compression ratio

General engine tech -- Drag Racing to Circle Track

Moderator: Team

CGT
Guru
Guru
Posts: 2063
Joined: Sun May 10, 2009 12:29 pm
Location:

Re: compression ratio

Post by CGT »

I agree with the heads being too big for that. Do you have epoxy etc in your vic jr to make it match up? What does the port alignment on that look like? I wouldnt bother with the half point compression myself.
Scotthatch
Pro
Pro
Posts: 311
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: compression ratio

Post by Scotthatch »

F-BIRD'88 wrote: Tue May 22, 2018 2:03 am The SBC victor JR is not a vortec head style intake. Thats why it lacks torque.
Get a "vortec" intake that matches the vortec heads raised intake port.
Change to a RPM air gap dual plane vortec intake. It will make a lot more torque and match the power of the MIS matched vic JR because the port alignment is better. Play with the valve lash a bit.
Looser valve lash shortens the duration. enhances low end torque
Move the cam to a 99deg intake C/L and adjust move >>>retard as needed.
Combine these changes with the compression ratio bump.

For what you are doing he is right the intake is the big problem the air gap should be very close to the height of the VR jr and it will recover better for you ...
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: compression ratio

Post by David Redszus »

mag2555 wrote: Tue May 22, 2018 6:25 am Please remember that that running any more octane then what you need to be safe only serves to cost you power and even slows the engines rev's in the extream!
A common racing folklore. Absolutely not true. Fuel octane alone has not effect on power. Its purpose is to avoid engine destruction.

Two fuels with the same octane can perform quite differently while two fuels with different octanes can perform the same.
88bluestar
Pro
Pro
Posts: 378
Joined: Sun May 21, 2017 7:39 pm
Location:

Re: compression ratio

Post by 88bluestar »

How much HP loss by running the air gap?
I know the Vic is just port matched, thats all.
Scotthatch
Pro
Pro
Posts: 311
Joined: Sat Apr 14, 2018 11:12 pm
Location: 7000 ft up

Re: compression ratio

Post by Scotthatch »

At peak maybe a few but going to be way better below the curve because torque will be better ... on short track you have little use of your peak number you need it to be more of a flat power curve so you can just drive and instead of fight to get up-to your peak..
88bluestar
Pro
Pro
Posts: 378
Joined: Sun May 21, 2017 7:39 pm
Location:

Re: compression ratio

Post by 88bluestar »

If I get an air gap, does it make sense to run a 1" spacer? I gather from previous post that on the duals you want to get a 4 hole spacer, mill the center out on the short side but on the tall side keep the primary and secondary side separated, allowing transition to be easier on the short side so its not going straight at the plenum floor.
F-BIRD'88
Guru
Guru
Posts: 9802
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: compression ratio

Post by F-BIRD'88 »

You will not "give up" power.. You will gain overall useful power, especially as you come off the corners. Faster car acceleration.
englertracing
Guru
Guru
Posts: 1547
Joined: Thu Sep 29, 2011 8:55 am
Location:

Re: compression ratio

Post by englertracing »

Can you raise the cr and advance the cam?
groberts101
Guru
Guru
Posts: 1980
Joined: Sat Mar 08, 2008 3:08 pm
Location: Minneapolis

Re: compression ratio

Post by groberts101 »

F-BIRD'88 wrote: Tue May 22, 2018 1:09 pm You will not "give up" power.. You will gain overall useful power, especially as you come off the corners. Faster car acceleration.
Sure about that? Obviously not a 355" street motor here with a 255 degree cam and the DP manifold has a sweet spot below where this engine will likely torque peak. Not to mention nobody even asked about current gearing and rpm range the engine typically operates in.

IMO, the OP will be down on average power in the 5000-7000 rpm range using a dual over a single plane. So unless the gearing and track length pulls the engine down much below about 4,500 rpm I think a dual plane is a mistake that no amount or type of spacer will ever be able to fix.
Tuner
Guru
Guru
Posts: 3184
Joined: Sun Apr 01, 2007 5:26 am
Location:

Re: compression ratio

Post by Tuner »

How much ignition timing? These heads are "fast burn" and do not need or want as much advance as is commonly used with other Chevy heads.
DrillDawg
Expert
Expert
Posts: 980
Joined: Tue Jul 06, 2010 11:03 am
Location:

Re: compression ratio

Post by DrillDawg »

