Compression ratio

[malc350]

I'm guessing that the reason my TT is so bad on fuel next to my 93 LT1 is mainly down to the inefficiency of the compression ratio (can't remember if it's 7.0 or 7.5:1 as opposed to the LT1's more than 10:1 ratio.

I know that this is for safety reasons, i.e. under boost a high compression ratio might melt pistons but I wondered if anyone is running higher compression without problems?

Interestingly someone pointed out to me the other day that the supercharger kits you can buy for C4s and C5s (with higher compression engines) are recommended to be fitted to stock motors with no internal mods. I'm thinking why can't we run 10:1 compression ratios, using high octane fuel and let our knock sensors take care of the detonation (or maybe use a modified chip?)

Despite my TT having 3.33:1 rear gears against my LT1's 2.59 the overall gearing in top gear (6th vs 4th on 700R4 auto) is about the same - 85 mph is about 2000 rpm.

I posted this sperately to my last post purposely...

 

[*89x2*]

Malc, I know Luigi's B2K was bumped in its cr. a bit when the 475 hp package was done - You are correct, upping it higher from 7.5 without taking other factors into consideration would indeed spell trouble (Luigi's has a whole host of things done to it as part of the upgrade )

the rpms @ 85 should be close - although the 6 speed trans has more of an "od" in 6th than the 700r4 does in its top gear, the difference of the gear ration makes up for it and thet's why they are close to the same.

Now Malc, what RPM are you pulling in 5th at 175 :eek?????

 

[8388]

In the world of boost it's much better to increase boost (air flow) rather than compression ratio. The reason for raising the compression ratio, would be if the current supercharger or turbos are maxed out and more power is needed without replacing them to larger units. In these cases, more compression is more of a bandaid effect to the solution.

As you raise boost or compression, higher octane, and reprogramming of the timming will become essential. while our Callaways have adjustable wastegates, they are not accessable while driving, so raising the boost or compression would require running higher octane *race* fuel all the times, or going with a different wastegate and or ecm, injectors etc..

Basically you can gain quite a bit of power without going to drastic measures, but once you go over the line, you open yourself to the reconfiguration on the entire system.

 

[malc350]

Actually my interest in increasing compression ratio was to do with efficiency rather than outright power as 7.5:1 sounds real inefficient nowadays.

Better steady speed fuel economy would be nice (who cares when you're going for it!?)
as the difference in economy between my Callaway and my old 93 seems a bit more massive than expected - I'd say maybe 6 or more mpg at a constant 75-80 mph.

When the speed gets up and I can hear the turbos spool up (85 mph+) I guess the extra injectors are contributing their 2c worth because the instant fuel readout falls off a cliff! Bit anoying because I used to enjoy a good 120 mph cruise in the LT1 with reasonable economy!

 

[SurfnSun]

Malc,

If you want to boost your cr I would go to 8.5 or 8.75:1. You'll definitely want to spend some money on a set of the best damn connecting rods you can find. Then go crazy

 

[*89x2*]

Connecting rods are part of the upgrade

 

[mangusta1969]

I would leave the CR alone. Even at the high UK prices, gas is still much cheaper than engine components...

Some high performance turbo cars like the Ruf Porsche have even lower compression ratios to allow for more boosted power. I think some of the Ruf units have engine compression ratios in the 5.5-6.5 range.

Several years ago I drove a super high performance 24 valve BMW turbo M635. It featured in-cockpit adjustable boost levels of 6/12/18 psi. To allow for the two higher levels of boost, it had something like a 5.8 compression ratio. It had horsepower in the mid-500s and ran like a scalded cat at all times. I don't think gas mileage was a real priority on this car, as it would sometimes shoot flames out the stainless exhaust pipe on up-shifts.

On your knock sensor question, the reason this approach is not used for normal engine detonation control is that it just does not operate quickly or reliably enough. You need to tune the engine for no knocking under normal boost conditions. The knock sensor is just there to help with abnormal conditions like a single injector going a bit lean or a fuel pump that is not generating enough pressure/capacity.

