7.5:1 too low???

Well I just walked in from the machine shop and after talking to him about my goals he suggested going 7.5:1 instead of 8.5:1.

A few people on here already know that I am building a 383 and going to be using a t-trim on it and want to basically max that blower out. He feels that over 15 pounds may be too much to run pump gas on 8.5:1 and said if we went to 7.5:1 that we could basically run as much boost as we can throw at it.

He was also considering doing a half-fill on the block for strength and by only doing a half-fill I should not run into any heating problems. Has anyone done this or do you know if it can be done on the LT1? He said he would have to do some more homework regarding the compression and also see if he can fill the block like that.

Partial fill is possible and I have done it. It may not be necessary, certainly not with a T-trim if the block is good. Consider going 0.010" over rather than the usual 0.030" over to preserve cylinder integrity.

My current combo is 7.8:1. With a zero deck block and readily available pistons and heads I don't think you can go lower anyway.

Rich Krause

thanks rich, I already told him to go .10 over he said he would try that first and see how it came out. Also I will probably have him go ahead and fill the block then. The plan is to eventually ditch the T-trim for a t88 or t91.

Remember that if you go the block fill route that you do NOT want the block completely filled. This will (obviously) make it unusable on the street.

Rich Krause

i would reccomend AGAINST the block fill..

Which heads are you planning on running? What size chambers? How will you get the compression that low on an LT1 383? Then again, is it an LT1 or a first gen small block?

I wouldn't go that low on compression.. I ran 17 plus pounds of boost on pump gas with 9.1:1 compression..

Remember, reverse cooled aluminum heads are worth at least a full point over conventional iron heads.. I would say you could run any amount of boost on an LT1 (properly setup) with 8.5:1 compression

You should also fill a block before machining, not after.

I also recommend that you not do this anyway on a street car.

You will sacrifice a lot of power with that low compression unless you plan to run big boost numbers.... like in the 20's. Your machinist is likely relating to roots blowers.

As Brady said 17# w/ 9.1 is OK intercooled

17# is also livable w/ 9.25 intercooled on pump fuel as well if you are careful with the tune.

You will likely need 18# w/ 7.5 to equal the power of 15# at 8.5 comp & your part throttle drivability will suffer as well.


p.s. Brady that big motor keeps trying to climb under the hood of my Camaro. LOL

I disagree with the comments made about "drivability" with a low CR boosted setup. I wonder if the comments are based on experience or heresay? No offense to anyone, but I do not agree that there is much power loss either. Boosted and NA motors are not the same in this regard.

Voice of experience here!

Rich Krause

Originally posted by rskrause
I disagree with the comments made about "drivability" with a low CR boosted setup. I wonder if the comments are based on experience or heresay? No offense to anyone, but I do not agree that there is much power loss either. Boosted and NA motors are not the same in this regard.


Voice of experience here!

Rich Krause

If you have an AIM name hit me up at I8YURLS1 so we can talk further about this. I trust your opinion over most on this board no offense to anyone else.

< 30thCamaroZ from my friends name

7.5:1 to low if asking me, block fill i highly recommend not doing on a street engine, even partial. if you run 15# 9:1 is best 20 psi I would drop it to 8.5-8.0 anything more then that i would go aslow as possible.
I guess rich k. likes playing on the safe side judging by the low cr suggested.

Rich, you've got my interest on this one.. Why would he need compression below 8.5:1? Many people have ran upwards of 20 pounds of boost on pump gas with 9:1 compression. Do you feel there are some gains somewhere (ie: more timing)? I ran 28* total timing with the twin turbo LT1 fwiw.

thanks!

Rich,

While I have no specific part throttle before/after dyno results, I do have experience in this area. I have witnessed better part throttle fuel efficiency & response with higher compression.
Regardless of boosted or N/A you are not cruising down the highway under boost. Low RPM heavy throttle differences may be negligible, but at low speed cruise, it is noticible.

Due to my credibility being at stake, I do not post heresay, but only things that I have actually done or witnessed. I'll leave the rumors to people trying to boost their post count.

I have seen cars go slower at the track after only a compression reduction. One LT1 Camaro in particular required 12# boost w/ 8:1 compression to eventually better the track performance that it had with stock compression & 8# boost. This was the typical "break the stock pistons & replace w/ TRW blower pistons" only to make it slower. Even after trying to reprogram, it needed more boost to regain the lost power.

Compression makes power & as long as you can avoid detonation without retarding timing, you will generally always gain power. I have done many head swaps to increase compression on low compression motors & it always added throttle response & generally required less ignition timing at part throttle. Maybe above a certain point, there are diminishing returns, but on pump fuel I have never had the need to experiment with above 11:1.

It is true that when comparing a 383 to a stock LT1, I have seen the 383 still make more torque even with a bigger cam & lower compression. Have you done any testing with only a compression change that disagrees with my theory? If so, i would like to see them to gain a new perspective.

Obviously running a low compression motor with a high dynamic compression ratio due to short cam timing will help to offset the low compression. Not many people run stock cams in rebulit blower motors though.

