when using e85. The BS isnt going to be worth the extra 5-10hp.

kgkern01

I'll probably end up going with what Lloyd suggests for the setup, since he'll be doing heads and cam, I'll just use his suggestion.

slomarao

Good idea.

WS6T3RROR

Modifying the chambers and swapping to thicker gaskets to run gasoline is a poor choice for several reaons. The first one is you end up opening up the quench which will make it more knock sensitive. The second one is that i am against pulling heads whenever it can be avoided. Its not an easy job it costs money and it introduces garbage into the engine 99% of the time. Also my whole philosophy when it comes to doing any type of work not just engine work is, do it correctly ONE time.

Its kind of like this, build a race car, or build a street car, but do not build one that is so-so at both. Just apply that to the type of fuel you plan to use. Either pump gas or e85 if you're swinging for the fences. I have helped a local get his carbed bb mopar stroker running on e85, he is running about 12.8 with indy heads.

If you want it to do both just build it on the edge for gas. Just ask lloyd what compression you should run if it was on 93 and run that, it'll still be fine on the e85 and you will still be faster on it. Ask him to get you a little more port volume than normal since your fuel is going to cut into the room for airflow and tell him what you're doing.

Compression is not the world beater of engine building. It helps but people make way too big a deal out of it. You want enough compression compression to match your cam and have the engine avoid fragging itself on whatever fuel you plan to run and thats about it.

Heres one for you, with more compession and the burn speed and intake charge all being the same raising the compression ratio will drop the peak temperature in the cylinder. The max temperature will also occur sooner due to heat transfer and the rate of temperature rise. Engine efficiency goes up as a result of this because the max temp is closer to the temp of the coolant and less heat is lost. Thermal efficiency or power out vs fuel (power) in goes up about 2.5% going from compression ratios of 8-12. Power increases more than this to the tune of about 3% per point in the lower 8-10 region and about 1.5-2% in the 10-12 region and it tapers off pretty quickly from there to the point you gain nearly nothing at around 20 where diesels run.

<Puck>

Sarcasm will not only get you nowhere in getting your point across, but does not help your reputability. You cannot blame the LT1 for the watered down gas in your area.

Of course it succumbs to the Law of Diminishing Returns once you get high enough, but just a half of point compression is worth over 5 peak hp at the flywheel. The more power the engine is making, the more every bit over stock will help. When we are talking a 12.1 CR Lt1 compared to a common 10.4CR(still baffles me why do people rebuild to LOWER CR then the stock shortblock takes from the factory?!? Usually ignorant machinists who are still stuck on SBC rules.) there is a healthy increase in free horsepower. I don't know about you, but on a high horsepower engine a free 15 hp or so sounds like an amazing deal to me! Any knowledable tuner can easily tune a 12.1:1 CR lt1 on pump gas for daily driver duty assuming the DCR is within a safety margin.

You do not need to change any hardware to go from gas to E85. Two seperate tunes can handle it, just switch to a pump gas tune with retarded timing when you want to run 93 and you will be fine. When you want to go back to E85 load up the more aggressive E85 tune to take advantage of the higher octane.

I do not know why you brought up injectors for compression ratio since that is flywheel horsepower dependant, but you do not have to worry about going too big on the injectors on most builds unless you go waaayyy overboard and the injector cannot open and close fast enough to get the required low pulse width to idle properly. Many people would be suprised what a 32# injector can support with a jump in fuel pressure anyway.

*ALL* the quick, well built NA LT1s are high compression, period. It is for a reason. When you have mix and matched crappy builds like a low compression 355 with an overcammed "N/A" shelf grind with not enough valve overlap, that is how people end up with these poor performing mid 12 second H/C LT1 setups.

WS6T3RROR

I was talking about the idle pulse width in ms and how large the % change in fueling will be when you get around 2ms or so when switching from e85 to gas. The same problem which you allude to in your post puck. I dont believe I said anything about different injectors for compression. Run the numbers assuming he will make 550fwhp with an 80% duty cycle on e85. Then run them for 12 g/s airflow on gasoline with the same injectors. Tell me if you see a problem there. That is why the suggested hardware change if he plans to swap back to gas, will it run? yep will it have good control of idle fueling? not a chance. Sure you can change pressures but with most of the fuel injectors going from 45-60psi is going to get you about 6pph or lets say 18% more fuel, but you need 30% for e85 so you either run at 92% duty cycle or you buy the bigger injectors and run low ms on gas.

We'll just have to agree to disagree on the compression thing. If I cant garentee it will work 100% of the time I dont stand by it, and you have no control over what comes out of the gas pump even from one time to the next. I tune my own car and several others, none of which complain too loudly on pump fuel, but things are alot more finicky the higher compression you add on pump fuel.

Sorry about the sarcasm, but I see people post the same thing all the time. Yes dcr and scr are relevant to the topic, but they are in no way the end all of engine building. I just do not see the fascination in chasing compression ratios around that most do. I tend to give everything else as hard a look as people do thier cam and thier compression, thats just how I do it.

No doubt all the really fast cars have high compression. They also have giant solid roller cams and huge stalls and super deep gearing with spherical rod end suspension links. I'm not interested in any of that for something I plan to cruise in. But it does have its place.