2k.civic.si

What is the limit on compression for 91 octane gas before you need to start pulling timing?
Im just curious if its even worth bothering trying to run my motor on 91 with 13+:1 c/r
My other engine is 10.4:1 c/r and it runs perfectly happy on 87

PyroProblem

Every engines different. Give it a try! If its gonna knock, its gonna be on the low cam most likely, but Im sure you know that.

2k.civic.si

True it will starting pinging before it knocks....im just curious how far people have been able to push 91 octane.

blacK_Dc4

a close friend's setup : b16 head / gsr block with 81.25 ctr pistons .... its roughly 12.8 compression I believe .... it makes 202 whp and 133 tq on 91 oct .... been daily driven for about 2 years now

d112crzy

12:1 h23vtec. 204/168 on 92. Stock cams. Had to retard a few degrees on the low cam map but runs great in vtec.

FlewByU352

I can relate more to this about motorcycle engines... but yes every setup is different. For instance the one air cooled motor runs 9.1:1 compression stock and will run on 87 but runs noticably better on 93. Now for this motor 11:1 is the max you can go on stock timing with 93 octane and that is the edge. Now another company with just about the same design but liquid cooling runs 11:1 compression stock and can go to 12.5:1 on 93 octance with stock timing. Cylinder temps to blame? Probably but I have seen engines on 13:1 being the edge of the 93 octane gas when the factory motor is built with 12.5:1 compression. Theres a lot involved, even load on the engine. Like ATV's that sand drag and are in high desert temps will sometimes have issues with those figures and actually need less compression or pull timing. Dont forget running on the limit can be catastrophic. If you get bad gas or gas that isnt the octane rating you tuned the limits on but lower you could run into problems. IDK good luck with whatever your building. Some specs atleast the motor might have helped.

hillssol95

i'd keep it under or around 11:1 just to be on the safe side... i'm pretty sure there are too many variables to give a definite answer just margins people can give you.

hood ride eg

Good thread i always wanted to read up on this

Uncle Dave

Camshaft centrelines are the key to high comp low octane fueled engines.

bambam

Other things come into factor too. If you were able to run a flat or dish type piston, as well as remove all sharp edges and shiny surfaces in the combustion chamber, you could effectively create a cooler combustion environment that has increased resistance to detonation. Thermal coating the pistons might help as well.....or anything else for that matter that lowers the cylinder operating temps as well as better heat distribution. I don't see why one couldn't build a motor that is centered around resistance to detonation and even achieve 12:1+ cr and run on 87 octane.

C_Rock77

The other thing you need to ask yourself: Is it WORTH it to run ~13:1 Dynamic CR on 91 octane?

Probably not. You're going to have to pull timing to do it to a point that you'd actually make MORE power running a lesser static CR and more timing.

I think your sweet spot for 91 octane is going to be no higher than 12:1.

BigBlock22

11.5:1 is said to be the ideal dynamic compression ratio limit for an engine running on 91 octane. the more you push it, the less room you have for tuning. but like mentioned before not all engines are created equal

EHondaJDM

Run CAM2 100 octane

cibao2ner

cam2 is way over priced at 8 buck a gallon

2k.civic.si

Yes thats very overpriced for 100 octane fuel.
I can get 100 octane fuel from the local airport for under 4 bucks a gallon.
I am thinking I may even try to tune the motor on 91 and see how that turns out and compare it to the 100 octane tune and see how much power I really lose. The motor is about 13.2-13.3:1 c/r so 91 will be a far stretch

ek9800

I had a gsr with CTR pistons, comp was roughly 12:9. Brought it out here to the west coast only to realize highest pump gas is 91. So i re-tuned and it ran good for a little while. Than i put a piston through my block.

