Highest compression for boost?

[na svt]

flat tops FTW
refer to my pretty red ones in sig pic lol

which is more like 10.7 psi then 10.5

flat tops with a 1.22CD provide a 10.4:1 CR when using a 52cc combustion chamber.

Bore 3.572
Gasket Bore 3.600
Piston TL Dia 3.537
Stroke 3.543
Crank throw 1.772
Con rod Length 5.933
Compression height 1.220
Deck height 8.937
Ring to top piston 0.300
Piston to deck 0.012
Gasket thickness 0.038
Cylinder volume 581.814
Volume above ring 0.961
Valve notches / Dish 0.000
Dome volume 0.000
Piston to deck 2.053
Gasket volume 6.338
Volume of head 52.000
Volume clearance 61.352


Compression ratio 10.483

The math doesn't lie.

 

[RPM4DAZ]

I usually don't post too much because of all the "pros" out there, but there are a couple of things nobody has said much about. The cams he states to have, are 234'/232' @ .050" I believe. That will help to bleed off some of the cylinder pressure when degreed to do so. Yes, he will lose some torque and yes it won't be the " most efficient " formula. But, could be done on 93 octane along with the proper tune. I would stay at or below about 5 PSI. A wider lobe separation and a zero (preferred) to say a +1'-2' advance on cams and a 115-116 l/sep will bleed off some cylinder pressure. Along with a larger tube header say 1 3/4" tubes. This will open the exhaust a bit earlier allow a quicker blow down. Nothing dramatic, but noticeable... Of course, a set of exhaust cams with more duration, say about 8 degrees would have similar results too. Again that's speaking of a centrifugal supercharger as opposed to a roots or turbos. Different dynamics on breathing. Food for thought maybe?

 

[na svt]

A wider lobe separation and a zero (preferred) to say a +1'-2' advance on cams and a 115-116 l/sep will bleed off some cylinder pressure.

He's running a 5.5" short runner so the last thing he needs/wants to do is widen the LSA. IMO, it's not worth going FI if the boost is to be kept at 5psi.

A wider LSA will bleed off cylinder pressure at low RPM but increase it at higher RPMs if the combo is working correctly.

 

[trxcobra]

He's running a 5.5" short runner so the last thing he needs/wants to do is widen the LSA. IMO, it's not worth going FI if the boost is to be kept at 5psi.

A wider LSA will bleed off cylinder pressure at low RPM but increase it at higher RPMs if the combo is working correctly.

I agree, just not worth it for the power gains I would see at ~5psi. I guess if nitrous will be reliable on my combo, that is the route I will go in the next year or so.

 

[SlowSVT]

10.2:1 isn't high for engines with efficient combustions chambers. Also, high compression is not a band-aid, it's necessary and adds power. No street driven mod motor should run low compression (8.5:1) when boosted, it flat kills power and off boost driveability. I've seen people go with low compression and lose 50hp/50tq by doing so. Going with flat tops is just plain smart.

Based on what your saying an NA Cobra engine converted to FI will make more hp than a 8.5:1 Terminator engine running the same octane fuel?

 

[na svt]

Based on what your saying an NA Cobra engine converted to FI will make more hp than a 8.5:1 Terminator engine running the same octane fuel?

yes, given the same cams and heads the n/a engine will make an additional 50rwhp and even more tq.

 

[AntiHeightPunk]

I know its a very rough guess and there are a ton of variables, but how much power at the same boost do you think my car will can going from 93 octane at 8.7:1 802 rwhp to 10.8:1 on E85?

 

[na svt]

i know its a very rough guess and there are a ton of variables, but how much power at the same boost do you think my car will can going from 93 octane at 8.7:1 802 rwhp to 10.8:1 on e85?

75

 

[SlowSVT]

yes, given the same cams and heads the n/a engine will make an additional 50rwhp and even more tq.

Thanks for clearing that one up for me :rollseyes

Not a world of difference between an NA Cobra and a Terminator engine other than the CR yet on pump gas you can boost the Terminator engine typically to 575 hp yet an FI Cobra engine will be in the 425 range.

Why's that?

 

[na svt]

Thanks for clearing that one up for me :rollseyes

Not a world of difference between an NA Cobra and a Terminator engine other than the CR yet on pump gas you can boost the Terminator engine typically to 575 hp yet an FI Cobra engine will be in the 425 range.

Why's that?

Because the n/a engines can't handle the higher psi needed to reach that level. At the the stock 03/04 boost levels the n/a engine with its higher cr will always make more power.

 

[SlowSVT]

Because the n/a engines can't handle the higher psi needed to reach that level.

Your whole argument just crashed against the rocks with this statement

At the the stock 03/04 boost levels the n/a engine with its higher cr will always make more power.

