question for anyone who has ever upped their compression ratio

[nando]

don't get me wrong, california gas sucks. But much of it comes down to the proper tune, and cam selection. For example, a stock cam with 11:1 isn't a good idea on pump gas. But with a more agressive cam, it would work well. Same reason you wouldn't want to run a schrick 304 on a stock 8.8:1 motor, only in reverse..

anyone who remembers Dave Length's 3 liter stroker - he had I think 10:1 or similar CR, a 272, and the stock chip. On 91 octane, there was no way that was going to work. He never made a lot of power, because it was detonating so badly, and his cam choice wasn't right for the motor. He never did get it tuned, either (car got rear-ended). I always used his engine as an example of how *not* to build an M20.

[LJ851]

New cars are in the 10's and 11's and run normal fuel. The fuel is fine, it's all about the tune/engine build.

And the shape of the combustion chamber and spark plug location.

[nando]

M20s with 885 heads don't have combustion chamber shape issues.

[CarsSuck]

hey nando, can you share some reading materials or discuss the compression/cam profile relationship a bit more? I get the basic idea, but what are the specific factors that cause detonation, assuming the ability to tune either setup properly?

[nando]

I don't have any links off hand.. but generally with a higher compression ratio, you want a more agressive cam. what you're after is a good dynamic compression ratio (which is different from a quoted static compression ratio), which is affected by the base static ratio plus the cam. If your dynamic compression ratio is too high, at peak torque the cylinder pressures will be too high and it will be hard not to ping. And if it's too low, low end torque will suffer. actually, dynamic compression ratio is a good search term to start with.

[blefevre]

And the shape of the combustion chamber and spark plug location.

Yes, that would be why I said "engine build". The point is that the fuel isn't the issue.

[frag]

nando: you're really exposing me as a noob enthusiast, cuz now i got a ton of new questions to ask. i do really appreciate the inputs though!

what's MBT? i did a quick goodle search, and there were several references to MBT with regards to engine tuning, and they were all different. i couldn't tell which was which. there's stuff about Minimum Best Timing, some talked about Maximum Brake Torque, and even Minimum advance for Best Torque. which is which?

as for the temperature, it's common to get up to about 38-39 degrees C (100-102 F) in the summers, and the daily gridlock traffic jams can last about an hour for a 15km (10 mile) drive in the city. that being said, what do you think about the cooling? i'm seriously considering doing it either way, since i'll be tracking the car from time to time. there are a couple of decent race tracks not too far from manila, and i'll definitely be doing more track days and hopefully a couple of run-what-you-brungs here and there.

i'll be using a 272 cam and we'll probably go with a WAR chip and tune it that way (i got a pretty good M20 tuner on the project). we get pretty good gas here - up to 100 octane, like Wanganstyle was saying. anything else i should look into?

thanks!

[nando]

MBT = minimum best timing (I guess maximum brake torque or maximum advance for best torque would be OK too). Basically, the least amount of timing you need to make the most amount of torque. This is in contrast with how most people think you tune an ignition map, which is by advancing until it knocks and then backing it off a few degrees. this is wrong. the knock threshold isn't neccesarily near the torque peak. that's why knock sensors are safety devices, not tuning devices.

I'd consider doing an electric fan on the front with PWM control, and get a new clutch fan. a new radiator for sure (whether it's upgraded or not), replace all the hoses etc. It sounds like it gets pretty damn hot there, you don't want shit breaking! :)

are you dead set on the 272? a 284/272 would be better suited to that CR, and it will still idle just fine. I'm pretty sure your 100 octane is RON, which would be like what, 94 here? It could be OK on the 272 but you could still end up having to retard the timing a bit at peak torque. I think a 272 is better suited to 9:1 or 9.5:1.

[frag]

thanks for the input nando!

i think i better start reading up on MBT... and ignition timing in general. though what little i have read on coincides with what you're saying - best to time it BTDC, and not base it on ATDC (where knocking occurs). but i think i'm in for a lot more studying.

as for the cooling, what's a PWM? and do you mean i'll have the clutch fan AND an electric fan? all my hoses are new, and the radiator is fairly new as well (less than 4 years old).

as for the cam, i'm not dead set. is that a rule of thumb? as CR goes up, the cam gets more aggressive?

