30thCamaroZ

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.

rskrause

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

30thCamaroZ

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.

rskrause

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

JordonMusser

i would reccomend AGAINST the block fill..

'68LT1camaro

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?

Brady

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

Lonnie Pavtis

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

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

2002_TAWS6

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

dnz28

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.

Brady

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!

Lonnie Pavtis

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.

[JFS]

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 ...

2002_TAWS6

< 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.