[Mike S]

I run a 10,0 index class. Lately I have been having a tuff time slowing the car. I have 200lbs of ballest and a 1.5 restrictor plate under the 1050. Car has a cage but not certified nor do I have a license to go faster. Last fall I ran it all out to the 1/8 with no weight or plate and ran a 6.04 with a 1,28 60' thru the mufflers.
My question is if I lower the compression from 12-1 to 10.5-1 How much will the car slow keeping everything the same? Cam might be to big with the lower compression. I would like to keep running 10.0's in the summer.

I was thinking of getting new heads with a bigger chambers and a little more flow. Right now heads are 72 cc and flow 330. Looking at 90 cc chambers and 340 flow.

[tom s]

What about a throttle stop to help you be where you want to be?Has to be cheaper than other options?Tom

[Stan Weiss]

Originally Posted by Mike S

I run a 10,0 index class. Lately I have been having a tuff time slowing the car. I have 200lbs of ballest and a 1.5 restrictor plate under the 1050. Car has a cage but not certified nor do I have a license to go faster. Last fall I ran it all out to the 1/8 with no weight or plate and ran a 6.04 with a 1,28 60' thru the mufflers.
My question is if I lower the compression from 12-1 to 10.5-1 How much will the car slow keeping everything the same? Cam might be to big with the lower compression. I would like to keep running 10.0's in the summer.

I was thinking of getting new heads with a bigger chambers and a little more flow. Right now heads are 72 cc and flow 330. Looking at 90 cc chambers and 340 flow.

Is it legal for you to run one of the MSD boxes where you can pull timing as needed? This way you still have all of your HP when needed.

Stan

[Mike S]

I should have been a little clearer. I am tired of the price of race fuel and would like to drive it more on the street more, without worrying about finding race fuel.
I would also like to do some pump gas races.
I was just wondering how much lowing the compression would slow it up.

[tom s]

So its really about being able to run 93 gas and not slowing it down.Going with more flo could make up for the loss of CR but you could always run the mentioned throttle stop.And as I learned from Joe Sherman the better the head less cam is needed to reach a target HP.Tom

[AG]

I run 93 pump in my race car that runs 10.0 in the 1/4 at 11.2 static CR. I think you are making more power than me, so you shouldn't take a big hit lowering your CR.

[R 70 Judge]

My old engine builder told me it was roughly 20hp per point of compression but that can vary. Very general rule of thumb

[aaronman]

From past experience I would say a 1.5 point drop in compression is 30-40 hp assuming no other changes. You could easily get this power back with improved heads, cam, intake and carb.

[turbo69bird]

Originally Posted by AG

I run 93 pump in my race car that runs 10.0 in the 1/4 at 11.2 static CR. I think you are making more power than me, so you shouldn't take a big hit lowering your CR.

I think dropping down to 10.0 you’ll lose (or gain)! .5 in er. Another .5 from 10.0 to 9.5 and gets really soggy lower than that .

Just what I’ve seen lowering compression for turbo set ups.

[blueghoast]

All my motors are pump gas engines, Any where from 10:0:1 to just
under 12:0:1. I don't think you would slow that much if your going to
go with more head flow. I drive mine on the street all the time.
Rule of refererance is below 200 lbs cranking compression your safe
for pump gas JMO.

GT

[25stevem]

Here's how the HP would change in terms of %, but depending on the Cam your running and where it makes peak torque, drivability may suffer once a certain level of cylinder pressure is gone.

[J.C.you]

Have u tried to run 93 the way it is?

[Mike S]

I did run it on 93 to load in the trailer a few times. Never tried to make a pass down the track with 93. Afraid of hurting parts with it. I have been using VP c12 but at $105 for 5 gallons is getting pricey. My cranking compression is 185psi. I am running the early 72cc E-heads with 36 degrees of timing on a stock block 455 .060 over. Compression works out to be right at 12-1.
What could be the worst to happen if I tried to make a pass on 93.

[Stan Weiss]

Originally Posted by Mike S

I did run it on 93 to load in the trailer a few times. Never tried to make a pass down the track with 93. Afraid of hurting parts with it. I have been using VP c12 but at $105 for 5 gallons is getting pricey. My cranking compression is 185psi. I am running the early 72cc E-heads with 36 degrees of timing on a stock block 455 .060 over. Compression works out to be right at 12-1.
What could be the worst to happen if I tried to make a pass on 93.

12:1 CR with a cranking compression of 185 psi seems like a pretty big cam. I get IVC at about 88 ABDC. Have you tried driving it on the street on 93 octane?

Stan

[turbo69bird]

Originally Posted by Mike S

I did run it on 93 to load in the trailer a few times. Never tried to make a pass down the track with 93. Afraid of hurting parts with it. I have been using VP c12 but at $105 for 5 gallons is getting pricey. My cranking compression is 185psi. I am running the early 72cc E-heads with 36 degrees of timing on a stock block 455 .060 over. Compression works out to be right at 12-1.
What could be the worst to happen if I tried to make a pass on 93.

How about running 100 LL from
your local airport? That’s what I do
If I want to lean on the tune a little bit, But I’m a pilot so it’s easy to get it.

You could knock the timing back fatten it way up, make some passes on pump gas and slowly sneak up on the tune on pump gas
So your testing is safe.

[25stevem]

I had a water injection system on my blower motor that when in use from half throttle on up allowed me to run 3 to 4 more psi of boost with a 9.2 comp 455 depending on the air temp of that particular day.

Why don’t you try adding one of these first since it’s a whole bunch cheaper then reworking the heads or the short block.

It also helps to keep carbon build up at bay.

You can even cut the water with Alky and get even a greater cooling effect.

Snow performance has a great kit for under $400.

[turbo69bird]

Originally Posted by 25stevem

I had a water injection system on my blower motor that when in use from half throttle on up allowed me to run 3 to 4 more psi of boost with a 9.2 comp 455 depending on the air temp of that particular day.

Why don’t you try adding one of these first since it’s a whole bunch cheaper then reworking the heads or the short block.

It also helps to keep carbon build up at bay.

You can even cut the water with Alky and get even a greater cooling effect.

Snow performance has a great kit for under $400.

That’s a great idea I used to run water / methanol
Injection on my turbo car I think of it as a boost thing but the Air Force used it on NA engines in WWII so no reason it couldn’t be used . Works really well. Great idea.

You can even set it up w just a universal washer pump for a cheap set up it’s pretty easy and CHEAP, mine ran off a Hobbs switch so when it saw boost it turned on,, but could use a throttle position type switch easy enough.

[Johnny406]

Mike, do you have pump E-85 where you live/race? I switched over last year and minus the PITA getting it to start when cold, it's been a great change.

[KEN CROCIE]

HP loss vs C/R chart. H-O published it in the TD-02 manual. I believe john wallace has the chart on his site