 

[SurfnSun]

In part I agree with you, but here's where I don't.....

On a b2k it isn't safe to boost over 14 or 15 lbs due to the supplemental injectors on the front of the intake. Too much boost will blow all the gas to the back of the plenum and toast the front end of the motor by going lean.

My thoughts on making power would be a modest raise in compression like 9.5:1 , then maybe a set of AFR heads with say a 64 or 68cc combustion chamber which would drop the cr a bit back to less that 9.0:1. Make 14lbs of boost with a 3" dual exhuast, AFPR, AZspeed runners, ported plenum, and extruded manifold....and stock in Goodyear tires

 

[91callaway]

8 1/2 to 1

when callaway upgraded engine the new pistons bumped it up from 7 1/2 to 8 1/2

 

[Ultra Slow]

This is a good topic and I think raising the compression on any callaway is a good idea if you have to rebuild it. If you are not inclinde to run race fuel, race the car, or try and set some speed record with it, I really think it is STILL a good idea to go back with more compression than the car has now. With higher compression, you can lower the boost and to achieve the same power for pump fuel, but your engine will be more efficiency and WAY more drivable car having increased throttle response and spool up. Something these cars are lacking as all of us owners know so well.

"When" I blow up my 89...... Knock on wood.... I will go back just as it is now, with better rods. I also will add a VERY small increase in camshaft and I will raise the compression to 10:5:1 using the best piston I can find to do the job. I dont recommend this for everyone, as 9:1 is a great ratio for a good factory minded rebuild on a callaway. I am certain that when I do this, it will make my callaway so much more drivable as it will come into boost quicker, along with more power overal for less boost. If you raise the compression any and are going to run more than 8-10PSI, you are definatly having to put good fuel in it. Currently on my untoched motor, at 12-13PSI, I have to run race fuel anyway, so I might as well take full advantage of it, and raising the compression will do just this.

As I said above, by raising the compression, I will run less boost to achieve the same if not a bit more power. Just that will definatly increase the much needed off the line torque before the turbos spool, the turbos will also spool faster from the raised compression, and the overall fun factor will be much more overall.... The less boost method with added compression also mean less heat in the intake because the turbos are not compressing the air as much, and this means more endurance and life. This will also alow the very undersized intercoolers to do more of the job they should, not having to try and cool a higher than normal air charge, due to the increased boost I have now, thus more power too.

Now if you got the fuel flow right, slap 15PSI of boost on a 10:5:1 engine, and you will have some 700+HP and probably 1000FT LBS...... I dont think the car will last to long at this level, but if you want to burn the candle real quick and be a hero for a day, 10:5:1 compression with 15PSI of boost will definatly do this if a person is so inclined.

I have thought about this topic for a long time and I think that anyone having to rebuild a callaway will benefit from increased compression, even if racing or maximum power is not the goal for that car.

For me the callaway is an answer for not having to do tons of fancy and usually unreliable porting and max-flow/extreme RPM level engines running aftermarket heads, etc to achive a ton of power. The turbos take care of all this for you, do it much lower RPM and more reliable. I have learned that if you Just work with the callaway in its bounderies, they can be very fast without re-invinting the wheel. Reinventing this car is just not a sensable thing to do because they are rare and a "re-invented" car will be valueless. Raising the compression a little is certianly within the cars bounderies if you are not running crazy boost and will be something that will improve the car.

I say do it if you are rebuilding one for any reason.

 

[malc350]

Cheers Ultra Slow and everyone else

looks like a bit more compression is overall a good idea. I was prompted to write this post as the compression ratio on our cars seems so low nowadays.

Of course I realise these cars are very old technology and engineering has moved on a lot since they were new and I think they can benefit from a little more efficiency. I am not crazy about loads of boost anyway as I'd like to keep my car together as long as possible as I enjoy driving it too much.

If it comes to a rebuild though I'll be studying the advice on this forum really carefully so I do the right things.

Wonder how quick I can pull the engine, build it up, and get it back in again?! Winter's the time for projects...