I welcome any discussion as maybe we can all learn from this.

My motor is 8.5:1 CR using dished piston and large CC heads. It has a 3200 stall and TH400 with a 9inch and locker in the back.

Driveability:
Using the stock ECM (1993 speed density), I was also to get the O2s within +/- 5% of staying at 128. For the most part I had the VE tables decently setup. You will notice a difference between a stock CR LT1/LS1 over 8.5:1. To me it is not a huge difference but noticable, mostly stop and go kind of driving. I would advise against 8.5:1 or lower if you would be driving the vehicle daily or even 3 out of 5 days.

Part Throttle Acceleration:
Again, comparing to a stock LT1/LS1, you'll part throttle not being as snappy. However, if fueling is setup right it should pull just as hard. You could makeup for a lot of this with a light weight rotating assembly ... but prepare to shell some $$$. In hind sight, having going this low in CR I would have bought some higher end / ligher rotating components. IMO, its worth the cost/investment.

Wide Open Throttle:
The overall motor combination with have an impact here. I'm 8:5.1, SpeedPro Dished Pistons, Scat 4130 rods, stock crank, 64CC AFR 190 heads with intake and exhaust port work, vortech T-trim, custom 4inch intake, ported LT1 intake, 58mm TB, 75lb injectors, mondo bypass, and aftercooler with ice chest.

I'm using the 75LB injectors in conjunction with a FAST system and WB02. As mentioned above with the stock ECU, I was running 42LB injectors. So far with driveability and part throttle I've had a better experience with the FAST.

I'm not expecting a big boost number on the guage but the t-trim should be able to flow most of the air it wants to on my setup. I'll be dynoing with 7/2.5 pulleys on pump gas with a shift point of 6300 rpm whcih maxes out the recomended impeller speed.

Conclusion:

8.5:1 is ok for a weekend warrior / cruising car who likes to bracket race. But spend the money to get the lightweight rotating stuff, its worth it.

9.5:1 is a good compromise if its something you want to drive daily. With the compromise comes higher octane requirements and watching the tuning more carefully.

9.1:1 is another good compromise in the situations above.

10.0:1 I think is something which would hardly be recomended unless it was a race motor being operated by a team effort.

I ended up at 8.5:1 as an afterthought. I planned my buildup around 9.1:1 but got a great deal on the 64CC AFRs. It just means I have a little more flexability with the tuning and overall setup. 7.5:1 is getting into the roots blower category, IMO.

... my thoughts at least ...

< 30thCamaroZ


What if I want to use a T88 or T91? You still think under 8:1 is too low? I found someone who I am choosing not to disclose so that there is no **** talking that is interested in doing a turbo setup for me.

Oh I also have a dual stage to get this thing up and going at the track. :cool:

Originally posted by 2002_TAWS6
What if I want to use a T88 or T91? You still think under 8:1 is too low?

Yes

Lonnie is a smart guy and experienced. I have read his posts for a long time and learned from them. So, none of this personal. Just a discussion of the facts as I see them and my own experiences with a large number of blower cars.

Let's take a hypothetical motor that has a displacement of 700cc/cylinder and a 70cc combustion space. The CR will be ~10:1. Operating NA, even if the VE is 100%, it can't take in more than a 700cc intake charge (yes, I do know that NA VE may exceed 100% but I am simplifying to make a point). If we raise the CR by decreasing the combustion space to 60cc it still takes in the same volume (and mass) of intake charge. The new CR is ~11.7:1. The mass of the intake charge is the same, so there is the same amount of fuel to burn. The combustion efficiency is higher due to the greater compression, and the power is therefore higher. In fact, it will be ~2% higher. But the mass of fuel available to burn is fixed by the displacement and the VE.

How is forced induction different? Well, I am going to have to ignore a lot of fine points, but bear with me. Under boost, the density of the intake charge is higher. And we are no longer limited to 100% VE. In the NA situation, you generallly cannot take in a volume of intake charge > the displacement, as that would mean a VE of >100% Yes, as I stated above, I know that the best NA motors do operate above 100% VE over a narrow rev band, but that's not relevant to the illustration and most street or street/strip motors never see 100% VE at all. So, let's take our hypothetical motor at 11.6:1 CR and put a blower on it. As we know, blower motors operate far above 100% VE. So, we fill the 700cc swept volume PLUS the 60cc combustion space with the intake charge. The intake charge is denser as well. So, the mass of the intake charge is MUCH larger (as is the cylinder pressure just before spark ignition) and with a given level of combustion efficiency, the motor will make a lot more hp. Now, let's drop the CR back to 10:1 by enlarging the combustion space to 70cc. The volume (and mass) of the intake charge is now going to be at least 10/760 = 1.3% greater. The actual mass of the intake charge at 130% VE would be at least 13/760 = 1.7% greater than with the lower CR and that's not even taking the higher charge density into account. Yes, the combustion efficiency will be lower than with a higher CR, but the mass of the intake charge will be greater. In fact, in the ranges we are talking, they nearly cancel out and there is minimal hp loss with decreased CR in a blower application.