Use a the rpm dual plane, non air gap, for use with a spread bore carb, mill the divider down to match the air gap, adapter to the holley, 1" open spacer, 1" 4 hole spacer, advance the cam to 106. Should have at least 20ftlbs in the middle and match or make more on the top.
BORN RIGHT THE FIRST TIME
novadude
Guru
Guru
Posts: 1500
Joined: Thu Oct 11, 2007 3:24 pm
Location: Shippensburg, PA

Re: compression ratio

Post by novadude »

Tuner wrote: Tue May 22, 2018 2:51 pm How much ignition timing? These heads are "fast burn" and do not need or want as much advance as is commonly used with other Chevy heads.
Not to de-rail this thread... I think this is on-topic....

In your experience, what total advance generally works best with Vortec / fast burn style chambers? I'm running mine at 33 total, and I guess I've never really tried using less total advance. Maybe I should?
Tuner
Guru
Guru
Posts: 3184
Joined: Sun Apr 01, 2007 5:26 am
Location:

Re: compression ratio

Post by Tuner »

I have found significant power increase with less than 30 degrees, in fact mid 20's at the torque peak. If you can get on a chassis dyno you may find your engine wants the timing to advance from the torque peak to the power peak. Recent experience is a dirt oval track 355, 13.8/1 comp ratio, 100LL w/MMT, GM marine DP spread bore intake that fits the large port head, Qjet carb, GM 140 cam, likes 24 at 4000 with the advance slope to 29 at 7000, the advance will go to 30 at 8000. With this advance setup the torque is flat like a billiard table and the power is still climbing at 7000 but it doesn't need to turn more than that to win. More than 24 at 4000 hurts the torque. All the stuff interwevbzsez don't work, ports are too big, dual plane has no power at high RPM, Qjet is junk, HEI has weak spark above 4000, 140 cam has too much duration and not enough lift, etc. etc. ….... ha... joke's on the intervbez and the rest of the field. They hate the Qjet. When they come over to see what yards them 1-1/2 lengths from the flag to corner entry they just walk off mumbling and shaking their heads. LOL Answer truthfully what cam is in it and get accused of lying. The internet has people's heads so screwed up it is almost impossible to have an intelligent conversation at the race track nowadays. Did you know race engines are supposed to idle all slobbery and stink up the shop and foul plugs if you drive them too much in the pits?
Newold1
Guru
Guru
Posts: 1963
Joined: Fri Apr 08, 2016 9:50 am
Location:

Re: compression ratio

Post by Newold1 »

Tuner =D>

I could not agree more!

In many racing situations the advise and comments always seem to be what a racer and how his engine should be changed to really work and not to pay any attention to things that don't go along with convention but ACTUALLY WORK !!

It must have been built by aliens !! HA!
The Older I Get, The Dumber I Get :wink:
zums
Guru
Guru
Posts: 1355
Joined: Sat Dec 08, 2007 10:57 am
Location: south jersey

Re: compression ratio

Post by zums »

Tuner wrote: Tue May 22, 2018 5:14 pm I have found significant power increase with less than 30 degrees, in fact mid 20's at the torque peak. If you can get on a chassis dyno you may find your engine wants the timing to advance from the torque peak to the power peak. Recent experience is a dirt oval track 355, 13.8/1 comp ratio, 100LL w/MMT, GM marine DP spread bore intake that fits the large port head, Qjet carb, GM 140 cam, likes 24 at 4000 with the advance slope to 29 at 7000, the advance will go to 30 at 8000. With this advance setup the torque is flat like a billiard table and the power is still climbing at 7000 but it doesn't need to turn more than that to win. More than 24 at 4000 hurts the torque. All the stuff interwevbzsez don't work, ports are too big, dual plane has no power at high RPM, Qjet is junk, HEI has weak spark above 4000, 140 cam has too much duration and not enough lift, etc. etc. ….... ha... joke's on the intervbez and the rest of the field. They hate the Qjet. When they come over to see what yards them 1-1/2 lengths from the flag to corner entry they just walk off mumbling and shaking their heads. LOL Answer truthfully what cam is in it and get accused of lying. The internet has people's heads so screwed up it is almost impossible to have an intelligent conversation at the race track nowadays. Did you know race engines are supposed to idle all slobbery and stink up the shop and foul plugs if you drive them too much in the pits?
So your comparing a 13.8 355 to the OPs 9.4 355 with close to the same in, duration @.05
Tom
Post Reply