compression ratio on pump gas

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Scamper

Scamper

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I have to make a decision quick, and I am told to run flat tops with r/t heads.
if I run 10.5 comp I will make better use of the quench vrs semi dished and 9.8 comp which may hurt my ability to run pump gas.
Need all the help I can get as I have to make a decision quick.:banghead:
 
What is the cam size and intended gear ratio?
 
Quench isn't something you just throw together and go. It has to be measured and actually blueprinted. In other words, parts have to be modified accross the board to acheive proper quench. Since you're talking about measurements in thousandths of an inch, each cylinder will be a little different and need to be addressed accordingly. It's not cheap. If it's not in your budget, you'd be better off building the lower compression motor. If you simply try to sling a motor together you think is quench, you might end up with a useless motor that will only run well on race gas.
 
cam is an xe268h and gear ratio is 4.10 and stall is 2400.
To be safe then am I better to drop forged all together and run stock pistons for a 9.3 comp ratio? I have no room left in budget for any more mistakes. I must end up being able to run on pump gas 91
 
Run the lower ratio. I also suggest looking at a thicker gasket than what is a normal unit in a kit. 9.8-1 and that cam is a area I would call dicey on 91 octane. Fel-Pro makes a .051 thick gasket.

I myself would look for a max of 9.5-1.
 
10.5 was mentioned in a recent Ford TV commercial. The engineers have only recently figured out how to run higher compression and get more hp than cid from their production engines. They're using electronics to control the throttle responce ( rpm ramp ). skew the fuel/air ratio, the valve timing, and the spark timing to efficiently burn todays puppy pee pump gas.
Those engines are supposed to last at least as long as it takes to pay off the loan too. Will the engine last that long ? remains to be seen.
Can we do the same with naturally asperated old school engines ? I dont think so.
 
What Scamper is trying to do is have an engine like mine or Dave Clements. LA 360 with magnum heads. Dave and I both have 10.58 to 1 compression and can run 89 octane. I prefer to run 91 just to be on the safe side. Scamper is using the same cam and rear ratio as I am.
I gave him some advice on his piston choice. It was my opinion and it works for me. I also gave him some advice that I think was more important and that was to talk to Dave or moper about it.

I run KB107's that are zero decked with a .045 thick head gasket. My personal opinion is that he doesn't need the forged pistons he is looking at and a set of hyper's would work perfectly good at a fraction of the cost.

Seeing as he is having this engine put together at a shop and was planning to have the block zero decked then he should be o.k.

I was unsure what the lower compression would do to the combustion chamber shape. I thought it may change it enough that he would end up with a car that wouldn't run on pump gas. That is why he is asking the question here.

Jack
 
We need to know what dish piston he's talking about though..

If its a d dish...then it has the flat top on the quench/closed chambered side of the head and will work just fine.

Along with proper quench....Its carefully modifying the timing curve, gears/car weight 'basically tuning' that will allow for 10.5 comp with int early closing and pump gas.

But that cam closes at 60*!!!!
It shouldnt be too hard to make it work on pump gas at all.imo
 
I will be running 10.2 compression on 91 with RHS magnum heads, BUT my dynamic compression will be from 8.7-8.9 which most say is race gas or 93, but Brian at IMM said it will work because of the custom cam I will be running from him i think it was 251-260 @50 in a 410 stroker.
 
Fst73, IDK if the OE poster is going to run a cam that big. Being that a big cam will help ......
 
I think I am going to rebuild motor with stock pistons and add my r/t heads for now should be about 9.3:1. I cannot afford anymore mistakes
 
rumblefish360 said:
Fst73, IDK if the OE poster is going to run a cam that big. Being that a big cam will help ......
Click to expand...

True. Was just saying that bigger cam can help depending on compression. If he wants to be sure of running 89-91 Xe274 with magnum heads and 10.7 compression with kb107 worked great for me and many others!

plus I think my Abdc is 53.5 to make 8.8-8.9.


Dont worry about being confused. I learned by reading and bugging all of these guys!
 
9.8-1 is doable. I had a 360 with a zero deck piston (KB-107's) and J heads. The cams were a bit healthy. Smallest being a 292 purple cam. Not exactly small in duration.

If your cam is not going to be above 236 @ 050, use the lower compression figures.

