High compression boosted motor question.
 

If I were to build a 461 ci motor with 11:3 to1 base compression ratio that was tuned for E85 with intercooler and aluminum heads. what could I expect? Would it live at low boost levels , say 12 to 15 pounds ? What different characteristics would there be compared to a 8:75 to 1 motor? Would it make more power at the same boost levels, or would it be less efficient and be detonating ? I guess higher air intake temps would be more inherent right? I'm throwing this around because I have this motor built and all I would need would be a cam to get it up an running.. It would have twin Precision PTE6870's on it.....

Feed it E98 when you turn the boost up

At some point Charlie, the compression ratio limits your boost level that you can run with available fuel. You seem to be on a Max Boost quest.

The fact that you were running 35 psi with a 4 cylinder engine and had good durability and now assuming the same guidelines (except for fuel economy) what would be the benefit of going 3 full points higher in compression ratio with the same fuel, a V-8 engine, and twin turbos? And thinking that it would not have combustion stability issues with the higher static compression ratio. Corvettes tried that deal in production with direct injection and would up running very moderate boost because it was a street car.

Tom V.

I'm only thinking of this because I have my motor in my Ventura that is built with all the good stuff but has flat tops which brings it to 11.3 to 1. So i was asking because it would be really easy to just change the cam and not have to go and take the whole motor apart for new dish tops. Maybe I could use a thicker head gasket ? You know me , I like to experiment with things Tom.. If you think it will be a time bomb then I wont even think about anymore .. Just wanted to see what everyone thought of this idea..

What are you at with your compression?

Charlie do a search on Yellow Bullet... maximum compression E85 in both the NA and turbo sections. See what the extremes are. I think you'll be surprised. I DONT advocate trying the extremes but I'd consider something between the minimums and extremes and adjust accordingly.

BTW inlet temps shouldnt change from compression but turbine may behave differently. (Compression affects EGT). HEAT of COMPRESSED air/fuel will increase!

That said Id keep manifold inlet temps cool(close to ambient) if boosted(A/W intercooler) With compression up where you're at I'd add water injection to slow combustion rate some.

Bigger concern is keeping cylinders sealed and stock block alive.

With your present boost success with E85 on your 4 cylinder you might want to consider staying NA on the 461 and upping compression 2-3 points.

Continue doing your homework!

I really want to go turbo with the 461.
I guess what I really would like to know is if some one can tell me what the cylinder pressure comparison would be with a 11 to 1 motor at 10 psi to a 8 to 1 motor at 35 psi, or is there more involved to think about? If its some of the same maybe it will be ok?

From Wallace site
http://www.wallaceracing.com/boost-c...ratio-calc.php

Note I used Altitude of zero!
Note from site says "2 Different Formulas Used For Answers
The first is the standard equation used by many,
The second may be the more reliable one "

IMO cam timing events will have effects on actual compression pressures seen! As will temperature. Plenty of unknown assumptions in the above calculator.

8:1 compression at 40 psi
"The Boosted Compression Ratio from your Boost of 40 psi is 29.77 : 1
The Effective Boosted Compression Ratio from your Boost of 40 psi is 15.43 : 1"

11.3 compression at 15 psi
"The Boosted Compression Ratio from your Boost of 15 psi is 22.83 : 1
The Effective Boosted Compression Ratio from your Boost of 15 psi is 16.06 : 1"

11.3 compression at 12 psi

"The Boosted Compression Ratio from your Boost of 12 psi is 20.52 : 1
The Effective Boosted Compression Ratio from your Boost of 12 psi is 15.23 : 1"

Thanks Bruce. I guess when I see my friend with 10:5 to 1 with his 2JZ motor having zero issues @ 30 psi with 11 degrees of timing, I think to myself that maybe this would be possible . He thinks ill be fine. But then again he's just a turner guy with a way better design of motor...

10.5 assumed compression ratio might actually be closer to 10 to 1 on a production engine. Pontiacs historically were lower than 10.75 to 1 on their engines.

That being said, a 10 to 1 engine with 30 psi with 11 degrees of timing basically means that the combustion cycle is starting late therefore the cylinder pressure is lower tna it would be at 27 degrees of timing. Your engine was probably not giving up near as much cylinder pressure at 30 psi of boost even with 8.75 to 1 compression ratio. Off boost I bet it drives nice with 10-1 compression ratio even with late timing.

Tom V.

ps don't give up on going with boost on the V-8 engine. Just buy the right CR pistons as adding a thick gasket to lower compression ratio make the engine want to knock much easier.

Tom V.

My compression is about 8.25 maybe 8.5 with the heads having been resurfaced about 3 times.
I do have a very small cam with no overlap so that must make some difference.
I run E85 most of the time on the street and run E98 at the strip just for some insurance in the 1/4 mile.

The Butler's 482" in this story is at 10:1 and at this point was at 44psi. They were on alcohol though. Just something to read.

There is twin turbo drag week car that runs on the street around our area Charlie, I won't use his name just incase he street races the car. It is a 540 c.i. Chevy, twin 88mm 10.25 to 1 runs 26psi on e85 with no issues per him.

Sounds like you have a good science experiment on your hands! A fast one too. My guess is at 10-12 psi the Ventura will not be able to handle the power with out upgrades!!!!!!!!!

Amount of ignition advance required is only relevant to combustion rate.

Peak power happens when peak cylinder pressure occurs @ 15 degrees AFTER TDC(some sources claim 16 or 17). It actually takes time from when the spark occurs until the flame has engulfed the chamber and created peak cylinder pressure.(Flame propagation in technical terms)

PCP is a critical point. If it occurs before 15 ATDC you risk damage from detonation. You dont want PCP occurring at TDC and certainly not before that. PCP occuring much later than 15-17 degrees ATDC WILL cost you power.

A whole bunch of factors determine actual combustion rate. To name a few...chamber design, piston design, quench, the fuels resistance to burn(octane), charge temps, heat of compression, cylinder head temp, water temp, air/fuel mix.

The amount of ignition lead required to acheive full flame propagation for Peak Cylinder Pressure(PCP) to occur is directly tied to how fast the combustion rate is.

E85 is a whole different animal than gasoline at same octane rating. Ethanol at a "race mixture" burns slower. The low octane gas portion of E85 is there to help light the fuel on cold starts. That said it still doesnt cold start as well as race gasoline. The ethanol has a cooling effect on charge temps in the cylinder, methanol in rich race mixtures more so.

Hope this helps you understand why your friends Toyota motor might only need 11 degrees of timing to your 27.

I know the higher compression yields a lot less boost capabilities. That's why I'm asking what would be the max level of boost with e85 and a 11:3 to 1 motor. If I could get away with 10 - 15 psi , what kind of power would we be talking about. Would it be more then a 8: to 1 motor at the same level of boost? This is the question that I would like to know the answer to. That and would the motor be able to live ....

I think the answer is an obvious yes. It would make more power than the 8:1 at the same level of boost. It would also be more responsive. There are lots of guys on the YellowBullet forums with similar compression and boost on E85.

Anyway, it sounds like you really, really want to give this a try. You already have the engine. I say go for it and log your progress here. I would love to follow along.

So if I ran something between 8 - 10 degrees of timing maybe this would be safe to try? I know with more compression its not going to have the same capable boost potential. Im just wondering if the amount I could get away with would be worth the effort....

Lol! You are right sir! I do want to try this and thank you for your support!

I think I can get through all the growing pains involved just didn't want to beat a dead horse if it was just going to be a time bomb waiting to go off... Haha

I like hearing what all the guys of knowledge over here think before I pull the trigger.... Still not sure yet..

Do you know this guy. I would like to talk to him if possible ..