EHondaJDM

Ya id at least use a booster

That things gonna ping like crazy

2k.civic.si

If it does i may just mix 91 and 100 from the airport...like a 50/50 mix

tuck1crx

good quench areas in your head will help out alot. 11.5 is said to be safe but with better quench areas ull get a more efficient burn


e85 is an option too

2k.civic.si

I wish...if i had e85 around here i would have used different pistons and gone higher on the c/r

C_Rock77

I'm just curious why you ask this question when it seems the engine is already built as a 13+:1 CR engine.....

neverstop

I've ran my ls vtec with shaved head on P30 pistons and B16 head for a long time on 91. It made 177whp with gsr cams too so it's pretty strong. I haven't cc'd the combustion chamber but it's probably 12:0+

2k.civic.si

just trying to get a feel for what people with 91 have been able to accomplish...ive had the motor in the car for about 4-5 years now and been strictly running it on 100. figured i would see if it was even a good idea to try 91...on 100 octane im able to do 30-32 degrees of timing...wondering how much i will have to pull timing if any to run it on 91

dj4monie

Bump this for running the thread of its rails with this post.

Big difference between Static CR (Compression Ratio), Dynamic CR and Effective CR.

The last two are effected by how long your intake valve is closed. Static is the only constant here, all else is effected by various things covered here as well, such as cylinder temp, gearing, load, even where you live.

With all the stuff I have found for Honda engines on this subject you think people would be better versed on this stuff, especially on HT, I guess not.

Use this - http://www.zealautowerks.com/

Read this - http://www.team-integra.net/forum/di...PagePosition=1 circa 2003 but still informative and helpful.

Read This - http://www.kb-silvolite.com/article....n=read&A_id=36

Finding specs for stock parts can be sketchy at times but spending enough time with Google will usually turn up with what you need, the question has likely been asked before.

Honda engines seemingly like 10:1 dynamic compression ratio. This is not Static Compression, big difference.

When you spec the typical high performance B engine (GSR, ITR, CTR), its running right at 9:1 or 10:1 Dynamic CR stock.

That should tell you plenty if you read the last link, that this is the limit of pump gas and it seems to be correct. I found a story by engine builder Dave Vizard and he said the cranking pressure limit for pump gas is around 210psi at sea level (14.7), but it seems small bore (under 4") engines with longer duration cams can have cylinder pressure around 220-230psi on pump gas before knock becomes an issue.

Resistance to Knock is a whole other ball of wax, but it safe to say that 4 valve aluminum head engines with center spark plug are far more resistance to knock than old wedge 2 valve V8 style heads.

Since I live in California as well, it became very apparent very quickly that to run as much compression as you can on pump gas, cam selection and range of operation were going to be very important. Since I don't have the gearing options Honda enjoy, it becomes a bit more critical.

ITR's were imported to the US I believe after 91 became available and since the cars were to be sold across all 50 States this is likely why the static comrpesssion was dropped from 11.1 to 10.6:1

All that said, on the stock computer and cams it seems 11.1-11.5 or so Static Compression Ratio. If you can tune, with more aggressive camshafts you can get up to 12.5:1-13:1 without any knock and without pulling any timing.

I say if you want the most reliable N/A setup, have the money and time, then take care to remove any sharp edges in the combustion chamber as mentioned and the pistons as well, you don't want to create any hot spots. Coating the pistons (and combustion chambers) makes sense to reduce surface temperature.

Cam selection is important as you don't want to under cam the engine (not likely) but never want to drop cylinder pressure below stock levels either (over cam).

Finally I would say, pick the cam you want to run, then use the calculators above to find your correct Static Ratio. Make sure your Dynamic CR stays around 10:1 and keep your cranking pressure to about 220-230psi. Dave Vizard also mention for his high psi number (210psi) that was based on 93 octane. In California we have two choices, 91 and E85. I left out 100 octane because its not practical for a daily driven car, its at least $9 a gallon. The OP is about 91 so we'll stick with that, you're pressure should be 5psi for every one octane point below 93, so using that as a guide I come up with 210-220psi being the max cylinder pressure for 91 octane.

I've been checking around the net and that is very consistent. I think you'll find everybody's setup on pump gas and that makes good power is around these numbers.

pr0honda

Does this mean you select a VTEC cam from the primary specs( where you check compression with a guage) or the VTEC specs where compression is not checked /Should the primary or VTEC lobe retard the IN closing? Should the center-lines be the same on the Primary and the VTEC lobes? Does VE have anything to do with dynamic compression? Is Vizard still alive? Has he progressed from a Lotus TC head?