Figure 1 point of compression is about 3% more (or less if lower) power in a street build of fair stoutness. I would rather run a lower ratio and be down on power (Big whoop! 3% down) than run something to high and have pump gas issues and have to retard the timing to get it to run.....
 
rumblefish360 said:
9.8-1 is doable. I had a 360 with a zero deck piston (KB-107's) and J heads. The cams were a bit healthy. Smallest being a 292 purple cam. Not exactly small in duration.

If your cam is not going to be above 236 @ 050, use the lower compression figures.

Figure 1 point of compression is about 3% more (or less if lower) power in a street build of fair stoutness. I would rather run a lower ratio and be down on power (Big whoop! 3% down) than run something to high and have pump gas issues and have to retard the timing to get it to run.....
Click to expand...

Another thing to look at is if you want to race it, retarding the timing will be a temp., fix for the street and then fix timing at track with better gas and instant power! Basically what I might do if my setup does not end up working on 91.
 
To the OP, you'll get it right if you turn everybody else of like a switch and just do this:


rumblefish360 said:
9.8-1 is doable. I had a 360 with a zero deck piston (KB-107's) and J heads. The cams were a bit healthy. Smallest being a 292 purple cam. Not exactly small in duration.

If your cam is not going to be above 236 @ 050, use the lower compression figures.

Figure 1 point of compression is about 3% more (or less if lower) power in a street build of fair stoutness. I would rather run a lower ratio and be down on power (Big whoop! 3% down) than run something to high and have pump gas issues and have to retard the timing to get it to run.....
Click to expand...
 
I was the one who originally recommended the 1st option with higher compression/flat-top pistons. Like my sig says I have a 10.5:1 360 with iron Magnum heads and the smallest Lunati Voodoo cam, and it runs great on mid-grade pump gas. I built this engine all by myself, and it was the first engine I built from the "block-up". If you take your time and buy a couple special tools (dial indicator is the main one) you can build an engine like this with not too much trouble.

My block had been previously decked and was already at spec for zero deck. I used OOTB KB-107 pistons and untouched factory iron 5.9L replacement Magnum heads with stock rocker gear, also stock crank and rods. I cc'd the heads and made sure the chambers matched, then used a Dremel grinder to round off the edges in the chambers and valve reliefs on the piston faces. I used off-the-shelf Mr. Gasket .039" head gaskets and when all was said and done I ended up with about .043" quench checked with both a dial indicator and modeling clay+depth calipers.

I'm at 5500 ft. above sea level and the timing is set to 15 initial, 34 final all in by ~2500 RPM and it runs with no pinging on 87-octane (mid-grade here). Might sound too good to be true for some, but I should have a few videos up here soon to show how responsive and smooth it is (not to mention decent on gas considering the power).

EDIT: I believe the OP is running RHS (or similar) heads which are definitely closed-chamber...
 
MOPEkidD-3 said:
I was the one who originally recommended the 1st option with higher compression/flat-top pistons. Like my sig says I have a 10.5:1 360 with iron Magnum heads and the smallest Lunati Voodoo cam, and it runs great on mid-grade pump gas. I built this engine all by myself, and it was the first engine I built from the "block-up". If you take your time and buy a couple special tools (dial indicator is the main one) you can build an engine like this with not too much trouble.

My block had been previously decked and was already at spec for zero deck. I used OOTB KB-107 pistons and untouched factory iron 5.9L replacement Magnum heads with stock rocker gear, also stock crank and rods. I cc'd the heads and made sure the chambers matched, then used a Dremel grinder to round off the edges in the chambers and valve reliefs on the piston faces. I used off-the-shelf Mr. Gasket .039" head gaskets and when all was said and done I ended up with about .043" quench checked with both a dial indicator and modeling clay+depth calipers.

I'm at 5500 ft. above sea level and the timing is set to 15 initial, 34 final all in by ~2500 RPM and it runs with no pinging on 87-octane (mid-grade here). Might sound too good to be true for some, but I should have a few videos up here soon to show how responsive and smooth it is (not to mention decent on gas considering the power).

EDIT: I believe the OP is running RHS (or similar) heads which are definitely closed-chamber...
Click to expand...

Lucky that you are higher up also.
 
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