[LJ851]

i think i better start reading up on MBT... and ignition timing in general. though what little i have read on coincides with what you're saying - best to time it BTDC, and not base it on ATDC (where knocking occurs). but i think i'm in for a lot more studying.



...as for the cam, i'm not dead set. is that a rule of thumb? as CR goes up, the cam gets more aggressive?

Yes you need to study some more.

A very general view of this is that you are trying to avoid stratospheric cylinder pressures with the high static compression ratio. The longer the duration of the cam is the more overlap it will have. Overlap bleeds off cylinder pressure so you are trying to avoid low compression with big cam (very low cyl pressures) and high compression with small cam (very high cyl pressures)

[frag]

LJ851: thanks! that actually made a lot of sense. so a simplified way of looking at it might be less overlap = shorter valve opening times = higher pressure/velocity of air while more overlap = longer valve opening times = lower pressure/velocity of air?

[digger]

Yes you need to study some more.

A very general view of this is that you are trying to avoid stratospheric cylinder pressures with the high static compression ratio. The longer the duration of the cam is the more overlap it will have. Overlap bleeds off cylinder pressure so you are trying to avoid low compression with big cam (very low cyl pressures) and high compression with small cam (very high cyl pressures)

its really the intake valve closing that "bleeds off pressure" most pressure. if you retard the intake valve the overlap decreases but the piston is closer to TDC when the intake valve closes which bleeds off pressure at low rpm.

[digger]

MBT = minimum best timing (I guess maximum brake torque or maximum advance for best torque would be OK too). Basically, the least amount of timing you need to make the most amount of torque. This is in contrast with how most people think you tune an ignition map, which is by advancing until it knocks and then backing it off a few degrees. this is wrong. the knock threshold isn't neccesarily near the torque peak. that's why knock sensors are safety devices, not tuning devices.

I'd consider doing an electric fan on the front with PWM control, and get a new clutch fan. a new radiator for sure (whether it's upgraded or not), replace all the hoses etc. It sounds like it gets pretty damn hot there, you don't want shit breaking! :)

are you dead set on the 272? a 284/272 would be better suited to that CR, and it will still idle just fine. I'm pretty sure your 100 octane is RON, which would be like what, 94 here? It could be OK on the 272 but you could still end up having to retard the timing a bit at peak torque. I think a 272 is better suited to 9:1 or 9.5:1.

272 with 10:1 should be fine with good fuel and ability to tune.

your 2.8L with 10:1 and 284/272 was 7.5:1 DCR (7.8:1 advanced 4 degrees), a 2.5-2.8L with 10:1 and 272 is ~7.8:1.

8.5:1 is rule of thumb for DCR of a street engine and applies to knock at low rpm below torque peak, at torque peak and slightly either side a bigger cam may introduce higher cylinder pressures even if the DCR is lower so it is not so simple.

[Wanganstyle]

2700cc m20+worked 731 head + 10.5:1 mahle pistons and 272 cam was the setup done by H&B for 91 octane Cali gas and "passing" emissions in 1985 on the 327s e30 model.

Alpina also did similar on their m20 in e30 m20 based alpina models

You should run BOTH mechanical clutch and electric fans. The clutch fan mechanism unit should be checked and a fresh unit ran, they do wear.

Oil cooler if not equipped would be an ideal item also.

Sometimes a thicker oil can help also, HKS routinely ran 20-50 oil in some cars in Asia climates, you could try the BMW TWS 10-60 castrol Motorsports oil also, it's expensive but good for heat and resisting breakdown.

Any elf/amsoil engine oil for your market should be setup similarly

[frag]

woah! now stuff is flying at me! i'll try to keep up...

digger: i just read up on DCR and SCR and i think i've got the basic concept. i am assuming that increasing CR (whether static or dynamic) means shrinking the size of the combustion chamber? and is there a way to compute DCR without having to actually line up the cam, valve and piston on the engine and simulate it? please note that i'm not going for a stroker kit.

Wanganstyle: how do you fit both fans? any pics i can see? also, i do have an oil cooler as standard, but i was thinking maybe there's an upgrade for this? and thanks for the tip on the oil. i'll look around for that locally.

thanks guys!