So, the way all this works in the dynamic environment of a running engine is a lot more complicated. But the above is the basic theory that supports the observation made by me and by many others: lowering the CR in a boosted motor has much less effect on power output than it does on an NA setup. A very simple way to explain this is that the cylinder pressure just before spark ignition in an NA motor decreases in direct proportion with CR but that in the case of forced induction, the decrease is much less due to the "decoupling" of displacement and the mass of the intake charge.

"Drivability" is subjective. It amuses me to see people worry about "drivability" on a board where nobody thinks there is such a thing as cam that is too big! My point is that lowering the CR by 1 - 1.5 will not make the car unpleasant to drive. The change, even in an NA motor, isn't huge though it's noticible. But if it's got a big blower, you WILL see increased MAP even at very light throttle. This partially compensates for the lower CR, and of course, you can always open the throttle a little more. And a lower CR allows even MORE peak boost. This, in turn, means that you will see more positive MAP at even lighter load/smaller throttle openings due to the pullied up blower.

I have never tested a combo where the only change was lowering the CR. Lonnie's example doesn't blow my argument however. A ~4.5:1 decrease in CR is huge. But even there, it did go faster with enough boost to compensate. We are talking about much smaller differences. So, there isn't a huge difference either way, but if you have a big enough blower and the correct cam choice, more boost and a lower static CR is a better choice to a point. I don't know "how low is too low". But we are running a bunch of pump gas combos with big boost and no intercooling reliably and making good hp with static CR in the ~8:1 range. Race cars that run race gas are a different animal as the boost limit for a given CR is much higher.

In the specific example being discussed. A lot depends on the blower being used. If you are already maxing out your blower, lowering the CR will hurt just as much as it does NA because the mass of the intake charge will be "fixed" by the blower capacity.

So, here's my guidelines for pump fuel.

6psi - stock compression
10psi - ~9.5:1
15psi - ~8.5:1
20psi - ~7.5 - 8.0:1

Rich Krause

Rich,
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.

But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).

If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible. The guys with a M6 will appreciate the low end as it is nice to actually use 6th gear besides at highway speeds. My car will accelerate from 900rpm in 6th (35mph) without hesitation in 6th w/ 3.42's where it was unhappy under 1200rpm when stock.

My main reason for this post was to say do not run less compression than you have to.

To reply to the original posted question..... consider 8.5:1 or higher if you only plan to have 15# boost & you should be happy.

As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.

9.6:1 on the LS1's also work well w/ 12# intercooled.

Originally posted by Lonnie Pavtis
Rich,
You are absolutely correct, without a doubt, that you can make more ultimate power with lower compression & higher boost.

But.... all things being equal, the higher compression will make a better driving car. Also, at lower boost levels (approx 12# & lower), the higher compression car will make more power (assuming that you do not repulley the blower to compensate).

If running a 230 or bigger cam, high stall converter & various other non-daily driver type components, drivability in my opinion is already less than ideal. I know that many view this type of drivability as "acceptable", but compared with the stock vehicle, it is a big step backwards. Most are a tolerable tradeoff for the level of performance the average enthusiast is trying to achieve. Given the opportunity for the best of both worlds, I'll take it when possible.

My main reason for this post was to say do not run less compression than you have to.

As you recommended 8.5:1 for 15#, if non-intercooled, I totally agree. I would say 9:1 at 15# intercooled on a LT1 is very safe. I wouldn't suggest under 8:1 unless planning well over 20#.

9.6:1 on the LS1's also work well w/ 12# intercooled.

Sounds like we are not in disageement then. I agree with your statement that you shouldn't run less compression than you have to. With a small blower, where pulleying it up is not even an option, the equation changes.

Rich

this is all on pump gas, right? what happens when you throw methanol into the mix replacing gasoline?

AdioSS,
You are now not talking about street motors for sure now.

N/A alky motors are usually between 13:1 to above 16:1

Forced induction is generally above 10:1. The evaporation rate of alcohol & the inherent cooling effect produced requires some additional compression & the corresponding heat to work well. Keeping fuel vaporized can be a problem without enough combustion chamber heat.

I have no direct experience w/ running on straight alcohol, but have seen some TA/FC motors with compression in the 10:1 range while running a big screw type charger.

Nice thought if you can afford it. Twice the fuel consumption & all the corrosion problems associated with alky prevent me from experimenting with it.

Well in all reality I am going to use a T-trim because I don't really want to wait on having the turbo setup, though it's not out of the equation. So say I want to run a T-trim on my 383 and I want to make out the blower and this will be intercooled. What compression would you suggest. I want it to make power until it is inefficient. Thanks

8.8:1

I'd suggest more or less that

I was running 9:1 and 10psi... I went down to a bit only 8.8 to gain a bit more top end when i strapped the ys-trim or even max out the t-trim...

The t-trim should max at around 700rwhp... The supercharger is noticeably bigger than a S-trim.. if the s-trim has done from 600-634 (intimid8) i think the T has 100HP more.