I like how you inserted "at the stock 03/04 boost level".

Why limit the boost level at the factory delivered 8 1/2 pounds when the engine can handle 16-17 lbs on pump gas? You would blow big holes in your pistons trying that on a 10:1 CR engine.

 

[snakeraper11b]

Your whole argument just crashed against the rocks with this statement



I like how you inserted "at the stock 03/04 boost level".

Why limit the boost level at the factory delivered 8 1/2 pounds when the engine can handle 16-17 lbs on pump gas? You would blow big holes in your pistons trying that on a 10:1 CR engine.

You're missing the point being made. Hes talking about literally a stock NA cobra motor, so boost would be limited to stock 03/04 levels before the hypereutectic pistons and powder metal rods come apart. However in those boost levels, the 9.85:1 stock 01 engine will outperform the 8.5:1 03/04 engine...Two identical engines, save for compression, lets say 8.5:1 and 10:1, will make different power, all other variables being the same, the higher compression will outperform the lower compression engine from idle to redline.

 

[SlowSVT]

You're missing the point being made. Hes talking about literally a stock NA cobra motor, so boost would be limited to stock 03/04 levels before the hypereutectic pistons and powder metal rods come apart.

The thread topic is the effect of higher compression in boosted application not ability of the bottom end to handle the load. That's a different thread.

However in those boost levels, the 9.85:1 stock 01 engine will outperform the 8.5:1 03/04 engine...Two identical engines, save for compression, lets say 8.5:1 and 10:1, will make different power

Just putting a smaller pulley on an 03/04 is not a big stretch of the imagination. Why handicap the low compression engine with the boost limit close to the NA engine? The whole point of FI is to make lots of power

all other variables being the same, the higher compression will outperform the lower compression engine from idle to redline. Once you pulley the low compression engine all bets are off!

I agree with this statement :beer:

 

[slow2v]

flat tops with a 1.22CD provide a 10.4:1 CR when using a 52cc combustion chamber.

Bore 3.572
Gasket Bore 3.600
Piston TL Dia 3.537
Stroke 3.543
Crank throw 1.772
Con rod Length 5.933
Compression height 1.220
Deck height 8.937
Ring to top piston 0.300
Piston to deck 0.012
Gasket thickness 0.038
Cylinder volume 581.814
Volume above ring 0.961
Valve notches / Dish 0.000
Dome volume 0.000
Piston to deck 2.053
Gasket volume 6.338
Volume of head 52.000
Volume clearance 61.352


Compression ratio 10.483

The math doesn't lie.

i was told 10.69 by manley, MMR, and Kesatie as confirmed here
i know your technical knowledge is far more vast then mine so maybe im missing something?

 

[na svt]

i was told 10.69 by manley, MMR, and Kesatie as confirmed here
i know your technical knowledge is far more vast then mine so maybe im missing something?

The math doesn't lie. Stock 4v pistons have a 3cc dish and a 9.8:1 cr and decreasing the dish by 3cc does not increase the cr by .9. Find a compression calculator and do the math yourself.

 

[slow2v]

The math doesn't lie. Stock 4v pistons have a 3cc dish and a 9.8:1 cr and decreasing the dish by 3cc does not increase the cr by .9. Find a compression calculator and do the math yourself.

so how would all these companies be wrong with the same exact numbers?

 

[slow2v]

im not saying you're wrong, im just confused lol
also in my case the stock pistons made 8.5 cr

 

[na svt]

Your whole argument just crashed against the rocks with this statement

I like how you inserted "at the stock 03/04 boost level".

Why limit the boost level at the factory delivered 8 1/2 pounds when the engine can handle 16-17 lbs on pump gas? You would blow big holes in your pistons trying that on a 10:1 CR engine.

Acutally I made my point with that argument which was that was higher compression results in more power at the same boost level. Yes, I did insert "at the stock 03/04 boost level" and that was to make the playing ground level. Sure more boost will add power...that's obvious. The problem is that people want to make all their power with boost. Why not start with an engine that makes good n/a power and add a little boost rather than a slug 8.5:1 engine that makes 275rw and throw high boost at it?

Do you have experience wiuth blowing holes in flat tops with 16-17psi? With a good tune a forged flat top will hold up fine. These engines are not the archaic engines of the 60s, they much better combustion chambers, better materials and hellish better engine management systems, 10:1 is absolutely safe with moderate boost.

 

[na svt]

so how would all these companies be wrong with the same exact numbers?

They are using a thinner head gasket and a shorter deck clearance.

 

[slow2v]

They are using a thinner head gasket and a shorter deck clearance.

i told them im using oem gaskets :shrug: