hey its me

ok im just wondering whats the highest compresssion can you do before you stop using pump gas cause the highest ive seen here in texas or in mcallan tx to be e xact is 93 oct so just wondering im running 11.5 right now on my ls.v

lilbuddjr

i think 11.5 is the limit but im not positive..dont hold it to me

Fabrik8

It depends on a lot of things, like the tuning, combustion chamber and piston top design, and air density and temperature. It's mostly engine design though. You're limited by detonation and pre-ignition, so if you can control that with the right air to fuel ratio and ignition timing, as well as a good knock sensor setup to retard timing, you can go pretty far. Ceramic coating the piston tops and combustion chambers can help too, and smoothing out everything with the slightest bit of an edge on it.. You're really trying as hard as you can to not make your spark ignited engine into a compression ignition engine (diesel engine). Running that high of a compression ratio means high cylinder pressure, and really fast changes in that pressure, so that's the enemy.
Speaking of that, stroke and other rod/crank geometry has an effect too, because that changes the expansion ratio (which is just the compression ratio on the other side of TDC, but is actually at least as important) and how fast the cylinder volume changes.

Motorcycle engines go over 12:1 on pump gas very easily, but that doesn't mean that the particular engine you have in mind will be able to do the same. Like I said, it's all about geometry and engine design.

So the answer is yes, you can do 12:1 compression, but it might not be easy, or cheap, and there isn't much safety margin for design and tuning if you're going to use pump gas. If you're looking to do something along those lines, I'd start with a lower compression build first, and do as much learning as possible with that, and then incrementally move up to higher compression. I really wouldn't recommend doing anything that high for a street build though; you're really walking on a knife edge for tuning and conditions, not to mention gas quality.

Jarrod

I HATE having a high compression motor as my daily driver.

My shit would PING loud if I got anything remotely considered bad gas.
When I got my pistons though, I wasn't planning to make it a street motor.

To the OP, 11.5:1 has gotten many over 200whp VERY easily on 81/81.5mm motors. No need to do a tear down for almost nothing.

hey its me

well thanks for the great feedbacks im planning to do another build either b16 bore it down to a b17 who knows or a b20 vtec boosted cause from what ive heard i dont really have to resleeve it but it is an option that i have though..thanks alot im keeping my ls/vtec on the compression is their any other way for me to get more powere without boosting...

Will's Fiero

The intake valve close event has a HUGE bearing on ability to run pump gas. Sport bike engines can run their sky high compression numbers because they have 300+ degree cams that make power into 5 digit revs. This puts the intake valve close event long after BDC and keeps the DYNAMIC compression low, even though the static compression is extremely high.

I've heard that 9.0 dynamic compression is an approximate limit for aluminum head engines on pump gas. You can go higher, but you really need to know what you're doing.

madsteez

I can vouch. My 11.5:1 81.5mm poor mans type r mande 212 whp.






On pump gas.

hey its me

yea mine is a 81.25 and were on the same pace 220 whp but im using the ls hydro tranny right now but its ok cause if i would have used the b16 tranny id be hot dogggin it like a mofo...im still breaking mine in for now im using 93 oct on my tank ..

BAD4DR

I'm going to over-simplify this a bit for some of these guys around here. As it's not a perfect logic, but it will apply in their case.
Almost every high-rev motorcycle engine that uses high compression has 2 items in common that differ drastically from a Honda 4-Cylinder - They are OVER SQUARE, and they have a high Rod/Stroke ratio, most of them above 2:1....

Also, Formula1 engines make 800 hp on a 2.4L V8, using 94 octane, and they also happen to be very OVER-SQUARE, with Rod/Stroke Ratios upwards of 2:1.... They don't have cams, so they have ultimate valve control, and some of them run higher than 15:1 compression....

They can do that due to having unilmited control of the valves, an engine design that is optimal, and combustion chamber shapes/volumes, piston designs, etc., that are so space age it'd make the average person just wonder....

So yes, it can be done.
For a Honda 4-cylinder of a B18 design, the GSR/TypeR will handle it slightly better than the B18B/B20 due to the shorter stroke and higher rod/stroke ratio.

As Fabrik8 pointed out, IF you're going through all the trouble to polish the chambers, de-burr everything, remove all sharp edges, and ceramic coat everything, you can actually gain power on a street 12:1 Honda engine, as you can keep the timing and make use of the compression.
Otherwise, you'll probably run into detonation, followed closesly by turning the timing down or destroying the motor, so you may actually lose power....

BTW, of all the B-series engines, the B16 is most tolerant of high compression from a design standpoint.

Will's Fiero

Rod ratio has a TINY effect compared to dynamic compression.
Small bore engines are actually more detonation resistant than large bore engines.
The bore/stroke ratio is basically irrelevant to detonation resistance.

F1 engines absolutely use camshafts. They are required to by regulations.

If anyone is wondering whether a given compression will work on a given fuel, I have three words for you: Cam, Cam, Cam.

Consider two engines with 12:1 static compression. One has a cam with 300 degrees of seat duration, one has a cam with 260 degrees of seat duration. With 110 degree lob separations, installed straight up, the engine with the 300 degree cam will have its intake valve close (IVC) event 80 degrees ABDC, while the 260 cammed engine will have IVC at 60 degrees ABDC.

With IVC at 80 ABDC, the 12:1 static compression becomes 7.45:1 (not accounting for rod angle) and with IVC at 60, it becomes 9.25:1. One will be easy to run on pump gas, one will be difficult.